Biological Significance of Sex

Types of Reproduction

Reproduction is a basic characteristic of all forms of life. There are two types: sexual and asexual. In sexual reproduction new individuals come about through the union of special sex cells known as gametes. It takes place in almost all plants and animals. Asexual reproduction does not involve gametes. In most one-cell microorganisms, for example, two individuals will be produced as a result of the splitting of one cell. Asexual reproduction is also involved when cuttings of stems of a plant are induced to produce roots. In asexual reproduction the new individuals are genetically identical to the parents. As we will see, this is not the case when sexual reproduction takes place.

Sexual Reproduction

Sexual reproduction follows the same basic pattern in almost all animals (Figure 1). A new generation starts when a sperm cell, the male gamete, unites with an egg, the female gamete. The joining of the male and female gametes is known as fertilization. The outcome of fertilization is the formation of a single cell, the zygote, the first cell of the new individual.

All types of cells, body cells as well as gametes, contain a set of chromosomes. These chromosomes are threadlike strands which contain genes, the units responsible for the determination of inherited traits.

With few exceptions, all forms of life have a characteristic number of chromosomes in each cell. For example, fruit flies have 8, frogs 26, rose plants 70, the California coast live oak 24, and humans 46. Chromosomes are found in pairs; therefore, roses have 35 pairs and humans 23 pairs. The two members of each pair are morphologically identical and are called homologous chromosomes. Differences in size and shape may be observed between the different pairs.

Mitosis

Cells are constantly dividing not only in the rapidly developing embryo but in every mature organism as well. Body cells divide by a process known as mitosis. During mitosis each chromosome will first duplicate forming two identical copies (Figure 2). The cell will then gradually divide in half forming two cells. Each new cell will contain one of the two duplicates which were initially formed from every chromosome. As a result two identical daughter cells are formed from a single mother cell. For this reason every cell of the human body contains 23 pairs of chromosomes for a total of 46. This double or paired number of chromosomes in body cells is referred to as the diploid or 2n condition.

Figure 1. Generalized life cycle of animals.

Figure 2. Schematic representation of the process of mitosis in a cell containing only two pairs of chromosomes.

Meiosis

The male and female gametes contain only half the number of chromosomes present in each parent. In the case of humans each sperm and egg contains 23 chromosomes. During fertilization 23 chromosomes from the mother will be combined with 23 chromosomes from the father. The new generation will then have a total of 46 chromosomes (23 pairs) combining genetic information from both parents. One member of each of the 23 pairs came from the mother and the other from the father.

Formation of gametes must then include some type of mechanism for reducing the number of chromosomes in half. This special type of cell division is called meiosis. It takes place only in the testes and ovaries during the formation of gametes. The most significant events taking place during meiosis and the differences between meiosis and mitosis are summarized in Figure 3.

Figure 3. Schematic representation of the process of meiosis in a cell containing only two pairs of chromosomes.

The most important differences are readily apparent. In mitosis two identical, diploid cells result from the division of one mother cell. In contrast, meiosis results in the production or four cells, each cell containing an unpaired number of chromosomes. This condition, half the normal (diploid) number of chromosomes, is known as the haploid or n condition.

During the production of eggs in the female, only one of the four cells will mature into an egg. The other three will develop into polar bodies which eventually disintegrate. During meiosis, four sperm cells are always produced from each mother cell in the testes of the male.

Sexual Reproduction and Variation

The biological significance of sex is that it accomplishes reproduction with variation. No two individuals resulting from separate fertilizations are genetically identical. This can be easily recognized by observing the behavior of chromosomes during meiosis (Figure 3).

Only one chromosome from each of the 23 pairs may pass into a gamete. Each pair of chromosomes consists of two morphologically similar but genetically different homologous chromosomes. This explained by the fact that one chromosome originally came from the mother and the other from the father. In Figure 3 this genetic difference is represented by using black and white chromosomes. The two pairs of chromosomes used in the example are differentiated by size. Only chance rules which of the two homologous chromosomes of a pair will enter a given gamete. Since each pair of chromosomes acts independently from each other pair, different types of gametes will always be produced by one individual. Chromosomes are therefore said to undergo independent assortment during meiosis.

The total number of possible chromosomal combinations in the gametes is equal to 2ⁿ, where n is equal to number of pairs of chromosomes. In the case of our hypothetical example (a cell with only two pairs of chromosomes) the total number of possible combinations of chromosomes in the gametes is equal to four (2² or 2 x 2 = 4). In the case of humans 2²³ (two times itself 23 times) is equal to 8,388,608. Each one of us has the potential of producing more than eight million combinations of chromosomes in the production of gametes.

The second important source of variation is the chance involved in the combination of one of the millions of sperm cells present in each ejaculation with a single egg during fertilization. The possible number of chromosomal combinations at fertilization is equal to the product of the possible number of different kinds of sperms and eggs that could combine: 8,388,608 x 8,388,608 or 70,368,744,177,664 in the case of humans. In other words, a couple can theoretically produce more than 70 trillion genetically different children! A more realistic way of expressing it is to say that chances of having two genetically identical children as a consequence of separate fertilizations is less than 1 to 70 trillion.

Additional sources of variation result from the fact that a great variety of gene combinations can be present in a particular chromosome. Changes in the genes (mutations) can also take place. These, as well as errors that may take place during meiosis (additional chromosomes, exchange of parts of chromosomes, and even the missing of sections or whole chromosomes), are usually deleterious and result in abortions, stillbirths, or birth defects.

Other terms discussed in lecture: 
    karyotype
    Down's syndrome (trisomy 21)
    pre-natal diagnosis
        amniocentesis
        chorionic-villi sampling (CVS)
    cloning
    stem cells

 
Human Heredity

The similarities and differences between parents and their offspring have been a source of much attention and speculation through the ages. It was once thought that inherited characteristics were transmitted to a new generation through the "blending of blood" from both parents. We now know that equal numbers of chromosomes from each parent are combined during fertilization. These chromosomes carry the units of inheritance called the genes. The maintenance of variation in the genetic material is the essential function of sexual reproduction. This is brought about by the random (independent) assortment of chromosomes during meiosis as well as by the chance fusion of gametes during fertilization.

Our understanding of the mechanisms of inheritance was initiated by the painstaking research of Gregor Johann Mendel (1822-1884). Mendel was an Augustinian monk born of German parentage in Silesia, now part of the Czeck Republic. Most of his life was spent in a monastery where he conducted breeding experiments with bees, mice, and a wide variety of plants. It was his work with garden pea plants, however, that eventually set the basis for genetics, the branch of biology which studies inheritance. In these experiments Mendel carefully analyzed the inheritance of seven pairs of contrasting and mutually exclusive characteristics; for example, the color of peas (yellow or green), the height of plants (tall or dwarf), and the shape of pods (inflated or constricted). Mendel clearly showed that each of these characteristics was transmitted to the offspring by distinct and independent "factors," later to be known as genes, derived from both parents. The appearance of these factors in new generations was found to follow simple mathematical predictions. Mendel's work was presented in a relatively obscure scientific publication in 1866 but it was not until 1900 (sixteen years after his death) that the significance of his work was understood.

Chromosomes, Genes, and Inheritance

Body cells are characterized by having a constant number of paired chromosomes. The two chromosomes of a pair can be identified under the microscope in specially stained preparations. They are known as homologous chromosomes. In humans there are 23 pairs of homologous chromosomes for a total of 46 chromosomes. You may recall that during meiosis and two members of a chromosome pair separate but only one of the two chromosomes enters each gamete (see Figure 3).

Genes can be best visualized at this time as specific spots or locations (loci) on chromosomes. Each chromosome contains thousands of genes. Although genes cannot be seen we know they are on the chromosomes because both behave in the same way. Like chromosomes, genes separate by chance during meiosis, something Mendel deduced without even knowing that chromosomes existed. Genes also occur in pairs, one on each homologous chromosome and each from a different parent.

Each gene determines or influences a particular character in an organism. Each individual then carries at least one pair of genes for each character. For convenience, pairs of genes are represented by a letter or symbol. Both genes may be the same in some individuals (a condition which we may represent as AA or aa or BB, for example) and different in others (Aa or Bb). Each of the two members of a pair of genes is referred to as an allele. In other words, an allele is one of the two alternative forms of a gene. In an individual carrying a pair of genes Aa, allele A is located on one of the two homologous chromosomes of a given pair and the allele a is on the other chromosome as shown below.

Alleles separate by chance during meiosis. The offspring will then receive one allele from each parent.

Albinism, a condition in which normal body pigmentation is absent, is known to be controlled by one pair of genes. In the case of albinism we can represent the two alleles of the pair as A and a. Three combinations are then possible: AA, Aa, and aa. One of these gene combinations is present in every human being. Individuals with two identical alleles (AA or aa) are said to be homozygous; those in which the two alleles are different (Aa) are heterozygous. When in the heterozygous condition one allele masks or prevents the expression of the other, it is called the dominant allele. The allele which is not expressed is called recessive. For convenience dominant alleles are represented by capital letters and recessive alleles by lower case letters. Albinism is a recessive condition. The dominant characteristic (normal pigmentation) will be expressed in individuals with the AA or Aa alleles; albinos can only have aa alleles. therefore, allele A dominates over allele a.

The specific combination of genes in an individual (in other word, its genetic constitution) is knows as the genotype. The expression of this genotype (in our example, being albino or having normal pigmentation) is known as the phenotype. Thus, in the inheritance of albinism there are three possible genotypes (AA, Aa, and aa), but only two possible phenotypes (albinism and normal pigmentation).

Albinism

The absence of pigmentation in the skin, hair, and eyes in albinos is the result of a deficiency in the manufacture of pigment (melanin) by the body. Albinism is a metabolic disorder resulting from the absence or inactivity of a specific enzyme. Enzymes are complex compounds which act as catalytic agents or mediators of chemical changes in living forms. This enzyme is involved in the formation of melanin. The condition is not restricted to humans and it has been found in many animals: snakes, salamanders, gorillas, rats, mice, Easter bunnies, and even ravens, to name a few.

Human albinos are characterized by white translucent skin and white hair. Because of the lack of pigment in the iris, the eyes are red due to blood vessels. There is no way to overcome albinism. The necessary information to produce the right enzyme has not been inherited and will never be acquired. The specific enzyme or the melanin pigment would have to be injected continuously into each and every pigment cell of the skin, scalp, and eyes in order to produce a normally pigmented individual.

Albinos need continuous protection from sunlight since they burn very easily. Their skin cannot develop a suntan since tans are nothing more than the accumulation of melanin as a response to an increase in the ultraviolet radiation of sunlight. They are also more susceptible to skin diseases and tend to have poor vision. They are otherwise perfectly normal people.

The condition is distributed through all human populations irrespective of normal skin color. Like other inherited traits and disorders the incidence of albinism is higher in isolated populations. Inbreeding, the mating of closely related individuals, increases the chances of "bringing out" recessive genes. The Cuna or San Blas Indians, the inhabitants of a group of small islands off the coast of eastern Panama, is a classical example. While the worldwide incidence of albinism has been estimated to be around one person in every 20,000 individuals, in the Cunas it is approximately one person in 140. In the United States it is about one person in 10,000. The incidence of heterozygous individuals is much higher.

Several hereditary conditions involving partial albinism have been described. The inheritance patterns involved in most of these cases are different from the total or classical albinism used in our example (see Table 1).

  TABLE 1 List of Selected Inherited Disorders in Humans  

* Most common of about ten known types of muscular dystrophies.

Crosses Involving One Characteristic

The probability parents have of passing a specific gene to their offspring can be predicted. Such a prediction can be made only after knowing the genotype (for the gene in question) of both parents. A person may be normal but heterozygous for a give condition, in other words, a "carrier." In most cases it is not yet possible to use diagnostic tests to detect a harmful gene in a heterozygous condition. We have to rely on a close examination of our family tree. Unfortunately, many normal couples discover they are carriers of a bad gene only after the birth of a child suffering from a hereditary disease.

Figures 4 and 5 summarize a hypothetical example in which the inheritance of albinism is followed through three generations. All of the children of a normally pigmented man (assumed to be homozygous dominant, AA) and an albino woman (homozygous recessive, aa) will be normally pigmented but heterozygous for albinism (Aa). The children of this first generation (first filial generation of F₁ for short) will be "carriers" of albinism, that is, they will all carry the albino allele (a). Albino children could have been born if the father were heterozygous for albinism (Aa).

Figure 4. Inheritance of albinism in humans: a cross between a homozygous, normal pigmented man and an albino woman.

When a son of this couple marries a normally pigmented woman who happens to be heterozygous for albinism (one of her parents was an albino) all three possible genotypes may occur among their offspring (Figure 4). A square (Punnet square) is used to determine the different genotypes and phenotypes that are possible in the F₂ (second filial generation). In this square all of the possible types of sperm and eggs in the parents are paired in order to predict all possible genotypes. In our example the genotypes show a theoretical ratio of 1 AA:2 Aa:1 aa. Therefore, the probability or chance that in this couple an a sperm will fertilize an a egg to produce an albino baby is one out of four (1/4) or three normally pigmented babies for every albino baby born (3:1).

Figure 5. Inheritance of albinism in humans: a cross between two heterozygous, normally pigmented individuals.

We can also say that the laws of inheritance tell us that about 25% of the babies expected by our couple will be albinos. The larger the number of offspring the closer we get to the predicted theoretical ratio. Ours is not the best species to carry out experimental work in genetics. Not only is the number of offspring limited but we cannot order two people to get together and have 25 babies just to see how many are born with albinism, hemophilia, or polydactyly. Nevertheless, the inheritance of albinism in humans (and in other animals where it has been investigated) follows the same pattern over and over.

The inheritance of many other traits also follows the same principles discovered by Mendel over a hundred years ago. As in albinism these characteristics: (1) are determined by a pair of genes consisting of two alleles, (2) there are three possible genotypes (AA, Aa, and aa), and (3) one of the alleles is always dominant over the other. This type of inheritance pattern is often called simple Mendelian inheritance because it follows the basic Mendelian principles outlines above.

This type of inheritance is responsible for cystic fibrosis of the pancreas, a serious cause of infant mortality in the United States. In some groups of people it appears in rates estimated as high as one in every 500 babies born. Babies suffering from cystic fibrosis may die early in life as a consequence of lung or intestinal obstruction due to an excessive production of mucus.

Like in albinism, the gene responsible for cystic fibrosis is recessive. Afflicted babies may be born to couples in which both individuals are normal but heterozygous for the gene. It has been estimated that there are at least five million heterozygous carriers in this country, so the chances of two heterozygous parents are not as remote as they might seem. Let us take a look at another hypothetical example.

A newly married couple is worried about their chances of having a baby suffering from cystic fibrosis. Both are normal for the condition but her baby brother died from the disease. We cannot tell their genotypes for cystic fibrosis but let us assume that she is heterozygous (Cc) and her husband is homozygous dominant (CC). What then are their chances of having an afflicted baby? We can predict that they do not have to worry (Figure 6).

Figure 6. Inheritance of cystic fibrosis of the pancreas in humans: a cross between a normal woman, heterozygous for cystic fibrosis, and a normal, homozygous man..

All of their children are expected to be normal, although there is a 50% chance (1:2 or two out of a total of four) of having a child that may carry the disease to future generations. What would happen if the husband were also heterozygous (Figure 7)? As expected, there is a one in four chance of their having a baby afflicted with cystic fibrosis. The expected phenotypic ratio is then 3:1 (three normal babies for every one with the disease). In simple Mendelian inheritance a 3:1 phenotypic ratio is always predicted when crossing two heterozygous individuals.

Figure 7. Inheritance of cystic fibrosis of the pancreas in humans: a cross between two individuals heterozygous for the disease.

Simple Mendelian Inheritance and Other Congenital Disorders

Besides albinism and cystic fibrosis of the pancreas simple Mendelian inheritance is responsible for disorders such as congenital cretinism, galactosemia, phenylketonuria (PKU), and Tay-Sachs disease (see Table 1). They are all recessively inherited. These and other hereditary conditions are commonly known as congenital diseases or birth defects.

The expression of the recessive genes is so drastic in some disorders that death results. These genes are known as lethal genes. Their lethal effects vary according to the organs or systems impaired. In some conditions death occurs during prenatal development (which may lead to the spontaneous abortion of the embryo or fetus) while in others death follows shortly after birth. Still other lethal genes are not manifested until later in life. Perhaps the most dramatic example is the case of Huntington's chorea or Huntington's disease.

A dominant gene is responsible for Huntington's chorea. Babies inheriting two dominant and lethal alleles (HH) die; normal individuals carry two recessive alleles (hh). The disease itself appears in heterozygous individuals (Hh). They develop normally but the symptoms that signal the inevitable and deadly degeneration of the brain begin after maturity. The dominant allele appears to control the production of a substance which causes the degeneration of brain cells. The gradual destruction of the brain is not usually noticed until after maturity when the growth and repair of brain cells actually stops. The mental disorders that develop will appear in almost all cases between 25 and 45 years of age, with an average of 35. The victims are sometimes diagnosed as suffering from insanity and are committed to insane asylums. There is no cure and the victims slowly deteriorate and die.

Genes, however, are not directly responsible for all birth defects. Brain damage during childbirth (a cause of congenital cerebral palsy), viral diseases (German measles, for example), and drugs taken by the expectant mother (such as thalidomide, a sleeping pill used from 1957 to 1961) are three examples of nongenetic causes of birth defects. Genes are indirectly involved in mother-fetal blood incompatibilities such as Rh disease.

Dominant genes are responsible for many other congenital disorders. All normal individuals are homozygous for the recessive gene. Individuals suffering from the condition are either heterozygous or homozygous for the dominant, defective gene. Achondroplasia, a form of dwarfism, is a well known example. The birth of an achondroplastic dwarf (Aa or AA) to normal parents (aa) is the result of a mutation, a change of a gene from one allele to another. Although mutations are rare, they usually have harmful effects. Other examples of dominant conditions are anonychia (absence of some or all nails), Ehlers-Danlos syndrome (individuals with very loose joints and highly elastic skin), Marfan syndrome (very long bones), and retinoblastoma (a malignant tumor in the retina of the eye).

Simple Mendelian Inheritance and Morphological Characters

Several morphological features appear to be controlled by a single pair of genes. For example, a cleft chin, ears with a free lower lobe, and the ability to curl the tip of the tongue are dominant characters. Red hair (natural, of course) is recessive to other hair colors.

Crosses Involving Two Characteristics (Not covered in BIO 110)

So far we have traced the inheritance of single traits only. In actual cases, however, thousands of genes affecting a multitude of traits are passed to every new generation. The same basic principles that we applied in crosses involving albinism or cystic fibrosis are also applied when following the inheritance of two (or more) characteristics at the same time.

Let us examine a hypothetical example in which albinism and the Rh blood type are traced through three generations. The Rh factor is one of several blood types of groups known in humans. Individuals having a particular protein in their red blood cells are known as "Rh-positive" (Rh+); those lacking the Rh protein are known as "Rh-negative" (Rh-). The Rh factor is involved in a serious type of mother-fetal blood incompatibility. The possession of the Rh protein is a dominant characteristic (RR or Rr), and its absence in Rh-negative individuals is recessive (rr). Although it has been found that more than two alleles may be involved, the simple Mendelian mechanism given here holds true in almost all cases.

The parental generation of our example involves an albino, Rh- man (Figure 8). His genotype is represented as aarr. The woman has normal pigmentation and has Rh+ blood. She is assumed to be homozygous for both characteristics, hence her genotype is AARR.

There is no evidence of any association between albinism and Rh blood type. The genes affecting these two characteristics are independent of each other and (as far as we know) are in separate chromosome pairs. Therefore, in the formation of gametes in the parental generation, the alleles for both pairs of genes will separate independently of each other. All children belonging to this couple (the F₁) are therefore expected to be heterozygous for both characteristics. Their genotype is represented as AaRr (Figure 8).

Figure 8. Inheritance of albinism and the Rh blood type in humans: a cross involving two pairs of genes.

Figure 8. Continued.

How many possible types of gametes may individuals of the first generation have? You may recall that in the production of gametes (meiosis) the chromosomes separate randomly. The same is, therefore, true of genes carried in these chromosomes. In other words, all possible gene combinations are expected to occur in the gametes. These arrangements are determined by chance. For the double heterozygous individuals in our example (AaRr) four different types of gametes are possible: AR, Ar, aR, and ar. Only one allele from each of the two pairs of genes will be present in a gamete. This is due to the fact that the two pairs of genes are located on different chromosome pairs and only one homologous chromosome from each pair enters a gamete. Each of these gene combinations will theoretically occur in equal proportions. In the case of a female each gene combination has equal chances (one in four, or 1/4, or 25%) of being carried in the ovum.

Similarly, an individual of the genotype BBDd will produce only two types of gametes as far as these two gene pairs are concerned: BD and Bd. The same principle applies when more gene pairs are considered. For example, only four different types of gametes (ABCd, aBCd, AbCd, and abCd) are expected from an individual of the genotype AaBbCCdd, and only one (ABCD) if the genotype is AABBCCDD. This of course applies only when each of the gene pairs are on separate chromosome pairs.

The genotypes which can theoretically be produced by a couple consisting of double heterozygous individuals are more numerous than if they are homozygous. In the second generation (F₂) of our example a maximum of nine different genotypes are possible (Figure 8). By adding each of these genotypes from the square, the phenotypes are arranged in a ratio of 9 A -- R -- (both dominant characteristics, normal pigmentation and Rh+):3 aaR -- (albinos and Rh+):3 A -- rr (normal pigmentation and Rh-):1 aarr (albino and Rh-).

Two Characteristic Cross: Another Example (Not covered in BIO 110)

Not all crosses involving two characteristics are as involved as our example. For instance, only two different genotypes can be expected from a couple consisting of a woman which is homozygous for normal pigmentation and normal, noncarrier (homozygous) for cystic fibrosis (AACC), and a man which is also homozygous for normal pigmentation but a carrier of cystic fibrosis (AACc):

Every egg from the mother will be identical as far as these two pairs of genes are concerned (AC); only half of the man's sperm will carry the recessive allele for cystic fibrosis (50 % AC and 50 % Ac). All of their children will be normal for both traits but half of them will be carriers of cystic fibrosis.

Incomplete Dominance: Intermediate Inheritance

The inheritance of many traits cannot be explained or predicted by the simple mechanism as previously applied to the inheritance of albinism, cystic fibrosis, and the Rh blood type (simple Mendelian inheritance). Such is the case of incomplete dominance, a pattern of inheritance in which there is a lack of complete dominance between the two alleles of a gene pair. As a consequence, the phenotype of the heterozygous individuals becomes an intermediate or "blend" of the two homozygous phenotypes.

The inheritance of sickle cell anemia is an excellent example of incomplete dominance. Individuals suffering from this disease are characterized by having an abnormal type of hemoglobin , the essential iron-containing pigment of blood. Hemoglobin is found in the red blood cells and it is responsible for transporting oxygen through the body. Every molecule of hemoglobin contains four chains of protein, but in the victims of sickle cell anemia two of the four chains have a minor but greatly significant change. Individuals who have inherited two recessive alleles (ss) will have the abnormal hemoglobin for the rest of their lives. This abnormal hemoglobin is still able to transport oxygen; however, it changes inside the red blood cells whenever the concentration of oxygen in the blood is low. As a result many of the red blood cells take the distorted, curved shape responsible for the name of the disease. These cells obstruct the small blood vessels, causing serious heart and circulatory disorders as well as numerous complications. The victim also suffers from anemia a deficiency in hemoglobin and/or red blood cells. In this case the anemia is a consequence of the rupturing of the abnormal red blood cells. Although some complications can be arrested or controlled, there is no cure. Victims usually die before reaching adulthood.

The hemoglobin of normal, homozygous individuals (Hb^AHb^A) contains four normal protein chains. Heterozygous individuals (Hb^AHb^S), however, show an intermediate condition with one of the four chains abnormal. Fortunately, the red blood cells do not take the sickle shape under normal oxygen concentrations and the person does not suffer any of the major symptoms of the disease. This situation changes, however, when the person is under some types of stress, such as exercise at high altitudes. Anemia may then develop. Complications due to the clumping of sickle blood cells may also take place. The carriers of sickle cell anemia are relatively normal even though the allele expresses itself in a partial, incomplete manner.

The predicted phenotypic ratios of genes expressing incomplete dominance is different from those previously studied. A couple in which both individuals show the sickle cell trait (Hb^AHb^S) can have children exhibiting three different phenotypes: normal homozygous dominant (Hb^AHb^A), normal carrier of the trait in which some of the red blood cells show the sickle characteristic (Hb^AHb^S), and those suffering from sickle cell anemia (Hb^SHb^S) (Figure 9). The predicted ratio of the three phenotypes (as shown by the square) is 1 normal: 2 normal intermediate: 1 with sickle cell anemia.

Figure 9. Incomplete dominance in the inheritance of sickle cell anemia: a cross between two heterozygous individuals.

The normal but heterozygous carriers of traits which exhibit incomplete dominance may sometimes be detected by special tests. A simple microscopic examination of a small blood sample can show if a person carries the sickle cell anemia trait. Other clinical tests have been designed to recognize the carriers of diseases such as galactosemia, phenylketonuria (PKU), and some types of thalassemia. Tests of this kind may be of great importance in genetic counseling. Couples can receive advice from trained personnel as to their chances of passing suspected recessive genes to their offspring. It is important for prospective parents to have this genetic information, but full evaluation of the ethical consequences of their decision requires skilled counseling and careful consideration on the part of the couple. Similar tests are gradually being developed for detecting small differences characteristic of heterozygous carriers of recessive disorders. Some of these differences have even been found in conditions where the harmful recessive gene was thought to be completely masked by its dominant allele.

Multiple Alleles

In this type of inheritance a characteristic is determined or affected by a gene having more than two alleles. Only two of these alleles are present in each individual. The inheritance of the ABO blood types (or groups) in humans is a well-known example.

Belonging to blood type A, B, AB, or O is determined by the expression of two out of three possible alleles: IA, IB, or i (see Table 2).

  TABLE 2 Genotypes Determining the ABO Blood Types    

The type A blood phenotype is characteristic of individuals carrying alleles IAIA or IAi; type B blood is the expression of the genotypes IBIB or IBi. Alleles IA and IB are both dominant. The combined presence in one individual (IAIB) is expressed as type AB blood. Type O blood is the phenotype resulting from the expression of the homozygous recessive genotype, ii.

Skin color is another trait which is controlled by multiple alleles. Although the mechanism has not been elucidated it appears that skin pigmentation results from the expression of several alleles belonging to at least two separate genes. Individuals showing extreme skin color (either very light or very dark) are probably homozygous for the "light" or "dark" alleles. The mixing of these alleles will determine the many degrees of pigmentation observed in intermediate individuals.

ABO Blood Types and Blood Transfusions

The ABO family is one of several genetically determined blood types which have been identified in humans. Belonging to a given blood type means having a characteristic antigen in the blood. Blood antigens are specific chemical entities (proteins) carried in the blood cells. Two antigens, known as A and B, are known in the case of the ABO blood types. Individuals belonging to blood type A possess antigen A in their red blood cells (see below). Others will have antigen B (blood type B), both antigen A and B (blood type AB), or neither (blood type O).

In addition to antigens A and B, various combinations of two antibodies, anti-A or protein. They act by recognizing and binding to antigens. This specific bond fits like a lock-and-key mechanism. A person cannot obviously have an antibody what reacts against its own antigen. Blood type A individuals, for example, are characterized by having anti-B antibodies only (see below).

Anti-A and anti-B antibodies, however, may readily attack the antigen contained in blood received as a result of a transfusion. The clumping (agglutination) of red blood cells that result may have fatal consequences. The important point to remember when matching blood types in transfusions is not to introduce antigens from a donor that can be agglutinated by antibodies in the serum of the recipient. Type AB individuals are the lucky "universal recipients" since they do not have any antibodies and will receive blood from persons of all types. The opposite situation is observed in type O individuals, the " universal donors." Theoretically, they can donate blood to everybody else (no antigens to be destroyed by the recipients' antibodies) but will be able to receive blood from O type individuals only. In practice, however, transfusions are only given from donors whose blood type matches that of the recipients. The Rh factor, another blood antigen, must also be taken into consideration in blood transfusions.

The occurrence of the four ABO blood types varies among different human populations. The incidence of each group in the United States in as follows: O (45 percent), A (41%), B (10%), and AB (4%). The blood type can be quickly determined by a simple blood test.

Sex-linked Characters

Fertilization not only brings about the combination of parental genes but also the determination of sex. In humans, as in many animals, sex is determined by one pair of chromosomes, the sex chromosomes. The remaining 22 pairs of chromosomes are called autosomes. In women the sex chromosomes consist of two X chromosomes; in males there is one X chromosome (identical to that of females) and a smaller Y chromosome. Sex is determined at the moment of fertilization. At that time the single X chromosome always carried in the egg is met by either an X or a Y chromosome carried in a sperm cell (Figure 10). Sex is, therefore, determined by the male in a process ruled by chance. It is also evident that the theory there are equal chances (1:1 or "50-50") of belonging to either sex.

The sex chromosomes are not just associated with the determination of sex. Genes responsible for several traits are known to be present in them, mostly in the X chromosome. Characteristics determined by genes carried in the X chromosomes are said to be sex-linked.

Figure 10. Sex determination in humans.

Hemophilia, a recessive deficiency in the blood clotting process, in a sex-linked condition. Hemophiliacs lack one of the 12 clotting factors needed for the normal clotting of blood. The gene responsible for hemophilia is present only in the X chromosome, thus only one allele (H or h) is carried by every male. The disease will be inherited by females possessing two recessive alleles (XhXh) and by males having only one recessive allele (XhY). It is therefore much more common in males. Figure 11 illustrates a cross involving a normal man and a normal woman who is a carrier of the disease.

Other examples of sex-linked inheritance in humans are red-green color blindness, some forms of muscular dystrophy, and the rare immunity disease, a lack of disease-fighting antibodies (see Table 1).

Figure 11. Sex-linkage in the inheritance of hemophilia: a cross between a normal heterozygous (carrier) female and a normal male.

Other Mechanisms of Inheritance

This section on human inheritance has covered only a few of the traits whose inheritance of several other characteristics is harder to explain. Blue eyes, for instance, are not always recessive to dark eyes. More than one set of the genes possibly influence this characteristic, a good example of interaction of genes. In addition, many genes are known to influence more than one characteristic. This phenomenon is known as pleiotropy.

The environment in which we develop is also very important in the expression of genetically determined or genetically influenced characteristics. This is the case of the traits such as intelligence, height, and weight. Identical twins raised in different environments show the same hair color and the same blood types but not necessarily the same scores in IQ tests. It is interesting to notice, however, that identical twins show a significantly higher similarity in IQs and police records than fraternal twins!

At one time or another everyone has seen a film or read a science fiction story about a somewhat "mad scientist" who has discovered a molecule that can control the characteristics of animals or perhaps even found the secret of life. This is not as far removed from reality as one might think. In 1953, two young investigators, James Watson from the United States and Francis Crick from Great Britain, elucidated the molecular structure of DNA. Their discovery was the key that unlocked the secret of the role DNA as the carrier of genetic information.

DNA, short for deoxyribonucleic acid, is found in the chromosomes of every living cell of all forms of life. It belongs to a group of compounds referred to as nucleic acids. The molecule can be described as a strand of genes. The very first cell in the development of every animal or plant contains a genetic message that will pass on information for that cell to develop into a giant redwood tree, a tiny hummingbird, of even a sexy blond. DNA is indeed the blueprint of life.

Molecular Structure

DNA molecules are relatively simple, considering their complex function. Each molecule consists of two long, parallel chains (composed of phosphate units alternating with a sugar, deoxyribose) twisted about each other like a spiral staircase. One of four nitrogen compounds, often referred to as a base, projects from each sugar into the inner surface of the chains (Figure 2.12). These four bases are adenine, thymine, cytosin, and guanine. Specific paring takes place between opposite bases in the chain: Adenine (A) is always paired with thymine (T) and cytosine (C) always with guanine (G). The two types of paired bases (A-T and G-C) arranged in sequences along the chain represent the genetic message carried by DNA. The genetic code is a sequence of base pairs containing coded messages.

Figure 12. Diagramatic representation of the process of replication in a DNA molecule.

DNA Replication

How is the genetic code duplicated? The accurate duplication of DNA must take place in order to carry the genetic message to new cells. Just prior to cell division (by mitosis in the case of new body cells, or meiosis in the production of eggs and sperm) the two chains uncoil and serve as templates for the formation of two new DNA molecules (Figure 2.12). The new replicas are built as a result of the specific attachment of new base units to the existing DNA molecule. The formation of two exact copies of the genetic code contained in DNA is know as replication.

Genetic Code

How is the genetic code expressed? How are the genes able to control or regulate the functions of the body? The answer lies in the fact that DNA controls the synthesis of proteins, essential components of all living systems. Proteins are both building blocks of cells and enzymes. Enzymes regulate all chemical reactions which take place in the body. A specific enzyme is needed for each chemical reaction to take place. Proteins are composed of complex chains of amino acids. There are only 20 amino acids but their number and specific sequence in chains of various lengths and complexity can produce an almost infinite variety of proteins. The genetic code contains the information needed for the determination of the correct sequence of amino acids in a protein. The nitrogenous bases spell out specific amino acids with three-letter words, the genetic code. It has been experimentally shown that a sequence of three bases in DNA specifies a particular amino acid. For example, the sequence cytosine, cytosine, guanine (CCG) is the code for the amino acid glycine and the sequence guanine, thymine, guanine (GTG) is the code name for the amino acid histidine. All possible triplet combinations of the four bases can theoretically form a total of 64 code words, more than enough for all 20 amino acids. In fact, it is known that most amino acids are represented by more than one code word. Glycine, for instance, has four. Each group of three bases coding for an amino acid is known as a triplet. The code appears to be universal. The same triplet codes for the same amino acid in man as well as bacteria and mice!

Translation of the Code: Protein Synthesis

The code contained in DNA moves into the rest of the cell by means of another molecule, RNA (ribonucleic acid). RNA, like DNA, is a nucleic acid. It consists of only one chain of bases, and its sugar is slightly different from that of DNA. DNA serves as the template for the formation of messenger RNA (mRNA), one of the two types of RNA. Messenger RNA is a one-chain "negative print" of the genetic code. For example, the CCG triplet for the amino acid glycine in DNA is translated into GGC in the messenger RNA. Another important difference between DNA and RNA is that the thymine (T) base of DNA is not translated into adenine (A) but a different base, Uracil (U). Thus the GTG triplet in DNA (the coded information for the amino acid leucine) is translated into CAC in messenger RNA.  The "recipe" for each specific protein is then carried in messenger RNA to small bodies in the cell known as ribosomes. It is here that synthesis of proteins takes place. In the ribosomes messenger RNA serves as the blueprint for the assembly of an amino acid chain. A second type of RNA, transfer RNA (tRNA), now enters into the picture. This shorter type of RNA brings the amino acids to the template in the messenger RNA. There are 20 different types of transfer RNA, one for each amino acid. Each type of transfer RNA carries a specific amino acid on one end. On the other end a sequence of three bases fits on a specific triplet in the messenger RNA template. The transfer RNA is then able to "read" the code contained in the template. For instance, the transfer RNA carrying the amino acid histidine will recognize the triplet CUC in the messenger RNA, bringing the amino acid into the correct sequence of the protein being made. The transfer RNA will then release its amino acid as it becomes attached to the previous amino acid in the growing chain. The process is repeated as messenger RNA keeps moving along the ribosome much like a type-writer ribbon. A series of amino acids linked in a sequence which has been dictated by the genetic code in DNA is then built up to form a specific protein.

Genes and Mutations

A gene can be visualized then as an arrangement of bases in the DNA molecule involved in the determination of the amino acid sequence of a protein. Even a small change in the genetic code can induce a larger and potentially harmful change by substituting the wrong amino acid in a protein. This abnormal protein may harm the organism. If the protein happens to be an enzyme, an essential chemical reaction may not take place. This is true of many hereditary diseases. The recessive gene for sickle cell anemia changes the sixth amino acid (valine instead of glutamic acid) in a chain of 146 amino acids. The chain is just one of the four (with a total of 542 amino acids) in hemoglobin, the oxygen-carrying protein in blood. This abnormal hemoglobin produces a disorder characterized, among other things, by sickle-shaped red blood cells. These alteration in the genetic code can be hereditary conditions (genes inherited from parents) or mistakes during the copying process of the gene. These mistakes are known as mutations and may be caused by certain chemicals, radioactivity, ultraviolet radiation, and in other ways.

Other terms discussed in lecture:
    retroviruses
    reverse transcriptase
    antiretrovirals
    genetic engineering
    transgenic organisms
    recombinant DNA
    gene therapy
    genetic fingerprinting

UNIT 7– BIOLOGY OF SEX

Gender determination
SRY gene
Variations in gender determination
    chromosomal anomalies:

Some birth defects have been discovered to be determined not by specific genes but as the result of an extra chromosome or the absence of a section of one. These abnormalities take place during meiosis in at least one of the parents. Down's syndrome (mongolism) is such a case. Individuals affected with this condition (characterized among other things by mental retardation) have a total of 47 chromosomes: one of the 23 pairs is represented by three homologous chromosomes instead of the normal pair. All cells of the body will have this abnormal number of chromosomes.

Extra sex chromosomes, also a consequence of errors during meiosis, are responsible for a number of sex anomalies. An XXY male (Klinefelter's syndrome), for example, is the product of a two-X egg fertilized by a Y-sperm. These individuals are characterized by underdeveloped testes, sterility, and enlarged breasts. Females with three X chromosomes (triple-X syndrome) are usually infertile and mentally retarded. When an X-bearing sperm fertilizes an egg lacking an X chromosome an "XO female" will result. Women lacking one of the two X chromosomes suffers from Turner's syndrome. They have thick folds of skin on the sides of the neck, reduced and nonfunctional ovaries, and undeveloped breasts. Some investigators have suggested that XYY males ('super-males" or XYY syndrome) tend to be more violent and therefore inherit an inclination to end up in trouble with the law. This, however, has been refuted by other investigations. The XYY syndrome has been shown to be far more common than once thought (estimated as one in every 300 to 400 males) but the only obvious effects are an above average height and a tendency to have lower than normal IQs.

    testicular feminization
    ambiguous genitalia (hermaphrodites
    gender alternatives

UNIT 9– EVOLUTION    

Charles Darwin--The Origin of Species (1857)
artificial selection
natural selection
population
gene pool and genetic variation

Examples of natural selection:
    sickle-cell anemia and malaria
    mosquitoes and pesticides
    malaria parasite and anti-malarial drugs

stabilizing selection
directional selection
disruptive selection
species
speciation
    geographic isolation
    adaptive radiation

UNIT 10– ECOLOGY 

community
ecosystem
biosphere
feeding interactions
producers (autotrophs)
consumers (heterotrophs)
decomposers
food chain
food web
trophic levels
competition

Human impact
    world population curve
    greenehouse effect
    global warming
    ozone layer
    ozone hole

Adopted from Biology and Man, 3rd edition, G.D. Brum, P. Castro, and R.D. Quinn

Study Guide Webpage Designed by: Tim Shoemaker, Biological Sciences Department

 
SELF-TEST

Questions 6 through 13--In humans, lobed ears (L) are dominant over nonlobed ears (l). A homozygous lobed-eared man married a nonlobed-eared woman and they have two children, a boy and a girl.

  father: LL; mother: ll; sperms: L and L; eggs: l and l

Assume that the son of the couple above marries a girl with a genotype for ear lobing like his sisters.

Hb^AHb^A, Hb^AHb^S--normal

Hb^SHb^S--sickle cell anemia

 
SELF-TEST ANSWERS

1. c

2. a

3. d

4. e

5. d

6. all Ll

7. all with lobed ears

8. L and l

9. LL, Ll, and ll

10. 75

11. 1 out of 3

12. all (100%)

13. 1:2 (50%)

14. 1:2 (50%)

15. d (parents: ii X I^BI^B or I^Bi; F₁: ii)

16. c (ii, I^Ai, I^Bi)

17. a (AB, aB)

18. e

19. d

20. a (parents: X^cX^c x X^CY)

21. e (parents: I^AI^B x ii, F₁: I^Ai and I^Bi)