Genetics Chapter 4

Although many genes contribute to coat color and pattern in domestic cats...

a single X-linked locus determines the presence of orange color. There are two possible alleles at this locus: X+, which produces non-orange (usually black) fur, and X0, which produces orange fur. Males are hemizygous and thus may be black (X+Y) or orange (X0Y) but not black and orange. (Rare tortoiseshell males can arise from the presence of two X chromosomes, X+X0y). Females may be black (X+X+), orange (X0X0), or tortoiseshell (X+X0), with the tortoiseshell pattern arising from a patchy mixture of black and orange fur. Each orange patch is a clone of cells derived from an original cell in which the black allele was inactivated, and each black patch is clone of cells derived from an original cell in which the orange allele was inactivated.

Turner syndrome

are female and often have underdeveloped secondary sex characteristics. This syndrome is seen in 1 of 3000 female births. Affected women are frequently short and have a low hairline, a relatively broad chest, and folds of skin on the neck. Their intelligence is usually within normal range. Many women who have this are sterile.

Sex-linked characteristics

A major extension of these Mendelian principles; characteristics determined by genes located on the sex chromosomes.

Let's consider what happens when a woman homozygous for normal color vision mates with a color-blind man.

All the gametes produced by the woman contain an allele for normal color vision. Half of the man's gametes receive the X chromosome, with the color-blindness allele, and the other half receive the Y chromosome, which carries no alleles affecting color vision. When an Xc bearing sperm unites with the X+ bearing egg, a heterozygous female with normal color vision (X+Xc) is produced. When a Y-bearing sperm unites with the X+bearing egg, a hemizygous male with normal color vision is produced (X+Y).

Pseudoautosomal regions

Although the X and Y chromosomes are not generally homologous, they do pair and segregate into different cells in meiosis. They can pair because these chromosomes are homologous in small regions.; carry the same genes. In humans, there are these regions at both tips of the X and Y chromosomes.

Problem from book: Red-green color blindness is an X-linked recessive trait. Betty has normal color vision, but her mother is color blind. Bill is color Blind. If Bill and Betty have a child together, what is the probability that the child will be color blind? Solution Steps #1

Because color blindness is an X-linked recessive trait, Betty's color-blind mother must be homozygous for the color blindness allele (XcXc). Females inherit one X chromosome from each of their parents, so Betty must have inherited a color-blindness allele from her mother. Betty has normal color vision, she must have inherited an allele for normal color vision (X+) from her father; thus, hemizygous for X-linked alleles, he must be (XcY).

To verify his hypothesis....

Bridges examined the chromosomes of his files and found precisely what he had predicted. The significance of Bridges's study is not that it explained the appearance of an occasional odd fly in his colony but that he was able to link the inheritance of specific gene (w) to the presence of a specific chromosome (X). This association between genotype and chromosomes provided unequivocal evidence that sex-linked genes are located on the X chromosome and confirmed the chromosome theory of inheritance.

1959

Charles Ford used new techniques to study human chromosomes and discovered that cells from a 14-year old girl with Turner syndrome had only a single X chromosome. This chromosome complement is usually referred to as XO. Many people with Turner syndrome have some cell s that are XX and other cells that are XO, a situation referred to a mosaicism.

Problem from book: Red-green color blindness is an X-linked recessive trait. Betty has normal color vision, but her mother is color blind. Bill is color Blind. If Bill and Betty have a child together, what is the probability that the child will be color blind? Step #2 punnet square

Cross X+Xc and Cross XcY -should get X+XC normal-vision female -X+Y normal vision male -XcXc color blind female -XcY color-blind male

The Use of Y-linked genetic markers

DNA sequences in the Y chromosome undergo mutation with the passage of time and thus vary among individual males. These mutations create variations in DNA sequence that, like Y-linked traits, are passed from father to son and can therefore be used as genetic markers to study male ancestry. Although the markers themselves do not encode any physical traits, they can be detected with the use of molecular methods. Mutations can readily accumulate in the Y chromosome because so much of it is nonfunctional. Many of these mutations are unique; they arise only once and are passed down through the generations. Individual males possessing the same set of mutations are therefore assumed to be related, and the distribution of these genetic markers on Y chromosomes provides clues about the genetic relationship of present-day people.

As a result of X inactivation

Female placental mammals are functionally hemizygous at the cellular level for X-linked genes. In females that are heterozygous at an X-linked locus, approximately 50% of the cells express one allele and 50% express the other allele; thus, in heterozygous females, proteins encoded by both alleles are produced, but not within the same cell. This functional hemizygosity means that the cells in an individual female are not identical with respect to the expression of the genes on the X chromosome; females are mosaics for the expression of X-linked genes.

Homogametic sex

Females, which produce gametes that are the same with respect to the sex chromosomes.

X-linked characteristics

Genes on the X chromosome

Morgan explaining the results

He proposed that the locus affecting eye color is on the X chromosome (i.e. that eye color is X linked). He also recognized that eye-color is on the X chromosome (i.e. that eye color is X linked). He also recognized that eye-color alleles are present on the X chromosome only; only homologous allele is present on the Y chromosome. Because the cells of females possess two X chromosomes, females can be homozygous or heterozygous for the eye-color alleles. The cells of males, on the other hand, possess only a single X chromosome and can carry only a single eye-color allele. Males, therefore, cannot be homozygous or heterozygous but are said to be hemizygous for X-linked loci.

1891

Herman Henking noticed a peculiar structure in the nuclei of cells from male insects. Understanding neither its function nor its relation to sex, he called this structure the X body. Later, Clarence E. McClung studied the X body in grasshoppers and recognized that it was a chromosome. McClung called it the accessory chromosome, but it eventually became known as the X chromosome, from Henking's original designation. McClung observed that the cells of female grasshoppers had one more chromosome than the cells of male grasshoppers, and he concluded that accessory chromosomes played a role in sex determination.

Triple-X syndrome

In 1 in 1000 female births, the infant's cells possess three X chromosomes. These individuals have no distinctive features other than a tendency to be tall and thin. Although a few are sterile, many menstruate regularly and are fertile. The incidence of intellectual disability among triple-X females is slightly greater than that in the general population, but most XXX females have intelligence within the normal range. Much rarer are females whose cells contain four or five X chromosomes. These females usually have normal female anatomy but are intellectually disabled and have a number of physical problems. The severity of intellectual disability increases as the number of X chromosomes increases beyond three.

The Lyon Hypothesis

In 1949, Murray Bar observed condensed, darkly staining bodies in the nuclei of cells from female cats; these structures became known as Barr Bodies.

Environmental Sex Determination

In a number of organisms, sex is determined fully or in part by environmental factors.

In species with XX-XY sex determination, differences between males and females in their number of X chromosomes present a present a special problem in development.

In females, there are two copies of the X chromosome and two copies of each autosome, so genes on the X chromosomes and on an autosomes are "in balance". In males, however, there is only a single X chromosome, while there are two copies of every autosome. Because the amount of protein produced is often a function of the number of gene copies encoding that encoded by autosomal genes. This difference can be detrimental because protein concentration often plays a critical role in development.

Example Genic Sex Determination

In mammals, for example, a gene (SRY ) located on the Y chromosome determines the male phenotype. In both genic sex determination and chromosomal sex determination, sex is controlled by indvidual genes; the difference is that with chromosomal sex determination, the chromosomes also look different in males and females.

XX-XY Sex Determination

In many species, teh cells of male and females have the same number of chromosomes, but the cells of females have two X chromosome (XX) and the cells of males have a single X chromosome and smaller sex chromosome, the Y chromosome (XY). In humans and many other organisms, the Y chromosome is acrocentric, not Y shaped, as is often assumed.

Z-linked characteristics

In organisms with ZZ-ZW sex determination, males are the homogametic sex (ZZ) and carry two sex-linked (usually referred to as Z-linked) alleles; thus, males may be homozygous or heterozygous. Females are the heterogametic sex (ZW) and possess only a single Z-linked allele. The inheritance of Z-linked characteristics, except that the pattern of inheritance in males and females is reversed.

Genic Sex Determination

In some organisms, sex is genetically determined, but there are no obvious difference in the chromosomes of males and females: there are no sex chromosomes. Genotypes at one or more loci determine the sex of an individual. Scientists have observed genic sex determination in some plants, fungi, protozoans, and fishes.

ZZ-ZW Sex Determination

In the ZZ-ZW sex determining system, found in the bearded dragons discussed in the chapter introduction, the female is heterogametic and the male is homogametic. After meiosis, half of the eggs have a Z chromosome and the other half have a W chromosome. Males are ZZ; all the sperm contain a single Z chromosome. The ZZ-ZW system is found in birds, some reptiles, butterflies, some amphibians, and some fishes.

Random inactivation of X chromosomes require two steps.

In the first step, the cell somehow assesses, or counts, how many X chromosomes are present. In the second step, one X chromosome is selected to become the active X chromosome, and all others are inactivated.

XXY XXYY XXXy XXXXY

Klinefelter syndrome Barr Bodies: 1, 1, 2, 3

Heterogametic sex

Males produce two different types of gametes with respect to the sex chromosomes.

XXY Males

Males with an extra Y chromosome (XXY) occur with a frequency of about 1 in 1000 male births. These individuals have no distinctive physical characteristics other than a tendency to be several inches taller than the average for XY males. Their IQ is usually within the normal range; however, some studies suggest that learning difficulties may be more common in this group than in XY males.

1909

Morgan switched his research to Drosophila melanogaster, a year later, he discovered among the flies of his laboratory colony a single male that possessed white eyes, in stark contrast to the red eyes of normal fruit flies. This fly had a tremendous effect on Morgan's career as a biologist and on the future of genetics.

Investigate the inheritance of the white eye trait in fruit flies....

Morgan systematically carried out a series of genetic crosses. First, he crossed pure-breeding red-eyed females with his white-eyed male, producing F1 progeny that had red eyes. Morgan's results from his initial cross were consistent with Mendel's principles: a cross between a homozygous dominant individual and homozygous recessive individual produced heterozygous offspring exhibiting the dominant trait. These results suggested that white eyes are a simple recessive trait. When Morgan crossed the F1 flies with each other, however, he found that all the female flies possessed red eyes, but half the male F2 flies possessed red eyes, but half the male F2 flies had red eyes and the other half had white eyes. This finding was clearly not expected result for a simple recessive trait, which should appear in one-fourth of both male an female F2 offspring.

1905

Nettie Stevens and Edmund Wilson demonstrated that in grasshoppers and other insects, the cells of females have two X chromosomes, whereas the cells of males have a single X. In some insects, they counted the same number of chromosomes in the cells of males and females but saw that one chromosome pair was different: two X chromosomes were found in female cells, whereas a single X chromosome plus a smaller chromosome, which they called Y, was found in male cells.

Genic sex determination

No distinct sex chromosomes. Sex determined by genes on undifferentiated chromosomes. Varies in Heterogametic sex.

Autosomes

Nonsex, same for males and females

Sequential hermaphroditism

Phenomenon in which the sex of an individual organism changes in the course of its lifetime; the organism is male at one age or developmental stage and female at a different age or stage.

XXXXX

Poly X female Barr bodies: 4

XXXX

Poly-X female Barr bodies: 3

Problem from book: Red-green color blindness is an X-linked recessive trait. Betty has normal color vision, but her mother is color blind. Bill is color Blind. If Bill and Betty have a child together, what is the probability that the child will be color blind? What information is provided to solve the problem?

Red-green color blindness is an X-linked recessive trait. The phenotypes of Betty, Betty's mother, and Bill.

Evolution of the Y chromosome

Research on se chromosomes has led to the conclusion that the X and Y chromosomes in many organisms evolved from a pair of autosomes. This first step in this evolutionary process took place when one member of a pair of autosomes acquired a gene that determines maleness, such as the SRY gene found in humans today. This step took place in mammals about 250 million years ago. Any individual organism with a copy of the chromosome containing this gene then became male. Additional mutations occurred on the proto-Y chromosome, affecting traits that are beneficial only in males, such as sperm development. The genes that encode these types of traits are advantageous only if they are present in males.

Environmental sex determination

Sex determined by environmental factors. Varies by Heterogametic sex

Dioecious

Species in which an individual organism has either male or female reproductive structures.

XY

Syndrome: None Barr Bodies: 0

XX

Syndrome: None Barr Bodies: 1

XO

Syndrome: turner Barr Bodies: none

Thomas Hunt Morgan

The first person to provide an explanation for sex-linked inheritance. Has began his career as an embryologist, but the discovery of Mendel's principles inspired him to begin conducting genetic experiments, initially on mice and rats.

Sex Determination in Drosophila melanogaster

The fruit fly Drosophila melanogaster has eight chromosome: three pairs of autosomes and one pair of sex chromosomes. Usually, females have two X chromosomes, and males have an X chromosome and a Y chromosome. In 1920s, Calvin Bridges proposed that sex in Drosophila was determined not by the number of X and Y chromosomes but rather by the balance of female-determining genes on the X chromosome and male determining genes on the autosomes. He suggested that a fly's sex is determined by the so-called X: A ratio of 1.0 produces a female fly and that an X: A ratio of 0.5 produces a male fly. He also suggested that an X: A ratio between 1.0 and 0.5 produces an intersex fly, with mixture of male and female characteristics. An X: A ratio of less than 0.5 or greater than 1.0 produces developmentally abnormal flies called metamales and metafemales, respectively.

Characteristics of the humanity y chromosome

The genetic sequence of most of the human Y chromosome was determined as part of Human Genome Project.

sex determination

The mechanism by which biological sex is established.

The Role of Sex Chromosomes

The phenotypes associated with sex chromosome anomalies allow us to make several inferences about the role of sex chromosomes in human sex determination: 1. The X chromosome contains genetic information essential for both sexes; at least one copy of an X chromosome is required for human development. 2. The male-determining gene is located on the Y chromosome. A single copy of this chromosome, even on the presence of several X chromosomes, usually produces a male phenotype. 3. The absence of the Y chromosome usually results in a female phenotype. 4. Genes affecting fertility are located on the X and Y chromosomes. A female usually needs at least two copies of the X chromosome to be fertile. 5. Additional copies of the X chromosome may upset normal development in both males and females, producing physical problems and intellectual disabilities that increase as the number of extra X chromosomes increases.

Problem from book: Red-green color blindness is an X-linked recessive trait. Betty has normal color vision, but her mother is color blind. Bill is color Blind. If Bill and Betty have a child together, what is the probability that the child will be color blind? What information is required in your answer to the problem?

The probability that Bill and Betty's child will be color blind.

Now consider the reciprocal cross between a color-blind woman and a man with normal color vision.

The woman produces only Xc bearing gametes. The man produces some gametes that contain the X chromosome and others that contain the Y chromosome. Males inherit the X chromosome from their mothers, so because both of the mother's X chromosome bear the Xc allele, all the male offspring of this cross will be color blind. In contrast, females inherit an X chromosome from each parent; thus, all the female offspring of this cross will be heterozygous with normal color vision.

Symbols for X-linked genes

There are several ways to record genotypes for X-linked characteristics. Sometimes these genotypes are written in the same way as those for autosomal characteristics. In this case, hemizygous males are simply given a single allele. -ww is the female Drosophila with white eyes, and the genotype of a white-eyed hemizygous is w. Another method is to include the Y chromosome, designating it as a slash (l).With this method, the white-eyed male's genotype is w/. Perhaps the most useful method is to record the X and Y chromosomes in the genotype, designation the Z-linked alleles with superscripts, as is done in this chapter. With this method, a white-eyed female is XwXw and white-eyed male is XwY. The use of X's and Y's in the genotype has the advantage of reminding us that the genes are X linked and the male must always have a single allele, inherited from the mother.

Y-linked genetic markers have been used to study...

Thomas Jefferson, principal author of the Declaration of Independence and third president of the United States. In 1802, a political enemy accused Jefferson of fathering a child by his slave Sally Hemings, but evidence was circumstantial. Hemings, who worked in the Jefferson household and accompanied Jefferson on a trip to Paris, had five children. Jefferson was accused of fathering the first child, but rumors about the paternity of the other children circulated as well. Descendants of Heming's children maintained that they were part of the Jefferson line, but some Jefferson descendants refused to recognize their claim.

X-Linked Color Blindness In Humans

To further examine X-linked inheritance, let's consider another X-linked characteristic: red-green color blindness in humans. The human eye perceives color through light-sensing cone cells that line the retina. Each cone cell contains one of three pigments capable of absorbing light of a particular wavelength: one absorbs blue light, a second absorbs red light, and a third absorbs green light. The human eye actually detects only three colors-blue, red, and green-but the brain mixes the signals from different cone cells to create a wide spectrum of colors that we perceive. Each of the three pigments is encoded by a separate locus; the locus for the blue pigment is found on chromosome 7, and those for the green and red pigments lie close together on the X chromosome.

XXX

Triple X syndrome Barr bodies 2

True or False: There are no known cases in which a person is missing both X chromosomes, an indication that at least one X chromosome is necessary for human development. Presumably, embryos missing both Xs spontaneously abort in the early stages of development.

True: they would be dead

Y-Linked characteristics

Y-linked traits-also called holandric traits -exhibits a distinct pattern of inheritance. These traits are present only in males, because only males possess a Y chromosome, and are always inherited from the father. Furthermore, all male offspring of a male with a Y-linked trait will display this trait, because every male inherits his Y chromosome from his father.

Researchers have located a number of genes on the....

X chromosome that affect sexual phenotype, but few autosomal sex-determining genes (required for the X: A ratio hypothesis) have been identified. New evidence indicates that genes on the X chromosome are the primary sex determinant. The influence of the number of sets of autosomes on sex is indirect, affecting the timing of developmental events and therefore how long sex-determining genes on the X chromosome are active.

For example:

XX flies with three autosomal sets (XX, AAA) have an X:A ratio of 0.67 and develop an intersex phenotype. In these flies, the presence of three autosomal sets causes a critical developmental stage to shorten, not allowing female factors encoded on the X chromosomes enough time to accumulate, with the result that the flies end up with an intersex phenotype. Thus, the number of autosomal sets of chromosomes influences sex determination in Drosophila, but not through the action of autosomal genes, as envisioned by Bridges.

Example of red-color blind

Xc to represent the allele for red-green color blindness and the symbol X+ to represent an allele for normal color vision. Female posses two X chromosomes, so there are three possible genotypes among females: X+X+ and X+Xc, which produce normal color vision, and XcXc, which produces color blindness. Males have only a single X chromosome and two possible genotypes: X+Y, which produces normal color vision, and XcY, which produces color blindness.

Y-linked characteristics

Y chromosome.

Although the palindromic sequences....

afford opportunities for recombination, which helps prevent the decay of the Y chromosome over evolutionary time, they occasionally have harmful effects.

When Morgan crossed his original white-eyed male with homozygous red-eyed females....

all 1237 of the progeny had red eyes except except for 3 white-eyed males. Morgan attributed these white-eyed F1 males to the occurrence of further random mutations. However, flies with these unexpected phenotypes continued to appear in his crosses. Although uncommon, they appeared far too often to be due to spontaneous mutation.

A female is color blind only when she inherits color-blindness....

allele from both parents.

A male is only color blind if he...

allele from his mother to be color blind; (reason why color blindness and most other rare X-linked recessive traits are more common in males than in females.

Steven and Wilson

also showed that the X and Y chromosomes separate into different cells in sperm formation: half of the sperm receive an X chromosome and the other half receive a Y. All eggs produced by the female in meiosis receive one X chromosome. A sperm containing a Y chromosome unties with an X-bearing egg to produce an XY male, whereas a sperm containing an X chromosome unites with an X-bearing egg to produce an XX female. This distribution of X and Y chromosomes in sperm accounts for 1:1 sex ration observed in most dioecious of X and Y chromosomes in sperm accounts for the 1:1 sex ratio observed in most dioecious organisms. Because sex is inherited like other genetically determined characteristics, Stevens and Wilson's discovery that sex is associated with the inheritance of a particular chromosome also demonstrated that genes are located on chromosomes.

Although the SRY gene is primarily determinant of sex in human embryos , several other genes influence sexual development...

as illustrated by women with androgen-insensitivity syndrome. These individuals have female external sexual characteristics. Most are unaware of their condition until they reach puberty and fail to menstruate. Examination by a gynecologist reveals that the vagina ends blindly and that the uterus, oviducts, and ovaries are absent. Inside the abdominal cavity, a pair of testes produce levels of testosterone normally seen in the males. The cells of a woman with androgen-insensitivity syndrome contain an X and a Y chromosome.

Gender is a category...

assigned by the individual or others based on behavior and cultural practices.

Hermaphroditism

both sexes are present in the same organism.

One's gender need not...

coincide with one's biological sex.

Mary Lyon

conducted significant research in mouse genetics, including important work on mutagenesis, chromosome inversions, and the t complex, a genetic element on mouse chromosome 17 that causes some chromosomes to be preferentially transmitted during meiosis. Lyon also developed many techniques that are used today in mouse genetics, helping to make the mouse an important model genetic organism.

Because the Y chromosome of many organisms...

contains little genetic information, most sex-linked characteristics are X linked. Males and female differ in their sex chromosomes, so the pattern inheritance for sex-linked characteristics differ from the exhibited by genes located on autosomes.

The most common types of human color blindness are caused by....

defects of the red and green pigments; we will refer to these as red-green color blindness.

Over millions of years, the Y chromosome slowly...

degenerated, losing DNA and genes until it became greatly reduced in size and contained little genetic information. This degeneration produced the Y chromosome found in males today. Indeed, the Y chromosomes of humans and many other organisms are small and contain little genetic information; therefore, few characteristics exhibit Y-linked inheritance. Some researchers have predicted that the human Y chromosome will continue to lose genetic information and will completely disappear from the species in about 10 million years, a disheartening prospect for those of us with a Y chromosome (perhaps some of those with two Xs). However, come to a halt and that no genes have been lost in the last 6 million years. The genes that remain only Y chromosome appear to be remarkably stable. Internal recombination within the Y chromosome may have aided in slowing down or preventing the complete decay of the human Y chromosome.

Humans are known as...

dioecious. Among dioecious species, sex may be determined chromosomally genetically, or environmentally.

In fruit flies...

dosage compensation is achieved by doubling of the activity of the genes on the X chromosome of males, but not that of females. In placental mammals, the expression of dosage-sensitive genes on the x chromosomes of both males and females has increased, coupled with inactivation of one of the X chromosomes in females, so that expression of X-linked and autosomal genes is balanced in both males and females.

Random X inactivation takes place...

early in development-in humans, it occurs within first few weeks development. After an X chromosome has been inactivated in a cell, it remains inactive in that cell and in all somatic cells that descend from the cell. Thus, neighboring cells tend to have the same X chromosome inactivated, producing a patchy pattern (mosaic) for the expression of an X-linked characteristic in heterozygous females. This patchy distribution of gene expression can be seen in tortoiseshell and calico cats.

The phenotype associated with these conditions probably arise because some X-linked genes...

escape inactivation. Indeed, the nature of X inactivation is more complex than originally envisioned. Studies of individual genes now reveal that only 75% of X-linked human genes are permanently inactivated. About 15% completely escape X inactivation, meaning that these gene produce twice as much protein in females as they do in males. The remaining 10% are inactivated in some females but not in others. The reason for this variation among females is not known. Furthermore, recent research indicates that X inactivation does not actually equalize dosage of many X-linked and autosomal genes in humans and mice.

About 10% of the time, the two X chromosomes in the females....

fail to separate in anaphase I of meiosis, a phenomenon known as nondisjunction.

Sexual reproduction

formation of offspring that are genetically distinct from their parents; most often, two parents contribute genes to their offspring, and those genes are assorted into new combinations through meiosis. Among most eukaryotes, sexual reproduction consists of two process that lead to an alternation of haploid and diploid cells: meiosis produces haploid gametes (spores in plants), and fertilization produces diploid zygotes.

Calvin Bridges

found exceptions arose only in certain strains of white-eyed files. When he crossed a white-eyed female from one those strains with a red-eyed male, about 5% of the male offspring had red eyes and about 5% of the female offspring had white eyes. In this cross, the expected result is that every male fly should inherit its mother's X chromosome and should have the genotype XwY and white eyes. Furthermore, every female fly should inherit a dominant red-eye allele on its father's X chromosome, along with white-eye allele on its mother's X chromosome; thus, all female progeny should be X+Xw and have red eyes. The continual appearance of red-eyed males and white-eyed females in this cross was therefore unexpected.

Mutations that produce defective color vision are...

generally recessive, and because the genes encoding red and the green pigments are located on the X chromosome, red-green color blindness is inherited as an X-linked recessive trait.

To resolve this long standing theory about Jefferson's possible children...

geneticists examined markers from the Y chromosomes of male-line descendants of Hemings's first son (Thomas Woodson), her last son (Eston Hemings), and a paternal uncle of Thomas Jefferson with whom Jefferson had Y chromosomes in common. Geneticists determined that Jefferson possessed a rare and distinctive set of genetic markers on his Y chromosome. The same markers were also found on the Y chromosomes of the male-line descendants of Easton Hemings. The probability of such a match arising by chance is less than 1%. The markers were not found on the Y chromosomes of the descendants of Thomas Woodson.

When nondisjunction of the Xs occurs...

half of the egg receive two copies of the X chromosome and the other half receive only a Y chromosome. When these eggs are fertilized by the sperm from the normal red-eyed male, four combinations of sex chromosomes are produced. An egg with two X chromosomes that is fertilized to by an X-bearing sperm produces a X+XwXw zygote, which usually dies. When a egg carrying two X chromosomes is fertlized by a Y-bearing sperm, the resulting zygote is XwXwY, which develops into a white-eyed female. An egg with only a Y chromosome that is fertilized by a Y bearing sperm, the resulting zygote has two Y chromosomes and no X chromosome, and dies.

The Y chromosome

in humans, and in all other mammals, is of paramount importance in producing a male phenotype. However, scientists have discovered a few rare XX males whose cells apparently lack a Y chromosome.

Environmental factors are also important in determining sex...

in some reptiles; the sexual phenotype of many turtles, crocodiles, alligators, and few birds is affected by temperature during embryonic development.

Androgen-insensitivity syndrome illustrates several points about the...

influence of genes on a person's sex. First, this condition demonstrates that human sexual development is a complex process, influenced not only by the SRY gene on the Y chromosome but also by other genes found elsewhere. Second, it shows that most people carry genes for both male and female characteristics, as illustrated by the fact that those with androgen-insensitivity syndrome have the capacity to develop female characteristics even though they have a Y chromosome. Indeed, the genes for most male and female secondary sex characteristics are present not on the sex chromosomes but on the autosomes. The key to maleness and femaleness lies not in the genes but in the control of their expression.

The fundamental difference between males and females

is gamete size: males produce small gametes, and females produce relatively large gametes.

Gender

is not the same as sex. Biological sex refers to the anatomical and physiological phenotype of an individual.

Example of a Z-linked characteristic

is the cameo phenotype in Indian blue peafowl (Pavo cristatus). In these birds, the wild-type plumage is a glossy metallic blue. The female peafowl is ZW and the male ZZ. Cameo plumage, which produces brown feathers, results from a Z-linked allele (Zca) that is recessive to the wild-type blue allele (Zca+). If a blue female (Zca+W) is crossed with a cameo male (ZcaZca), all the F1 females are cameo (ZcaW) and all the F1 males are blue (Zca+Zca). When F1 are interbred, 1/4 of the F1 are blue males (Zca+Zca), 1/4 are bllue females (Zca+W), 1/4 are cameo males ZcaZca, and 1/4 are cameo females (ZcaW). The reciprocal cross of a cameo female with a homozygous blue male produces an F1 generation in which all offspring are blue and an F2 generation consisting of 1/2 blue blues (Zca+Zca+ and Zca+Zca), 1/4 blue females (Zca+W), and 1/4 cameo females (ZcaW).

Lyon's hypothesis

led to important insights into the process of development, the expression of X-linked traits, and X-linked genetic diseases.

Most organisms have two sexual phenotypes

male and female

Humans, like Drosophila, have XX-XY sex determination, but in humans...

maleness is primarily determined by the presence of a particular gene (SRY) on the Y chromosome. The phenotypes that results from abnormal numbers of sex chromosomes, which arise when the sex chromosomes do not segregate properly in meiosis ro mitosis, illustrate the importance of the Y chromosome in human sex determination.

The lack of crossing over led to (and continues to lead to) an accumulation of...

of mutations on and a loss of genetic material from the Y chromosome.

She suggested that within each female cell....

one of the two chromosomes is inactivated. Which X chromosome is inactivated is random; if a cell contains more than two X chromosomes, all but one of them are inactivated. The number of Barr bodies present in human cells with different complements of sex chromosomes.

Sometimes an individual organism has chromosomes or genes that are normally associated with one sex but an anatomy corresponding to the...

opposite sex. For instance, the cells of human females normally have two X chromosomes, and the cells of male have one X chromosome and one Y chromosome. A few rare individuals have male anatomy although their cells contain two X chromosomes. Even though these people are genetically female, we refer to them as male because their sexual phenotype is male.

Monoecious

organisms that bear both male and female reproductive structures.

Recall that the X and Y chromosome....

pair in meiosis because they are homologous at the small pseudoautosomal regions. Genes in these regions of the X and Y chromosomes are homologous, just like those on autosomes, and they exhibit autosomal patterns of inheritance rather than the sex-linked inheritance seen for most genes on the X and Y chromosomes.

The function of most Y-linked genes is...

poorly understood; many appear to influence male sexual development and fertility. Others play a role in gene regulation and protein stability. Some are expressed throughout the body, but many are expressed predominantly or exclusively in the testes. Although the Y chromosome has relatively few genes, research with Drosophila suggest that it carries genetic elements that affect the expression of numerous genes on autosomes and X chromosomes.

Bridge's hypothesis predicted that the white eyed females from his crosses would...

possess two X chromosomes and one Y chromosome and that the red-eyed males would possess a single X chromosome.

The presence of sex chromosomes does not always produce...

problems of gene dosage, and dosage compensation of X-linked genes is not universal. A number of animals do not exhibit obvious mechanisms of dosage compensation; these animals include butterflies and moths, birds, some fishes, and even the duck-billed platypus.

Recent research has revealed that recombination between the palindromes can lead to...

rearrangements of the Y chromosome that cause anomalies of sexual development. In some cases, recombination between the palindromes leads to deletion of the SRY gene, producing an XY female. In other cases, recombination deletes other Y-chromosome genes that take part in sperm production. Sometimes recombination produces a Y chromosome with two centromeres; such a chromosome may break as the centromeres are pulled in opposite directions in mitosis. The broken Y chromosomes may be lost in mitosis, resulting in XO cells and Turner syndrome.

Sex

refers to sexual phenotype.

In organisms with ZZ-ZW sex determination...

the female always inherits her W chromosome from her mother, and she inherits her Z chromosome, along with any Z-linked alleles, from her father. In this system, the male inherits Z chromosomes, along with any Z-linked alleles, from both his mother and his father. This pattern of inheritance is the reverse of that of X-linked alleles in organisms with XX-XY sex determination.

Human Genome Project

revealed that about two-thirds of the Y chromosome consists of short DNA sequences that are repeated many times and contain no active genes. The other third consists of just a few genes. Only about 350 genes have been identified on the human Y chromosome, compared with thousands on most chromosomes, and only about half of those identified encode proteins. Some of the protein-endcoding genes found on the Y chromosome have homologous genes on the X chromosome.

A palindrome

sentence or word that reads the same backward and forward. In DNA, this is a sequence that reads the same on both strands of the double helix, creating two nearly identical copies stretching out from a central point.

Among animals...

sex chromosome have independently evolved from autosomes many times. This phenomenon has even been observed in mammals: the sex chromosomes of monotremes (egg-laying mammals such as the duck-billed platypus) evolved from different autosomes than did the sex chromosomes of placental and marsupial mammals.

Sex chromosomes

sex in many organisms is determined by a pair of chromosomes.

Chromosomal sex-determining systems

sex is actually determined by individual genes.

The chromosome theory of heredity

states that genes are located on chromosomes, which serve as vehicles for the segregation of genes in meiosis. Prove of this theory was provided by the discovery that the sex of certain insects is determined by the presence or absence of particular chromosomes.

Although some details of X-chromosome inactivation remain unknown, several genes and sequences...

that participate in the process have been identified. Foremost among them is a gene called Xist (for X-inactive specific transcript). On the X chromosomes destined to be inactivated, the Xist gene is active, producing a 17,000-nucleotide-long RNA molecule that coats the X chromosome and inactivates the genes on it by recruiting protein complexes that alter chromatin structure. On the X chromosome destined to be active, other genes repress the activity of Xist so that the Xist RNA never coats the X chromosome, and genes on this chromosome remain active.

In the human embryo with a Y chromosome....

the SRY gene causes the gonads to develop into tests, which produce testosterone. Normally, testosterone stimulates embryonic tissue to develop male characteristics. But for testosterone to have its effect, it must bind to an androgen receptor. This receptor is defective in females with androgen-insensitivity syndrome; consequently, their cells do not respond to testosterone, and female characteristic develop. The gene for the androgen receptor is located on the X chromosome, so people with this condition always inherit it from their mothers.

In these crosses for color blindness we notice that an affected woman passes....

the X-linked recessive trait to her sons but not to her daughters, whereas an affected man passes the trait to his grandsons through his daughters but never to his sons. X-linked recessive characteristics therefore appear to alternate between sexes, appearing in females in one generation and in males in the next generation.

Bridges hypothesized that...

the exceptional white-eyed females of this strain actually possessed two X chromosomes and a Y chromosome (XwXwY). In Drosophila, flies with XXY sex chromosomes normally develop as females, in spite of possessing a Y chromosome. About 90% of the time, the two X chromosomes of the XwXwY females separate from each other in Anaphase I of meiosis, with a X and a Y chromosome entering one gamete and a single X entering another gamete.

In placental mammals...

the expression of dosage-sensitive genes on the X chromosomes of both males and females has increased, coupled with inactivation of one of the X chromosomes in females, so that expression of X-linked and autosomal genes is balanced in both male and females.

In the XX-XY sex determining system,

the male is the heterogametic sex-half of his gametes have an X chromosome and half have a Y chromosome. The female is the homogametic sex-all her eggs contain an X chromosome. Many organisms, including some plants, insects, and reptiles, and all mammals (including humans), have the XX-XY sex-determining system. Other organisms have variations of the XX-XY system; for example, the duck-billed platypus has an interesting system in which females have five pairs of X chromosomes and males have five pairs of X and Y chromosomes.

A surprising feature revealed by sequencing is...

the presence of eight massive palindromic sequences on the Y chromosome.

The Lyon hypothesis suggests that...

the presence of variable numbers of X chromosomes should not affect the phenotype in mammals because any X chromosomes in excess of one should be inactivated. However, people with Turner syndrome (XO) differ from XX females, and those with Klinefelter syndrome (XXY) differ from XY males.

In the XX-XO systems....

the sex of an individual is therefore determined by which type of male gamete fertilizes the egg.

The X: A ration correctly predicts what?

the sexual phenotype, research now suggests that the mechanism of sex determination is not a balance between X-linked genes and autosomal genes.

Nondisjunction of X chromosomes among XwXwY white-eyed females ....

therefore produces a few white-eyed females and red-eyed males, which is exactly what Bridges found in his crosses.

Together with circumstantial historical evidence...

these matching markers suggest that Jefferson (or a male-line relative of Jefferson) was the father of Eston Hemings but not Thomas Woodson.

If an animal has more than one X chromosome....

they can overcome this problem by evolving mechanisms to equalize the amounts of protein produced by the single X and by two autosomes in the heterogametic sex. These mechanisms are referred to as dosage compensation.

Natural selection favors the suppression of crossing over for most of the length of the X and Y chromosomes because...

this suppression prevents genes that encode male traits from appearing in females. Crossing over can still take place between the two X chromosomes in females, but there is little crossing over between the X and Y chromosomes, except for the small pseudoautosomal regions in which the X and the Y chromosomes continue to pair in meiosis, as stated earlier.

Y-chromosome sequences have also been used extensively...

to examine past patterns of male migration and the genetic relationships among different human populations. Female lineages can be traced through sequences on mitochondrial DNA, which are inherited from the mother.

Palindromic sequence in DNA appears

twice, very much like the two copies of DNA sequence that are found on two homologous chromosomes. And just as it does with homologous chromosomes, recombination can take place between the two palindromic sequences on the Y chromosome. As already mentioned, the X and the Y chromosomes are not homologous at most of their sequences, and most of the Y chromosome does not undergo crossing over with the X chromosome. This lack of interchromosomal recombination leads to an accumulation of deleterious mutations on the Y chromosome and the loss of genetic material. Evidence suggests that the two arm of Y chromosome recombine with each other, which partly compensates for the absence of recombination between the X an Y chromosomes. This internal recombination may help to maintain some sequences and functions of genes on the Y chromosomes and prevents its total degeneration.

Early in development all humans possess...

undifferentiated gonads and both male and female reproductive ducts. Then, about six weeks after fertilization, a gene on the Y chromosome becomes active. This gene causes the neutral gonads to develop into the testes, which begins to secrete two hormones: testosterone and anti-Mullerian hormone. Testosterone and other androgens induce the development of male characteristics, and anti-Mullerian hormone causes the degeneration of the female reproductive ducts. In the absence of this male-determining gene, the neutral gonads become ovaries, and female features develop. In mice, the regression of male reproductive ducts in females is promoted by another protein called chicken ovalbumin upstream promoter transcription factor 2 (COUP-TFII).

X-bearing sperm

unite with X-bearing eggs to produce XX zygotes, which develop into females. Sperm lacking an X chromosome unite with X-bearing eggs to produce XX zygotes, which develop into females. Sperm lacking an X chromosome unite with X-bearing eggs to produce XO zygotes, which develop into males.

Close examination eventually revealed a smart part of the Y chromosome attached to another chromosome....

usually the X. This finding indicates that it is not the entire Y chromosome that determines maleness in humans; rather, it is a gene on the Y chromosome.

Sex-Determining region Y (SRY) gene

was discovered in 1990. This gene is found in XX males and is missing from rare XY females; it is also found on the Y chromosome of other mammals. Definitive proof that SRY is the male-determining gene came when scientists placed a copy of this gene in XX mice by means of genetic engineering. The XX mice that received this gene, although sterile, developed into anatomical males.

Klinefelter syndrome

which has a frequency of about 1 in 1000 male births, have cells with one or more Y chromosomes and multiple X chromosomes. The cells of most males with this conclusion are XXY, but the cells of a few males with Klinefelter syndrome are XXXY, XXXXY, or XXYY. Men with this condition frequently have small tests and reduced facial and pubic hair. They are often taller than normal and sterile; most have intelligence within normal range.

When these gametes fertilize by sperm from a normal red-eyed male,...

white-eyed males and red-eyed females are produced.