Chapter 4

klinefelters syndrome

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 condition are XXY but the cells of a few males with .... are XXXY, XXXXY, or XXYY. frequently have small testes and reduced facial and pubic hair. They are often taller than normal and sterile; most have intelligence within the normal range. *people with Klinefelter syndrome have a Y chromosome and two or more X chromosomes in their cells.

evolution of the y chromosome

a mutation of a gene on one chromosome causes maleness Mutations at other genes affect male characteristics Supression of crossing over keeps genes for male traits linked to a male-determining gene Over time, lack of crossing over b/n the X and Y chromosomes leads to degeneration of the Y.

sequential hermaphroditism

a type of sexual development called .... each individual animal can be both male and female, although not at the same time. sex is determined environmentally by the limpet's position in the stack.

pseudoautosomal region

-the x and y chromosomes are homologous only at pseudoautosomal regions, which are essential for x and y chromosome pairing in meiosis in the male. -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 called the ... ... in which they carry the same genes. In humans, there are .... at both tips of the X and Y chromosomes.

environmental factors

... are also important in determining sex in some reptiles; the sexual phenotype of many turtles, crocodiles, alligators, and a few birds is affected by temperature during embryonic development. in turtles warm incubation temperatures produce more females, whereas cool temperatures produce males. In alligators, the reverse is true In a few species, sex chromosomes determine whether individuals are male or female, but ..... can override this chromosomal sex determination. ex: bearded dragons are normally male when ZZ and female when ZW, but when the eggs are incubated at high temperatures, ZZ individuals develop into phenotypic females

gender

... is not the same as sex is a category assigned by the individual or others based on behavior and cultural practices. One's ... need not coincide with one's biological sex.

role of sex chromosomes

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 in 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.

palindrome

A surprising feature revealed by sequencing is the presence of eight massive ... sequences on the Y chromosome. A ... is a sentence or word, such as rotator, that reads the same backward and forward. A ... sequence in DNA 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. still in the 5' to 3' direction. thus a ... sequence in DNA appears twice very much like the two copies of a DNA sequence that are found on two homologous chromosomes. recombination can take place between the two palindromic sequences on the Y chromosome. the lack of inter chromosomal recombination between x and y leads to an accumulation of deleterious mutations on the Y chromosome and the loss of genetic material. theres evidence that the two arms of the y chromosome recombine with each other this compensates for the lack of recombination between x and y This internal recombination may help to maintain some sequences and functions of genes on the Y chromosome and prevent its total degeneration.

y chromosome in animals

Among animals, sex chromosomes 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.

heterogametic sex

Because males produce two different types of gametes with respect to the sex chromosomes, they are said to be the ... ...

bridges explanation

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 an X and a Y chromosome entering one gamete and a single X entering another gamete When these gametes are fertilized by sperm from a normal red-eyed male, white-eyed males and red-eyed females are produced 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. When nondisjunction of the Xs occurs, half of the eggs receive two copies of the X chromosome and the other half receive only a Y chromosome. When these eggs are fertilized by sperm from a normal red-eyed male, four combinations of sex chromosomes are produced. An egg with two X chromosomes that is fertilized by an X-bearing sperm produces an X+XwXw zygote, which usually dies. When an egg carrying two X chromosomes is fertilized 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 an X-bearing sperm produces an X+Y zygote, which develops into a normal RED EYED MALE. If the egg with only a Y chromosome is fertilized by a Y-bearing sperm, the resulting zygote has two Y chromosomes and no X chromosome, and dies. Nondisjunction of the 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

confirmation of bridges hypothesis

Bridges'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 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 a 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.

y linked genetic markers

DNA sequences in the Y chromosome undergo mutation with the passage of time and thus vary among individual males. 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. 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 relationships of present-day people. have been used to study the offspring of Thomas Jefferson. 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 Eston 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. Together with the 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

homogametic sex

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

x linked characteristics

Genes on the X chromosome determine ...

environmental sex determination

In a number of organisms, sex is determined fully or in part by environmental factors an example is the common slipper limpet Each limpet begins life as a swimming larva. The first larva to settle on a solid, unoccupied substrate develops into a female limpet. It then produces chemicals that attract other larvae, which settle on top of it. These larvae develop into males, which then serve as mates for the limpet below. After a period of time, the males on top develop into females and, in turn, attract additional larvae, which settle on top of the stack, develop into males, and serve as mates for the limpets under them. Limpets can form stacks of a dozen or more animals; the uppermost animals are always male.

not equal

In females, there are two copies of the X chromosome and two copies of each autosome, so genes on the X chromosomes and on 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 a protein produced is often a function of the number of gene copies encoding that protein, males are likely to produce smaller amounts of a protein encoded by X-linked genes than of a protein encoded by autosomal genes. This difference can be detrimental because protein concentration often plays a critical role in development. For unknown reasons, 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.

xyy males

Males with an extra Y chromosome (XYY) occur with a frequency of about 1 in 1000 male births individuals have no distinctive physical characteristics other than a tendency to be several inches taller than the average for XY males. 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.

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 is the same as that of X-linked characteristics, except that the pattern of inheritance in males and females is reversed. example would be The female peafowl is ZW and the male is ZZ. Cameo plumage, which produces brown feathers, results from a Z-linked allele (Zca) that is recessive to the wild-type blue allele (ZCa+). 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.

genic sex determination

In some organisms, sex is genetically determined, but there are no obvious differences in the chromosomes of males and females: there are no sex chromosomes. These organisms have ...; genotypes at one or more loci determine the sex of an individual. found in some plants, fungi, protozoans, and fishes.

hermaphroditism

In some species, both sexes are present in the same organism, a condition termed ....

zz zw sex determination

In the ...., found in the bearded dragons discussed in the chapter introduction, the female is heterogametic and the male is homogametic. To prevent confusion with the XX-XY system, the sex chromosomes in this system are called Z and W, but the chromosomes do not resemble Zs and Ws. Females in this system are ZW; after meiosis, half of the eggs have a Z chromosome and the other half have a W chromosome. Males are ZZ; all sperm contain a single Z chromosome. is found in birds, some reptiles, butterflies, some amphibians, and some fishes.

bridges

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 a white-eye allele on its mother's X chromosome; thus, all the 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.

chromosomal sex determination

It is important to understand that even in ... sex is actually determined by individual genes. In mammals, for example, a gene (SRY) located on the Y chromosome determines the male phenotype. In both genic sex determination and ..., sex is controlled by individual genes; the difference is that with ..., the sex chromosomes also look different in males and females.

lyon hypothesis

Lyon proposed, in 1961, that the Barr body was an inactive X chromosome; her hypothesis (now generally accepted for placental mammals) has become known as the She suggested that within each female cell, one of the two X 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. led to important insights into the process of development, the expression of X-linked traits, and X-linked genetic diseases. you always want to leave one x with no barr body the rest contain barr body.

morgans experiment

Morgan's results from this initial cross were consistent with Mendel's principles: a cross between a homozygous dominant individual and a homozygous recessive individual produced heterozygous offspring exhibiting the dominant trait. 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 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 the expected result for a simple recessive trait, which should appear in one-fourth of both male and female F2 offspring. To explain this unexpected result, Morgan 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 alleles are present on the X chromosome only; no 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. To verify his hypothesis that the white-eye trait is X linked, Morgan conducted additional crosses. He predicted that a cross between a white-eyed female and a red-eyed male would produce all red-eyed females and all white-eyed males. When Morgan performed this cross, the results were exactly as predicted In all these crosses, the results were consistent with Morgan's conclusion that the white-eye trait is an X-linked characteristic.

barr bodies

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

effects of palindromic sequences

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 /n palindormic sequences leands to rearrangements of the Y chromosome that cause anomalies of sexual development. sometime recombination 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 determining systems

SYSTEM - MECHANISM - HETEROGAMTEIC SEX -ORGANISMS XX-XO - Females XX - Males X - Male - Some grasshoppers and other insects XX- XY - Females XX - Males XY - Male - Many insects, fishes, amphibians, reptiles; mammals, including humans ZZ-ZW - Females ZW - Males ZZ - Female - Butterflies, birds; some reptiles and amphibians Genic sex determination - No distinct sex chromosomes Sex determined by genes on undifferentiated chromosomes - Varies - Some plants, fungi, protozoans, and fishes Environmental sex determination - Sex determined by environmental factors - None - Some invertebrates, turtles, and alligators

dosage compensation

Some animals have 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 .... In fruit flies, dosage compensation is achieved by a 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.

symbols for x linked genes

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: for example, the genotype of a female Drosophila with white eyes is ww, and the genotype of a white-eyed hemizygous male is w. Another method is to include the Y chromosome, designating it with a diagonal slash (/). With this method, the white-eyed female's genotype is still ww, and the white-eyed male's genotype is w/. Perhaps the most useful method is to record the X and Y chromosomes in the genotype, designating the X-linked alleles with superscripts

dioecious

Species in which an individual organism has either male or female reproductive structures are said to be .... ("two houses") -humans are .... -among .... species, sex may be determined chromosomally, genetically, or environmentally.

hemizygous

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 ... for X-linked loci.

sex determining region y

The male-determining gene in humans, called the .... (SRY) gene. 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. The SRY gene, located on the Y chromosome, causes the development of male characteristics. Found this out by injecting gene to mice that were XX, they later developed male anatomy this gene is Y linked because it is found only on the Y chromosome SRY gene encodes a protein called a transcription factor which binds to DNA and stimulates the transcription of genes that promote the differentiation of the testes. SRY is the primary determinant of maleness in humans, but a number of other genes (some X linked, others Y linked, and still others autosomal) also have roles in fertility and the development of sexual phenotypes.

sex determination

The mechanism by which biological sex is established is termed ... .... We define the sex of an individual organism in reference to its phenotype. 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 males have one X chromosome and one Y chromosome. A few rare individuals have male anatomy although their cells each contain two X chromosomes. Even though these people are genetically female, we refer to them as male because their sexual phenotype is male.

androgen insensitivity syndrome

These individuals have female external sexual characteristics. Indeed, most are unaware of their condition until they reach puberty and fail to menstruate. 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 males. The cells of a woman with ...... contain an X and a Y chromosome. In a human embryo with a Y chromosome, the SRY gene causes the gonads to develop into testes, which produce testosterone. Normally, testosterone stimulates embryonic tissues 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 characteristics develop. The gene for the androgen receptor is located on the X chromosome, so people with this condition always inherit it from their mothers.

male determining gene

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

y linked genetic markers

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

turner syndrome

are female and often have underdeveloped secondary sex characteristics. 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. intelligence is usually within the normal range Many women who have .... are sterile had only a single X chromosome; this chromosome complement is usually referred to as XO. people with ... have some cells that are XX and other cells that are XO, a situation referred to as mosaicism 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.embryos missing both Xs spontaneously abort in the early stages of development.

poly x females/ triple x syndrome

about 1 in 1000 female births, the infant's cells possess three X chromosomes, a condition often referred to as ... 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. 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.

in early development

all humans possess undifferentiated gonads and both male and female reproductive ducts. about six weeks after fertilization, a gene on the Y chromosome becomes active. This gene causes the neutral gonads to develop into testes, which begin to secrete two hormones: testosterone and anti-Müllerian hormone. Testosterone and other androgens induce the development of male characteristics, and anti-Müllerian 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.

y linked characteristics

are also called holandric traits- exhibit a 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

x inactivation

as a result of .... 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.

other animals

birds, butterflies, and some reptiles, sex is still chromosomal but it is the male that possesses two identical sex chromosomes (called Z chromosomes) and the female that has two different sex chromosomes (called Z and W).

sex linked characteristics

characteristics determined by genes located on the sex chromosomes Because the Y chromosome of many organisms contains little genetic information, most sex-linked characteristics are X linked. Males and females differ in their sex chromosomes, so the pattern of inheritance for .... differs from that exhibited by genes located on autosomes.

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. also developed many techniques that are used today in mouse genetics, helping to make the mouse an important model genetic organism.

y linked characteristics

genes on the Y chromosome determine ....

sex determination in humans

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 result from abnormal numbers of sex chromosomes, which arise when the sex chromosomes do not segregate properly in meiosis or mitosis, illustrate the importance of the Y chromosome in human sex determination.

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. 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

xx xo sex determination

is one of the simplest mechanisms of chromosomal sex determination in this system females have 2 X chromosomes (XX) and males posses a single X chromosome (XO). There is no O chromosome—the letter O signifies the absence of a sex chromosome. In meiosis in females, the two X chromosomes pair and then separate, and one X chromosome enters each haploid egg. In males, the single X chromosome segregates in meiosis to half the sperm cells; the other half receive no sex chromosome the sex of an individual is therefore determined by which type of male gamete fertilizes the egg. 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 XO zygotes, which develop into males.

sexual reproduction

is the 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. consists of two processes that lead to an alternation of haploid and diploid cells: meiosis produces haploid gametes (spores in plants), and fertilization produces diploid zygotes 1)Meiosis produces haploid gametes 2)Fertilization (fusion of gametes) produces diploid zygote

x and y chromosomes

like other pairs of chromosomes, the X and Y sex chromosomes pair in the course of meiosis and segregate, but throughout most of their length, they are not homologous (their gene sequences do not encode the same characteristics): most genes on the X chromosome are different from genes on the Y chromosome. Consequently, males and females do not possess the same numbers of alleles at sex-linked loci. This difference in the number of sex-linked alleles produces distinct patterns of inheritance in males and females.

colorblindness

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 may therefore appear to alternate between the sexes, appearing in females in one generation and in males in the next generation. Recall that the X and Y chromosomes 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.

monoecious

organisms that bear both male and female reproductive structures are said to be ... (meaning "one house")

x linked color blindness

red-green color blindness in humans. The human eye actually detects only three colors—blue, red, and green—but the brain mixes the signals from different cone cells to create the 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 the red pigments lie close together on the X chromosome. Mutations that produce defective color vision are generally recessive, and because the genes encoding the red and the green pigments are located on the X chromosome, red-green color blindness is inherited as an X-linked recessive trait. Females possess 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. normal color vision female X+X+ * color blind male XcY = normal color vision female and male color blind female XcXc* normal color vision male X+Y = normal color vision female and color blind male A female is color blind only when she inherits color-blindness alleles from both parents, whereas a male need only inherit a color-blindness allele from his mother to be color blind; for this reason, color blindness and most other rare X-linked recessive traits are more common in males than in females.

sex

refers to sexual phenotype. Most organisms have two sexual phenotypes: male and female The fundamental difference between males and females is gamete size: males produce small gametes, and females produce relatively large gametes

biological sex

refers to the anatomical and physiological phenotype of an individual.

random x inactivation

requires 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. 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.

characteristics of the y chromosome

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-encoding genes found on the Y chromosome have homologous genes on the X chromosome. 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 suggests that it carries genetic elements that affect the expression of numerous genes on autosomes and X chromosomes.

sex determination

sex can be determined by genetics (sex chromosomes) or through the environment or both like in beaded dragons.

sex chromsomes

sex in many organism is determined by a pair of chromosomes called the ... .... We think of sex in organisms with sex chromosomes as being determined by the presence of the sex chromosomes but in fact, the individual genes located on the sex chromosomes, in conjunction with the genes on the autosomes, are usually responsible for the sexual phenotypes.

humans

sex is determined by chromosomes in ....: females possess two identical sex chromosomes (X chromosomes), and males possess a single X and a different chromosome, called Y.

turtles and alligators

sex is not determined by sex chromosomes at all but is primarily a function of the environment sex is influenced by the temperature at which the eggs are incubated, with males produced at one temperature and females at another.

the chromosomes theory of inheritance

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

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. The phenotypes associated with these conditions probably arise because some X-linked genes escape inactivation. the nature of X inactivation is more complex than originally envisioned. Studies of individual genes now reveal that only about 75% of X-linked human genes are permanently inactivated. About 15% completely escape X inactivation, meaning that these genes 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

random x inactivation

takes place early in development—in humans, it occurs within the first few weeks of development. After an X chromosome has become inactivated in a cell, it remains inactive in that cell and in all somatic cells that descend from that 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. Although many genes contribute to coat color and pattern in domestic cats, a single X-linked locus determines the presence of orange color. Rare tortoiseshell males can arise from the presence of two X chromosomes, X+XoY. 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 a clone of cells derived from an original cell in which the orange allele was inactivated. The patchy distribution of color on tortoiseshell cats results from the random inactivation of one X chromosome in females.

evolution of the y chromosome

the X and Y chromosomes in many organisms evolved from a pair of autosomes The 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 happened 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. Natural selection suppresses crossing over for most of the length of the X and Y chromosomes, preventing genes that encode male traits from appearing in females. crossing over can still happen between two female XX chromosomes but there is little to none between X and Y, except for the small psudoautosomal regions in which the X and Y continue to pair in meiosis.

x and y chromosomes

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 unites 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 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. XX*XY= 1:1 male and female

sex determination in fruit fly

the X: ration predicts sexual phenotype of fruit flies correctly however there is research that suggests that the mechanism of sexx determination is not a balance between X linked genes and autosomal genes. 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. 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. the number of autosomal sets of chromosomes influences sex determination in Drosophila, but not through the action of autosomal genes, as envisioned by Bridges

xx xy sex determination

the cells of males and females have the same number of chromosomes, but the cells of females have two X chromosomes (XX) and the cells of males have a single X chromosome and a 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. In this 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. is found in many organisms including some plants, insects, and reptiles, and all mammals (including humans)

sex determination in fruit fly

the fruit fly has 8 chromosomes. three pairs of autosomes and one pair of sex chromosomes. -females will usually have 2 X chromosomes and males have an X chromosome and a Y chromosome. -sex was determined not by the number of X and Y chromosomes but 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 that is the number of X chromosomes divided by the number of haploid sets of autosomes. -Normal flies possess two haploid sets of autosomes and either two X chromosomes (females) or one X chromosome and a Y chromosome (males). an 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 a mixture of male and female characteristics. An X : A ratio of less than 0.5 is metamales greater than 1.0 produces metafemales which are developmentally abnormal flies

sex linked

the inheritance of characteristics encoded by genes located on the sex chromosomes, which often differ between males and females. These characteristics, and the genes that produce them, are referred to as ... ....

autosomes

the nonsex chromosomes, which are the same for males and females

nondisjunction

the two X chromosomes in the females fail to separate in anaphase I of meiosis, a phenomenon known as ... when .... of the Xs occurs, half of the eggs receive two copies of the X chromosome and the other half receive only a Y chromosome

male

they have an X and a Y chromosome

female

they have an X and an X chromosome

lack of crossing over

this has lead to an accumulation of mutations on and a loss of genetic material from the Y chromosome. 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 in males today. the y chromosomes is small in humans and many other animals therefore it carries a small amount of genetic information. therefore a few characteristics exhibit y linked inheritance. some predict that the human Y chromosome will continue to lose genetic information in the future and will completely disappear from the species in about 10 my. however new research suggest that decay of the human y chromosome has come to a halt and that no genes have been lost is the past millions of years. those genes that remain on the y chromosome are very stable. internal recombination may have aided in slowing down or preventing complete decay of the human y chromosome.