Ch.6: Sexual Identity Genetics

Males (XY) are the heterogametic sex. Females (XX) are homogametic sex. X chromosome has over 1,000 genes. Y chromosome is much smaller and are not sure how many genes it carries. Primary sex ratio (fertilization)-160 males for every 100 females Secondary sex ratio (at birth)-105 males for every 100 females Tertiary sex ratio- (20-25 yrs of age) 1:1 Thereafter, females outnumber males.

All embryos contain the same 2-sided unspecialized gonads until the 6 th week of pregnancy. (See Fig.6.1, p.114) Mullerian ducts-potential female structures Wolffian ducts-potential male structures On the Y chromosome is the male determining gene called the SRY gene which stands for the sex-determining region of the Y. If the SRY gene is not expressed, the gonad develops into an ovary. If the SRY gene is expressed, the gonads specializes as testes.

Once the SRY gene has directed the testes to form: 1)Sustentacular cells (Sertoli cells) in the testes secrete anti-Mullerian hormone that stops those ducts from forming female structures. 2)Interstitial cells (Leydig cells) in testes secrete testosterone which stimulates development of internal male structures. 3)Some testosterone is converted to DHT (dihydrotestosterone) which directs the development of external male structures.

Genetic Abnormalities 1. Testicular feminization- (X-linked trait)- chromosomal males develop as females. Testes formation is normal & testosterone & MIH production begins as expected. MIH (mullerian - inhibiting hormone) brings about degeneration of mullerian duct so no internal female traits are produced. Mutation in X chromosome gene blocks ability of cells to respond to DHT or testosterone. Individuals develop female structures. They are often attractive females, but with little pubic hair and no menstrual cycle.

Male Pseudohermaphroditism- (autosomal recessive gene) testes are present (indicate that SRY gene is functioning) and Anti-mullerian hormone is produced b/c the female set of tubes has degenerated, no male external structures till age 12.

Homosexuality Researchers believe that homosexuality is 50% genetically controlled and 50% environmentally controlled. Evidence presented 1.Feelings at a young age before knowing meaning of term. 2.Timing of hormonal surges during prenatal development show impaired mating ability. 3.Twin studies suggest genetic influence. 4.Two brain areas are of different sizes in homosexual men vs. heterosexual men. 5.Studies by Hamer suggest that genes causing homosexuality reside on X chromosome. 6.Studies have shown that if 2 brothers are homosexual and 1 brother is heterosexual that the heterosexual brother does not share the X chromosome markers. 7.Alterations in male fruit flies’ genes have led to homosexual behavior.

Sex-linked traits in humans are determined by genes that are carried only on the X chromosome with no apparent alleles on the Y chromosome. Males are considered hemizygous for sex-linked traits because they have half the number of sex-linked genes that the female has. If a trait is sex-linked, a male will pass it on to all his daughters who may be heterozygous or homozygous for the condition. A female heterozygote will pass a recessive trait to 50% of her sons, each of whom will express the recessive phenotype.

Lyon Hypothesis Simply stated: compensation is accomplished by inactivating or turning off ALL the genes on ONE OF THE X chromosomes in females. 1. In the somatic cells of the female mammals, one X chromosome is active and the second X chromosome is inactive and tightly coiled to form the Barr Body. A normal male has NO barr bodies b/c his one X chromosome remains active. 2.The inactive chromosome can be from the father, or from the mother. In different cells of the same individual, a female expresses the X chromosome genes inherited from the father in some cells and those from her mother in other cells. 3.Inactivation takes place early in development. After 8-10 rounds of mitosis following fertilization, each cell of the embryo randomly inactivates one X chromosome. This inactivation is permanent in somatic cells but not in the germ line cells which will become the oocytes.

Lyons hypothesis also offers the idea that females are mosaics, constructed of two different cell types: one with the maternal X chromosome active and one with the paternal X chromosome active. Ex: tortoise-shell cat and calico cats; Almost always female, the only way a male cat can have the coat color pattern is if he inherits an extra X chromosome.

X inactivation has no effect on females homozygous for sex-linked genes. However, for heterozygous sex-linked genes X inactivation leads to the expression of one allele or the other. Manifesting heterozygote-a female carrier of a sex-linked recessive gene who expresses the phenotype because the wild type allele (normal) is inactivated in some affected tissues. X inactivation also has valuable medical application in detecting carriers of some sex- linked disorders.

Sex-limited Traits-affects a structure or function of the body that is present in only males or females. Ex: breast development, facial hair growth and heavy beard Sex-influenced Traits-traits that are expressed in both sexes but the results of the expression from allele is dominant in one sex but recessive in the other. Ex: Male pattern baldness

Genes on the X chromosome are classified as dominant or recessive based on the patterns of inheritance in the females. X-linked Recessive traits: hemophilia, Duchennes muscular dystrophy, color blindness. 1.Affected males pass to all daughters. 2.Never transmitted from father to son but from grandfathers to grandsons. 3.Phenotypic expression more frequent in males than females.

X-linked Dominant Inheritance-a female who inherits a dominant sex-linked allele has the illness or trait. A male who inherits the dominant allele is usually more severely affected because he has no other corresponding allele. 1.Affected males produce all affected daughters and no affected sons. 2.A heterozygous affected female will transmit the trait to ½ her children, with males and females equally affected. 3.A homozygous female will transmit the trait to all her offspring.