6.1 Sexual Development
1. The male determining gene, SRY, has been found on the Y chromosome.
2. Female is not the "default" sex, multiple genes determine the development of female structures.

Sex Chromosomes
1. Human females are homogametic (XX) and males are heterogametic (XY).
2. The Y chromosome contains few identified genes, while the larger X chromosome contains over 1,500 genes.
3. The Human Genome Project has facilitated study of the Y chromosome.
4. Pseudoautosomal regions at both tips of the Y chromosome contain genes that have counterparts on the X chromosome.
5. The male-specific region (MSY) lies between the pseudoautosomal ends of the Y chromosome.

The Phenotype Forms
1. The SRY gene is a single copy transcription factor unique to the Y chromosome.
2. The SRY protein triggers a cascade of gene action that initiates development of male features while suppressing development of female features.
3. Factors necessary for male development include Anti-Müllerian Hormone, testosterone, and dihydrotestosterone (DHT).
4. In the absence of these male factors, the embryo follows the female developmental pathway.
5. Pseudohermaphrodites are chromosomally one sex, but look phenotypically like the other sex.

Is Homosexuality Inherited?
1. Evidence is accumulating that homosexuality is partially inherited.
2. The biological basis of homosexuality is being studied using twin studies and model organsisms, such as the fruit fly.

6.2 Traits Inherited on Sex Chromosomes
1. Y-linked traits are passed on the Y chromosome, and X-linked traits on the X.
2. Males are hemizygous for X-linked traits, expressing genes on the X chromosome. Females express homozygous recessive alleles on the X chromosome.

X-Linked Recessive Inheritance
1. X-linked recessive traits pass from carrier mothers to sons with a 50% probability, and can be passed from hemizygous men to their daughters, if the phenotype is not severe. There is no father to son transmission of X-linked traits.

X-Linked Dominant Inheritance
1. X-linked dominant conditions are expressed in both males and females.
2. These conditions are generally more severe in males.

6.3 X Inactivation Equalizes the Sexes
1. Early in female development, the maternal or paternal X chromosome is turned off in each cell.
2. The X inactivation center is responsible for shutting down most of the chromosome.
3. X inactivation compensates for differences between males in females in the number of gene copies on the X chromosome.
4. The XIST gene encodes RNA that inactivates these genes.
5. The inactivated X takes up stain and appears as a Barr body.
6. The Lyon hypothesis proposed that the Barr body was the inactive X chromosome.
5. A female may express a sex-linked trait if the mutant allele is on the active X in affected tissues. This is called a manifesting heterozygote.

6.4 Sex-Limited and Sex-Influenced Traits
Sex-limited Traits
1. A sex-limited trait (which may be autosomal or sex-linked) affects body parts or functions present in only one gender.

Sex-influenced Traits
1. In a sex-influenced trait, an allele is dominant in one sex but not the other.

6.5 Genomic Imprinting
Silencing the Contribution From One Parent
1. Imprinting is an epigenetic alteration, in which a layer of meaning is stamped upon a gene without changing its DNA sequence.
2. As oocyte and sperm form, the protective groups shielding their imprinted genes are stripped away, and new patterns are set down, depending on whether the fertilized ovum is male or female.
3. Genomic imprinting may explain incomplete penetrance.

Imprinting Disorders in Humans
1. 600 genes in humans are known to be imprinted, with 30 known diseases resulting from imprinting problems.
2. Prader-Willi and Angelman syndromes are examples of genomic imprinting.

A Sheep with a Giant Rear End
1. Genomic imprinting is easier to observe in a species that can be bred so traits can be followed over several generations.
2. "Solid Gold," a ram with overly muscled hindquarters offered researchers further study of inherited traits. This is due to a gene called callipyge, which is also found in humans.