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| 6.1 Sexual DevelopmentSex Chromosomes 1. Sex can be considered at chromosomal, gonadal, phenotypic, and gender identity levels. 2. Human females are homogametic (XX) and males are heterogametic (XY). 3. The Y chromosome contains few identified genes, while the larger X chromosome contains several thousand genes. Pseudoautosomal regions at both tips of the Y chromosome contain genes that have counterparts on the X chromosome. 4. The male determining gene, SRY, has been found on the Y chromosome. The Phenotype Forms 1. The SRY gene is a single copy gene 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. Gender Identity-Is Homosexuality Inherited? 1. Evidence is accumulating that genes contribute to homosexuality. 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. X inactivation compensates for differences between males in females in the number of gene copies on the X chromosome. 3. The XIST gene encodes RNA that inactivates these genes. 4. The inactivated X takes up stain and appears as a Barr body. 5. A female may express a sex-linked trait if the mutant allele is on the active X in affected tissues. 6. The Lyon hypothesis proposed that the Barr body was the inactive X chromosome. 6.4 Gender Effects on Phenotype 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. A sex-influenced allele is dominant in one sex but recessive in the other. Genomic Imprinting 1. In genomic imprinting, the phenotype differs depending on which parent transmits a gene. Methyl groups may suppress gene expression in a pattern determined by sex. |