Human Genetics: Concepts and Applications (Lewis), 9th Edition

Chapter 6: Matters of Sex

Chapter Outline

CHAPTER OVERVIEW

1. Maleness and femaleness is a trait that is genetically determined at conception
2. Sexual identity and sexual feelings are complex traits sculpted over time by the interaction of biological and environmental factors.

1. Around the sixth week of embryonic development, sexual differentiation begins to transform indifferent gonads into testes or ovaries
2. A gene on the Y chromosome called SRY is activated in males

1. Human females are homogametic (XX) and males are heterogametic (XY).
2. In some species, such as birds and snakes, females are heterogametic and males are homogametic.
3. The Y chromosome contains few identified genes, while the larger X chromosome contains several thousand genes.
4. Pseudoautosomal regions at both tips of the Y chromosome contain genes that have counterparts on the X chromosome.
5. The male determining gene, SRY, has been found on the Y chromosome.

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. Abnormalities in these male factors may disrupt the developmental pathway. Individuals may be chromosomally one sex, but look phenotypically like the other sex.
5. The term hermaphrodite has traditionally been used for individuals with both male and female sexual structures. Pseudohermaphrodites exhibit male and female sexual structures at different ages.

1. Evidence is accumulating that genes and environment contribute to homosexuality.

1. Mendelian principles predict approximately equal numbers of males and females at birth.
2. Sex ratio is the proportion of males to females in a population. The primary sex ratio is defined as the sex ratio at conception.
3. In human populations, boys are expected to slightly outnumber girls at birth. This ratio may be altered by societal practices including sex selection, abortion and infanticide.
4. Sex ratios change later in life reflecting differential environmental and medical influences on males and females.

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. They have one copy of X-linked genes and these are expressed in the phenotype.
3. Females express X-linked traits like those that are autosomally inherited. Two copies of homozygous recessive alleles are required for expression.

1. Heterozygous women who lack symptoms are called carriers.
2. X-linked recessive traits pass from carrier mothers to sons with a 50% probability.
3. The allele can be passed from hemizygous men to their daughters, if the phenotype is not severe.
4. There is no father to son transmission of X-linked traits.

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

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

1. A sex-influenced allele is dominant in one sex but recessive in the other.
2. Hormonal differences between males and females typically cause the differential expression.

1. X inactivation compensates for the difference in the number of X chromosomes between males and females.

1. Early in female development, the maternal or paternal X chromosome is mostly turned off in each cell.
2. The XIST gene encodes RNA that inactivates these genes.
3. The inactivated X takes up stain and appears as a Barr body.
4. Geneticist Mary Lyon proposed that the Barr body was the inactive X chromosome.

1. A female may express a sex-linked trait if the mutant allele is on the active X in affected tissues.
2. A carrier of an X-linked trait who expresses the phenotype due to X inactivation is referred to as a manifesting heterozygote.
3. A number of human and animal conditions demonstrate the effect of X inactivation on phenotype. Tortoise shell and calico cats are a recognizable example.

1. X inactivation of either the maternal or paternal X chromosome leads to two different cell populations in the female.

1. In genomic imprinting, the phenotype differs depending on which parent transmits a gene.
2. Methyl groups suppress gene expression in a pattern determined by sex.

1. Genomic imprinting is epigenetic and does not alter the DNA sequence. It is maintained by mitosis but not meiosis.
2. The function of this mechanism is poorly understood.

1. About 156 human genes are imprinted.
2. Around 60 of these may affect health when abnormally expressed.
3. Prader-Willi and Angelman syndromes are examples of imprinting disorders in humans.