LECTURE CONNECTIONS 4 | Sex Determination and Sex-Linked © 2009 W. H. Freeman and Company Characteristics

Sex Determination Mechanisms Hermaphroditism: both sexes in the same organism Monoecious: both male and female reproductive structures in the same organism Dioecious: either male or female reproductive structures in one organism

Chromosomal Sex-Determination Systems: Sex chromosomes and non-sex chromosomes (autosomes) XX-XO system: XX – female XO – male Example: grasshoppers XX-XY system: XX – female XY – male Example: some plants, insects, reptiles, and all mammals

Concept Check 1 How does the heterogametic sex differ from the homogametic sex? a.The heterogametic sex is male; the homogametic sex is female. b.Gametes of the heterogametic sex have different sex chromosomes; gametes of homogametic sex have the same sex chromosome. c.Gametes of the heterogametic sex all contain a Y chromosome. d.Gametes of the homogametic sex all contain an X chromosome.

Chromosomal Sex-Determination Systems ZZ-ZW system: ZZ – male ZW – female Examples: Birds, snakes, butterflies, some amphibians, and fishes Which one is the homogametic?

Haplodiploidy system: Haploid set – male Diploid set – female Examples: Bees, wasps, and ants

Genic Sex-Determining System In some plants and protozoans, sex is genetically determined, but there are no obvious differences in the chromosomes of males and females: there are no sex chromosomes. Genic sex determination: genotypes at one or more loci determine the sex of an individual plant or protozoan. No sex chromosomes, only the sex-determining genes

Environmental Sex Determination Environmental factors : Limpet’s position in the stack Temperature in turtles

Sex Determination in Drosophila melanogaster D. melanogaster has 8 chromosomes: 3 pairs of autosomes + 1 par of sex chromosomes Genic balance system: in D. melanogaster, the dosage of X chromosomes determines sex. For example, both XY and XO flies are phenotypically male. In contrast, both XX and XXY flies are phenotypically female. X : A ratio (X, number of X chromosomes; A, number of haploid sets of autosomes)

Concept Check 2 What will be the sexual phenotype of a fruit fly with XXYYY sex chromosomes and two sets of autosomes? a.male b.female c.Intersex d.metamale

Sex Determination in Humans XX-XY SRY gene on the Y chromosome determines maleness Turner syndrome: XO; 1/3000 female births Klinefelter syndrome: XXY, or XXXY, or XXXXY, or XXYY; 1/1000 male births Poly-X females: 1/1000 female births, XXX

The Role of Sex Chromosomes The X chromosome contains genetic information essential for both sexes; at least one copy of an X is required. The male-determining gene is located on the Y chromosome. A single Y, even in the presence of several X, still produces a male phenotype. The absence of Y results in a female phenotype.

The male-determining gene in humans About 6 weeks after fertilization: testosterone and Mullerian-inhibiting substance are secreted by a gene in the Y chromosome. Sex-determining region Y (SRY) gene

The male-determining gene in humans Androgen-insensitivity syndrome Caused by the defective androgen receptor

4.2 Sex-Linked Characteristics Are Determined by Genes on the Sex Chromosomes X-Linked White Eyes in Drosophila Nondisjunction and the Chromosome Theory of Inheritance X-Linked Color Blindness in Humans Symbols for X-Linked Genes Dosage Compensation Z-Linked Characteristics Y-Linked Characteristics

X-Linked Characteristics X-linked white eyes in Drosophila