Inheritance Patterns Related to Gender Determination Mechanisms of gender determination X-linkage Sex-limited and sex-influenced traits

A. Mechanisms of Gender Determination Chromosomal mechanisms Gender determined by the presence or absence of certain chromosomes called sex chromosomes Other “non-sex” chromosomes are called autosomes Not all genes on a sex chromosome are necessarily related to gender or reproduction Genic mechanisms Gender determined by the activity of genes located on several chromosomes

A. Mechanisms of Gender Determination X-O mechanisms In grasshoppers and related insects A single sex chromosome (X) Females are have two copies of the X chromosome (XX); each ovum produced will carry an X chromosome (females are homogametous) Males have only one copy of X, and no other sex chromosome (XO); half the sperm carry an X, and the other half have no sex chromosome (males are heterogametous)

A. Mechanisms of Gender Determination X-Y mechanisms Females are XX (homogametous gender) males are XY (heterogametous gender) In mammals: The embryonic gonads develop into testes in response to the expression of a “testes determining factor” gene on the Y chromosome; Otherwise, the gonads develop into ovaries The Y chromosome also has a region that has sequence homology with the X chromosome, allowing pairing and segregation during meiosis

A. Mechanisms of Gender Determination X-Y mechanisms In Drosophila and similar insects, gender is determined by the ratio of X chromosomes and autosomes In both mammals and Drosophila, the X chromosome carries many genes not related to gender or reproduction Genes located on the X chromosome are called X-linked genes

A. Mechanisms of Gender Determination Z-W mechanisms In birds and some reptiles Z chromosome is similar to the X; W similar to Y Except . . . Males are homogametous (ZZ) and females are heterogametous (ZW)

A. Mechanisms of Gender Determination Diploid-haploid mechanisms In bees and wasps Females (queen and worker bees) develop from fertilized eggs and have diploid cells Males (drones) develop from unfertilized eggs and have haploid cells

B. X-linkage Discovered by T.H. Morgan around 1910 First X-linked gene to be discovered: white eye gene in Drosophila Morgan’s students found a single rare, mutant white eyed male fly and, of course, wanted to characterize it by a monohybrid cross

B. X-linkage P: White eyed male X wild type female  F1: All wild type, males & females F1 X F1 F2: ½ wild type females ¼ wild type males ¼ white eyed males

B. X-linkage Obviously, the gene for white eye is recessive to its allele for normal eye color What was unusual was that all of the F2 recessive offspring were males – no females Morgan would have expected to find equal numbers of males and females in the F2 phenotypes With autosomal genes, one expects an F2 ratio of 3/8 dominant females: 1/8 recessive females: 3/8 dominant males: 1/8 recessive males

B. X-linkage Morgan deduced that the white eye results could be explained if the gene for white eye color is located on the X chromosome This was the first direct evidence that genes are located on chromosomes

B. X-linkage Genotypic explanation of Drosophila white eye: One gene, X-linked, with two alleles, w+ & w Different genotypes in males and females: w+ w+ : Homozygous wild type females w+ w : Heterozygous wild type females w w : Homozygous white-eyed females w+ Y : Hemizygous wild type males w Y : Hemizygous white-eyed type males

B. X-linkage Note that, with respect to X-linked genes, males are neither homozygous nor heterozygous, but hemizygous Also note that the “Y” designates the Y chromosome in males During spermatogenesis, ½ the sperm get the X chromosome (that has the X-linked genes), and ½ the sperm get the Y chromosome

B. X-linkage So, for the white-eye monohybrid cross: P: w Y male X w+ w+ female  F1: w+ Y males & w+ w females F1 X F1 F2: ¼ w+ w+ females ¼ w+ w females ¼ w+ Y males ¼ w Y males

B. X-linkage Morgan found that a testcross would produce white-eyed (w w) females: w Y male & w+ w females  ¼ w+ w females ¼ w w females ¼ w+ Y males ¼ w Y males

B. X-linkage There are many X-linked traits in humans Example: Red-green color blindness Normal female (N N) X Colorblind male (n Y)  ½ N n (Carrier females) & ½ N Y (Normal males) Carrier female (N n) X Normal male (N Y) ¼ N N, ¼ N n, ¼ N Y, ¼ n Y

B. X-linkage Z-linked traits in birds exhibit similar inheritance patterns, except that it is the male bird that is homogametous Example: barred feathers in chickens

C. Sex-limited & Sex-Influenced Traits Sex-limited trait: A trait, produced by an autosomal gene, in which the expression of a specific genotype is limited to only one gender Example: Hen-feather vs. rooster-feather patterns in chickens

C. Sex-limited & Sex-Influenced Traits A trait, produced by an autosomal gene, in which an allele is dominant in one gender but is recessive in the other gender Example: Pattern baldness in humans