Inheritance Patterns Related to Gender Determination

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

2. 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

3. 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)

4. 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

5. 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

6. 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)

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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 femalesw+ w: Heterozygous wild type femalesw w: Homozygous white-eyed femalesw+ Y: Hemizygous wild type malesw Y: Hemizygous white-eyed type males

13. 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 theX chromosome (that has the X-linked genes), and ½ the sperm get the Y chromosome

14. 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 wY males

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

16. 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

17. 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

18. 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

19. C. Sex-limited & Sex-Influenced Traits • Sex-influenced trait: • 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