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 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

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 the X 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 w Y males

15. 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

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
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