In many organisms, sex is determined by chromosomes. Focusing on human XY sex determination, let us study various sex-determination mechanisms and patterns of sex-linked inheritance. Because sex-linked genes produce traits at different frequencies in males and females, they frequently appear in genetics problems.
Basic Knowledge
XY sex determination in humans: Human somatic cells contain 46 chromosomes (23 pairs), one of which is the pair of sex chromosomes. Females = XX; males = XY. The SRY gene on the Y chromosome induces testis differentiation, and testosterone secreted by the testes causes male secondary sexual characteristics.
Other methods of sex determination: ZW type (birds and some reptiles) = ZZ → male, ZW → female. X0 type (grasshoppers and others) = XX → female, X0 → male (no Y chromosome). Sex reversal (sex determined by the environment: clownfish and the incubation temperature of turtles). Sex determination in plants is diverse.
Sex-linked inheritance: The inheritance of genes on the X chromosome. Because males have only one X chromosome (hemizygous), X-linked recessive traits are more likely to appear in their phenotypes. Examples: red-green color vision deficiency (8% of males, 0.5% of females) and hemophilia (X-linked recessive).
For a son of a carrier mother of an X-linked recessive trait: 50% are affected. For a daughter of an affected father of an X-linked recessive trait: all are carriers.
X-chromosome inactivation (Barr body): In female somatic cells, one of the two X chromosomes is randomly inactivated (lyonization) and can be observed in the nucleus as a Barr body. Females are X-chromosome mosaics, providing the basis for the coat-color pattern of calico cats.
Find the proportions of genotypes and phenotypes among children born to a carrier female (Xᴬxᵃ) for red-green color vision deficiency (X-linked recessive) and a normal male (Xᴬ Y).
Answer: The mother's gametes are Xᴬ or xᵃ (each 1/2). The father's gametes are Xᴬ or Y (each 1/2). The combinations are XᴬXᴬ (normal female, 1/4), Xᴬxᵃ (carrier female, 1/4), XᴬY (normal male, 1/4), and xᵃY (male with color vision deficiency, 1/4). Half of the sons are affected. All daughters are normal (including carriers).
- Humans = XY type (XX female, XY male); the SRY gene induces testis development
- ZW type = birds (ZZ male, ZW female)
- X0 type = grasshoppers and others (XX female, X0 male)
- Sex-linked inheritance = X-linked genes; males are hemizygous
- X-linked recessive traits = appear more often in males (red-green color vision deficiency, hemophilia)
- Barr body = an inactivated X chromosome; one occurs in female somatic cells
- Calico cats = a mosaic resulting from random X-chromosome inactivation (lyonization)
Points to Note
① An affected father with an X-linked recessive condition does not pass the condition to his sons because he passes on the Y chromosome. ② For a female to be affected by an X-linked recessive condition, recessive alleles must be present on both X chromosomes (XX). ③ Number of Barr bodies = number of X chromosomes - 1 (normal male = 0; normal female = 1).
Practice
- Explain the role of the SRY gene in XY sex determination.
- When a carrier female for hemophilia (X-linked recessive) marries a man with hemophilia, find the probabilities that their daughters and sons will be affected.
- What is a Barr body? State the relationship between its number and the sex chromosomes.