Exercises: Sex-linked inheritance

Explain why genes on the X chromosome are inherited differently in men and in women, and clarify what it means that, for those genes, a man is hemizygous.

In the sex-determination system characteristic of mammals, explain why it is the father who determines the sex of the offspring and what the probability of having a son or a daughter is in each pregnancy. Reason your answer from the gametes produced by each parent.

What is holandric inheritance? Explain why a trait linked to the Y chromosome is always transmitted from father to son and why no woman can show it or transmit it.

Common baldness is a sex-influenced trait: the allele B behaves as dominant in men but as recessive in women, so that only BB women are bald, while BB and Bb men are bald. A heterozygous bald man has children with a heterozygous non-bald woman. a) State the genotypes of both. b) Calculate the probability that a son will be bald. c) Calculate the probability that a daughter will be bald. d) Explain the difference between sex-influenced inheritance and sex-linked inheritance.

Auricular hypertrichosis (abundant hair growth on the ears) has been described as a trait linked to the Y chromosome. A man who shows it has four sons and three daughters. a) State what proportion of his sons would be expected to show the trait. b) State what proportion of his daughters will show it. c) Justify both answers from the transmission of the sex chromosomes.

The phenomenon of gene dosage compensation through random inactivation of one of the X chromosomes (Lyon hypothesis) leads to complex phenotypic situations. Analyse the following three independent cases:

1. a) A tortoiseshell female cat (a mosaic of black and orange patches on a white background) is cloned by somatic cell nuclear transfer. Although the resulting clone is genetically identical to the donor animal, the distribution and shape of its patches are completely different. Justify this fact biologically at the cellular and embryonic development levels.
2. b) A female heterozygous for haemophilia A exhibits abnormally low levels of clotting factor VIII (7% of normal) and suffers frequent bleeding, whereas her monozygotic twin sister, with the same genotype, exhibits normal levels (around 50%) and is completely asymptomatic. Explain the mechanism responsible for this phenotypic disparity between identical twins.
3. c) A Barr body is observed in most cell nuclei in a buccal smear from a male tortoiseshell cat. Reasonably deduce the chromosomal abnormality responsible for its phenotype and justify why a male cat with a normal chromosomal complement (XY) cannot display tortoiseshell fur.

In industrial poultry farming, the ZW chromosomal sex-determination system is used for the immediate identification of the sex of chicks right after hatching (autosexing). Barred plumage is determined by a dominant allele (Z^B) and non-barred by its recessive allele (Z^b).

1. a) Design the exact parental cross that must be carried out on a farm so that 100% of male chicks hatch with barred plumage and 100% of females hatch non-barred, indicating the genotypes and phenotypes of both parents. Justify why the reciprocal cross would not work for universal phenotypic autosexing at hatching.
2. b) If the females obtained from this cross are subsequently crossed with a rooster heterozygous for this trait, determine the expected genotypic and phenotypic proportions in the offspring broken down by sex.
3. c) In a poultry line, an early recessive lethal mutation linked to the Z chromosome (Z^l) is detected that prevents egg hatching. If a healthy carrier rooster (Z^B Z^l) is crossed with normal non-barred hens (Z^b W), predict and justify the sex ratio (male : female ratio) observed among the live chicks that manage to hatch.