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.

Colour blindness is due to a recessive allele linked to the X chromosome ($X^d$), while normal vision depends on the dominant allele ($X^D$). A carrier woman has children with a man with normal vision. State the possible genotypes and phenotypes of the sons and of the daughters, with the probability of each one.

Haemophilia is a recessive disease linked to the X chromosome ($X^h$). Calculate the probability that a carrier woman and a healthy man will have a haemophilic son, stating the genotypes of the parents.

In cats, the gene for orange or black coat colour is on the X chromosome: the allele $X^O$ produces orange and $X^B$ black; $X^B X^O$ females have a tortoiseshell coat, with patches of both colours. a) Explain why male cats are almost never tortoiseshell. b) State the expected phenotypes, separated by sex, of the cross between a tortoiseshell female and a black male.

A colour-blind man has children with a woman with normal vision who is not a carrier. Reason whether they can have colour-blind sons, colour-blind daughters or carrier daughters, stating the genotypes of all the offspring.

The square in the diagram represents the cross between a carrier woman for colour blindness and a man with normal vision. a) Complete the four cells with the genotypes of the offspring. b) State the probability that a daughter will be a carrier. c) State the probability that a son will be colour-blind. d) Explain why no son inherits the X chromosome from his father.

In Drosophila, red eye colour ($X^{w+}$) is dominant over white ($X^w$), the trait being linked to the X chromosome. A heterozygous female is crossed with a white-eyed male. a) State the genotypes of both parents. b) Determine the phenotypic ratios expected in the total offspring and separated by sex. c) Compare these ratios with those of an autosomal test cross and point out the essential difference.

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.

In a sheep breed, the inheritance of the presence or absence of horns is studied. A farmer crosses a horned ram with a hornless ewe, both coming from pure-breeding lines. In the F1 offspring, 100% of the males have horns and 100% of the females lack them.

An agricultural technician states: "The gene responsible is located in the differential segment of the X chromosome and the horned allele is recessive, so the ram transmitted its Y chromosome and the ewe its X chromosome."

1. a) Refute the technician's proposal by formally demonstrating, with the corresponding parental genotypes, why the X-linked model is incompatible with the F1 results.
2. b) Formulate an alternative hypothesis based on an autosomal sex-influenced gene (H allele horned, h allele hornless), defining the dominance relationship in each sex and verifying that it accounts for both the parents and the F1.
3. c) If individuals of this F1 are intercrossed, predict the exact genotypic and phenotypic proportions expected in the F2 broken down by sex (among males and among females).
4. d) Justify why a sex-influenced trait differs biologically from a sex-limited trait (such as milk production), reasoning whether a hornless ewe can transmit the presence of horns to her male offspring.

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.

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.

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.

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 Drosophila melanogaster, yellow body colour (y) and white eyes (w) are recessive traits determined by two genes located on the X chromosome. A pure-breeding female with a grey body and red eyes is crossed with a male with a yellow body and white eyes. All the F1 offspring exhibit the wild-type phenotype (grey body and red eyes).

Next, a test cross is carried out by crossing the heterozygous F1 females with yellow-bodied, white-eyed males, yielding the following counts in the offspring:

• 427 grey-bodied, red-eyed flies.

• 433 yellow-bodied, white-eyed flies.

• 68 grey-bodied, white-eyed flies.

• 72 yellow-bodied, red-eyed flies.

1. a) Deduce the genotype of the initial parents and of the F1 females, indicating with justification whether the mutant alleles in the F1 female are in coupling (cis) or repulsion (trans) phase.
2. b) Calculate the recombination frequency between both loci and the genetic distance in centimorgans (cM).
3. c) If F1 males are crossed with yellow-bodied, white-eyed females, predict in detail the expected genotypic and phenotypic proportions in the offspring broken down by sex, justifying your answer according to the characteristic meiotic behaviour of males in this species.

A woman with Turner syndrome (karyotype X0, a single X chromosome) is colour-blind, while both her parents have normal vision. a) State the genotype of the mother and of the father for the colour blindness trait. b) Reason from which of the two parents the woman inherited her single X chromosome. c) Explain what meiotic error (non-disjunction), and in which parent, could have given rise to this karyotype.