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.

Color 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.

A woman is color-blind ($X^d X^d$). State, with reasoning, what her father's genotype and phenotype must necessarily be for this trait, and what can be stated about her mother's genotype.

Haemophilic women are extremely rare. State what genotype a woman must have to suffer from haemophilia and what genotypes her parents would have to have for a haemophilic girl to be born, reasoning why that combination is so infrequent.

A color-blind man has children with a woman with normal vision who is not a carrier. Reason whether they can have color-blind sons, color-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 color 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 color-blind. d) Explain why no son inherits the X chromosome from his father.

A healthy woman whose father was haemophilic has children with a healthy man. a) State the woman's genotype, justifying why she is an obligate carrier. b) Calculate the probabilities of haemophilic offspring among her sons and among her daughters. c) Reason whether the woman's mother also needed to be a carrier to explain her genotype.

A couple in which both have normal vision have a color-blind son. a) Deduce the genotypes of the parents and of the son. b) Calculate the probability that the couple's next daughter will be color-blind. c) Calculate the probability that that daughter will be a carrier of the allele.

A woman carrier of haemophilia has children with a haemophilic man. a) State the genotypic ratios expected in the offspring. b) Calculate the probability of having a haemophilic daughter. c) Explain why this cross can indeed produce haemophilic daughters, unlike the cross between a carrier and a healthy man.

In cats, the gene for orange or black coat color 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 colors. 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.

In Drosophila, red eye color ($X^{w+}$) 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 testcross and point out the essential difference.

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.

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.

A woman carrier of the color blindness allele and with the normal number of fingers (polydactyly being an autosomal dominant trait, pp in her case) has children with a man with normal vision who is heterozygous polydactylous. a) State the genotypes of both parents. b) Calculate the probability that they will have a color-blind son. c) Calculate the probability of having a child who is polydactylous and color-blind at the same time. d) Calculate the probability of having a daughter who is healthy for both traits.

In a medical genetics consultation, the case of a family with a history of haemophilia A (an X-linked recessive disease) is studied:

• A haemophilic male (I-1) and a healthy, non-carrier female (I-2) have a healthy daughter (II-2).

• II-2 marries a healthy male (II-1) and they have three healthy offspring: two males and one female (III-3).

• Female III-3 plans to have children with a healthy man (III-4). A genetics student claims: "Since the mother (II-2) had two healthy sons and never had symptoms, it is not certain that she is a carrier; and since III-3 is a healthy female, there is no risk of her male offspring suffering from the disease."

1. a) Refute the student's claim in a justified manner by demonstrating why II-2 is necessarily an obligate carrier.
2. b) Calculate the probability that female III-3 is a carrier of the mutated allele and the probability that her first son is born with haemophilia.
3. c) If the first son of III-3 and III-4 turns out to be completely healthy, reason whether this fact modifies the probability that a second son will be haemophilic.
4. d) Design a different hypothetical family situation in which a female is an obligate carrier of haemophilia A without her father having been haemophilic, justifying your answer.

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, the genes for eye color (white w, recessive) and body color (yellow y, recessive) are both on the X chromosome. A double heterozygous female carries the two mutant alleles on one X chromosome and the two normal alleles on the other, and is crossed with a normal male. a) State what types of gametes the female will produce if there is no crossing over. b) Determine the phenotypes and ratios expected in the offspring in that case. c) Explain what new types of offspring would appear if crossing over occurred between the two genes during meiosis and why they would be in the minority.

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

In a genetic counseling consultation, a healthy couple wishing to have offspring is being assisted. The woman's mother was the biological daughter of a male affected by hemophilia A (X-linked recessive) and a healthy homozygous woman. The couple has already had two consecutive, completely healthy sons.

1. a) Calculate the initial ("a priori") probability that the woman is a carrier of the hemophilia allele before having had her children.
2. b) A student claims that, since two healthy sons have already been born, the probability that the mother is a carrier has not changed and remains 0.5 (50%). Reasonably refute this claim by calculating, using conditional probability (Bayes' theorem), the actual updated probability that the woman is a carrier.
3. c) Using the probability obtained in the previous section, calculate the actual risk that the third child the couple is expecting will be a male affected by the disease.
4. d) The couple requests a conventional fetal karyotype via amniocentesis to rule out hemophilia. Reason whether this cytogenetic technique is suitable for diagnosing this pathology and propose a valid diagnostic alternative.