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3.5.4. Sex-linked inheritance (problems solved)

Sex-linked inheritance (problems solved). (Page 16)

In the case of sex-linked inheritance, the character we are studying is located on a sex chromosome. After seeing some examples, we will focus on seeing problems about two diseases, very common in genetics exercises. You have more solved exercises of this type on the following pages:

Nomenclature for genetic problems

For problems of inheritance linked to sex we will use this nomenclature:

  • In the female genotype, the two alleles of the gene are represented as superscripts or subscripts of the X chromosome, such as XAXA, XAXa or XaXa.
  • In the male genotype, the only allele X is represented as subscript or superscript of the single X chromosome having, in addition to the other chromosome Y. For example, XAY or XaY.
  • Y-linked inheritance:
    • In the female genotype there is no Y chromosome.
    • In the male genotype, the single allele is represented as a subscript or superscript of the Y chromosome. Possible genotypes are XYA, XYa.

Genetics problem 123

In humans, the presence of a fissure in the iris is regulated by a recessive sex-linked gene (Xf). From a marriage between two normal people, a daughter was born with this anomaly. The husband files for divorce alleging the wife's infidelity.
It explains the mode of inheritance of the character, indicating the genotypes of the marriage and what conclusion the judge must reach in relation to the possible infidelity of the wife, taking into account the birth of the daughter who presents the fissure.

Genetics problem 124

In a cross between a white-eyed fruit fly and a red-eyed male, what genotypes do their descendants have? (white eyes are X-linked, recessive)

Genetics problem 125

Duchenne muscular dystrophy is a disease that causes gradual deterioration of the muscles. It only appears in children born to apparently normal parents and generally, they tend to die in adolescence.

A) Is this disease caused by a dominant or recessive allele?

B) Is it an autosomal inheritance or is it linked to sex? Why?


Genetics problem 126

Neither Luis nor Maria have Duchenne muscular dystrophy (sex-linked disease), but their first-born son does.

a) Indicates if the allele responsible is dominant or recessive, and the genotypes of the parents and the child.

b) If they have another male child, what is the probability that he will suffer from this disease? What if it's a daughter? Reason for the answers.

Genetics problem 127

A researcher finds that a spontaneous mutation has occurred in a male among the mice in his laboratory. After crossing it with a normal female, he verifies that in the offspring, no male presents the mutation, but instead, all females present it. Indicate what type of mutation has been produced. What percentages of mutant individuals would be expected in the offspring if a mutant female (from the previous cross) is crossed with a normal male? Reason for the answers.

Genetics problem 128

A female has a recessive lethal allele on one of her X chromosomes and the normal dominant allele on the other. What will the sex ratio be in this woman's offspring if she has children with a man who does not carry the lethal gene?

Genetics problem 129

Congenital nystagmus is a condition known as intermittent vision disease. This is due to the involuntary, rapid and repetitive movement of the eyes due to alterations in regions of the brain. This disease is controlled by a recessive allele linked to sex.
A man and a woman, both with normal vision, have four descendants. They all marry normal individuals.

  • The first male child has nystagmus, but has a normal daughter;
  • the second son is normal and has a normal daughter and son;
  • the first daughter is normal and has 8 children all normal;
  • the second daughter has two normal daughters and two sons, one normal and one affected.

Make a scheme that shows the previous cross indicating the genotypes of both the parents and all the members of the family

Genetics problem 130

A. Represents the cross between a female who is a carrier of an X-linked disease and a healthy male.
Regarding the previous cross, answer the following questions:

B. What percentage of offspring will suffer from the disease?
C. What percentage of offspring will not suffer from the disease but could pass it on to their male children?
D. Could any of the daughters of the cross (A), married to a man with the disease, have a son or daughter with this disease? (0.5 points per section)

Genetics problem 131

The figure indicates the transmission of a certain phenotype (individuals in black) in a family (men are represented by a square and women by a circle).

1.- Indicate if this phenotype represented with black is dominant or recessive, and if the gene that determines this character is autosomal or is linked to sex. Reason for the answer. (Maximum score 1 point)

2.- Indicate the probability that the woman indicated with an arrow, in the pairing indicated in the graph, has a descendant with the phenotype represented in white: (1) yes it is about a child; (2) if it is a girl (maximum score 1 point)

Genetics problem 132

In the Drosophila melanogaster fly the body color can be brown or yellow. The appearance of flies with a yellow body depends on a recessive character linked to the X chromosome. In this species the sex determination is the same as that of the human being.
A brown-bodied female is crossed with a yellow-bodied male and they have offspring of 11 female flies, 5 with a yellow body and 6 with a brown body, and 12 males, half a yellow body and the other half a brown body.


a. Represents the genotype of both parents.
b. Please indicate the genotype of your offspring.


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