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Ch. 4 - Modification of Mendelian Ratios
Klug - Essentials of Genetics 10th Edition
Klug10th EditionEssentials of GeneticsISBN: 9780135588789Non è quello che usi tu?Cambia libro di testo
Tutti i libri di testoKlug 10th EditionCh. 4 - Modification of Mendelian RatiosProblema 27
Capitolo 4, Problema 27

Two mothers give birth to sons at the same time at a busy urban hospital. The son of mother 1 is afflicted with hemophilia, a disease caused by an X-linked recessive allele. Neither parent has the disease. Mother 2 has a normal son, despite the fact that the father has hemophilia. Several years later, couple 1 sues the hospital, claiming that these two newborns were swapped in the nursery following their birth. As a genetic counselor, you are called to testify. What information can you provide the jury concerning the allegation?

Guida verificata passo dopo passo
1
Step 1: Understand the inheritance pattern of hemophilia, which is an X-linked recessive disorder. This means the gene causing hemophilia is located on the X chromosome, and males (XY) who inherit the affected X chromosome will express the disease because they have only one X chromosome.
Step 2: Analyze the genetic status of the first family. Since the son of mother 1 has hemophilia but neither parent shows the disease, the mother must be a carrier (X^H X^h) and the father must have a normal X chromosome (X^H Y). The son inherits the affected X chromosome (X^h) from the mother and a Y chromosome from the father, resulting in hemophilia.
Step 3: Analyze the second family. The father has hemophilia (X^h Y), so he can only pass the affected X chromosome to his daughters and the Y chromosome to his sons. Since mother 2 has a normal son, this son must have inherited the Y chromosome from the father and a normal X chromosome (X^H) from the mother, meaning the mother is not a carrier.
Step 4: Use this information to assess the claim of swapped babies. The son with hemophilia must have inherited the affected X chromosome from his mother, so the mother of the hemophiliac son must be a carrier. The normal son born to the father with hemophilia must have inherited a normal X chromosome from his mother, who is not a carrier. This genetic evidence supports that the babies were not swapped.
Step 5: Summarize for the jury that the genetic patterns of X-linked inheritance and the presence or absence of carrier status in the mothers provide strong evidence against the claim that the babies were swapped in the nursery.

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X-linked Recessive Inheritance

X-linked recessive traits are caused by mutations on the X chromosome. Males, having only one X chromosome, express the disease if they inherit the affected allele, while females must inherit two copies to be affected. Carrier females typically do not show symptoms but can pass the allele to their sons.
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X-Inactivation

Genetic Probability and Carrier Status

In X-linked recessive diseases like hemophilia, an unaffected mother can be a carrier if she has one mutated X chromosome. A son of a carrier mother has a 50% chance of being affected, while a son of a non-carrier mother will be unaffected. Fathers with hemophilia cannot pass the disease to sons but pass the mutated X to all daughters.
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Probability

Paternity and Maternity Testing Using Genetic Markers

Genetic testing can determine biological relationships by comparing DNA markers. Since hemophilia is X-linked, testing the mother’s carrier status and the child’s genotype can confirm maternity. Similarly, paternity can be confirmed by comparing autosomal markers, helping resolve disputes about infant identity in hospital mix-ups.
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Maternal Effect
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Five human matings (1–5), identified by both maternal and paternal phenotypes for ABO and MN blood-group antigen status, are shown on the left side of the following table:

Each mating resulted in one of the five offspring shown in the right-hand column (a–e). Match each offspring with one correct set of parents, using each parental set only once. Is there more than one set of correct answers?

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Pigment in mouse fur is only produced when the C allele is present. Individuals of the cc genotype are white. If color is present, it may be determined by the A, a alleles. AA or Aa results in agouti color, while aa results in black coats. In three crosses between agouti females whose genotypes were unknown and males of the aacc genotype, the following phenotypic ratios were obtained:

(1) 8 agouti

(2) 9 agouti

(3) 4 agouti, 8 white 10 black, 5 black, 10 white

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In Dexter and Kerry cattle, animals may be polled (hornless) or horned. The Dexter animals have short legs, whereas the Kerry animals have long legs. When many offspring were obtained from matings between polled Kerrys and horned Dexters, half were found to be polled Dexters and half polled Kerrys. When these two types of F₁ cattle were mated to one another, the following F₂ data were obtained:

3/8 polled Dexters

3/8 polled Kerrys

1/8 horned Dexters

1/8 horned Kerrys

A geneticist was puzzled by these data and interviewed farmers who had bred these cattle for decades. She learned that Kerrys were true breeding. Dexters, on the other hand, were not true breeding and never produced as many offspring as Kerrys. Provide a genetic explanation for these observations.

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Pigment in mouse fur is only produced when the C allele is present. Individuals of the cc genotype are white. If color is present, it may be determined by the A, a alleles. AA or Aa results in agouti color, while aa results in black coats. What F₁ and F₂ genotypic and phenotypic ratios are obtained from a cross between AACC and aacc mice?

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