Question

For the Week 6 Critical Thinking Exercise, you will answer the following questions using your critical thinking and reasoning skills:

Why do you think females are typically the carriers of sex-linked traits, but those traits are not expressed? Cystic fibrosis (CF) is an autosomal recessive disease. Jane and John do not have CF but are worried about having a child who will have it, because John had a brother with the disease. While meeting with a genetic counselor it was determined that John’s parents were not affected with CF, yet his brother had it.

What is immediately known about the genotypes of John’s parents?

Is the genotype of John immediately known?

Under what circumstances could John and Jane have a child with CF?

Answer 1

1.

Females have two X chromosomes (XX). Males have one X chromosome and Y chromosomes (XY). For the X-linked dominant trait, the presence of one copy of the mutated gene is sufficient to cause the disorder in males and females.

For the X-linked recessive trait, one copy of the mutated gene is sufficient to cause the disorder in males since they have only one X chromosome. In the case of females, two copy of mutated is required to exhibit the trait. Females with one copy of the mutated gene do not exhibit the trait, but they are carriers. Carriers transfer the one copy of the mutated gene to their children

2.

Cystic Fibrosis (CF) is an autosomal recessive trait. In autosomal recessive trait, two copies of recessive alleles are required to cause the disorder. Individuals with one recessive allele do not exhibit the trait, but they are carriers. If both parents are carriers, then among their offsprings 25% is normal, 50% are carriers and 25% is affected. So, in autosomal recessive inheritance, an affected child can have unaffected parents (carriers)

i) A child with Cystic Fibrosis receives one copy of the mutated gene from his mother and one copy of the mutated gene from his father. Both parents of John do not have CF, but his brother has CF, then John's parents are carriers.

Let C be the dominant allele for normal

Let c be the recessive allele for CF

then,

genotype of John's mother = Cc (carrier)

genotype of John's father = Cc (carrier)

parent cross: Cc (carrier father) * Cc (carrier mother)

gametes of father: C,c

gametes of mother: C,c

Offspring: CC (normal), Cc (carrier), Cc (carrier), cc (affected)

ii)

John does not have CF. So John may be normal (having two dominant alleles) or carrier (having one dominant allele and a recessive allele) Therefore, the genotype of John could be CC or Cc

iii)

Both John and his wife do not have CF. So a child with CF can be born to John and his wife only if both are carriers of Cystic Fibrosis. Cystic fibrosis is an autosomal recessive trait and two copies of recessive alleles should be present to exhibit the trait. If both parents are carriers, then the child can have CF, inheriting one copy of the mutated gene from the father (John) and one copy of the mutated gene from the mother (John's wife)