Pedigree analysis problem with solution (example 15)
Q) The following pedigree illustrates the inheritance of Nance- Horan syndrome, a rare genetic condition in which affected persons have cataracts and abnormally shaped teeth.
(a) On the basis of this pedigree, what do you think is the most likely mode of inheritance for Nance-Horan syndrome?
Answer: X-linked recessive mode of inheritance.
- Skipping of generations observed
- Only males have the condition
- Unaffected female carriers have affected sons.
(b) If couple III-7 and III-8 have another child, what is the probability that the child will have Nance-Horan syndrome?
Answer: The probability is ¼.
Genotype of III – 7 female (Carrier): XNXn
Genotype of III – 8 male: XnY
- The female III-7 is a carrier, so there is a ½ probability that the child will inherit her X chromosome with the Nance-Horan allele.
- Female child of these parents will not have the disease. Female need to inherit two copies of the disease allele, one from each parent to show the disease since it is an X linked recessive trait.
- So we have to find the probability of their male child to have the disease.
- The probability of having a male child for these parents is 1/2
Probability of that male child to have Nance – Horan syndrome = ½ X ½ = ¼
(c) If III-2 and III-7 mated, what is the probability that one of their children would have Nance-Horan syndrome?
Answer: The probability is 1/2
Genotype of III – 2 affected male: XnY
Genotype of III – 7 female: XNXn (Carrier)
In this case there is a chance of having affected female child and an affected male child.
Genotype of Affected female offspring: XnXn (For this genotype female need to inherit two copies of the disease allele, one from each parent)
- Half the boys will inherit the Nance-Horan allele from the III-7 carrier female. ½ of the boys will inherit normal allele from carrier mother.
- All the girls will inherit one Nance-Horan allele from the III-2 affected male.
- Half of them will get a second Nance-Horan allele from the III-2 female, so half the girls will also have Nance-Horan syndrome.
(That means ½ of female offspring will receive Nance-Horan allele from the III-7 carrier female and III – 2 affected male and they will show the trait. Other half females will have one Nance-Horan allele from the III – 2 affected father and normal allele from III – 7 carrier mother).