Genetic problem Schilder's disease, which causes a progressive degeneration of the nervous system...


Genetic problem Schilder's disease, which causes progressive degeneration of the nervous system leading to death at about two years of age, behaves as a simple autosomal recessive trait. If a couple loses their first two children due to Schilder's disease, what is the chance that their third child will develop the disease?

Answer and Explanation:

It would be a 25% chance of a third child inheriting this disorder. The parents have passed the trait on to two other children, but do not express it themselves. We know this because both the parents lived passed two years of age. So each parent would be heterozygous for this. While the mother has had two children already affected by the disorder, it is typically calculated as the chance per pregnancy. So the chances do not necessarily go down just because she has already had to other children with the disorder.

We can use the letter "B" to represent dominant trait and "b" to represent the recessive trait for Schidler's disease. Since both parents are heterozygous, their genotypes would be Bb x Bb.

B b
b Bb bb

There would be a 25% chance of child expressing the "bb" genotype for the disease, a 50% chance of being a carrier "Bb" and a 25% chance of being homozygous dominant "BB". So there is a also a 75% chance that the child would not be affected by the disorder.

Learn more about this topic:

Pedigree Analysis in Human Genetics: Inheritance Patterns

from Biology 102: Basic Genetics

Chapter 6 / Lesson 3

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