1-6 Mendelian Patterns of Inheritance

Question 1

What are the three types of genetic heterogeneity?

Answer 1

1. Locus heterogeneity: same phenotype can result from mutations at different genes
2. Allelic heterogeneity: different mutations at the same gene
3. Clinical/phenotypic heterogeneity: variation in phenotype occuring in patients with mutations in a specific gene; could be variation in expression, penetrance, or even diseases

Question 2

What are mosaicism and chimerism?

Answer 2

Mosaicism occurs when a mutation event results in a second population of cells differing from the first ONLY in that mutation; could be a germline mosaicism.

Chimerism is the result of two different zygotes, which can occur naturally (and very rarely) or, for example, as the result of an organ donation.

Question 3

What is the difference between expression and penetrance?

Answer 3

Expression is the variation in a condition, such as mild or severe.

Penetrance is the presence or absence of a phenotypic effect (e.g., disease penetrance).

Question 4

What is Haldane’s Rule?

Answer 4

An explanation for why not all mothers of affected boys with X-linked recessive disorders are carriers.

[( 1 - f ) x u] / ( 2u + v )
f = effective fertility, u = female mutation rate, v = male mutation rate

There are three conditions:

1. The disease is X-linked recessive
2. There are no other affected maternal relatives
3. The disease reduces fitness (the likelihood of the affected boys having offspring)