MadSci Network: Genetics

Re: Can both homozygous forms be expressed together

Date: Thu Apr 6 09:59:02 2006
Posted By: Paul Szauter, Staff, Mouse Genome Informatics
Area of science: Genetics
ID: 1143647402.Ge

Exam questions like this one assume that the disease in question is a Mendelian
factor, meaning 
a single gene that determines a particular trait, with a strong difference
between the two alleles. 
One allele is usually assumed to be wild type, while the other is a clear
departure from wild type 
that is recognizable as a disease.

You are expected to assume, if the disease would prevent survival to adulthood and 
reproduction, that it is recessive.

Therefore the parents are both A/a, with A/A and A/a wild type, and a/a affected
by the 
disease. You have correctly identified the risk for each child as 25%.

You have asked whether there is a case in which A/A and a/a are affected, and
A/a is normal. I 
can think of an example of this kind, but it is not entirely due to genetics, as
there is an 
environmental factor.

In sickle-cell anemia, there is a recessive variant of the beta globin gene that
causes a severe 
disease in homozygotes. Heterozygotes are mildly affected. People homozygous for
the wild-
type allele are susceptible to malaria, while heterozygotes are resistant.
Therefore, in areas with 
a high incidence of malaria, heterozygotes are strongly selected, with
homozygotes for the wild-
type allele facing malaria, and homozygotes for the sickling allele facing
severe anemia.

Please see:

It is difficult to think of a genetic mechanism in which only individuals that
are heteroallelic at a 
given locus are normal. This would require that there be no wild-type allele. If
the condition 
caused by homozygosity for the two alternative alleles strongly affects fitness,
there would be 
strong selection for a wild-type allele.

I'd recommend reading a good genetics, book for more information, for example:


Paul Szauter
Mouse Genome Informatics

Moderator's Note:

There is a similar situation called heterosis, which occurs in some organisms.
This is a case where a cross between two related inbred strains of a species
produces offspring that are more healthy than either of the parents. This is
presumably because each parent line is homozygous for certain alleles that cause
lower fitness, while the heterozygote has complementing alleles from both
parent, so is more "wild-type" than either parent. 

e.g. Say A and B are two genes that when mutant (a or b) cause mild phenotypes,
the organism is alive but not optimally healthy. Strain X is AAbb with
homozygous little b causing mild sickness, while strain Y is aaBB with
homozygous little A also causing a very mild phenotype. The offspring of the two
strains is then AaBb and contains a good copy of each gene, and so is more healthy. 

The exact genetic mechanisms for heterosis are not known at present, probably
involve many different genes and gene interactions.

This Wiki article has a
brief overview of heterosis.

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