6) How is it possible for two alleles of a heterozygote to have both a dominant/recessive and a co–dominant relationship?

Question

Please help me search for the answer in Modern Genetic Analysis textbook by Griffiths et al Modern Genetic Analysis, 2nd Ed.

Modern Genetic Analysis, 2nd Ed.

Modern Genetic Analysis, 2nd Ed.

Answer 1

This question is likely asking you to check your assumptions. When we introduce Genetics, we make a TON of simplifying assumptions, and then slowly introduce more realistic situations. In Mendelian cases, each trait is controlled by only one gene (or one gene under study), each gene has only two alleles, one allele is cleanly dominant, etc.

Another assumption is: each gene contributes to only one phenotype. A lovely simplification, but not always true (indeed, like a lot of the above assumptions, rarely true). Consider sickle cell anemia--there are many phenotypes associated with the single mutant allele (a change of one nucleotide which cause a change of one amino acid in one protein). There is the decrease in ability to carry oxygen there are many other phenotypes.

and each one of these may manifest differently in terms of dominance.

For example, in terms of anemia, heterozygotes are pretty healthy (though I believe I have read that at high altitude, they are somewhat impaired) so you might score the anemic allele as 'recessive'. On the other hand, the sickle cell allele provides great resistance to malaria--those who are homozygous for the sickle cell allele are almost totally resistant to malaria, those with a wild-type allele are very vulnerable to malaria, and heterozygotes show intermediate resistance...

Answer 2

if the trait you are considering is decided by just 1 gene, this could be due to mutations during development. However in traits that are decided by a combination of genes (particularly genes on X chromosome) this could arise from the relative expression and interaction with in this gene network.