1) Many deleterious genes are recessive and therefore are expressed only in the

Question

1) Many deleterious genes are recessive and therefore are expressed only in the homozygote. Many of them are also utterly lethal. Many of them have also been present in the human gene pool for hundreds of thousands and possibly even millions of years. How do you explain the persistence of these genes over such long periods of time in the face of such intense selective pressure against them?

2) Suppose the environment in which a population was living changed in some way and the new conditions were much more harmful to homozygous dominant individuals and to heterozygotes than to homozygous recessives? What would happen to p and q as the years went by? If this selective pressure against homozygous dominants and heterozygotes suddenly ceased and no other factors changed, what would happen to p and q?

Explanation / Answer

1)

In two words- heterozygote advantage. This is where carriers of a genetic condition have some kind of benefit over non-carriers. For example, people who carry but do not express the sickle cell anemia gene (are heterozygous) are resistant to malaria, and carriers for cystic fibrosis are less likely to get typhoid or tuberculosis.

They persist because they are recessive. If they were dominant than all of the people carrying the allele would display the trait. If the condition were truly debilitating then those people would probably be less likely to reproduce.

But people who carry the trait (heterozygotes) still reproduce and continue to pass on the gene

2)

Population genetics is the quantitative study of the distribution of genetic variation in a population and of how the frequencies of its genotypes, alleles, and phenotypes are maintained or changed. It seeks answers to such practical questions as why the frequency of PKU in Caucasians is so much greater than in Japanese, or why the frequency of the sickle cell allele varies markedly in people from different West African countries. The mathematical cornerstone of population genetics is the Hardy-Weinberg law or principle. The law has two parts. First, it states that in a large, randomly mating population with two alleles at a locus (for example, A and a), there is a simple relationship between these allele frequencies (frequency of A = p; frequency of a = q) and the genotype frequencies (p2, 2pq, or q2) they define. Second, it holds that this relationship between allele and genotype frequencies, constructed simply on the binomial expansion of (p + q)2, does not change from one generation to the next. When a population conforms to this two-part law, it is in Hardy-Weinberg equilibrium. In such populations, the law is of great value in showing why dominant traits do not increase in frequency from one generation to the next and why recessive traits do not decrease. Further, the law is regularly used in genetic counseling settings where estimates of genotype, allele, and carrier frequencies are calculated from limited phenotypic information in small families, such estimates then being employed to estimate specific genetic risk.

Hardy-Weinberg equilibrium is never fully realized in human populations because it is perturbed by one or more deviations. First, individuals do not usually mate randomly. Mating is more often assortative(mate choice depends on geographic proximity), stratified (within an ethnic subset), or inbred (among relatives or a small group). Second, allele frequencies do not remain constant for a number of reasons: random or chance events producing major changes in population size and composition (called

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