Selection for Single-gene Disorder

Question 1

This natural selection operates on the ____, w/c in turn is determined by the ____, & ____ is simply a change in gene frequencies as a result of selection, w/ _____ thus being a prerequisite for evolution.

Answer 1

phenotype; genotype; evolution; genetic variation

Question 2

Selection is a powerful means of altering gene frequencies for single - gene disorders and it can operate to reduce, by ____ or increase, by ____, a particular phenotype & hence its genotype.

Answer 2

negative selection; positive selection

Question 3

In equatorial Africa, the birth frequency of individuals affected with sickle-cell anemia is 1 in 40, and 1 in 3 of the population carry the mutant gene.This high gene frequency has arisen due to a selective advantage for the mutant gene carriers with respect to malaria due to ____.

Answer 3

Plasmodium falciparum

Question 3

Many populations tend to increase in size over time, but w/o disturbing factors, such as selection, the relative gene frequencies tend to remain constant. This can be demonstrated mathematically by the _____.

Answer 3

Hardy – Weinberg equilibrium

Question 4

Selection for a particular allele also influences the allele frequencies at adjacent closely linked loci due to ____.

Answer 4

linkage disequilibrium

Question 5

A selective advantage w/ respect to malarial infection also operates for heterozygotes for ____ and ____, &, as a consequence, these conditions are also prevalent in regions where malaria is (or was) common.

Answer 5

β - thalassaemia; glucose - 6 - phosphate dehydrogenase deficiency

Question 6

A heterozygote advantage is also evident for carriers of _____ w/ respect to ____ infection, but for the majority of conditions where a natural selective pressure is (or was) operating, its basis is as yet unknown.

Answer 6

congenital adrenal hyperplasia; Haemophilus influenzae type B

Question 7

T/F. Advances in treatment can reverse this disadvantage, w/ the frequency of the condition then rising to a new equilibrium but the rate of change depends on the mode of inheritance. For an autosomal dominant trait where reproduction was previously impossible, restoration to normality would double the frequency of the disorder in a single generation, whereas for an X - linked recessive trait, doubling of the birth frequency would take about 4 generations.

Answer 7

True.

Question 8

T/F. Advances in treatment can reverse this disadvantage, w/ the frequency of the condition then rising to a new equilibrium but the rate of change is change depends on the mode of inheritance. For an autosomal recessive trait, it would take about 50 generations, as most of the mutant alleles in the population would be present in carriers whose reproductive fitness would be unchanged by the availability of treatment.

Answer 8

True.