Question

1.) Describe what type of alleles natural selection works most strongly on, and explain why.

2.) How does Dawson’s experiment demonstrate the concept referred to in question 1?

3.) Differentiate between overdominance and underdominance and give an example of when overdominance can occur.

4.) Explain how frequency-dependent selection works.

5.) Describe the potency of mutation as an evolutionary force, and explain why.

6.) What happens when mutation is combined with selection and how does cystic fibrosis exemplify this?

I need help with mainly 3-6

Answer 1

Answer 3:-

Overdominance is said to have occurred when a heterozygous offspring exhibit phenotype that is outside the range of both parents phenotype (homozygous). The condition of overdominance is also known as heterozygous advantage, as the heterozygous offspring shows greater fitness than homozygous offspring.

An example of overdominance is that of sickle cell anemia. The sickle cell anemia is recessive trait. The homozygous SS individuals have normal blood, however, the heterozygous Ss individual instead of exhibiting normal blood phenotype, exhibits both normal and sickle haemoglobin. An Individual homozygous for the sickle-cell allele i.e. ss, shows sickle cell haemoglobin itself.

Underdominance is opposite of overdominance. In underdominanace, selection occurs against the heterozygous. Underdominance leads to disruptive selection and divergent genotypes. The heterozygous offspring shows lesser fitness than the homozygous offspring.

An example of underdominence is observed in case of polymorphism.

Answer 4:-

Frequency dependent selection is of two types, positive and negative. As the name suggest, frequency dependent selection occurs based on the frequency of the particular phenotype relative to other phenotypes in a given population. The fitness of the phenotype is dependent on its frequency.

Positive selection means the fitness of particular phenotype increases as it becomes more frequent in the population.

Negative selection means the fitness of particular phenotype decreases as it becomes more frequent in the population.

Answer 5:-

Mutations means changes in the nucleotide sequence in the DNA, RNA i.e. genome of an organism. Mutations are either harmless or they can cause beneficial or deleterious effects. The beneficial mutations usually get fixed in the population due to natural selection and hence are carried forward. Mutations are, thus considered as a source of genetic variation. The fixing of beneficial mutations serves as a raw material for evolution. Thus, mutations act as evolutionary force.

Answer 6:-

Cystic fibrosis is caused by mutation in CFTR gene. It is an autosomal recessive disease i.e. the disease occurs if an individual inherits mutant gene from both the parents. Such an individual has two mutant CFTR genes and no normal CFTR gene. The mutant gene has deleterious effect and would have wiped out due to natural selection but selective advantage of the mutant gene has still kept it in the population.