Question

1. Explain why Hardy-Weinburg equilibrium represents a null model of population genetics.

2. What forces can act to produce deviations from a Mendelian population? What forces can act to disrupt panmixia?

Answer 1

1. Hardy Weinberg Equilibrium presents a null model of population Genetics because it takes into account an ideal population which is not evolving. It tells us the structure of population taking into account, allelic and genotypic frequency in the population across generations, when no evolutionary force is acting on the population.

As per Hardy weinberg equilibrium the population is said to be known evolving when the following evolutionary forces are not acting on the population -

a. Mutation - the sequence of nucleotides on DNA and RNA as well as sequence of amino acids on proteins remains the same in population across generations because no mutation is acting on the population.

b. Migration - the number of individuals in the population increases or decreases only because of birth and death and not because of migration. No individuals from other places are coming to the given population nor individuals of that population are going outside. Hence no immigration or emigration are taking place in the population.

c. Natural selection - there is no selection of individuals on the basis of certain criteria because there is no mutation to be selected.

d. Genetic drift - genetic drift is defined as loss of a large segment of a population due to some natural calamity. Genetic drift results in an increase in number of individuals with certain genotypes where as decrease in number of individuals with some other genotype. No such genetic drift is occurring in population when it is not evolving.

e. The population should be very large and should show random mating.

As per HWE,

p (frequency of dominant allele) + q (frequency of recessive allele) = 1

2. As per mendelian inheritance three features are possible in a population -

A. A gene is controlled by two alleles where one of the alleles is dominant over the recessive allele.

B. During cell division, each chromosome in a homologous pair segregate independently of each other.

C. During cell division 1 chromosome assort independently of the Other chromosome.

But there are certain deviations from these laws. They are

1. Codominance - in case of codominance two alleles of a gene are are dominant to each other and we get a variegated phenotype in this case, for example when we cross a true breeding white Camellia with a true breeding red Camellia, we get a a progeny which is having both white and red patches.

2. Incomplete dominance - in this case the dominant allele is not completely dominant to the recessive allele and it is not able to express completely therefore we get a blended phenotype in the progeny which is an intermediate of both the parents. For example, when we cross a true breeding red Snapdragon flower with the true breeding white Snapdragon flower we get a pink Snapdragon flower in the progeny.

3. Polygenic inheritance - in case of polygenic inheritance more than one genes are controlling the inheritance of a trait, for example Inheritance of skin colour in humans which is controlled by 3 genes.

4. Lethal allele - some combination of results in the formation of a Lethal phenotype. For example coat colour in Mouse can be Yellow (Aa) or agouti (AA). Agouti is homozygous and yellow is heterozygous. Yellow, if homozygous, is lethal.

5. Linkage - according to the phenomenon of linkage two or more than two alleles to not segregate independently of each other during gamete formation, in fact, they are linked tightly so, that they are most of the times inherited together.

6. Crossing over - it is the process by which non sister chromatids of homologous chromosomes exchange certain genetic material with each other resulting in formation of chromosomes which resemble both paternal and maternal chromosome.

7. Epistasis - it is the interaction between more than one genes where presence of one gene inhibit the expression of another gene.

8. Sex linked inheritance - certain traits are limited to the type of sex. For example, patterned baldness is seen in males and not in females.

Panmixia is the population which shows random mating. Therefore it is the population which will be in Hardy weinberg equilibrium, that is it will be non evolving. So forces which can disrupt random mating or panmixia will be those that disrupt Hardy weinberg equilibrium. These forces will be natural selection, mutation, migration and genetic drift.

please rate.