HARDY-WEINBERG PRINCIPLE NEET

7.7 HARDY-WEINBERG PRINCIPLE NEET 2016, 15, 14,13, 05

• In a given population one can find out the frequency of occurrence of alleles of a gene or a locus. This frequency is supposed to remain fixed and even remain the same through generations. 
• Hardy-Weinberg principle stated it using algebraic equations. This principle says that allele frequencies in a population are stable and is constant from generation to generation. 
• The gene pool (total genes and their alleles in a population) remains a constant. 

·         This is called genetic equilibrium. Sum total of all the allelic frequencies is 1.

·         In a diploid, if p represents the frequency of allele A (dominant) and q represent the frequency of and allele a (recessive). p+q=1

• The frequency of AA (homozygous dominant) individuals in a population is simply p².
• The frequency of aa (homozygous recessive) individual in population is q²,
• The frequency of Aa (heterozygous) individual in population is 2pq.
•  Hence, p²+2pq+q²=1. This is a binomial expansion of (p+q)².
•  When frequency measured, differs from expected values, the difference (direction) indicates the extent of evolutionary change.

Problem 1. In a population of 1000 individuals 360 belongs to genotype AA, 480 to Aa and the remaining 160 to aa. Based on the data, the frequency of allele A in the population is-

a) 0.4           b) 0.5           c) 0.6           d) 0.7

solution-  total no. of individuals are -1000

• problem 2- at a particular locus, frequency of allele A is 0.6 and that allele a is 0.4. what would be the frequency of heterozygous in a random mating population at equilibrium?

        solution- 

        p+q=1

     frequency of allele A i.e., p= 0.6   

     frequency of allele a i.e., q= 0.4

     frequency of heterozygote i.e., Aa = 2 X p X q = 2X0.6X.4X=0.48

problem 3- in Hardy-weinburg population there are 8000 individuals, 80 individuals are recessive for a trait, then find out the followings

a.  frequency of recessive allele

b. frequency of dominant allele

c. number of homozygous dominant individual

d. number of heterozygous dominant individual

 solution- 

a. frequency of recessive allele if q2 (aa)  80/8000= 0.01, q=0.1

b. frequency of dominant allele  if p+q=1, then p=1- 0.1 = 0.9

c. number of homozygous dominant individual i.e., p2=  0.81 ,  0.81x8000=6480

d. number of heterozygous dominant individual i.e., 2pq= 2x0.9x0.1= 0.18, 0.18x8000= 1440

(AA+Aa+aa=8000) or (648+1440+80=8000)

tricks 

• in any case of HW equilibrium based problem first find out the p & q (allelic frequency)
• to find the number of individual (homozygous dominant and recessive) simply multiply the frequency i.e., p2 or q2 to the total number of individuals present in the population respectively, to find out heterozygous i.e., 2pq individual in the given population multiply the product of 2pq to the total population.
• in question if you get values of frequencies such as 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 then take it as allelic frequency of either p or q 
• in question if you get values of frequencies such as 0.01, 0.04, 0.09, 0.16, 0.25, 0.36, 0.49, 0.64, 0.81 then take it as frequency of individual either p2 or q2 

Disturbance in genetic equilibrium, or Hardy- Weinberg equilibrium, i.e., change of frequency of alleles in a population would then be interpreted as resulting in evolution.

• Five factors are known to affect Hardy-Weinberg equilibrium. These are gene migration or gene flow, genetic drift (also known as allelic drift or the Sewall Wright effect), mutation, genetic recombination and natural selection.
• When migration of a section of population to another place and population occurs, gene frequencies change in the original as well as in the new population. New genes/alleles are added to the new population and these are lost from the old population.
• There would be a gene flow if this gene migration, happens multiple times.
• The original drifted population becomes founders and the effect is called founder effect.
• If the same change occurs by chance (natural calamities- earth quack, storm, flood, volcanic eruption), it is called genetic drift (bottleneck effect).
• Sometimes the change in allele frequency is so different in the new sample of population that they become a different species (speciation).