PLANT BREEDING AND GENETICS, Study notes of Genetics

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Hardy- Weinberg Equilibrium

KEY CONCEPT

Hardy-Weinberg equilibrium provides a framework for understanding how populations evolve.

Hardy-Weinberg equilibrium describes populations that are not evolving.

• (^) Biologists use models to study populations.
• (^) Hardy-Weinberg equilibrium is a type of model.

Hardy-Weinberg equilibrium describes populations that are not evolving.

• (^) Genotype frequencies stay the same if five conditions are met. - (^) very large population: no genetic drift - (^) no emigration or immigration: no gene flow - (^) no mutations: no new alleles added to gene pool - (^) random mating: no sexual selection - (^) no natural selection: all traits aid equally in survival

The Hardy-Weinberg equation is used to predict genotype frequencies in a population.

• (^) Predicted genotype frequencies are compared with actual frequencies. - (^) used for traits in simple dominant-recessive systems "The Hardy-Weinberg equation is based on Mendelian genetics. It is derived from a simple Punnett square in which p is the frequency of the dominant allele and q is the frequency of the recessive allele." - (^) must know frequency of recessive homozygotes - (^) p 2 + 2pq + q 2 = 1

The Hardy–Weinberg model uses the Mendelian principles of segregation along with simple probability to explain the relationship between allele and genotype frequencies in a population.

Example

• If only 6% of the population displays

pale eyes (recessive gene e). What

is the frequency of genotype Ee in

this population?

q

2

= 0.06 ---> q = 0.

p + q = 1 ---> p = 0.

Ee = 2pq = 2(0.76)(0.24) = 0.

• (^) Natural selection selects for traits advantageous for survival.

if all genotypes do not have equal rates of survival or do not leave equal numbers of offspring, then allele frequencies may change from one generation to the next. To see why, let’s imagine a population of 100 individuals in which the frequency of allele A is 0.5 and that of allele a is 0.5. genotype frequencies in the present generation are (0.5)2 = 0.25 for AA 2(0.5)(0.5) = 0.5 for Aa, and (0.5)2 = 0.25 for aa. we have 25 AA individuals, 50 Aa individuals, and 25 aa individuals. Now suppose that individuals with different genotypes have different rates of survival

All 25 AA individuals survive to reproduce, 90 percent or 45 of the Aa individuals survive to reproduce, 80 percent or 20 of the aa individuals survive to reproduce. When the survivors reproduce, each contributes two gametes to the new gene pool, giving us 2(25) +2(45)+2(20)=180 gametes. What are the frequencies of the two alleles in the surviving population? We have 50 A gametes from AA individuals, plus 45 A gametes from Aa individuals, so the frequency of allele A is (50 + 45)/180= 0.53. We have 45 a gametes from Aa + 40 a gametes from aa. So the frequency of a allele is (45+40)/180=0.47.

• (^) Mutations produce the genetic variation needed for evolution.

For certain dominant mutations, however, a direct method of measurement can be used. To ensure accuracy, several conditions must be met: 1.The allele must produce a distinctive phenotype that can be distinguished from similar phenotypes produced by recessive alleles.

2. The trait must be fully expressed or completely penetrant so that mutant individuals can be identified.
3. An identical phenotype must never be produced by nongenetic agents such as drugs or chemicals. Mutation rates can be defined as the number of new mutant alleles per given number of gametes.

• (^) Gene flow moves alleles from one population to another.

Migration occurs when individuals move between the populations. Imagine a species in which a given locus has two alleles, A and a. There are two populations of this species, one on a mainland and one on an island. The frequency of A on the mainland is represented by p m , and the frequency of A on the island is p i

If there is migration from the mainland to the island, the frequency of A in the next generation on the island (p i ’) is given by p i ’ =(1m)p i +mP m where m represents migrants from the mainland to the island.