10.1.2 An Introduction to Hardy-Weinberg Theory

Question 1

Hardy-Weinberg theory

Answer 1

• When Gregor Mendel’s theory was brought to light in the early 1900s, there was considerable conflict about whether it applied to Darwinian selection.
• The Hardy-Weinberg theory (published in 1908) showed mathematically that Mendelian genetics and Darwinian selection were not mutually exclusive.
• According to the Hardy-Weinberg theory, in a randomly mating population with no evolution or selection, the relative gene frequency will stay the same from generation to generation.
- Mendel’s research paper was rediscovered and reevaluated in the early 1900s. Mendel had performed a series of experiments on pea plants whose characteristics varied in an either-or manner. For example, a flower-color variety produced either red or blue flowers with no gradations in color. Mendel’s first and second laws of inheritance demonstrated a simple dominant/recessive relationship between discrete units of inheritance. Mendel’s theory of inheritance provided no basis for understanding how variation in traits could be selected for.
- In the early 1900s, scientists debated over whether Mendel’s law of inheritance applied to Darwin’s theory of natural selection:

Question 2

Udney Yule

Answer 2

• argued that if Mendel’s theory was correct,

recessive traits should disappear from a population altogether, leaving only dominant traits.

Question 3

Godfrey Hardy

Answer 3

• mathematically demonstrated that in a
  nonevolving population, the relative frequency of dominant and recessive alleles should not change from generation to generation. Hardy’s theorem demonstrated that Mendel’s discrete units could be maintained from generation to generation.
• Hardy’s theorem came to be known as the
  Hardy-Weinberg theory because another scientist, Wilhelm Weinberg, independently came to the same conclusion as Hardy.

Question 4

Hardy-Weinberg

Answer 4

• According to the Hardy-Weinberg theory, the sexual shuffling of alleles because of meiosis and random fertilization does not affect the overall allelic composition of a population.
• The example on the left shows a hypothetical population consisting of two individuals. In the parental cross, an individual with the AA genotype produces offspring with an individual of the aa genotype. At this point, the dominant A allele and the recessive a allele each have a 0.5 frequency in the population.
• The F1 offspring all have the same Aa genotype. Note that the gene frequency has not changed and that the frequency of A and a is still 0.5.
• The F1 generation produces offspring, and four different genotypes are produced in the F2 generation. The relative frequency of the A and a alleles still has not changed; however, each accounts for half of the total alleles.
• In the F2 generation, there are four different genotypes, AA, aA, Aa (which produces the same phenotype as aA), and aa.
• To determine the possible outcome in the F3 generation, construct a Punnett square involving all the possible crosses among the three genotypes. The resulting nine squares are the possible genotypes in the F4 generation. Has the relative frequency of A and a changed? To determine this, count the total number of A and a alleles in the square. There are 16 each, meaning that the relative frequency of each is still 0.5.
• The conclusion is that sexual recombinationdoes not change the relative frequency of alleles in a population.

Question 5

True or false?
According to the Hardy–Weinberg theory, the sexual shuffling of alleles due to meiosis and random mating does not affect the overall allelic composition of the population.

Answer 5

• true

Question 6

Why is the theory of genetic frequency distribution called the Hardy–Weinberg theory and not the Hardy Theory?

Answer 6

• Weinberg published his mathematical solution about the same time Hardy published his.

Question 7

One of the historical arguments suggested that short fingers should become the phenotype in a selected population of humans after several generations. On what hypothetical basis was this prediction made?

Answer 7

• It was argued that if the allele for short fingers was dominant it would quickly spread throughout the population.

Question 8

Which of the following statements regarding Gregor Mendel’s findings is true?

Answer 8

• Gregor Mendel proposed that discrete units contain information that is inherited.

Question 9

What condition is assumed to be in place for the Hardy–Weinberg theory to work?

Answer 9

• the population is not evolving.

Question 10

A parental cross involves a cross between an individual who is homozygous dominant for a trait and an individual who is homozygous recessive for the same trait. What is the genetic frequency distribution of dominant and recessive single alleles in the F2 generation?

Answer 10

• 50% dominant and 50% recessive

Question 11

Although Hardy’s mathematical solution was a way to explain why dominant genes do not “take over” a population, why is it not realistic?

Answer 11

• The solution depends on a non-evolving population and in reality all populations are evolving to some extent.

Question 12

During evolution

Answer 12

• individuals are selected, while populations evolve

Question 13

What is the allelic frequency of the F1 generation from the cross AA × aa?

Answer 13

• A = 0.5, a = 0.5

Question 14

True or false?
According to the Hardy–Weinberg theory, in a randomly mating population with no evolution or selection, the relative gene frequency will change from generation to generation.

Answer 14

• false