Stubble bristles (Sb) is dominant to wildtype bristles (Sb ) in Drosophila melanogaster. In a population where there are 137 flies with wildtype bristles and 836 flies with stubble bristles, what are the allele frequencies for the Sb and Sb alleles (assuming the population is in Hardy-Weinberg equilibrium)

Respuesta :

Generation after generation, populations in H-W equilibrium have the same allelic and genotypic frequencies.

f(Sb) = p = 0.626. f(Sb+) = q = 0.374. F(SbSb+) = 2pq = 0.468

What is Hardy-Weinberg Equilibrium?

According to the Hardy-Weinberg equilibrium theory, assuming a diallelic gene.

The dominant allele p(X) has a frequency of p.

The recessive allele p(x) has a frequency of q.

After one generation, the genotypic frequencies are

p² (Homozygous dominant genotypic frequency),

2pq (Heterozygous genotypic frequency),

q² (Homozygous recessive genotypic frequency).

In a population that is in H-W equilibrium, genotypic and allelic frequencies don't change over generations.

The allelic frequencies added together equal 1.

p + q = 1.

The sum of genotypic frequencies equals 1

p² + 2pq + q² = 1

In the exposed:

Sb is dominant

Sb+ is recessive ⇒ wild type

Population size ⇒ 137 + 836 = 973

wildtype bristles ⇒ 137  ⇒ Sb + Sb+

stubble bristles ⇒ 836 ⇒ SbSb and SbSb+

Phenotypic frequency:

F(wildtype) = q² = 137/973 = 0.14

F(stubble) = p² + 2pq = 836/973 = 0.86

To determine the genotypic and allelic frequencies, we will now use the frequency of the recessive trait.

F(Sb+Sb+) = q² = 137/973 = 0.14 ⇒ Recessive Genotypic frequency

f(Sb+) = q = √0.14 = 0.374 ⇒ Recessive Allelic frequency

We will now utilize q (recessive allelic frequency) to determine the dominant genotypic frequency and the dominant allelic frequency since this population is in H-W equilibrium. We will solve the following equation to achieve this.

p + q = 1

p + 0.374 = 1

p = 1 - 0.374

p = 0.626 ⇒ Dominant Allelic frequency

F( SbSb) = p² = 0.626² = 0.392 ⇒ Dominant Genotypic frequency

The heterozygous frequency will be the last thing we learn. There are 2 possibilities, The dominant genotypic frequency is subtracted from the dominant phenotypic frequency.

F(SbSb + SbSb+) - F(SbSb) = 0.86 - 0.392 = 0.468

p² + 2pq + q² = 1

0.392 + 2pq + 0.14 = 1

2pq = 1 - 0.392 - 0.14

2pq = 0.468

Answers:

- f(Sb) = p = 0.626 ⇒ Dominant Allelic frequency

- f(Sb+) = q = 0.374 ⇒ Recessive Allelic frequency

- F(SbSb+) = 2pq = 0.468 ⇒ Heterozygous frequency

Learn more about Hardy-Weinberg Equilibrium here:

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