Question

In Drosophila, gray body color is dominant to ebony body color, while long wings are dominant to vestigial wings. Assuming that the \(P_{1}\) individuals are homozygous, work the following crosses through the \(\mathrm{F}_{2}\) generation, and determine the genotypic and phenotypic ratios for each generation. (a) gray, long \(\times\) ebony, vestigial (b) gray, vestigial \(\times\) ebony, long (c) gray, long \(\times\) gray, vestigial

Short answer

Answer: The phenotypic ratio for the F2 generation is 9 gray, long : 3 gray, vestigial : 3 ebony, long : 1 ebony, vestigial. The genotypic ratio for the F2 generation is 1 GG, LL : 2 GG, Ll : 1 GG, ll : 2 Gg, LL : 4 Gg, Ll : 2 Gg, ll : 1 gg, LL : 2 gg, Ll : 1 gg, ll.

Step-by-step solution

(a) Cross: Gray, long x Ebony, vestigial

First, we need to identify the genotypes of the parent organisms: Gray body (G) is dominant, so its genotype would be GG. Ebony body (g) is recessive, so its genotype would be gg. Long wings (L) are dominant, so their genotype would be LL. Vestigial wings (l) are recessive, so their genotype would be ll. Now we can represent the given cross as (GG, LL) x (gg, ll). (a)

1. F1 generation: Cross between P1 individuals

We will use the Punnett square method to find the genotypes of the F1 generation. For each trait, we will cross the genotype of each parent: For body color: GG x gg = Gg, Gg, Gg, Gg All offspring will have the genotype Gg. For wing type: LL x ll = Ll, Ll, Ll, Ll All offspring will have the genotype Ll. Thus, the F1 generation will have the genotype Gg, Ll (gray, long).

2. F2 generation: Cross between F1 individuals

Now we will perform a cross between two individuals of the F1 generation, both with the genotype Gg, Ll. For body color: Gg x Gg = GG, Gg, Gg, gg (1 GG: 2 Gg: 1 gg) For wing type: Ll x Ll = LL, Ll, Ll, ll (1 LL: 2 Ll: 1 ll) The possible genotypic combinations are: 1. GG, LL (Gray, long) 2. GG, Ll (Gray, long) 3. GG, ll (Gray, vestigial) 4. Gg, LL (Gray, long) 5. Gg, Ll (Gray, long) 6. Gg, ll (Gray, vestigial) 7. gg, LL (Ebony, long) 8. gg, Ll (Ebony, long) 9. gg, ll (Ebony, vestigial) Now we need to find the phenotypic ratios for the F2 generation. We have 3 different phenotypes: Gray, long (1+2+4+5); Gray, vestigial (3+6); Ebony, long (7+8); and Ebony, vestigial (9). The phenotypic ratio for the F2 generation would therefore be 9 gray, long : 3 gray, vestigial : 3 ebony, long : 1 ebony, vestigial. The genotypic ratio for the F2 generation would be 1 GG, LL : 2 GG, Ll : 1 GG, ll : 2 Gg, LL : 4 Gg, Ll : 2 Gg, ll : 1 gg, LL : 2 gg, Ll : 1 gg, ll. For (b) and (c), the process will be similar to (a). Identify the genotypes for each cross and then perform the cross for F1 and F2 generations to find the genotypic and phenotypic ratios.

Key concepts

Punnett Square

The Punnett square is a simple graphical method used to predict the outcome of crossbreeding in genetics. Named after its creator, Reginald Punnett, this square grid helps students understand how alleles combine from the contributing parents to produce various genotypes in their offspring.

For example, in Drosophila studies, if you have a homozygous dominant gray body parent (GG) and a homozygous recessive ebony body parent (gg), the Punnett square allows you to visually combine each allele from the mother and father to construct the genotypes for their offspring. A standard Punnett square for a single trait has four cells, each representing a potential genotype of the offspring—this expands to 16 cells when analyzing two traits simultaneously.

Using a Punnett square improves comprehension of genetics by making it easier to visualize the probability of inheriting specific traits, especially when explaining complex situations involving multiple alleles and gene interactions.

Genotypic Ratio

The genotypic ratio describes the proportion of genotypes that appear in the offspring of a particular genetic cross. It is expressed in the simplest whole number ratio. In essence, this ratio provides an expected count of how many offspring will have each genotype.

For an F2 cross in Drosophila involving two heterozygous parents (Gg, Ll), you might see a genotypic ratio such as 1 GG, LL : 2 GG, Ll : 1 GG, ll : 2 Gg, LL : 4 Gg, Ll : 2 Gg, ll : 1 gg, LL : 2 gg, Ll : 1 gg, ll. This expresses the predicted occurrences of each genotype among the offspring. It's important to note that due to independent assortment and random fertilization, the actual offspring might not precisely reflect this ratio, but it provides a solid predictive framework.

Phenotypic Ratio

Conversely, the phenotypic ratio relates to the physical characteristics (phenotypes) observable in the offspring of a genetic cross. While the genotypic ratio is concerned with alleles regardless of whether they're visible, the phenotypic ratio considers only observable traits.

For the aforementioned Drosophila cross, despite there being nine different genotypic outcomes, there are only four different phenotypes due to dominance and recessiveness of the alleles. This gives us a phenotypic ratio for the F2 generation of 9 gray, long: 3 gray, vestigial: 3 ebony, long: 1 ebony, vestigial. Understanding this helps students to correlate the presence of alleles with their actual expression in the organism.

Dominant and Recessive Alleles

Traits in organisms are influenced by alleles, which are different forms of a gene. Dominant alleles are those that express their traits even when only one copy is present, while recessive alleles require two copies to exhibit their traits.

In Drosophila, the gray body color allele (G) is dominant, meaning only one copy of this allele is necessary for the gray color to be expressed. Conversely, the ebony body color allele (g) is recessive, and the ebony color will only show up if the organism has two copies of this allele (gg).

Understanding the nuanced dance between dominant and recessive alleles is crucial when predicting the phenotype of offspring. It also becomes evident why certain traits, such as gray body color in Drosophila, are more common—simply because it takes only one dominant allele for the trait to appear, making its transmission more probable than a recessive trait.