Question

Drosophila females homozygous for the third chromosomal genes pink eye (p) and ebony body (e) were crossed with males homozygous for the second chromosomal gene dumpy wings (dp). Because these genes are recessive, all offspring were wild type (normal). \(F_{1}\) females were testcrossed to triply recessive males. If we assume that the two linked genes ( \(p\) and \(e\) ) are 20 mu apart, predict the results of this cross. If the reciprocal cross were made (F1 males-where no crossing over occurs-with triply recessive females), how would the results vary, if at all?

Short answer

Answer: The results of testcrosses between F1 dihybrid females and triply recessive males differ from the reciprocal cross involving F1 males with triply recessive females because of the differences in recombination rates and the number of possible phenotypes. The F1 female testcross has 4 possible phenotypes and a 20% chance of recombination, while the reciprocal cross involving F1 males has only 2 possible phenotypes and no crossing over occurs.

Step-by-step solution

Analyze the P generation cross

We are given the following initial cross: - Female: pink eye (p), ebony body (e), homozygous for these genes - Male: dumpy wings (dp), homozygous for this gene Since pink eye, ebony body, and dumpy wings are all recessive, the offspring of this cross will be wild type with normal eyes, body color, and wings. These offspring make up the F1 generation.

Determine characteristics of the F1 females

The F1 females will be heterozygous for all three genes: - Pink eye: Pp - Ebony body: Ee - Dumpy wings: Dpdp (since the female will have chromosomes with only normal alleles for the dumpy wing gene) The genotypes of the F1 females will be Pp Ee Dpdp.

Perform the F1 female testcross with triply recessive males

Now, the F1 heterozygous females Pp Ee Dpdp are crossed with males that are triply recessive (pp ee dpdp). Due to the 20 mu distance between the linked genes p and e, there will be a 20% chance of recombination between them. Therefore, the expected phenotype percentages of the offspring are as follows: - 40% wild type (normal eye, body, and wings) - P_E_ Dpdp - 20% pink eye, ebony body, dumpy wings (pp ee dpdp) - 20% normal eye, normal body, dumpy wings (P_E_ dpdp) - 20% pink eye, ebony body, normal wings (pp ee Dpdp)

Perform the reciprocal cross and predict the results

In the reciprocal cross, F1 males (Pp Ee DpDp) are crossed with triply recessive females (pp ee dpdp). Since no crossing over occurs in male Drosophila, there will be no recombination for the p and e genes. Therefore, the expected phenotype percentages of the offspring are as follows: - 50% normal eye, normal body, dumpy wings (P_E_ dpdp) - 50% pink eye, ebony body, dumpy wings (pp ee dpdp)

Compare the results of the two crosses

Comparing the results of the F1 female testcross with the reciprocal cross, we can see that the outcomes do vary: - In the F1 female testcross, there are 4 possible phenotypes and a 20% chance of recombination due to crossing over. - In the reciprocal cross with F1 males, there are only 2 possible phenotypes and no crossing over occurs. Thus, the results do vary between the two crosses because of the differences in recombination rates and the number of possible phenotypes.

Key concepts

Recessive Genes

In this context, recessive genes refer to those which require two copies of the same allele to express a trait. These genes, like pink eye (p), ebony body (e), and dumpy wings (dp) in Drosophila, will not affect the phenotype, or visible traits, if paired with a dominant allele.

• The recessive traits are only visible when both alleles in the gene pair are recessive.
• In a Drosophila genetics scenario, the recessive alleles of pink eye and ebony body observed in the female parent result in offspring that do not show these traits unless crossed with another recessive carrier.

Understanding recessive genes is key in predicting the offspring's traits, as shown when both parents carry these traits, potentially leading to three visible recessive traits in their offspring.

Testcross

A testcross involves breeding an organism showing a dominant phenotype with an organism having a recessive phenotype. It helps in determining the genotype of the former. Generally, the unknown genotype is crossed with a homozygous recessive.

• In this exercise, the F1 heterozygous females (Pp Ee Dpdp) are crossed with triply recessive males (pp ee dpdp).
• This allows us to see what traits are carried and expressed, as the recessive traits will only appear if the F1 females are heterozygous for all genes.

A testcross is instrumental in mapping out genetic inheritance and understanding the recombination possibilities, as seen when predicting phenotype possibilities of the offspring by identifying the dominant and recessive gene pairs in the parents.

Recombination Frequency

Recombination frequency is the percentage indicating the likelihood of genes crossing over and exchanging between homologous chromosomes during meiosis. It's crucial in genes that are linked closely, like pink eye (p) and ebony body (e) in this Drosophila genetics example.

• A recombination frequency of 20% indicates that in 20% of cases, these genes will recombine.
• This affects the variety in offspring phenotypes, leading to combinations of traits not present in the parents.

For instance, in the testcross of F1 heterozygous females, the presence of recombination allows for more diversified offspring phenotypes compared to the reciprocal cross scenario.

Phenotype Prediction

Phenotype prediction is the process of forecasting the observable traits of offspring based on their genetic information. In genetics exercises, this involves determining the alleles for traits and understanding how they combine to produce visible characteristics.

• In our example, the phenotypes of the offspring from the F1 female testcross show 40% wild type, 20% fully recessive, and two other combinations owing to recombination.
• However, in the reciprocal cross, the phenotypes are limited to just two, due to no recombination in male Drosophila.

Predicting phenotypes accurately requires understanding both the genetic makeup of parents and the interaction of linked genes, which guide the expression of traits seen in offspring.