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Ch. 18 - Developmental Genetics
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 18 - Developmental GeneticsProblem 29b
Chapter 18, Problem 29b

In Drosophila, recessive mutations in the fruitless gene (fru) result in males courting other males, and recessive mutations in the Antennapedia gene (Ant⁻) lead to defects in the body plan, specifically in the thoracic region of the body, where mutants fail to develop legs. The two genes map 15 cM apart on chromosome 3. You have isolated a new dominant Antdᵈ mutant allele that you induced by treating your flies with X-rays. Your new mutant has legs developing instead of antennae on the head of the fly. You cross your newly induced dominant Antᵈ mutant (a pure-breeding line) with a homozygous recessive fru mutant (which is homozygous wild type at the Ant⁺ locus), as diagrammed below: Your cross results in the following phenotypic proportions:
Legs on head, normal courting behavior 75
Normal head, abnormal courting behavior 25
Legs on head, abnormal courting behavior 0
Normal head, normal courting behavior 0
Provide a genetic explanation for these results and describe a test for your hypothesis

Verified step by step guidance
1
Step 1: Begin by analyzing the phenotypic proportions provided in the problem. The phenotypes observed are 'legs on head, normal courting behavior' (75 flies), 'normal head, abnormal courting behavior' (25 flies), and the absence of the other two phenotypes ('legs on head, abnormal courting behavior' and 'normal head, normal courting behavior'). This suggests a specific genetic interaction between the Ant and fru loci.
Step 2: Recall that the two genes, Ant and fru, are located 15 cM apart on chromosome 3. This distance indicates that recombination between the two loci occurs at a frequency of 15%. The dominant Antᵈ allele causes the 'legs on head' phenotype, while the recessive fru allele causes 'abnormal courting behavior.' The absence of certain phenotypes suggests that the two loci are linked and that specific allele combinations are not viable.
Step 3: Hypothesize that the Antᵈ and fru alleles are in a coupling phase in the parental generation (i.e., Antᵈ is linked with the wild-type fru⁺ allele, and Ant⁺ is linked with the fru allele). The observed phenotypes can be explained if the double mutant combination (Antᵈ fru) is lethal, preventing the development of flies with 'legs on head, abnormal courting behavior' and 'normal head, normal courting behavior.'
Step 4: To test this hypothesis, perform a test cross. Cross the F1 progeny (heterozygous for both loci: Antᵈ fru⁺ / Ant⁺ fru) with a homozygous recessive tester (Ant⁺ fru / Ant⁺ fru). Analyze the phenotypic ratios in the offspring. If the hypothesis is correct, you should observe a 1:1 ratio of the viable phenotypes ('legs on head, normal courting behavior' and 'normal head, abnormal courting behavior'), with no offspring displaying the lethal double mutant phenotype.
Step 5: Additionally, confirm the genetic map distance between the two loci by calculating the recombination frequency from the test cross data. If the observed recombination frequency is approximately 15%, it will support the hypothesis that the two loci are linked and separated by 15 cM. This will further validate the proposed genetic explanation for the observed phenotypic proportions.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Gene Interaction and Epistasis

Gene interaction refers to the way different genes influence each other's expression and the resulting phenotype. In this case, the interaction between the Antennapedia gene (Ant) and the fruitless gene (fru) is crucial. Epistasis occurs when the effect of one gene is masked or modified by another gene, which helps explain why certain phenotypes appear in specific combinations, such as the dominance of Antdᵈ affecting leg development.
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Dominance and Recessiveness

Dominance and recessiveness are fundamental concepts in genetics that describe how alleles express traits. A dominant allele, like Antdᵈ, can mask the effect of a recessive allele, such as Ant⁻. In the given cross, the presence of the dominant Antdᵈ allele leads to the development of legs on the head, while the recessive fru allele influences courting behavior, illustrating how these genetic principles affect phenotypic outcomes.
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Phenotypic Ratios and Genetic Crosses

Phenotypic ratios are the relative frequencies of different phenotypes resulting from a genetic cross. The observed ratios in this experiment (75:25) suggest a specific inheritance pattern, likely indicating a 3:1 ratio typical of a monohybrid cross involving a dominant and recessive allele. Analyzing these ratios helps in understanding the genetic basis of the traits and can guide further testing, such as backcrossing to confirm the dominance relationships.
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Related Practice
Textbook Question

Zea mays (maize, or corn) was originally domesticated in central Mexico at least 7000 years ago from an endemic grass called teosinte. Teosinte is generally unbranched, has male and female flowers on the same branch, and has few kernels per 'cob,' each encased in a hard, leaf-like organ called a glume. In contrast, maize is highly branched, with a male inflorescence (tassel) on its central branch and female inflorescences (cobs) on axillary branches. In addition, maize cobs have many rows of kernels and soft glumes. George Beadle crossed cultivated maize and wild teosinte, which resulted in fully fertile F₁ plants. When the F₁ plants were self-fertilized, about 1 plant in every 1000 of the F₂ progeny resembled either a modern maize plant or a wild teosinte plant. What did Beadle conclude about whether the different architectures of maize and teosinte were caused by changes with a small effect in many genes or changes with a large effect in just a few genes?

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Textbook Question

In C. elegans there are two sexes: hermaphrodite and male. Sex is determined by the ratio of X chromosomes to haploid sets of autosomes (X/A). An X/A ratio of 1.0 produces a hermaphrodite (XX), and an X/A ratio of 0.5 results in a male (XO). In the 1970s, Jonathan Hodgkin and Sydney Brenner carried out genetic screens to identify mutations in three genes that result in either XX males (tra-1, tra-2) or XO hermaphrodites (her-1). Double-mutant strains were constructed to assess for epistatic interactions between the genes (see table). Propose a genetic model of how the her and tra genes control sex determination.

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Textbook Question

In Drosophila, recessive mutations in the fruitless gene (fru) result in males courting other males, and recessive mutations in the Antennapedia gene (Ant⁻) lead to defects in the body plan, specifically in the thoracic region of the body, where mutants fail to develop legs. The two genes map 15 cM apart on chromosome 3. You have isolated a new dominant Antdᵈ mutant allele that you induced by treating your flies with X-rays. Your new mutant has legs developing instead of antennae on the head of the fly. You cross your newly induced dominant Antᵈ mutant (a pure-breeding line) with a homozygous recessive fru mutant (which is homozygous wild type at the Ant⁺ locus), as diagrammed below:

What phenotypes, and in what proportions, do you expect in the F₂ obtained by interbreeding F₁ animals?

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Textbook Question

In Drosophila, recessive mutations in the fruitless gene (fru) result in males courting other males, and recessive mutations in the Antennapedia gene (Ant⁻) lead to defects in the body plan, specifically in the thoracic region of the body, where mutants fail to develop legs. The two genes map 15 cM apart on chromosome 3. You have isolated a new dominant Antdᵈ mutant allele that you induced by treating your flies with X-rays. Your new mutant has legs developing instead of antennae on the head of the fly. You cross your newly induced dominant Antᵈ mutant (a pure-breeding line) with a homozygous recessive fru mutant (which is homozygous wild type at the Ant⁺ locus), as diagrammed below:

Provide a molecular explanation for the reason your new Antᵈ mutant is dominant and for its novel phenotype.

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