In a haploid organism, loci A/a and D/d are 8 map units apart. Given the cross Ad × aD, the proportion of each progeny class can be calculated using the recombination frequency. The correct answer is (1) 92% parental (Ad) and 8% recombinant.

Explanation of the Problem

• The distance between loci A and D is 8 map units (8 cM), meaning 8% recombination.

• The parental haploid genotypes in the cross are Ad and aD.

• Recombination frequency (r) = 8% = 0.08.

• Frequencies of parental gametes = (1 – r) = 92% = 0.92.

• Frequencies of recombinant gametes = r = 8% = 0.08.

Calculations

• Parental classes arise from non-recombinant gametes:

  • Proportion of parental: 0.92 or 92%

• Recombinant classes arise from crossover gametes:

  • Proportion of recombinant: 0.08 or 8%

Explanation of Each Option

1. 92%, 8% (Correct): Matches the expected theoretical proportions given 8 cM distance. Parental types constitute 92%, recombinant types 8%.

2. 46%, 8%: 46% is half of 92%. This might confuse parental genotype frequencies with gamete frequencies. Incorrect for total progeny class proportions.

3. 92%, 4%: 4% recombinants is half of 8%, which is incorrect since recombination frequency directly translates to 8% recombinants.

4. 46%, 4%: Both values are halved; incorrect for total progeny proportions.

Introduction

In haploid genetics, understanding how recombination affects progeny genotype proportions is key. When loci A and D are linked 8 map units apart, a cross of Ad × aD produces specific proportions of parental and recombinant progeny. This article explains how to calculate these proportions accurately in haploid organisms.

Detailed Explanation

Distance of 8 map units between linked loci A and D means there is an 8% chance of recombination occurring between them. In the cross Ad × aD, the parental haplotypes (Ad and aD) are present in 92% of gametes, and recombinant haplotypes (AD and ad) occur in 8% of gametes.

Thus, the offspring class proportions are:

• Parental class (Ad): 92%

• Recombinant class: 8%

This calculation fits fundamental genetics mapping principles where recombination frequency equals the percentage of recombinant progeny.

Other options suggesting half or differing percentages confuse gamete versus progeny frequencies or reflect incorrect mapping interpretations.

Understanding these proportions helps in genetic linkage analysis, mapping gene loci, and predicting inheritance patterns in haploid organisms.