An organism of the genotype AaBbCc was testcrossed to a triply recessive organism (aabbcc). The genotypes of the progeny are presented in the following table.

What can you conclude from the actual data about the location of the three genes in relation to one another?

Verified step by step guidance

1. Step 1: Identify the parental and recombinant genotypes from the progeny data. The parental genotypes are expected to be the most frequent classes, while recombinant genotypes appear less frequently. Here, the genotypes with counts of 20 are likely parental types, and those with counts of 5 are recombinant types.
2. Step 2: Group the progeny into parental and recombinant classes. Parental genotypes: AaBbCc, AaBbcc, aabbCc, aabbcc (each with 20 progeny). Recombinant genotypes: AabbCc, Aabbcc, aaBbCc, aaBbcc (each with 5 progeny).
3. Step 3: Determine which genes are linked by analyzing the frequency of recombination between gene pairs. Calculate recombination frequencies for each gene pair by summing the recombinant progeny involving those genes and dividing by the total progeny number.
4. Step 4: Use the recombination frequencies to infer gene order. The gene pairs with the lowest recombination frequency are closest together, while the pair with the highest recombination frequency is farthest apart. This helps establish the linear order of the three genes on the chromosome.
5. Step 5: Conclude whether the genes are linked or assort independently. If recombination frequencies are significantly less than 50%, the genes are linked. If close to 50%, the genes assort independently. Based on the data, interpret the linkage relationships and gene order.

Key Concepts

Testcross and Its Purpose

A testcross involves crossing an organism with a dominant phenotype but unknown genotype with a homozygous recessive organism. This helps reveal the genotype of the first organism by analyzing the phenotypes and genotypes of the progeny, especially useful for studying gene linkage and recombination.

Gene Linkage and Recombination

Genes located close together on the same chromosome tend to be inherited together, a phenomenon called linkage. Recombination occurs when crossing over during meiosis exchanges genetic material between homologous chromosomes, producing new allele combinations. The frequency of recombinant offspring indicates the distance between genes.

Interpreting Progeny Ratios to Map Genes

By comparing the number of parental-type progeny to recombinant types in a testcross, one can infer gene order and relative distances. Higher numbers of parental genotypes suggest linkage, while recombinant genotypes indicate crossover events. Mapping genes involves calculating recombination frequencies to determine gene positions.