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.

Answer part (a) again, assuming the three genes are so tightly linked on a single chromosome that no crossover gametes were recovered in the sample of offspring.

Verified step by step guidance

1. Step 1: Understand the testcross setup. The organism with genotype AaBbCc is crossed with a triply recessive organism aabbcc. The progeny genotypes reflect the gametes produced by the AaBbCc parent combined with the aabbcc parent's gametes.
2. Step 2: Recognize that if the three genes are tightly linked with no crossover, only the parental (non-recombinant) gametes will be produced by the AaBbCc parent. This means the progeny will only show genotypes corresponding to the original parental haplotypes.
3. Step 3: Identify the parental haplotypes from the AaBbCc parent. Since the parent is heterozygous at all three loci, the two parental haplotypes are the combinations of alleles found together on each chromosome. For example, one chromosome might carry ABC and the other abc.
4. Step 4: Determine which progeny genotypes correspond to these parental haplotypes. Only progeny with genotypes matching these haplotypes (combined with the recessive aabbcc parent) will be observed if no crossover occurs.
5. Step 5: Conclude that the progeny genotypes that do not match the parental haplotypes represent recombinant types and would not be present if no crossover occurred. Therefore, only the progeny with genotypes corresponding to the parental haplotypes should be counted.

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 dominant parent by analyzing the phenotypes and genotypes of the offspring, especially useful for studying linked genes and recombination.

Genetic Linkage and Recombination

Genetic linkage occurs when genes are located close together on the same chromosome and tend to be inherited together. Recombination through crossing over can separate linked genes, producing new allele combinations. If no crossover occurs, only parental allele combinations appear in offspring.

Interpreting Progeny Ratios in Linked Genes

Progeny genotype frequencies from a testcross can indicate linkage and crossover events. Equal numbers of parental types and fewer recombinants suggest linkage with some crossing over. If no recombinants are observed, all offspring genotypes correspond to parental haplotypes, indicating very tight linkage.