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. 20 AaBbCc 20 AaBbcc 20 aabbCc 20 aabbcc 5 AabbCc 5 Aabbcc 5 aaBbCc 5 aaBbcc 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.

Key Concepts

Testcross

A testcross is a genetic cross between an individual with an unknown genotype and a homozygous recessive individual. This method helps determine the genotype of the unknown individual based on the phenotypes of the offspring. In this case, the organism with genotype AaBbCc is crossed with aabbcc, allowing the observation of the segregation of alleles in the progeny.

Linkage and Crossover

Linkage refers to the tendency of genes located close together on the same chromosome to be inherited together. Crossover is a process during meiosis where homologous chromosomes exchange genetic material, leading to new allele combinations. In this scenario, the assumption of no crossover means that the alleles for the three genes are inherited together without recombination, affecting the expected progeny ratios.

Genotypic Ratios

Genotypic ratios represent the relative frequencies of different genotypes produced in a genetic cross. In this case, the progeny genotypes from the testcross can be analyzed to determine the ratios of dominant and recessive traits. Understanding these ratios is crucial for predicting the inheritance patterns and assessing the effects of linkage on the offspring's genetic makeup.