A test cross between a plant of genotype PpSs and the tester of genotype ppss gi

Question

A test cross between a plant of genotype PpSs and the tester of genotype ppss gives the following numbers of progeny in four phenotypic types. 14:86:84:16 (purple flower + smooth seed coat: purple flower + wrinkled seed coat: white flower + smooth seed coat: white flower + wrinkled seed coat).

a) What is the expected ratio of progeny phenotypes assuming independent assortment of alleles?

b) Explain how ratios of progeny show that the two genes are linked. (Plz highlight which part of the question is not clear )

c) How many map units separate the purple and smooth genes? Show your calculations.

Explanation / Answer

The best way to determine the genotypes of the gametes of a dihybrid diploid is to make a cross to a tester, an individual that carries only recessive alleles for the genes under investigation. This cross is called a Testcross. A tested must be fully homozygous recessive, for example a/a ; b/b. Since the gametes of the tester carry only recessive alleles, the genotypes of the gametes of the dihybrid will be expressed in the phenotypes of the testcross progeny.

The expected ratio of progeny phenotypes assuming independent assortment of alleles(9:3:3:1) is 1:1:1:1

The ratios of progeny show that the two genes are linked by linkages and crossing over. Linked alleles, for example, A-b and a-B, form the gametes A-b and a-B, which maintain the linkage of the alleles.

The map units separate the purple and smooth genes are of Purple flower+ smooth =14 and purple flower=86/2=43 and Smooth=84/2=42

RF=86+84/14+86+84+16×100%=85%

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