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.

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

Explanation / Answer

a) The expected ratio of progeny phenotypes assuming independent assortment of alleles can be calculated as follows,

PpSs x ppss

The expected ratio of progeny assuming independent assortment of alleles as shown in the above punnett square is 1:1:1:1

b) The ratio given in the question is 14:86:84:16. When the genes are linked, independent assortment of gametes will not take place and the expected ratio of 1:1;1:1 will not occur.

c) Since all 4 phenotypes are in equal ratio, the map units that separate purple and smooth genes is 0.25 or 25%.

gametes PS Ps pS ps ps PpSs Ppss ppSs ppss ps PpSs Ppss ppSs ppss ps PpSs Ppss ppSs ppss ps PpSs Ppss ppSs ppss

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