In tomatoes, tall (D) is dominant to dwarf (d) and smooth fruit is (H) is domina

Question

In tomatoes, tall (D) is dominant to dwarf (d) and smooth fruit is (H) is dominant to hairy (h) fruit. A farmer has two tall and smooth tomato plants, A & B. The farmer performs a testcross with each of these plants & obtains the following results:

Progeny of

Plant A

DdHh 6

Ddhh 86

ddHh 78

ddhh 8

Plant B

139

10

14

150

(A) What are the genotypes of plants A & B? (B) If the loci are linked, what is the map distance between these loci? (C) Explain why different proportions of progeny are produced in each test cross?

Explanation / Answer

For Plant A

Since this result does not follow 9:3:3:1 ratio so has a deviation from independent assortment law of Mandel.

It means there is linkage.

So in this case the highest no. of progeny are parental type.

means the genotype of parents are Ddhh and ddHh

For plant B,

the parental genotype are

DdHh and ddhh

B. Yes genes are linked because they do not follow the mandel rule of indeendent assortment. Which is later proved with distance calculation of alleles.

Total progeny = 6+86+78+8= 178

Linkage Distance= No. of recombinants/total no. of offsprings X100

= 14/178X100

=0.07*100 = 7 CM

Distance is very much less than 50% so alleles are on same gene and linked

For plant linkage distance

10+14/178X100

=24/178X100

=13.48 CM

Distance is very much less than 50% so alleles are on same gene and linked

c. Different progeny is produced because thesetwo parents have different genotypes as well as different gene distance. test cross with different genotype whose genes are linked will lead to different types of progenies.

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