In corn, the recessive gene s produces shrunken endosperm, while the dominant ge

ID: 64739 • Letter: I

Question

In corn, the recessive gene s produces shrunken endosperm, while the dominant gene S produces full, plump kernals. Color of the endosperm is determined by a locus that has dominant allele C producing colored endosoerm, or by recessive allele c producing colorless endosperm.

Two homozygous plants are crossed, producing F1 that are all phenotypically plump and colored. These F1 plants are then test crossed, producing the following offspring:

Shrunken, colored 149

Shrunken, colorless 4035

Plump, colored 4032

Plump, colorless 152

a. What are the phenotypes of the original homozygous P1 plants used?

b. How are the genes linked in the F1? (i.e. what alles occur together on a chromosome?)

c. Estimate the map distnace between S and C

Explanation / Answer

1). A) Given that the parental cross produced all phenotypically plump and colored endosperm, and the parents are homozygous (means, this is a test cross), the parental cross is as follows.

SSCC* sscc = 100% SsCc. (all are phenotypically plump and colored) --------------> F1 generation.

F1 test cross is, SsCc* sscc = SsCc, Sscc, ssCc, sscc.

It is given that, ssCc (Shrunken, colored) ------------------------> 149

sscc (Shrunken, colorless) ---------------------------------------------> 4035

SsCc (plump, colored) -----------------------------------------> 4032

ssCc (plump, colorless) -------------------------------------> 152

b). If the genes are located on two different chromosomes, the genotypic ratio of SsCc, Sscc, ssCc, sscc must be 1: 1:1:1. But it is not true in the given case, so the genes are lined and hey are present on the same chromosome.

c). From the F2 generation, we can say that SsCc and sscc are parental and ssCc and Sscc are recombinant.

The map distance between S and C chromosomes is = recombinant frequency/ total genotypic frequencies* 100 =149+152/ 8368 *100 = 3. 597 or approximately 3.6 map units.

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In corn, the recessive gene s produces shrunken endosperm, while the dominant ge

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