Genetic problem

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Genetic problem

Post by Alla on Tue Mar 18, 2008 12:50 am

georgina009 wrote:Can anyone help me with this question? I am very confused!
In corn-
coloured kernal - R
colurless - r
green plant colour - Y
yellow plant colour - y

The R and Y genes are linked. Two different plants (1 and 2) that were heterozygous for each trait - RrYy - were test crossed to plants that were homozygous for reccessive alleles. this info was gotton from that:
Coloured green -plant 1 145 plant 2- 18
colured yellow - 1-13 2-175
colourless green - 1 - 17 2- 185
colourless yellow - 1- 125 2- 22

what phenotypic ratio is expected in each if the genes were unlinked
and
plant 1 and 2 were generated by crossing true breeding plants. Crossing RRYYx rryy plants generated plant 1. The parents of plant 2 had genotypes that were different from the genotypes of the parents of plant 1. What are the genotypes of parents from plant 1?

Very confusing question. Any help is much appreciated.

Let’s start with simple part:

1. True breeding plants are homozygous. So if Plant1 was produced by RRYY x rryy cross and parents of Plant2 have different genotype, then they have to be RRyy x rrYY – the only other possible combination. Both crosses would produce RrYy offspring.

2. If the genes for plant and kernel color were unlinked, you expect RrYy to produce RY, Ry, rY, and ry gametes in equal frequency. Therefore, if you crossed RrYy x rryy you would expect a ratio of 1 RrYy (colored/green): 1 Rryy (colored/yellow): 1 rrYy (colorless/green): 1 rryy (colorless/yellow).

3. However, in this case those genes are linked (on the same chromosome) and those gametes do not occur with equal frequency – most of the time the original combination of genes is inherited together. So for Plant1, most gametes would be RY and ry because those were the combinations inherited by it from homozygous parents and only a few would be Ry and rY gametes produced by recombination of the sister chromatids.

http://i42.servimg.com/u/f42/12/12/20/07/recomb11.jpg

So for Plant1 RrYy x rryy cross you would expect majority of the progeny to be close to 50% colored/green and 50% colorless/yellow and only a few plants with other phenotypes.

The same reasoning applies to Plant2. It inherited Ry and rY gene combinations from its parents and, therefore, when crossed to rryy it would produce mostly colored/yellow and colorless/green progeny (in the ration close to 1:1) and only few of the other phenotypes produced by reconbination.

If you need more help, let me know.

I would appreciate it, if you registered to this site and used it directly.
Good luck!

Alla
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