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Home Q & A Board Biology-Related Homework Help Genetics and Developmental Biology
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Dave4Bio Dave4Bio
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11 years ago
i don't understand
i never really got it and i need help
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#1
11 years ago
When you look at the answers you have from making the crosses:
.. the genotypic ratio is the ratio of "letter combinations" in the Punnett square.  For instance, if a monohybrid cross produces RR, Rr, rR, rr, then the genotypic ratio is 1 RR: 2 Rr: 1 rr.
.. the phenotypic ratio is the ratio of traits expected among the offspring.  For instance, if R = red and r=white, then the RR, Rr, rR, rr outcome from the previous example would yield 3 reds and 1 white.  The phenotypic ratio is 3 red: 1 white.

Dihybrid crosses are done the same way.  It's just that dihybrid crosses have a 4x4 Punnett square and the ratio will be more complicated, like 9 green, round: 3 green, wrinkled: 3 yellow, round: 1 yellow, wrinkled.
wrote...
#2
11 years ago
The monohybrid cross has a GENOTYPIC ration of 1:2:1 and a PHENOTYPIC ratio of 3:1.

The dihybrid cross is more complicated with things such as  epistasis and haplo-sufficiency and insufficiency, etc. The general ratio is 9:3:3:1. However, this coul vary and be 9:3:4 in the case of epistasis for example.

To better explain the monohybrid cross since it has less variables, the 1:2:1 GENOTYPIC ratio can be determined using a simple punnet square. Do a cross of A/a vs. A/a. You will get the following results:
A/A, A/a, A/a, a/a. There is 1 A/A, 2 A/a, and 1 a/a in the punnet square getting a 1:2:1 GENOTYPIC ratio.

However, because in genetics the upper case letter represents dominance, the A/A and A/a individuals will all represent the A phenotype. Therefore there are 3 individuals with the A phenotype and 1 individual with the a/a for the a phenotype, or a 3:1 ratio.

Hope this helps.
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tomochatomocha
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11 years ago
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