An experiment is set up to study two genes. The two genes assort independently and for each gene the expression of alleles involves complete dominance. In the genetic cross RrTt x RrTt, a number of different phenotypes are seen. What is the difference between this number of phenotypes and the number of phenotypes seen when the cross RrTt x rrtt is carried out?

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Have a look through this presentation. Let's focus on the last slide:

Dihybrid cross

First, let's understand what the letters mean:

R = red allele
r = white allele

T = tall allele
t = short allele

There are two of each because the organism is presumably diploid, i.e. contains two alleles for each genetic locus, one from the father and one from the mother. In the case of RRTT or rrtt, it means the mother and father alleles are identical and the organism can be said to be homozygous ("same alleles") for these traits. In the case of RrTt, or rRtT, or rRTt, or RrtT (they are all the same, basically), the organism can be said to be heterozygous ("different alleles") for both genes.

The two genes assort independently and for each gene the expression of alleles involves complete dominance.

So we are told the alleles are completely dominant. By convention, we use capital letters to indicate dominant traits. That means that if a capital letter is present, the organism's trait will be determined by the dominant allele. A few examples of possible genotypes (left) and their phenotypes (right), to help you understand:


RrTt -> RED & TALL

rrTt -> white & TALL

rRtt -> RED & short

rrtt -> white & short

You first make your Punnett squares, as in the picture above. In your case, one for RrTt x RrTt and one for RrTt x rrtt. You fill both in. You will know all possible genotypes to result from the cross. Now - count how many phenotypes each cross will produce! You'll be able to say which cross will produce more, equal or fewer red, white, tall and short offspring. There's your answer!

Personal tip: Always visualize to make sense of the steps. When you see the cross RrTt x rrtt, imagine a red, tall individual carrying both red/white and tall/short alleles mating with a white, short indivual carrying only white and short alleles. When you make a Punnett square, you are simply predicting all the possible different babies to come from this cross. The amazing thing is that you can predict the ratio of traits among the babies. This was a big leap in genetics!

Keep in mind you are making a few assumptions. Every time you make a Punnett square, you are assuming the laws of Mendelian inheritance apply. You are assuming that the genes assort independently (which is not always true, especially for genes which are close to each other in the genome; they could travel together when they are being mixed), are not sex-linked (e.g. "only males can become red"), that the phenotype is determined solely by one gene ("monogenic"), and that your theoretical prediction will match reality. In practice, some alleles may confer a competitive advantage (and seem over-represented when you check the offspring), or may be lethal (expected offspring would be dead and you wouldn't even see them - they would be under-represented).

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