UNIT GENETICs PROBLEMs #2: TEST CROSSES 1. In peas, green pods are dominant and
ID: 89440 • Letter: U
Question
UNIT GENETICs PROBLEMs #2: TEST CROSSES 1. In peas, green pods are dominant and yellow You have pea pods are the recessive trait. plants with green pods but you are unsure of the parentage of these plants. How would you determine the genotype of your green pod plants? 2. Two black mice mate. are black and two are white. (Black fur is Six of their offspring dominant to white.) a. What are the genotypes of the parents? b. For which offspring are you sure of the genotypes? 3. In rabbits, the allele for brown coat is dominant, the allele for white coat is Suppose you have a group of rabbits, some brown and some white. you want to find out the genotype of a brown rabbit, what color rabbit would you mate it with? 4. A brown buck (male) is mated with a white doe (female). In their litter of 11 young six are white and five are brown. Brown is dominant to white. a. What is the genotype of the buck? b. What ratio of brown and white offspring would have been produced by the above mating if the brown buck had been homozygous?Explanation / Answer
Hi,
1. The green and yellow pods are the two colors of the pea. These are called phenotypes. The gene or allele coding for this coloration / phenotype is called Genotype. In the present question, the green pea is dominant i.e. it is called by a dominant allele. Let us call the allele as G. The yellow pod is recessive so its genotype will be g.
Since green allele G is dominant, presence of only one copy of G is sufficient to cause the green color. The yellow pod is recessive, so two copies of g are required. As we know that the pea plant is diploid, so there will be two copies of allele, so the combination for green pod will be either GG or Gg. The combination for yellow pod is ‘gg’.
Let us assume that we have pea plants with green pods, but no idea about its genotype. Genotype can either be ‘GG’ (Homozygous) or ‘Gg’ (heterozygous) both of which will give green pod. To find out the genotype of these plants, we can do something called a ‘Test cross’. Here we cross our plants with a yellow pod plants ( recessive). We know that the yellow is ‘gg’. The outcome of such cross are:
IF the green pods are ‘GG’, then mating will be, GG x gg = Gg = All green pods.
If the green pods are Gg, then mating will be , Gg x gg = Gg and gg . 50% green 50% yellow.
So if the ratio of test cross is 1, and all the plants are green then the green pod parent is heozygous GG.
If the ratio is 1:1 , with equal number of green and white plants, then the green pod parent is heterozygous, Gg.
2. Black fur is dominant so let the genotype of Black mice à BB, or Bb.
White fur is recessive, so the genotype of white mice be, à bb.
As we saw in previous question, the mating between BB and bb will produce all Bb ( heterozygous, black) mice. But in the question, some mice are white. So the parent black mice is heterozygous with Bb combination . Another way of answering it logically is, if the white mice is born to the parents, and white can only arise with each parent providing one copy of ‘b’, so atleast one of the parent is heterozygous.
So the answer is
a. Genotype of the parents à Bb and bb
b. Since all the black offsprings are Bb and all white one are bb, we are sure of all the offsprings genotype.
3. This question is connected to the previous ones. The brown is dominant so it can either be caused by homozygous or heterozygous condition. The white coat on the other hand in recessive , so only caused by homozygous recesive condition. In order to find out the genotype of the genotype of the brown rabbit, we can mate it with unknown recessive rabbit. I.e mating with white coat rabbit.
As per our regulation, I have answered first full question with a couple of more.
Please solve the rest with similar logic. If you find any difficulty in any step, get back to me in comments section.
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