punnet square of inherited indipendently traits
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Monohybrid Cross
The typical example of the test cross is the origin experiment Mendel conducted himself, to determine the genotype of a yellow pea. As seen in the image below, the alleles Y and y are used for the yellow and green versions of the allele, respectively. The yellow allele, Y, is dominant over the y allele. Therefore, in an organism with the genotype Yy, only the yellow allele is seen in the phenotype. Mendel had a yellow pea, and he wanted to know whether it was YY or Yy.
This was important to Mendel as it is to many seed producers and farmers today. The quality of a seed is determined by the plant it produces. A YY plant, if self-fertilized, would produce only yellow peas, in all of its offspring. There are many traits which are desirable to reproduce, and a homozygous plant is the obvious choice to do reproduce it with. However, in a dominant/recessive relationship, it is impossible to distinguish between a homozygous dominant plant (YY) and a hybrid, or heterozygous plant (Yy). Both would produce yellow seeds. However, if the Yy plant self-fertilizes, there is a chance of an offspring with the (yy) genotype, which would make green peas. Mendel sought to sort this out once and for all, so he devised the following test cross.
Mendel bred the unknown yellow pea (Y?) with a green pea, being homozygous recessive (yy). The chart below shows the two possible outcomes of the test.
Either the offspring would be all yellow, or around half of them would be green. This is based on the results of the two Punnett squares shown. The top square shows the results if the unknown yellow pea is (YY). In this case, the pea has no recessive allele to pass to the offspring. Therefore, 100% of the offspring receive one Y allele and one y allele, making them all yellow.
In the second case, if the unknown yellow pea has the genotype Yy, half of the offspring will receive this allele. The other allele will be from the green pea, and will also be a green allele (y). In this case, half of the offspring will produce green peas. The test cross itself occurs when the two plants are bred together, by taking pollen from the recessive plant, and carefully placing it on the flowers of the yellow pea plant. Mendel would then carefully rear all of the beans produced (which would be yellow) into plants of their own. The color of peas that these plants produced would determine the genetics of the original plant, which produced the yellow (Y?) seeds.
