What is the probability to get specific genotype as result of the cross of the parent's genotypes. Punnett Squares are a useful tool for predicting what the offspring will look like when mating plants and animals. Reginald Crundall Punnett, a mathmatician, came up with these in 1905, long after Mendel's experiments. Lets take a look at how these work using the yellow and green peas examples from Mendel's garden experiments.
For every gene, many different versions called alleles exist. Alleles control things like pea color or the presence of dimples on your face. Children inherit two alleles for each gene from their parents, one from the mother and one from the father.
The genotype refers to which two alleles an organism has. Sometimes both alleles are the same, and sometimes they are different. The phenotype refers to the visible trait that results from the combination of alleles present.
Mendel began his experiments with true breeding strains, meaning plants that have offspring of only one phenotype when mated. In true breeding strains, both alleles are the same for a gene. Since there is only one kind of allele present, mating two plants from the same strain will produce offspring that have the same phenotype and genotype as their parents. Plants or animals with two identical alleles for a gene are said to be homozygous.
Mendel first crossed two true different true breeding strains together, one that produced yellow peas and one that produced green peas. We'll use letters to represent alleles. Capital "A" will represent the yellow pea allele and lowercase "a" will represent the green pea allele.
The yellow pea phenotype has a genotype of AA.
The green pea phenotype has a genotype of aa.
AA x aa
When Mendel looked at the results of this mating, he saw that all of the offspring had yellow seeds. How did this happen? If one of the parent plants had green peas, why didn't a single one of the children plants have green peas? We can answer these questions and understand what's happening to the alleles in this crossing with the help of a Punnet Square.
Begin by writing the parent's genotype along the top and side of the Punnet Square.
Punnett Square, aa x AA
Next, fill in each cell with two allele, one from the parent along the top and one from the parent along the side. The letters in the middle show you all possible combinations of alleles that can happen from mating these two genotypes.
Punnett Square, Aa all yellow
In this case, all offspring have the same genotype and phenotype. The order of the letters doesn't make a difference in the phenotype (aA is the same as Aa) but the capital letter is usually written before the lowercase one. These offspring are said to be heterozygous, meaning that they have two different alleles for pea colors.
Despite the fact that both alleles are present in the offspring, the traits did not blend together to result in yellowish-green peas. Instead, only one phenotype was visible and all peas were yellow. The yellow pea phenotype is said to be dominent, meaning that it is visible in the heterozygous individual.
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