To figure out the outcome of a cross, set up a Punnett square.a. Put the gametes of one parent at the top and those of theother on the left. To determine the allele(s) in each gamete fora given genotype, set up a systematic way to list all the possibilities. (Remember, each gamete has one allele of each gene.)Note that there are 2npossible types of gametes, where n is thenumber of gene loci that are heterozygous. For example, anindividual with genotype AaBbCc would produce 23 = 8 typesof gametes. Write the genotypes of the gametes in circlesabove the columns and to the left of the rows.b. Fill in the Punnett square as if each possible sperm werefertilizing each possible egg, making all of the possibleoffspring. In a cross of AaBbCc * AaBbCc, for example, thePunnett square would have 8 columns and 8 rows, so thereare 64 different offspring; you would know the genotype ofeach and thus the phenotype. Count genotypes and phenotypes to obtain the genotypic and phenotypic ratios. Becausethe Punnett square is so large, this method is not the mostefficient. See tip 5.
To figure out the outcome of a cross, set up a Punnett square.
a. Put the gametes of one parent at the top and those of the
other on the left. To determine the allele(s) in each gamete for
a given genotype, set up a systematic way to list all the possibilities. (Remember, each gamete has one allele of each gene.)
Note that there are 2n
possible types of gametes, where n is the
number of gene loci that are heterozygous. For example, an
individual with genotype AaBbCc would produce 23 = 8 types
of gametes. Write the genotypes of the gametes in circles
above the columns and to the left of the rows.
b. Fill in the Punnett square as if each possible sperm were
fertilizing each possible egg, making all of the possible
offspring. In a cross of AaBbCc * AaBbCc, for example, the
Punnett square would have 8 columns and 8 rows, so there
are 64 different offspring; you would know the genotype of
each and thus the phenotype. Count genotypes and
the Punnett square is so large, this method is not the most
efficient. See tip 5.
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