When allele A is dominant over allele A', write out the parental phenotypes and genotypes. Then write out the offspring phenotypes and genotypes as well as the number of each.

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mr-mayonnaise eNotes educator| Certified Educator

If we were to complete Punnet squares for all of the potential parents' genotypes and phenotypes there are 3 different possible genotypes for children and parents with 2 possible phenotypes for each. 


  • AA, homozygous dominant
  • AA', heterozygous
  • A'A', homozygous recessive


  • AA & AA', showing dominant allele A
  • A'A', showing recessive allele A'

All options for crosses are as follows:

  • AAxAA: Both parents hom. dom. with phenotypes showing dominant allele A. This would give 4 hom. dom. children like the parents. 
  • AAxAA': One hom. dom. other het., both phenotypes showing dominant allele A
  • AAxA'A': One parent hom. dom. other parent hom. rec., one parent showing phenotype for allele A, other showing allele A'. Children will be 2 hom. dom. and 2 het. all with phenotype dominant allele A.
  • AA'xAA': Both parents het. showing phenotype for allele A. 1 child will be hom. dom. with phenotype for allele A, 2 children will be het. with phenotype for allele A, and 1 hom. rec. with phenotype for allele A'. 
  • AA'xA'A': Both parents het. showing phenotype for allele A. 2 children will be het. with phenotype for allele A. 2 children will be hom. rec. with phenotype showing allele A'.
  • A'A'xA'A': Both parents are hom. rec. showing phenotype with allele A'. All 4 children will be just like their parents. 

There are only 6 unique Punnet squares that can be completed using only two traits, one dominant and one recessive. Use the above information to complete any Punnet square you may need to solve your problem. Hope it helps! 

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