Genetics and Mutation Notes
Genetics is based on predictable patterns of inheritance. Understanding genetics depends on
understanding these patterns, which are based on possible combinations of gametes. Punnett
squares provide a way to determine these combinations. Thus, it is possible to predict offspring
traits.
MONOHYBRID CROSSES
In Mendelian genetics, each trait has two alleles. In monohybrid crosses, one trait is involved.
In a monohybrid cross, each parent contributes two alleles, producing four possible combinations
for the one trait.
Dihybrid crosses involve two traits. A dihybrid cross involves two alleles per trait for two traits,
for a total of four alleles. The gamete combinations predict which offspring (F2 generation) can be
produced from these F1 parents. The phenotypic ratio (relative count of trait combinations) can
now be determined. It is recommended that phenotypes be written in each square when working
a Punnett square problem. Remember that phenotype is determined by genotype. Therefore, the
phenotypes can be determined by the alleles in each offspring square: an uppercase letter
represents a dominant allele. Any offspring with one uppercase letter for a particular trait will have
that trait.