If you assume that a single gene controls a trait and that there are two alleles, one from each parent, you can use Mendelian genetics to determine the probability of genotype and phenotype of the offspring by using Punnett squares. For example if wrinkled peas are recessive and smooth peas are dominant you may assign a big letter for the dominant trait and a small letter for a recessive trait
P - Dominant - smooth
p - Recessive - wrinkled
Assume that each plant has two alleles. You can have the following combinations
PP, Pp, pP, pp
In situations of complete dominance, the dominant trait will always show if there is at least one big P allele. The phenotypes are as follows:
Genotype
|
Phenotype
|
PP - Homozygous
|
Smooth Peas
|
Pp - Heterozygous
|
Smooth Peas
|
pP - Heterozygous
|
Smooth Peas
|
pp - Homozygous
|
Wrinkled Peas
|
If we know the genotype of the parents we can use a Punnett square to determine the probability for the genotype and and phenotype of the offspring as follows
crossing PP x PP gives us the following:
100% of the offspring will be homozygous and will have a genotype of PP and a phenotype of smooth.
crossing Pp and Pp gives the following:
25% off the offspring would be homozygous dominant and smooth.
50% would be heterozygous dominant and smooth.
25% would be homozygous recessive and wrinkled.
Hope this helps.