Monohybrid
 Single-gene crosses.
 TT x tt
Law of Segregation
 Each allele inherited separately.
 The alleles for each gene segregate from each other, so
that each gamete carries only one allele for each gene.
 Ex: A Tt (tall) plant can pass on either its dominant T
allele, or its recessive t allele to each sex cell (gamete)
 Tt
Dihybrid
 Two gene cross
 Shows two different genes getting passed from one
generation to another.
Mendel wondered if the segregation
of one pair of alleles affects another
pair
 For example, does the gene that determines the gene
that determines the shape of the seed affect the gene
for seed color?
Dihybrid practice
 Lets cross two pea plants that are heterozygous for
height and pod color.
 Parents: TtGg x TtGg
 First, lets determine the possible gametes these
parents can create:
 TtGg
TtGg
 Now use these 4 gametes in a dihybrid punnett square
to see what offspring they would create
 Since each parent now makes 4 possible gametes,
there are going to be more boxes in our punnett
square.
Try one on your own:
 In guinnea pigs, short hair is dominant to long hair.
Also in guinnea pigs, black eyes are dominant to red
eyes. A male guinnea pig that is heterozygous for both
traits is crossed with a female that is long haired and
red eyed. What are the expected phenotypes of their
offspring and in what proportion?
Independent assortment
 Genes for different traits can segregate independently
during the formation of gametes.
END OF MENDELIAN GENETICS