Question

In some bees, thin stripes (S) are dominant over thick stripes (s) and black eyes (B) are dominant over gray eyes (b). Complete a dihybrid cross for parents with the genotypes: SsBB × ssBb and answer the following in complete sentences.

Describe how you would set up a Punnett square for this cross.
List the likelihood of each possible offspring genotype.
List the likelihood of each possible offspring phenotype.

Answer 1

In mice, black hair (H) is dominant over white hair (h) and brown eyes (E) are dominant over green eyes (e).

HH : homozygous dominant (have same allele that is dominant)
hh : homozygous recessive (have same allele that is recessive)
Hh : heterozygous (have one dominant allele and one recessive allele)

Same concept applies for the eye color gene.

Eye color is a gene. Brown and green are alleles (or variants) of that gene.

Genotype is the allele make-up and phenotype is the resulting characteristic. For instance if someone was heterozygous for black hair, they would have the genotype Hh and the phenotype black hair.

Parents : SsBB x ssBb

Top row is one parent’s contributing alleles
First column is the other parent’s contributing alleles

SB SB sB sB
sB SsBB SsBB ssBB ssBB
sb SsBb SsBb ssBb ssBb
sB SsBB SsBB ssBB ssBB
sb SsBb SsBb ssBb ssBb


SB SB sB sB
sB Black Black White White
sb Black Black White White
sB Black Black White white
sb Black Black White white

all have brown eyes.

This di-hybrid cross (two genes) will have a resulting phenotypic ratio of 8:8. That is, 8 individuals from the cross will have black hair and brown eyes (the two dominant alleles) and 8 individuals will have white hair and brown eyes. There will be no individuals with green eyes. Therefore no individuals will be homozygous recessive (ssbb). This makes sense since one parent is homozygous dominant for brown eyes.

So what did you get for your di-hybrid cross??