9
4) To figure the phenotypic ratio, count the number of individuals with either the dominant or recesssive
phenotype for both traits! Then that ratio would be something like 4:4:4:4 or 9:3:3:1
Dihybrid Crosses. Set up the crosses using the rules and the letters from the other page.
1. If a woman who is a non-PTC taster (recessive) with heterozygous hitchhikers thumb has children with a man who is a
heterozygous PTC taster with straight thumbs (recessive), what is the probability of them having each of the following types
of children? (Fill in the Punnett Square and the blanks).
Parents’ genotypes ____________ X ____________
a. How many PTC taster, Hitchhikers thumb
b. How many PTC taster, straight thumb
c. How many Non-PTC taster, Hitchhikers thumb
d. How many Non- PTC taster, straight thumb
e. What is the phenotypic ratio?
2. If a woman who has no hair on her mid-digit (recessive)and is homozygous attached earlobes (dominant) has children with a
man who has hair on his mid-digit and has attached earlobes (heterozygous for both traits), what is the probability of them
having each of the following types of children? (Fill in the Punnett Square and the blanks).
Parents’ genotypes ____________ X ____________
a. How many hair, attached earlobes
b. How many hair, not attached earlobes
c. How many hairless, attached earlobes
d. How many hairless, not attached earlobes
e. What is the phenotypic ratio?
3. John Doe and Jane Doe want to have children and are thinking about how their childrens’ hands might look. What would
their children look like if they are both heterozygous for straight pinky and hitchhikers thumb? (Fill in the Punnett Square and
the blanks).
Parents’ genotypes ____________ X ____________
a. Straight pinky, hitchhikers thumb
b. Straight pinky, Straight thumbs
c. bent pinky, hitchhikers thumb
d. bent pinky, Straight thumbs
e. What is the phenotypic ratio?
PTC-taster- TT, Tt Attached earlobes- EE, Ee Can roll tongue- RR, Rr
Non-PTC taster – tt Free earlobes – ee Can’t roll tongue - rr
Hitchhikers thumb- HH, Hh Straight pinky- PP, Pp
Straight thumb – hh Bent pinky- pp
Hair on mid-digit – MM, Mm Widow’s peak- WW, Ww
No hair on mid-digit- mm No widow’s peak- ww