Simple Genetics Practice Problems KEY

This worksheet will take about 20 minutes for most students, I usually give it to them after a short lecture on solving genetics problems. I don't normally take a grade on it, instead just monitor progress of students as they work and then have them volunteer to write the answers #5-15 on the board.

1. For each genotype, indicate whether it is heterozygous (HE) or homozygous (HO)

AA _HO__
Bb _HE__
Cc _HE___
Dd _HE___

Ee _HE___
ff _HO___
GG _HO__
HH _HO__
Ii __HE__
Jj __HE__
kk _HO___
Ll _HE___
Mm _HE___
nn _HO__
OO _HO__
Pp _HE___

2. For each of the genotypes below, determine the phenotype.

Purple flowers are dominant to white flowers
PP ______purple____________
Pp ______purple_______________
pp _____white________________

Brown eyes are dominant to blue eyes
BB _______brown________________
Bb _______brown________________
bb _______blue________________

Round seeds are dominant to wrinkled
RR ______round__________________
Rr ______round________________
rr _______wrinkled_______________

Bobtails are recessive (long tails dominant)
TT ________long________________
Tt _________long__________________
tt _________bobtails_____________

3. For each phenotype, list the genotypes. (Remember to use the letter of the dominant trait)

Straight hair is dominant to curly.
_____SS______ straight
_____Ss_____ straight
_____ss_____ curly
Pointed heads are dominant to round heads.
____PP____ pointed
____Pp______ pointed
____pp_______ round

4. Set up the square for each of the crosses listed below. The trait being studied is round seeds (dominant) and wrinkled seeds (recessive)

Rr x rr
  What percentage of the offspring will be round? ___1/2 or 50%___
Rr x Rr
  What percentage of the offspring will be round? ___75% or 3/4_
RR x Rr
  What percentage of the offspring will be round? __all, 100%_____

Practice with Crosses.

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I've only included a couple of squares as samples here, most of these are very straightforward. Given enough practice, students will learn to do most of them without the squares.

5. A TT (tall) plant is crossed with a tt (short plant).
What percentage of the offspring will be tall? _____all tall_____

TT x tt

6. A Tt plant is crossed with a Tt plant. What percentage
of the offspring will be short? ___25%__

TT x tt

7. A heterozygous round seeded plant (Rr) is crossed with a
homozygous round seeded plant (RR). What percentage of
the offspring will be homozygous (RR)? ____1/2 or 50%______

8. A homozygous round seeded plant is crossed with a homozygous
wrinkled seeded plant. What are the genotypes of the parents?
_____RR____ x ___rr______

What percentage of the offspring will also be homozygous? _____0%_____

9. In pea plants purple flowers are dominant to white flowers.
If two white flowered plants are cross, what percentage of their
offspring will be white flowered? _____all white_______

If students are stuck on this one, advise them to make a "key" to help them sort it out.

PP = purple, Pp = purple, pp = white

10. A white flowered plant is crossed with a plant that is
heterozygous for the trait. What percentage of the
offspring will have purple flowers? _____pp x Pp, 50% purple_____

11. Two plants, both heterozygous for the gene that controls
flower color are crossed. What percentage of their offspring
will have purple flowers? Pp x Pp , 75% purple______________
What percentage will have white flowers? ____25% white_______

12. In guinea pigs, the allele for short hair is dominant.
What genotype would a heterozygous short haired guinea pig have? __Hh___
What genotype would a purebreeding short haired guinea pig have? __hh____
What genotype would a long haired guinea pig have? __HH____

Why did I use H instead of S for short hair. Students may discover the hard way that capital and lower case S's are hard to tell apart. This is a good time to talk to them about how to choose their letters. You can choose the letter of the dominant trait, or you can chooe the letter for the trait itself (H is for hair).

13. Show the cross for a pure breeding short haired guinea pig
and a long haired guinea pig. HH x hh
What percentage of the offspring will have short hair? ____all____

14. Show the cross for two heterozygous guinea pigs. Hh x Hh
What percentage of the offspring will have short hair? __75%____
What percentage of the offspring will have long hair? __25%_____

Hh x Hh

15. Two short haired guinea pigs are mated several times. Out of 100
offspring, 25 of them have long hair. What are the probable
genotypes of the parents? Hh x Hh Show the cross to prove it! most students just point to the cross right in #14