Everyone’s favorite homework: Word problems!

Activity two—Lesson 5

Objective: To familiarize yourself with genetics questions; be able to use your reasoning skills to solve such questions; and prepare for the assignment to turn in.

Items needed:

Pencil

eraser

patience

faith in yourself

Procedure:

You will be given 13 genetics problems to solve. I will show you one example. You are free to use any resource you want in solving the problems, but remember, you need to do these on your own on the test, so try to do them on your own now.

1. Example: In dogs, short hair is dominant over long hair. Two short-haired dogs are the parents of a litter of eight puppies. Six puppies have short hair and two have long hair.

What are the genotypes of the parents?

First choose the letters for the problem, let’s use s for long hair and S for short hair.

The end result of the litter was 6:2 ratio for the puppies, in favor of the dominant allele. Divide both sides by two, and you get a 3:1 ratio. Hmmmm, where have you seen that before? Before using the Punnett square, we know that at least one of the parents had to have a recessive allele because of long haired puppies. Taking that a step further, the recessive allele appeared, and the only way a recessive allele appears is if both parents were carriers of the allele. So let’s try our hypothesis that both of the parents had to be heterozygous for the alleles.

Testing Punnett Square / parent # 1 S / s
parent #2 S / SS / Ss
s / Ss / ss

The results of our hypothesis show that the phenotypes of the offspring, SS, Ss; and Ss are all short haired puppies. The ss is the recessive alleles. The ratio of these phenotypes is 3 short haired puppies to one long haired puppy. This is what we were given at the beginning of the problem so we were successful in our efforts.

2. Two black mice are mated and have eight offspring. Some of the baby mice are white. How can this happen? You need to show this on a Punnett square.

Testing Punnett Square / parent # 1 S / s
parent #2 S / SS / Ss
s / Ss / ss

3. One parent is homozygous for a certain trait, and the other is heterozygous. What fraction of their offspring would you expect to be heterozygous?

4. What genotypic ratio could you expect to get from a cross between an organism having the genotype Dd and an organism with the genotype DD?

5. In sheep, white wool (W) is dominant over black wool (w). How would you determine the genotype of a white male sheep?

6. If the pea plant you are working with is Green, with the dominant allele expressed as S, how could you find out if it is homozygous?

a. Run a test cross with a plant that was SS.

b. Run a test cross with a plant that was Ts.

c. Run a test cross with a plant that was tt.

d. Run a test cross with a plant that was ss.

7. What are the phenotypes expected among the offspring in a cross between a hybrid tall pea plant with a pure short?

a. 3:1

b. All short

c. All tall

d. 1:1

8. What are the phenotypes expected among the offspring in a cross between a purebred tall pea plant with a pure short?

a. 3:1

b. All short

c. All tall

d. 1:1

Okay, for the next two questions you may want to use a Punnett square. The questions are all based upon the following three-factor cross:

Parental Genotypes: AaBbDd X AaBbDd

9. What is the odds of an offspring having the genotype aabbdd?

a. 1/4

b. 1/16

c. 1/64

d. 1/256

10. What fraction of the offspring will be purebreds for the dominant alleles: AABBDD?

a. 1/4

b. 1/16

c. 1/64

d. 1/256

11. Meiosis differs from mitosis in that:

a. In mitosis the tetrads form during anaphase one.

b. In meiosis tetrads form during prophase one.

c. In mitosis the crossing over is more frequent.

d. In meiosis the end result is diploid.

12. How are genes related to chromosomes?

a. Chromosomes are the codes for various traits, genes are where the chromosomes are found.

b. Genes code for various traits and are found on the chromosomes.

c. Genes are locations on the RNA, and chromosomes are location on the DNA.

d. Genes are located on the sex chromosomes and alleles are located on the autosomes.

13. Gene linkages (where genes are linked to other genes on the same chromosome) are sometimes broken by:

a. Nondisjunction

b. Duplication

c. Crossing-over

d. Polyploidy