Chapter 4

Review

Understanding Concepts 1. Explain why Mendel’s choice of the garden pea for his experiments was especially appropriate. 2. Explain why, under normal circumstances, an individual can carry only two alleles of a gene. 3. Differentiate between codominance and incomplete dominance. 4. Test crosses are valuable tools for plant and animal breeders. (a) Provide two practical examples of why a cattle rancher might use a test cross. (b) Why are test crosses most often done on bulls rather than on cows? 5. Cystic fibrosis is regulated by a recessive allele c. Explain how two normal parents can produce a child that has the disorder. 6. Cats with 6 toes carry a dominant allele. Draw a pedigree showing the mating of a male cat with 6 toes to a normal female. (Assume that the male cat had a normal mother.) Include this information in your pedigree chart: • The cats produce 6 offspring (4 females and 2 males). • Only one of the female offspring mates and produces a litter (3 males and 2 females). 7. Two different genes control the expression of kernel colour in Mexican black corn. Gene B produces black coloration. Gene B influences the expression of gene D, which produces a dotted pigmentation. The dotted variation appears only when gene B is homozygous. A colourless variation arises when both genes are homozygous recessive. (a) What is (are) the possible genotype(s) for corn with dotted pigmentation? (b) What would kernels with a genotype of BBdd look like? 8. For shorthorn cattle, the mating of a red bull and a white cow produces a roan calf that has intermingled red and white hair. Many matings between roan bulls and roan cows produce cattle in the following ratio: 1 red, 2 roan, 1 white. Is this an example of codominance or multiple alleles? Explain your answer.

Applying Inquiry Skills 9. For pea plants, long stems are dominant over short stems. Determine the phenotype and genotype ratios of the F1 offspring from the cross-pollination of a heterozygous long-stem plant with a short-stem plant.

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10. For horses, the trotter characteristic is dominant over the pacer characteristic. A male, who is described as a trotter, mates with three different females and each female produces a foal. The first female, who is a pacer, gives birth to a foal that is a pacer. The second female, also a pacer, gives birth to a foal that is a trotter. The third female, a trotter, gives birth to a foal that is a pacer. Determine the genotypes of the male, all three females, and the three foals sired. 11. For ABO blood groups, the A and B alleles are codominant, but both A and B are dominant over type O. Indicate the blood types possible from the mating of a male with blood type O to a woman with blood type AB. Could a female with blood type AB ever produce a child with blood type AB? Could she ever have a child with blood type O? 12. The following information was gathered on blood types in one family (Figure 1). I 1

2

II 1

2

3

4

5

1

2

3

4

6

7

III 5

female male

female male

blood type A

blood type AB

blood type B

blood type O

Figure 1

(a) Indicate the genotypes for individuals 1 and 2 in generation I. (b) Would it ever be possible for individuals 2 and 3 in generation II to have a child with blood type O? Explain why or why not. (c) If individuals 6 and 7 had another child, what would be the probability of the child having blood type O? 13. Thalassemia is a serious human genetic disorder which causes severe anemia in the homozygous condition (T mT m). People with thalessemia die before sexual maturity. The heterozygous condition (T mT n) causes a less serious form of anemia. The genotype T nT n causes no symptoms of the disease. Indicate the

possible genotypes and phenotypes of the offspring if a male with the genotype T mT n married and had children with a woman of the same genotype. 14. In guinea pigs, black coat colour is dominant over white. Short hair is dominant over long hair. A guinea pig that is homozygous for white and for short hair is mated with a guinea pig that is homozygous for black and for long hair. Indicate the phenotype(s) of the F1 generation. If two hybrids from the F1 generation are mated, determine the phenotype ratio of the F2 generation. 15. The diabetes allele is recessive. Use the phenotype chart (Figure 2) to answer the following questions. (a) How many children do parents A and B have? (b) Indicate the genotypes of parents A and B. (c) Give the genotypes of M and N.

(c) Should amniocentesis be performed even if there is no strong evidence suggesting genetic problems? Give your reasons. (d) Should pedigrees be made public? Identify pros and cons before coming to a conclusion. Father K’s Family Tree I B

A II C

D

G

F

E

H

I

J

Q

R

S

III K

L

N

M

O

P

I female diabetic

B

A

female normal

II C

E

D

III I

J

K

G

F

L

?

?

M

N

H

Mother O’s Family Tree I A

male diabetic male normal

Figure 2

II C

16. In Canada, it is illegal for individuals to marry their own immediate relatives. Using the principles of genetics, explain why inbreeding is discouraged. 17. Amniocentesis is a common prenatal procedure that is used to obtain cells to test for genetic abnormalities, such as cystic fibrosis. The test is usually done in the 15th to 18th week of pregnancy on a woman who has an increased risk of having children with genetic abnormalities. Cystic fibrosis is caused by a recessive allele found on chromosome 7. (a) See Figure 3. Woman (O), who has cystic fibrosis in her family history through marriage, is carrying a child. The lineage of her husband (K) is also linked with cystic fibrosis. On the basis of the information provided, would you recommend amniocentesis? Keep in mind that, like all invasive procedures, some risk, although small, is associated with amniocentesis. Provide reasons for your response. (b) Would you recommend the procedure if man K married woman O’s cousin, woman J? Give your reasons.

D

G

F

E

H

I

III J

Making Connections

B

K

L

M

N

O

Figure 3

18. During the 19th century, individuals with genetic disorders were often shunned. As the 20th century emerged and society began to understand that many of these conditions were genetic, a movement emerged that aimed to eliminate defective genes or less desirable genes from the human population. (a) How might defective genes be eliminated from a population? (b) Identify moral and ethical issues associated with a policy that attempts to eliminate genes considered less advantageous.

Exploring 19. Reread “The Plant Breeders” in section 4.3. Find out about other Canadian contributions to plant genetics. What technologies were used or created in the discovery process? Follow the links for Nelson Biology 11, Chapter 4 Review. GO TO

www.science.nelson.com

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