HUMAN BIOLOGY

iNtroDUCtioN to GeNetiCs 383

CritiCaL tHinKinG

1. One gene has alleles A and a. Another has alleles B and b.
   For each genotype listed, what type(s) of gametes can be
   produced? (Assume independent assortment occurs.)
   a. AABB c. Aabb
   b. AaBB d. AaBb


2. Still referring to Problem 1, what will be the possible
   genotypes of offspring from the following matings? With
   what frequency will each genotype show up?
   a. AABB 3 aaBB c. AaBb 3 aabb
   b. AaBB 3 AABb d. AaBb 3 AaBb


3. Go back to Problem 1, and assume you now study a third
   gene having alleles C and c. For each genotype listed, what
   type(s) of gametes can be produced?
   a. AABBCC c. AaBBCc
   b. AaBBcc d. AaBbCc


4. The young woman shown at
   right has albinism—very pale
   skin, white hair, and pale blue
   eyes. This phenotype is due to
   the absence of melanin, which
   imparts color to the skin, hair,
   and eyes. It typically is caused by
   a recessive allele. In the following
   situations, what are the probable
   genotypes of the father, the
   mother, and their children?
   a. Both parents have normal
   phenotypes; some of their
   children are albino and others
   are not.
   b. Both parents and all their
   children are albino.
   c. The mother is not albino, the father is albino, and one of
   their four children is albino.


5. When you decide to breed your Labrador retriever Molly
   and sell the puppies, you discover that two of Molly’s four
   siblings have developed a hip dis order that is traceable to
   the action of a single recessive allele. Molly herself shows
   no sign of the disorder. If you breed Molly to a male
   Labrador that does not carry the recessive allele, can you
   assure a purchaser that the puppies will also be free of the
   condition? Explain your answer.


6. The ABO blood system has been used to settle cases of
   disputed paternity. Suppose, as a geneticist, you must
   testify during a case in which the mother has type
   A blood, the child has type O blood, and the alleged
   father has type B blood. How would you respond to the
   following statements?
   a. Man’s attorney: “The mother has type A blood, so the
   child’s type O blood must have come from the father.
   Because my client has type B blood, he could not be
   the father.”
   b. Mother’s attorney: “Further tests prove this man is hetero-
   zygous, so he must be the father.”

seLf-Quiz Answers in Appendix VI

1. Alleles are.
   a. alternate forms of a gene
   b. different molecular forms of a chromosome
   c. always homozygous
   d. always heterozygous


2. A heterozygote has.
   a. only one of the various forms of a gene
   b. a pair of identical alleles
   c. a pair of contrasting alleles
   d. a haploid condition, in genetic terms


3. The observable traits of an organism are its.
   a. phenotype c. genotype
   b. pedigree d. multiple allele
   system


4. Offspring of a cross AA 3 aa are.
   a. all AA d. 1/2 AA and 1/2 aa
   b. all aa e. none of the above
   c. all Aa


5. We say a gene is expressed when.
   a. a parent passes it to offspring
   b. a cell makes the protein coded by the gene’s chemical
   instructions
   c. its chemical instructions are “read out” in a cell more
   rapidly than usual
   d. both a and c


6. Assuming complete dominance, offspring of the cross
   Aa 3 Aa will show a phenotypic ratio of.
   a. 1:2:1 c. 9:1
   b. 1:1:1 d. 3:1


7. Which statement best fits the principle of segregation?
   a. Units of heredity are transmitted to offspring.
   b. Two genes of a pair separate from each other during
   meiosis.
   c. Members of a population become segregated.
   d. A segregating pair of genes is sorted out into gametes
   independently of how gene pairs located on other
   chromosomes are sorted out.


8. Crosses involving parents that are heterozygous for two
   traits (AaBb 3 AaBb) lead to offspring with phenotypic
   ratios close to.
   a. 1:2:1 c. 3:1
   b. 1:1:1:1 d. 9:3:3:1


9. Reasons that many phenotypes can’t be predicted with
   certainty include:
   a. There is continuous variation in a gene’s effects.
   b. A trait is shaped by more than one gene and by some
   aspect of a person’s environment.
   c. The gene responsible is recessive.
   d. The trait is polygenic.


10. Match each genetic term appropriately.
    cross involving two traits a. Aa 3 Aa
    cross involving one trait b. Aa
    homozygous condition c. AABB 3 aabb
    heterozygous condition d. aa

© Rick Guidotti, Positive Exposure

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