1. Modifications of Mendelian Ratios
Active Lecture PowerPoint® Presentation for Essentials of Genetics Seventh Edition Klug, Cummings, Spencer, Palladino
Chapter 4
Modifications of Mendelian Ratios
Copyright © 2010 Pearson Education, Inc.

2. 1/4 blue: 1/2 light blue: 1/4 white 1/2 blue: 1/2 white All light blue
04.01Q In a plant species, if the B allele (blue flowers) and the b allele (white flowers) are incompletely dominant (Bb is light blue), what offspring ratio is expected in a cross between a blue-flowered plant and a white-flowered plant?
1/4 blue: 1/2 light blue: 1/4 white
1/2 blue: 1/2 white
All light blue
3/4 blue: 1/4 white
1/3 blue: 1/3 light blue: 1/3 white
3. All light blue

3. 04.01A In a plant species, if the B allele (blue flowers) and the b allele (white flowers) are incompletely dominant (Bb is light blue), what offspring ratio is expected in a cross between a blue-flowered plant and a white-flowered plant?
Answer: 3. All light blue
Explanation: The cross is BB  bb, which would yield all Bb (light blue) offspring.

4. 04.02Q If a mother has type A blood and her son has type O blood, what are the possible blood types of her son’s father?
Type O only
Types A or O
Types B or O
Types A, B, or O
Any blood type
4. Types A, B, or O

5. 04.02A If a mother has type A blood and her son has type O blood, what are the possible blood types of her son’s father?
Answer: 4. Types A, B, or O
Explanation: The father must carry the IO allele, so he could be IOIO (type O), IAIO (type A), or IBIO (type B).

6. two alleles are codominant. there is a recessive lethal allele.
04.03Q A cross between two short-tailed mice results in offspring in the ratio of 2/3 short-tailed and 1/3 long-tailed. The best explanation for this result is that
two alleles are codominant.
there is a recessive lethal allele.
there are at least three alleles.
there is simple dominance between two alleles.
there is a dominant lethal allele.
2. there is a recessive lethal allele.

7. 04.03A A cross between two short-tailed mice results in offspring in the ratio of 2/3 short-tailed and 1/3 long-tailed. The best explanation for this result is that
Answer: 2. there is a recessive lethal allele.
Explanation: A 2:1 ratio indicates a “missing” phenotypic class, suggesting a homozygous (recessive) lethal allele. The cross could be represented as Tt  Tt  1/4 TT (long-tailed), 1/2 Tt (short-tailed), and 1/4 tt (lethal, not observed).

8. 04.04Q In a cross between two double heterozygous parents (AaBb), an offspring ratio of 9:3:4 suggests
epistasis.
epigenesis.
sex-linkage.
incomplete dominance.
codominance.
1. epistasis.

9. 04.04A In a cross between two double heterozygous parents (AaBb), an offspring ratio of 9:3:4 suggests
Answer: 1. epistasis.
Explanation: Epistasis involves a nonreciprocal impact of one genetic locus on another. An example is the coat color in mice, as a dihybrid cross would yield 9/16 A-B- agouti, 3/16 aaB- black, and 4/16 (3/16 A-bb, 1/16 aabb) albino.

10. could have inherited it from either parent.
04.05Q Duchenne muscular dystrophy is caused by a recessive X-linked allele. A man with this disorder
could have inherited it from either parent.
must have inherited it from his mother.
must have inherited it from both parents.
would pass it along to all of his children.
would pass it along to only his sons.
2. must have inherited it from his mother.

11. 04.05A Duchenne muscular dystrophy is caused by a recessive X-linked allele. A man with this disorder
Answer: 2. must have inherited it from his mother.
Explanation: The man must carry only one copy of the allele on his X chromosome that he must have inherited from his mother. He would have inherited a Y chromosome from his father. He would pass that X chromosome on to only his daughters.