Section 5.2: Variations in Heredity
Section 5.2 Questions
(Page 196)
1. In dominance, one allele (the dominant allele) is fully expressed in the phenotype of an individual,
regardless of whether the individual is homozygous or heterozygous for that allele. In incomplete
dominance, two different alleles are both partially expressed in cells of the phenotype of a heterozygous
individual, creating a third phenotype. The third phenotype is a blend of the two homozygous phenotypes..
In codominance, both alleles are fully expressed in a heterozygous individual, creating a third phenotype
that is unlike either homozygous phenotypes and is not a blend of the two. In the third phenotype, both
homozygous phenotypes are intermingled, not blended.
2. (a) The genotype of black chickens is FBFB.
(b) The genotype of white chickens is FWFW.
(c) The genotype of erminitte chickens is FBFW.
(d) Students’ diagrams should be similar to the following:
There is a ¼ probability of a black chick, and a ¼ probability of a white chick.
3. The F1 heterozygous generation were all oval radishes, which is intermediate between round and long
shape. Also, the F2 generation has a phenotypic ratio of 1:2:1 ratio. The alleles for radish shape therefore
show incomplete dominance. This can be demonstrated by following the cross of oval radishes by a
Punnett square analysis.
Punnett squares will vary depending on the symbols chosen by students.
Sample answer:
Let S stand for shape, SR for the round allele and SL for the long allele. The genotype of long radishes is
SRSR, of round radishes is SLSL, and of oval radishes is SRSL. The cross gives a F2 genotypic ratio of 1 SRSR
: 2 SRSL: 1 SRSL . This theoretical genotypic ratio would give rise to a theoretical F2 phenotypic ratio of 1
round: 2 oval: 1 long, which is the same as the observed F2 phenotypic ratio. Students’ diagrams should be
similar to the following:
4. If Mendel worked with alleles that exhibited incomplete dominance, he would not have been able to
observe the mathematical patterns that lead him to identify that each gene (factor) had two alleles (forms)
and to formulate the law of segregation. Instead, his conclusions might have supported the theory that
inheritance was a blend of parental traits.
Copyright © 2011 Nelson Education Ltd.
Chapter 5: Mendelian Genetics—Patterns of Inheritance
5.2-1
5. Since there are three distinct phenotypes for a single gene trait with only two alleles, thalassemia is an
example of incomplete dominance. The types of genotypes that students propose will vary with the
symbols they choose.
Sample answer: Let T represent non-thalassemia allele and t the thalassemia allele. Since minor
thallesemia is a blend between major anemia and no anemia, it must be the heterozygous trait. Therefore,
the genotype for major anemia is tt, for minor anemia is Tf and no anemia is TT.
6. There are two possible genotypes for type A blood: IAIA and IAi.
7. Blood type O has only one genotype: ii. Therefore, the child must have received an i allele from both
parents. This is possible only if the parents are both heterozygous. Therefore, the genotype of the father is
IAi and the mother’s genotype is IBi.
8. Blood type O has only one genotype: ii. Therefore, the genotype of the mother must be ii, and the child
must have received an i allele from its father as well. The father must therefore be heterozygous, so his
genotype iss IBi.
Based on the Punnett square analysis, there is a ½ (50 %) chance that the couple’s next child will have
blood type O, and a ½ (50 %) chance that their next child will have blood type B. There is no chance (0 %)
that their next child will have either blood type A or AB. Students’ diagrams should be similar to the
following:
9. The alleles IA and IB are examples of codominance, since heterozygous individuals produce both types of
sugars, giving them a blood type AB. However, the alleles IA and IB are completely dominant to the i
allele, since heterozygous individuals (genotypes IA i and IBi) produce only the sugars for A and B
respectively (phenotype Type A and Type B respectively).
10. Answers will vary, but should contain all the information given in the sample answer.
(a) Sample Answer: Individuals with genotypes RR or Rr are Rh positive, rr individuals are Rh negative.
This is an example of complete dominance.
(b) Sample Answer: If a woman is Rh negative and a man is Rh+, there may be problems during a
pregnancy if the child is Rh+. When the child is Rh+, the mother might make antibodies against the Rh+
blood type. This is usually not a problem during a first such pregnancy. However, in a subsequent
pregnancy, the antibodies present in the mother may attack the baby’s blood, causing hemolytic disease of
the baby. This can usually be avoided by having blood tests of the parents to evaluate if they have different
Rh factors and, if they do, giving the woman injections to reduce her immune response.
Copyright © 2011 Nelson Education Ltd.
Chapter 5: Mendelian Genetics—Patterns of Inheritance
5.2-2