Module 6 Lesson 6 - 5

Work through the following problems to solidify your understanding of polygenetic inheritance.

1. Most chickens do not have feathers on their legs. This phenotype is the result of two genes interacting and having only recessive alleles for both genes. The presence of a dominant allele for either gene or for both genes causes feathers. What is the unfeathered leg : feathered leg ratio in the offspring of chickens that are heterozygous for both genes?
   
   
   1. 9:7
   2. 12:4
   3. 13:3
   4. 15:1
      
      
2. In corn plants, a dominant allele (I) inhibits kernel colour, but the recessive allele (i) permits colour when homozygous. At a different locus, the dominant gene P causes purple kernel colour, and the homozygous recessive genotype pp causes red kernels. If plants that are heterozygous at both loci are crossed, what will be the phenotypic ratio of the F1 generation?
   
   

Diploma Connection

Answer the following questions from a previous Biology 30 Diploma Examination.

Use the following information to answer the next two questions.


Two different genes control the expression of kernel colour in Mexican black corn: a black pigment gene (B) and a dotted pigment gene (D). Gene B influences the expression of gene D. The dotted phenotype appears only when gene B is in the homozygous recessive state. A colourless variation occurs when both genes are homozygous recessive.


After pure-breeding black-pigmented plants were crossed with colourless plants, all offspring were black-pigmented.
- from Griffiths et al., 1993


1. The genotypes of the parents of these F₁ offspring could be
   
   
   1. BBDD x bbdd
   2. BbDD x bbdd
   3. Bbdd x bbDD
   4. bbDD x BBdd
      


2. Plants of the F₁ generation are suspected of being heterozygous for both genes. A test cross of colourless plants with the heterozygote plants should produce a genotypic ratio in the offspring of
   
   
   1. 1:0
   2. 3:1
   3. 2:1:1
   4. 1:1:1:1




Use the following information to answer the next question.

In Labrador retriever dogs, two alleles (B and b) determine whether coat colour will be black (B) or brown (b). Black coat colour is dominant. A second pair of alleles, E and e, are on a separate chromosome from B and b. The homozygous recessive condition, ee, prevents the expression of either allele B or b, and produces a dog with a yellow-coloured coat. Some examples of genotypes and phenotypes for Labrador retrievers are shown below.



Genotype	Phenotype
BBEe	black
bbEe	brown
Bbee	yellow





Numerical Response


3. Two dogs, each with the genotype, BbEe, were crossed. What is the percentage probability that their offspring will have yellow coat colour? (Record your answer as a whole number percentage)




Use the following information to answer the next question.

Sickle cell anemia is an autosomal recessive genetic disorder. Because individuals affected by sickle cell anemia have defective hemoglobin proteins, their blood cannot transport oxygen properly. There appears to be a relationship between the incidence of malaria and sickle cell anemia. Individuals with sickle cell anemia and carriers of the sickle cell allele have some resistance to malaria. Malaria is caused by the parasite Plasmodium and is transmitted among humans by mosquitoes.





4. If scientists are successful in reducing significantly or eliminating malaria, the best prediction for what will happen to the allele for sickle cell anemia in the population is that it will
   
   
   1. not be affected by the elimination of malaria
   2. increase as its selective advantage is increased
   3. be reduced as its selective advantage is decreased
   4. disappear quickly as its selective advantage is increased




Use the following information to answer the next question.

Melanin pigments range in colour from yellow to reddish-brown to black. The amount and the colour of melanin in the skin account for differences in human skin coloration.

Albinism is a genetic disorder that results in unpigmented skin and other tissues. About 1 in 20 000 humans has albinism. In humans, it can be caused by an autosomal recessive allele (a). Its dominant allele (A) results in normal pigmentation.





5. The fact that exposure to sunlight increases melanin production in many humans and produces a tan demonstrates that
   
   
   1. some people have mutations that prevent melanin production
   2. the expression of some genes is influenced by the environment
   3. the environment causes mutations that increase the chance of survival
   4. the environment causes mutations that have no effect on the chance of survival





Use the following information to answer the next question.

Sickle cell anemia is caused by the sickle cell allele (Hb^S) of a gene that contributes to hemoglobin (Hb) production. The abnormal hemoglobin (hemoglobin-S) produced causes red blood cells to become deformed and block capillaries; tissue damage results. Affected individuals homozygous for the sickle cell gene rarely survive to reproductive age. Heterozygous individuals produce both normal hemoglobin and a small percentage of hemoglobin-S. These individuals are more resistant to malaria than are individuals who are homozygous for the allele for normal hemoglobin (Hb^A). Their red blood cells are prone to sickling when there is a deficiency of oxygen.



6. Which of the following conclusions can be drawn from the information provided on sickle cell anemia?
   
   
   1. The sickle cell gene eventually will disappear because of its interaction with malaria.
   2. Malaria causes heterozygous individuals to be less fertile than homozygous individuals.
   3. In Africa, sickle cell anemia will disappear because it is lethal in the homozygous condition.
   4. In Africa, carriers for sickle cell anemia have an advantage over homozygous individuals