California State University Northridge Biology Questions

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Inheritance of Albinism

To learn more about how genetic traits are inherited, we will consider a specific example -the gene that controls whether or not a person can produce the pigment melanin which contributes  to  the  color  of  skin,  eyes  and  hair.    Some  people  have  the  hereditary condition, albinism; they are not able to produce melanin and have little or no pigment in their skin and hair.  Two different versions of the same gene are called alleles.  One allele of this gene codes for melanin production and normally pigmented skin and hair; it is symbolized by A. Another allele of this gene codes for albinism; it is symbolized by a.

        

AA

Aa

 

Aa

aa

Based on the above Punnett square,

1. What proportion of this couple’s children would you expect to be AA?   

2. What proportion of this couple’s children would you expect to be Aa?   

3. What proportion of this couple’s children would you expect to be aa?    

The next question is: What pigmentation will be observed for the children who have Aa alleles? This type of combination of two different alleles is called heterozygous. One allele in a heterozygous pair of alleles is dominant and the other allele is recessive; this means that the dominant allele determines the observable characteristic (phenotype expression) of the heterozygous individual.  Typically, the dominant allele is symbolized by a capital letter, in this case A for the allele for pigmentation. Thus, heterozygous (Aa) individuals will have normal pigmentation.

Based on the above Punnett square,

4. What proportion of this couple’s children would you expect to have normal pigmentation?   

5. What proportion of this couple’s children would you expect to have albinism?   

6. Two normally pigmented parents have an albino child. What are the parents’ genotypes?

      Mother __________________    Father___________________________

The child grows up and marries another albino. What is the probability of their having an albino child?

Show your work.

             

8. The genotype refers to the genetic makeup of an individual.  The phenotype refers to the observable physical and physiological characteristics of an individual.   Give an example of two individuals who have the same phenotype, but different genotypes for the albinism gene.   Explain how two individuals with the same phenotype can have different genotypes.

9. Suppose a father has aa alleles and a mother has Aa alleles. Complete a Punnett Square below to describe this mating and determine the genotypes, and what fraction of this couple’s children would be expected to have albinism.

List the genotypes:  

Fraction expected to have albinism:  

Ratio expected to have albinism:  

10. In a population of 500 people, 160 are homozygous dominant for tongue rolling and 90 cannot roll their tongue. 

             a. How many people are heterozygous for tongue rolling? Use the space below for your calculations.

     

b. List the three possible genotypes and the number of people that have each.

c. Keeping in mind that each individual has two alleles for the trait, how many dominant alleles for tongue rolling are in the gene pool?

d. How many recessive ones are there?

California State University Northridge Biology Questions

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California State University Northridge Biology Questions

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California State University Northridge Biology Questions

  

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