Question

Mendelian Genetics

The gene involved in the disease Sickle Cell Anemia (SCA) is on human chromosome 11. Allele “A” is the normal form of the gene and codes for a part of the protein complex called hemoglobin. Hemoglobin is required for your blood cells to carry oxygen. Allele “a” is an abnormal form of the gene. The hemoglobin protein made from the “a” allele is defective. Red blood cells containing the defective protein are very fragile. This disease is recessive–meaning that you have to inherit two copies of the recessive allele in order to be affected with the disease SCA. The gene involved in the disease Cystic Fibrosis (CF) is on human chromosome 7. Allele “B” is the normal form of the gene and codes for a channel protein that allows chloride to pass through a cell. Allele “b” is the abnormal form of the gene. The channel protein made from the “b” allele is defective and doesn’t insert into the cell’s plasma membrane, resulting in an inability of sodium and chloride to cross the membrane. CF is also recessive therefore only people with two defective copies of the gene (“bb”) are affected with the disease.

1. Chris and Emma are phenotypically normal but carriers for SCA (gene A).

a. What are their genotypes?

b. With respect to gene A, what are the possible genotypes of the gametes that they can produce.

Chris: _____ Emma: _____

  _____ _____

In front of each gamete genotype above write probability of producing that gamete.

c. What is the probability that Chris and Emma will have a child affected with SCA? (affected=has the disease=“aa” =homozygous recessive)

d. Write the cross and the Punnett square that demonstrates this. ___ ___ x ___ ___

e. The phenotypic ratio is ____ normal : ____ affected

The genotypic ratio is ____ AA : ____ Aa : ____ aa

2. Chris and Emma are also carriers for CF (gene B is on chromosome 7). Both are genotype Bb.

a. What is the probability that they will have a child affected with CF? (affected = has the disease = “bb” =homozygous recessive)

b. Write the cross and the Punnett square that demonstrates this. ___ ___ x ___ ___

3.

a. Chris and Emma both have genotype AaBb. Genes A and B are on separate chromosomes. List the genotypes of the possible gametes that each parent can make. Remember, you should have only ONE allele from EACH chromosome in each gamete (haploid). Write the probabilities for each gamete

c. Now demonstrate random fertilization by combining gametes from each parent to create possible offspring in the white squares. Note: not all of these offspring are actually made in any given human fertilization—only ONE is made! Each of the white squares below represents one potential offspring; the set of white squares together represents all the possible offspring that these two individuals could have with respect to these two traits.

d. What is the probability of each white box in the Punnett Square above? ______________

e. There are 4 different phenotypes in the Punnett Square. (Capital letters in genotypes are dominant so AA and Aa both show a big A phenotype.) Using 4 different colors, or shapes, group together the 4 phenotypes on the figure in 3b. What is the probability of each? ______ A B ______ A b ______ a B ______ a b

4. Instead of one big square it is much easier to make two small Punnett squares, one for each gene. In the space at the bottom of page, draw the two squares (from 1d and 2b on previous page). Indicate the phenotypic ratios for each square and then multiply to get the combined probabilities (like 3e). For example, the probability of having both diseases requires multiplying the probability of each event.

a. Is the inheritance of an “A” or “a” allele dependent on (or affected by) the inheritance of a “B” or b” allele? Why or why not? ________________________________________________________________ (hint: Which of Mendel’s Laws is involved?)

b. What is the chance that their child will be phenotypically normal (for both genes)? _________________

c. What is the chance that their child will be genotype aabb (affected with both diseases)? ____________

d. What is the chance that they will have a boy who has genotype AaBb (carrier for both diseases)?

Answer 1

Answer to question 1 and its subparts

SCA is an autosomal recessive disorder