Apply What You’ve Learned

Review

15.1 Use a definded DNA sequence to illustrate how the following occur and describe their effect upon phenotype: transitions, transversions, missense mutations, nonsense mutations, frame-shift mutations, and mutations outside the coding region of a gene.

15.2 Genetic diseases often result from mutations that render proteins nonfunctional.

15.2 Mutations that cause disease include the full range of affected DNA from point mutations to entire chromosomes.

Hemoglobin, composed of four polypeptide subunits and an oxygen-binding heme group, carries O2 in human blood, delivering it to tissues. Because of the importance of O2 for cellular respiration, a genetic mutation that alters the structure and function of hemoglobin can have severe consequences. In an adult, hemoglobin has two types of polypeptide subunits: alpha-globin (α-globin), consisting of 141 amino acids, and beta-globin (β-globin), consisting of 146 amino acids (see figure below).

People with thalassemia typically make little to no α- or β-globin and so have anemia—inadequate quantities of normal red blood cells. They need blood transfusions to survive. Most common in Mediterranean countries, thalassemia is typically inherited as an autosomal recessive. While homozygotes with the mutant alleles have severe symptoms, people who are heterozygous carriers for thalassemia often have mild anemia, as they have only one of two functional alleles for the globin gene in question.

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Consider the following scenario: Two people have a mild, persistent anemia. Because each has relatives who suffered from thalassemia, they are concerned that they may be carriers for a thalassemia mutation. Tissue samples of both people are taken and DNA for globin genes sequenced. Here is a region for the normal β-globin gene in the coding strand of DNA:

Codon # 36    37    38    39    40    41  

Normal: CCT TGG ACC CAG AGG TTC

Below are the DNA sequences of the coding strands for the same region of the β-globin gene in the two people with mild anemia. Person 1 has two different sequences in this region:

Sequence 1: CCT TGG ACC CAG AGG TTC

Sequence 2: CCT TGG ACC TAG AGG TTC

Person 2 also has two different sequences in this region:

Sequence 1: CCT TGG ACC CAG AGG TTC

Sequence 2: CCT GGA CCC AGA GGT TCT

Questions

Question 1

Analyze the DNA sequences to identify the type of mutation (silent, nonsense, missense, or frame-shift) in these two patients. Then evaluate each mutation’s effect on the protein chain.

Person 1 is heterozygous, with one normal allele and one mutant allele. The normal allele translates to the amino acids: pro-trp-thr-gln-arg-phe. The mutant allele has a stop codon at position 4 in the mRNA (a nonsense mutation): Pro-trp-thr-(stop), and the polypeptide is short, only 38 amino acids instead of 146. So this globin would not be functional.

Person 2 is heterozygous, with one normal and one mutant allele. In this case, the mutant allele is a deletion of the first T in the second codon. This causes a frame shift: Pro-Gly-Pro-Arg-Gly-Ser…, and the resulting polypeptide has a very different sequence and is most likely nonfunctional.

Question 2

Describe a mutation that would have an effect similar to that of the two mutations described but that would involve a change outside the coding region for the β-globin gene.

One example is a mutation that affects the promoter such that RNA polymerase cannot bind to initiate transcription. Another example is a mutation that affects mRNA splicing sites in the DNA that leads to abnormal deletions or insertions in the mature mRNA transcript.

Question 3

If person 1 were to have a child with person 2, what are the risks involved for this couple’s children? Draw a pedigree chart to illustrate the possible outcomes.

The couple has a 1 in 4 chance of producing a child who is homozygous with two normal alleles, a 1 in 2 chance of producing a child who is heterozygous with one normal allele and one mutant allele, and a 1 in 4 chance of producing a child who is homozygous with two mutant alleles. A child with two mutant alleles would suffer severe anemia and would have to receive regular blood transfusions throughout life.