Why can't you use mitochondrial DNA to distinguish between these two suspects?

Question

  You are a forensic technician working on a DNA sample obtained from a crime scene. Your job is to compare the unknown DNA sample with known DNA samples collected from five different suspects. Preliminary analysis using only a few DNA markers revealed that the unknown sample could possibly match two of the suspects. After consulting the case file, you find out that these two suspects are brothers (but not twins). You realize that you will have to do a more detailed analysis on the samples so that you can distinguish between the brothers and determine which brothers' DNA matches the unknown sample.

A.  The sequence of mitochondrial DNA has not yet been determined.
B.  The brothers share the same mitochondrial DNA.
C.  There are no molecular techniques available that allow one to analyze mitochondrial DNA.
D.  Because mitochondrial DNA is inherited in a paternal pattern.

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
· What key words does the question contain?

  Gather Content
· What do you already know about mitochondrial DNA?

  Consider Possibilities
· Consider the different answer options. Which can you rule out?

  Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?

  Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
 

Answer 1

B.  The brothers share the same mitochondrial DNA.

Clarify Question
· What is the key concept addressed by the question?
        o This question addresses mitochondrial inheritance.
· What type of thinking is required?
        o Although you may not have seen this example before, you know enough about the concept to Apply your knowledge and understanding to this unfamiliar situation.
· What key words does the question contain?
        o MItochondrial DNA is the DNA that encodes some of the proteins used by mitochondria (other mitochondrial proteins are encoded by regular nuclear DNA). The mitochondrial DNA is carried within the mitochondria.

  Gather Content
· What do you already know about mitochondrial DNA?
        o Mitochondrial DNA is carried within mitochondria, and mitochondria are inherited only from the mother – the sperm have too little cytoplasm to carry mitochondria.
        o There is generally only one type of mitochondrial DNA in any individual. That is, all the mitochondria in an individual have the same mitochondrial DNA.

  Consider Possibilities
· Consider the different answer options. Which can you rule out?
        o Mitochondrial genomes are small and easy to sequence – they have been sequenced.
        o Mitochondrial DNA is inherited maternally, not paternally.
        o Mitochondrial DNA is the same as nuclear DNA, and regular molecular biology techniques work fine.

  Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o The correct answer is left – since the two men are brothers, they both inherited the same mitochondrial DNA from their mother. Thus, that information will not help distinguish the samples.

  Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o This question required you to Apply your knowledge and understanding to this unfamiliar situation.
        o Did you remember that mitochondrial DNA is maternally inherited?         o Did you recognize that brothers will have inherited the same mitochondrial DNA?