Achondroplasia is a common form of dwarfism caused by an autosomal dominant mutation in the fibroblast growth factor receptor 3 (FGFR3) gene. Two copies of the mutant gene are invariably fatal before or shortly after birth. If a person with achondroplasia marries a person of normal height, what is the probability that both their first child and second child will have achondroplasia?

Question

A.  1/8
B.  1/4
C.  1/2
D.  1

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 phenotype ratios from crosses?

Consider Possibilities · What other information is related to the question? Which information is most useful?

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.  1/4

Clarify Question
· What is the key concept addressed by the question?
        o This question requires two steps: figuring out the likelihood of an affected offspring in a single pregnancy, then calculating the probability of the first and second children both being affected. Although each pregnancy results in just one child, you can calculate the expected phenotype ratio from the cross to determine the probability of the dwarfism phenotype.
· What type of thinking is required?
        o You need to Analyze the information to determine the probability of the two children being affected.
· What key words does the question contain?
        o Achondroplasia (dwarfism)may be unfamiliar, but just treat it like any autosomal dominant mutation – that is, an allele on a non-sex chromosome, with the trait exhibited in the heterozygote.

Gather Content
· What do you already know about phenotype ratios from crosses?
        o What is the genotype of the parent with normal height?
        o What is the genotype of the parent with achondroplasia? (Hint: Remember that the question mentioned that two copies of the mutant gene are fatal.)
        o Treat each pregnancy as a cross. The expected phenotype ratio will tell you the likelihood that the child will have the dwarfism phenotype. What method have you learn for keeping track of genotypes and phenotypes from a cross?

Consider Possibilities · What other information is related to the question? Which information is most useful?
        o Punnett squares are a useful way to determine both genotype and phenotype ratios.

Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o Try drawing a Punnett square. Write the possible gametes on the top and the side.
        o The only gametes for the normal height parent are FGFR3+.
        o The gametes for the parent with dwarfism are FGFR3+ and FGFR3-.
        o There are 2 combinations of offspring : FGFR3+/ FGFR3- and FGFR3+ / FGFR3+, in equal ratio. So there is a 50% chance that a child will have dwarfism.
        o If the first child has dwarfism (50%), there is again a 50% chance that the second child will have dwarfism. The second child is unaffected by the result for the first child. So the total probability that both child 1 and 2 will be affected is 1/2 x 1/2, or 1/4.

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 Analyze the information given, using logic, to dissect the problem and determine the answer.
        o Did you see the connection between phenotype ratios and probability for one child? Now you may appreciate another aspect of how Punnett squares can be useful.
        o Remember, in genetics treat each child as an independent event – like a coin flip.