Free response · Unit 5 Beta

Reading a pedigree: a bleeding disorder

Analyze Model or Visual Representation · 4 points

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The question

Name: ______________________   Date: ____________   Reading a pedigree: a bleeding disorder (4 points)

Model

A family with a bleeding disorder

The pedigree shows three generations of a family in which some members have a rare bleeding disorder caused by an allele of a single gene. Genetic testing later showed that the gene is on the X chromosome. II-4 married into the family and has no relatives with the disorder. Use XH for the allele for normal clotting and Xh for the allele that causes the disorder.

Pedigree of a family with a bleeding disorder Three generations. Generation I: I-1, an unaffected man, and I-2, an unaffected woman. Their children in generation II, left to right: II-1, an unaffected man; II-2, an affected man; II-3, an unaffected woman. II-3 has children with II-4, an unaffected man from outside the family. Their children in generation III, left to right: III-1, an affected boy; III-2, an unaffected girl; III-3, an unaffected boy; III-4, an unaffected girl. I II III I-1 I-2 II-1 II-2 II-3 II-4 III-1 III-2 III-3 III-4 affected unaffected

  1. (a) Using evidence from the pedigree, explain why the disorder cannot be caused by a dominant allele. [1 point]

  2. (b) Identify the genotypes of I-2 and II-3, and describe the evidence in the pedigree for each. [1 point]

  3. (c) Calculate the probability that III-2 is a carrier of the disorder. [1 point]

  4. (d) III-2 plans to have children with a man who does not have the disorder. Predict the probability that their first son will have the disorder, and justify your prediction. [1 point]