Question

example 2: tracing the path of an autosomal trait: neurofibromatosis forms of the trait: the dominant form is neurofibromatosis, caused by the production of an abnormal form of the protein neurofibromin. affected individuals show spots of abnormal skin pigmentation and non - cancerous tumors that can interfere with the nervous system and cause blindness. some tumors can convert to a cancerous form. the recessive form is a normal protein - in other words, no neurofibromatosis. a typical pedigree for a family that carries neurofibromatosis is shown below. note that carriers are not indicated with half - colored shapes in this chart. use the letter “n” to indicate the dominant neurofibromatosis allele, and the letter “n” for the normal allele. analysis questions: 1. is individual #1 most likely homozygous dominant or heterozygous? explain how you can tell. 2. what is the genotype of individual #3? 3. can you be sure of the genotypes of the affected siblings of individual #3? explain.

Explanation:

Step1: Analyze offspring of individual #1

Individual #1 has offspring with different phenotypes. If #1 was homozygous dominant (NN), all offspring would be affected (have the dominant - trait). But since some offspring are normal (nn), #1 must be heterozygous (Nn).

Step2: Determine genotype of individual #3

Individual #3 has two affected parents (Nn). Since #3 is normal, its genotype must be nn as it needs two recessive alleles to show the normal phenotype.

Step3: Consider genotypes of affected siblings of #3

The affected siblings of #3 have one parent with genotype Nn and the other with genotype Nn. The affected siblings could have genotypes NN or Nn. We cannot be sure because both genotypes would result in the affected phenotype.

Answer:

1. Heterozygous. Has normal offspring, so must carry a recessive allele.
2. nn
3. No. Could be NN or Nn as both genotypes show the affected phenotype.