Question 3. You are a clinician in a small hospital and are monitoring the progression of myclonic epilepsy (MERRF) in twin boys. One twin remains only mildly affected by the condition, but the other twin develops other symptoms of full blown MERRF, including deafness, ragged red muscle fibers, and ataxia (loss of ability to control muscles). Use a diagram that includes chromosomes, genes, alleles to explain the phenotypic dissimilarity in these identical twins. In respect to MERRF what is the genotype of the mother?
Question 5. In the following question we are dealing with a diploid organism. You have just
isolated a new embryonic lethal mutation (a single mutation in one gene) that is a little peculiar
because three distinct phenotypes have been associated with this mutation:
Phenotype 1: viable, minor anterior deformity.
Phenotype 2: more severe than above, anterior defect and is embryonic lethal.
Phenotype 3: like phenotype 2 it is an embryonic lethal, but halts development even earlier.
They appear in the following crosses:
Cross 1: male +/- x female +/- : 1/4 phenotype #1; 3/4 normal
Cross 2: male +/- x female -/- : 1/2 phenotype #2; 1/2 phenotype #3
Cross 3: male -/- x female +/- : 1/2 phenotype #1; 1/2 normal
Cross 4: male -/- x female -/- : all display phenotype #3
a) Explain the difference between phenotype #1 and phenotype #2. Which phenotype represents the maternal effect phase, the zygotic phase and why?
b) Why does this gene have both a maternal and a zygotic phase? How does phenotype #3 arise?