GENETICS TUTORIAL II
Monday, September 24, 2012
- pedigree is very common on midterms/exams
- hardy-weinberg eq’b question might be there
- Another factor NOT included: diploid organism
- Examples to look at: Cystic fibrosis
- PKU = causes intellectual disability but is treatable
- What if there’s more than one gene? – you’d have to use a different equation
a) See loose leaf paper
b) Autosomal recessive. Risk of having, it’s ¼
c) 2/3 (1/3 is AA, 2/3 is Aa, 1/3 is aa)
d) It’s too damn high! But we need numbers! Answer is 1/22.
Charles’ chances of being a carrier is ½ (many unaffected kids with Bob and Betty.
Betty’s sister’s son + her own daughter have affected child, so she’s likely the carrier.
Bob is likely not)
For Charles’ wife, we assume the general population. 1/2000 for CF, so q^2 = 1/2000, q
= 1/44. p = 1 – q = 43/44 ~1. So 2pq = (2)(1/44)(1) = 1/22. So Debby’s risk is
½ is likelihood Charles is a carrier. 1/22 is how likely Charles’ wife is a carrier. ¼ is the
likelihood that Debbie will have CF if both are carriers.
- Likelihood that Debbie is a carrier = [(1/2)(1/2)(21/22)(1)] + [(1/22)(1/2)(1/2)(1)]
e) Barbara, Calvin, Cathy, Betty
f) It could be the same family mutation (same nucleotide)
- Very likely dominant – autosomal dominant. It’s passed on to every generation
- NOT X-LINKED RECESSIVE: not X because a male passed it onto his son
- Not Y-linked because an affected male did not pass it on to a son
- Disease = testicles don’t descend + urethra doesn’t position properly
- THEREFORE – AUTOSOMAL DOMINANT, SEX-LIMITED
- IF autosomal dominant – she has 50% chance (father Aa, mother aa)
- IF AD sex-limited: she’s female – so she won’t be affected
- Expressivity = the degree to which a phenotype is expressed. e.g.
neurofibromitosis = kids over time will eventually develop bum