Kuru is a prion disease of the Fore people of highland New Guinea. It was once transmitted by the consumption of deceased relatives at mortuary feasts, a ritual that was ended by about 1960. Using archived tissue samples, Mead et al. (2009) investigated genetic variants that might confer resistance to kuru. The data in the accompanying table are genotypes at codon 129 of the prion protein gene of young and elderly individuals all having the disease. Since the elderly individuals have survived long exposure to kuru, unusually common genotypes in this group might indicate resistant genotypes.

a. Illustrate these data with a grouped bar graph. Which genotype(s) are especially prevalent in the elderly compared with young individuals?

b. Test whether genotype frequencies differ between the two age groups.

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Age Elderly Elderly Young Young MM 1313 2222 Genotypes at codon 129 MV 7777 1212 VV 1414 1414