Given that Tay-Sachs disease is caused by an autosomal recessive allele (n): 1. Determine Charly's phenotype, Portia's genotype, and Bill's genotype. 2. Explain why Patrick is normal but a carrier.

2b (i) Chas's phenotype Step 1: Identify Chas in the pedigree. Chas is a white square, indicating an unaffected male. Step 2: Tay-Sachs disease is autosomal recessive, so unaffected individuals do not express the disease. Chas's phenotype is normal (does not have Tay-Sachs disease). normal 2b (ii) Ellen's genotype Step 1: Father is affected (tt), so passes t allele to all offspring. Step 2: Ellen is unaffected (white circle), so must have at least one T allele. Step 3: Ellen received t from father and T from mother. Ellen's genotype is Tt. Tt 2c Explain why Patrick is normal, but a carrier of Tay-Sachs. Step 1: Determine Seth's genotype. Seth is unaffected son of affected father (tt) and unaffected mother. Seth received t from father and T from mother, so Seth is Tt. Step 2: Patrick's mother (Seth's wife) is affected (black circle), so genotype tt. Step 3: Punnett square for Seth (Tt) × wife (tt): c|c|c & t & t \\ T & Tt & Tt \\ t & tt & tt \\ Offspring: 50\% Tt (normal), 50\% tt (affected). Step 4: Patrick is unaffected (white square), so genotype Tt. Step 5: Patrick is normal because the dominant T allele masks the recessive t allele (heterozygous advantage). Step 6: Patrick is a carrier because he has one t allele, which he can pass to his offspring (risk of affected children if mate is carrier).