A polygenic model for a threshold trait

Any allele designated as 1 confers genetic liability, any allele designated as 2 confers no liability, and the 1 alleles are additive. (a) In Cross 1, the couple has a 1/32 chance of producing an affected child. (b) In Cross 2, the couple has a 7/64 chance of producing an affected childAny allele designated as 1 confers genetic liability, any allele designated as 2 confers no liability, and the 1 alleles are additive. (a) In Cross 1, the couple has a 1/32 chance of producing an affected child. (b) In Cross 2, the couple has a 7/64 chance of producing an affected child

(a) Cross 1: A A B B C C x A1AZB,BZC,C2 121222 (Two liability alleles) (Three liability alleles) Number of liability alleles 0 1 2 3 4 5 6 0.35 Unaffected Affected 0.30 0.25 020 —Threshold 0.15 of liability 0.10 0.05 Progeny frequency i i 1 1 5 i Q 32 32 32 32 32 32 32 Progeny proportion (b)CrossZ:AABBCC x AABBCC 121212 121212 (Three liability alleles) (Three liability alleles) Number of liability alleles 0 1 2 3 4 S 6 U naffected Affected — Threshold of liability Progeny frequency .L.§.l§lQl§_§._1_ 64 64 64 64 64 64 64 Progeny proportion