Which one of the followings is caused by a recessive allele of a gene on chromosome 7?
Phenylketonuria (PKU)
Tay-Sachs disease
Cystic fibrosis (CF)
Huntington disease
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ABO blood groups in humans are controlled by the gene l. It has three alleles – lA, lB and i. Since there are three different alleles, six different genotypes are possible. How many phenotypes can occur?