Thinking Scientifically

1. Not all individuals with Down syndrome have equally severe mental retardation or other physical abnormalities. Explain on the basis of a difference in the expression of alleles.
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2. The particular recessive allele that causes cystic fibrosis (CF) can vary. Seventy percent of patients with CF are homozygous recessive for one particular allele, but 30% are homozygous recessive for any number of different alleles that also cause CF. The screening test for heterozygotes detects only the most common allele for CF. If a person with CF relatives, tests negative for the common allele how might she/he determine that she/he need not worry about the possibility of having a rarer form of the CF allele?
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