Genetics — 8 Common Exam Traps

1.Trap one: dominance does NOT mean better or more common. Dominance only means the heterozygote phenotype matches one homozygote. Frequency in the population is unrelated. For example, Huntingtons disease is a dominant genetic disorder despite being rare.

2.Trap two: confusing q and q squared in Hardy-Weinberg. q squared equals frequency of homozygous recessive aa — that is the phenotype frequency. q equals frequency of the recessive allele.

3.When the problem says disease rate is one percent, that means q squared equals zero point zero one — NOT q equals zero point zero one. You then take the square root to get q equals zero point one.

4.Trap three: Hardy-Weinberg for X-linked genes works differently by sex. Males have only one X, so color blindness frequency in males equals q — not q squared. Females have two X, so color blindness frequency in females equals q squared. Common mistake: treating male q as q squared and taking the square root.

5.Trap four: confusing epistasis with dominance. Dominance acts within the same locus — A versus little a. Epistasis acts between different loci — A locus versus B locus, where one gene masks the expression of another. These two concepts are entirely different.

6.Trap five: test cross ratios for linked genes. Independent assortment gives test cross offspring ratio of one to one to one to one. Linked genes give offspring where parental types far outnumber recombinant types. Common mistake: seeing a non one-to-one ratio and assuming you calculated wrong.

7.Trap six: maximum recombination frequency RF is fifty percent. RF equals fifty percent does NOT mean two genes are on different chromosomes — they could be on the same chromosome but very far apart. RF does not equal physical distance, especially at greater distances, because of double crossovers reducing apparent recombination.

8.Trap seven: pedigree analysis assumptions. Do not assume an inheritance pattern — use process of elimination across all possibilities. Note whether the problem says the disease is rare. If rare, individuals marrying into the family are usually assumed homozygous normal. The term carrier applies only to recessive inheritance.

9.Trap eight: ABO blood type paternity testing. Two type O parents cannot produce type A or type B children. Two type AB parents cannot produce type O children. But type A times type B CAN produce type O — if both parents are heterozygous I-A i and I-B i, the child can inherit two i alleles.