Question

the pedigree below tracks duchenne muscular dystrophy (dmd) through several generations. dmd is an x - linked recessive trait. what is the genotype of individual ii - 2? a normal female b heterozygous male c homozygous dominant d heterozygous female

Explanation:

Step1: Recall X - linked recessive inheritance

In X - linked recessive inheritance, males have one X and one Y chromosome (\(X^Y\)), and females have two X chromosomes (\(X^X\)). A recessive trait is expressed when there are two recessive alleles (for females, \(X^rX^r\)) or one recessive allele on the X - chromosome for males (\(X^rY\)) since Y has no corresponding allele. A carrier (heterozygous female) has one dominant and one recessive allele (\(X^RX^r\)).

Step2: Analyze individual II - 2's phenotype and family

Individual II - 2 is a normal female (white circle in pedigree). Her mother (I - 2) is affected (has DMD, so genotype \(X^rX^r\)) and her father (I - 1) is normal (genotype \(X^RY\)). When we look at the offspring of I - 1 and I - 2: I - 2 (affected female, \(X^rX^r\)) can only pass on \(X^r\) alleles. I - 1 (normal male, \(X^RY\)) passes on \(X^R\) to daughters. So daughters of I - 1 and I - 2 will have one \(X^R\) (from father) and one \(X^r\) (from mother), so genotype \(X^RX^r\) (heterozygous female, carrier). Also, II - 2 has a son (III - 3) who is affected (black square, so \(X^rY\)). Since the son gets his X - chromosome from his mother (II - 2) and Y from his father (II - 1, normal male \(X^RY\)), the mother (II - 2) must have passed on the \(X^r\) allele. Since II - 2 is normal (does not show the disease), she must have a dominant allele (\(X^R\)) as well. So her genotype is \(X^RX^r\), which is a heterozygous female (carrier). Now let's analyze the options:

• Option a: "normal female" is a phenotype description, not a genotype. Also, a normal female with a affected son (who has the recessive allele) cannot be homozygous dominant (\(X^RX^R\)) because then she couldn't pass on the recessive allele to her son. So a is incorrect.
• Option b: "heterozygous male" is wrong because II - 2 is a female (circle), not a male (square). Males have XY, females XX. So b is incorrect.
• Option c: "homozygous dominant" (\(X^RX^R\)) would mean she can't pass on the recessive allele, but her son (III - 3) is affected (\(X^rY\)), so she must have passed on \(X^r\). So c is incorrect.
• Option d: "heterozygous female" (genotype \(X^RX^r\)) fits because she is a female, normal (due to \(X^R\)), but carries the recessive allele (\(X^r\)) which she passed on to her affected son.

Answer:

d. heterozygous female