AP Biology Unit 5: Heredity

Expected counts are 90, 30, 30, and 10. Chi-square = 4/90 + 4/30 + 36/30 + 36/10 = approximately 4.98.

The stem asks for a numerical or quantitative judgment: "A dihybrid cross is expected to produce a 9:3:3:1 phenotype ratio. The observed counts for 160 offspring are...". The safe route is to identify the counted event, use the full denominator, and then interpret the number biologically. Here the worked reasoning is Expected counts are 90, 30, 30, and 10, so choices based on 1.20; only the third class contribution or 0.18; only the first class contribution approximated lose the required setup.

Recombinant offspring total 61 + 59 = 120 out of 1000 total offspring. The recombination frequency is 120/1000 x 100% = 12%.

The stem asks for a numerical or quantitative judgment: "In a testcross for two linked genes, the offspring counts are shown. What is the approximate recombination frequency". The safe route is to identify the counted event, use the full denominator, and then interpret the number biologically. Here the worked reasoning is Recombinant offspring total 61 + 59 = 120 out of 1000 total offspring, so choices based on 5.9%; using only recombinant type 2 or 44%; using one parental class as the numerator lose the required setup.

A son receives his X chromosome from his mother and Y chromosome from his father. A carrier mother has a 1/2 chance of passing the recessive X-linked allele to a son.

The stem asks for a numerical or quantitative judgment: "A recessive X-linked allele causes a trait. A carrier female and an unaffected male have a son. What is the...". The safe route is to identify the counted event, use the full denominator, and then interpret the number biologically. Here the worked reasoning is A carrier mother has a 1/2 chance of passing the recessive X-linked allele to a son, so choices based on 1/4; the probability from an autosomal carrier cross or 3/4; the probability of not expressing the trait in some autosomal... lose the required setup.