4. An agricultural geneticist who uses Nepenthes distillatoria (the carnivorous

Question

4. An agricultural geneticist who uses Nepenthes distillatoria (the carnivorous pitcher plant) as a model plant makes the following cross. She proceeds to take a wild type pure-breeding female pitcher plant trisomic for chromosome 2 and crosses it to a normal diploid male pitcher plant that is homozygous for a recessive mutation (v) that makes it hungry all the time (the super hungry mutant). A trisomic F1 plant is then back-crossed to the super-hungry male parent. From this cross:

A] What is the ratio of normal pitcher plants to those that are super hungry when you assume that v is located on chromosome 2?

B] What is the ratio of normal pitcher plants to those that are super hungry when you assume that v is not located on chromosome 2?

Explanation / Answer

Trisomic *   homozygous recessive mutation (parents)

Ch2 represents an allele of the wild-type gene on chromosome 2

v represents the mutated allele

A)

If the mutation is in chromosome 2

Ch2Ch2Ch2 * vvv

Ch2v +v

Ch2v +v

Ch2v +v

Ch2v +v

Ch2v +v

Ch2v +v

Ch2Ch2 +v

Ch2Ch2 +v

Ch2Ch2 +v

Two F1s back crossed (no homozygous recessive to v are seen in this generation)

First cross between F1 and homozygous recessive parent

Ch2Ch2v * vvv

Ch2vv, Ch2vv, vvv, Ch2vv, Ch2vv, vvv

3:2

Second cross between F1 and homozygous recessive parent

Ch2vv * vvv

Ch2vv, vvv, vvv, Ch2vv, vvv, vvv

2:1

B)

If the mutation is not in chromosome 2

Ch2Ch2Ch2 * vv

Ch2vv, Ch2vv, Ch2vv, Ch2v, Ch2v, Ch2v - F1

First cross between F1 and recessive parent

Ch2vv * vv

Ch2v, Ch2v, vv, vv, vv, vv

2:1

Second cross between F1 and recessive parent

Ch2v * vv

Ch2v, Ch2v, vv, vv

1:1

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