An agricultural geneticist who uses Nepenthes distillatoria (the carnivorous pit

Question

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

The genes are located on chromosome 2, i.e. an autosome. Trisomic means 2n+1 chromosome number. Please note that trisomics produce two types of gametes, ‘n’ and ‘n+1’.

Wild type pure breeding trisomic (+++) female is crossed with normal diploid male (vv)

It is given that ‘v’ is a recessive mutation.

Gametes of female: In female, (2n+1) chromosome are there. One gamete will receive ‘+’ of chromosome 2. The other gamete will also get “+” of the chromosome 2. Now, the third extra chromosome will go to either of the gametes. So, 50% gametes will be ‘++’ and the rest 50% will be ‘+’.

Gametes of male will be “v”

Result: ++v and +v in F1 generation. All will be wild type because ‘v’ is recessive mutation in F1 generation.

Now, trisomic F1 (++v) is backcrossed (vv)

Gametes of trisomic F1 will be ‘v’, ‘+’ ‘++’ and ‘+v’..

These will produce ‘vv’ (25% super hungry plants). Rest of the plants ++, ++v, and +vv will be normal. Ratio will be 1:3.

(Answer to Part ‘A’)

Part B

If ‘v’ is not located on chromosome 2, then the ratio depends on percentage crossing over between chromosome 2 and the other chromosome. If the other chromosome is chromosome 3, then homologous recombination and crossing over will occur. I mean, the information is less. For exact ratio, you need to tell, which chromosome the ‘v’ gene is present on.

Note: Generally, trisomics are non-viable.

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