When true-breeding mice with brown fur and short tails (BBtt) were crossed to truebreeding mice with white fur and long tails (bbTT), all F1 offspring had brown fur and long tails. The F1 offspring were crossed to mice with white fur and short tails. What are the possible phenotypes of the F2 offspring? Which F2 offspring are recombinant, and which are parental? What are the ratios of the F2 offspring if independent assortment is taking place? How would the ratios affected by linkage?

Explanation:

From the question, brown fur is dominant, long tail is dominant; while white fur is recessive, short tail is recessive. Dominant traits are represented in capital while recessive traits are represented in lower case.  

When true-breeding mice with brown fur and short tails (BBtt) were crossed to truebreeding mice with white fur and long tails (bbTT),

Using Punnet’s square to show the crossing:

bT           bT           bT            bT

BtBbTtBbTtBbTtBbTt

BtBbTtBbTtBbTtBbTt

BtBbTtBbTtBbTtBbTt

BtBbTtBbTtBbTtBbTt

All F1 had brown furs and long tails: BbTt

F2

Brown fur and long tails: BbTt

White fur and short tails: bbtt

Using Punnett’s square to show the crossing of the two:

bt          bt              bt  bt

BTBbTtBbTtBbTtBbTt

BTBbTtBbTtBbTtBbTt

btbbtt         bbtt          bbttbbtt

btbbtt  bbtt          bbttbbtt

F2 offspring: 8 BbTt and 8 bbtt

Parental offspring have the same alleles as the parents while recombinant do not. Recombinant alleles are due to a crossover of the gametes of the two parents. Given that each parent has BbTt and bbtt, all the offspring are parental.

Independent assortment states that the two alleles are inherited individually, as shown above in the punnet square. Therefore, the ratio of the F2 offspring is 8:8.

In reality, genes are not always sorted independently. In fact, many traits are linked to other traits. Therefore if the traits were linked, e.g. if BT were linked and bt were linked, the alleles would stick together and the offspring would have an equal proportion, e.g. 50% BT and 50% bt.

In reality, in linked gametes, crossing is not so common (but happens), so in rare instance t could cross over to B and an offspring can inherit Bt. In any case the ratios of the offspring would be equal.

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