In roses, Red (R) is dominant over white (r) flowers, and Thorny (T) is dominant

Question

In roses, Red (R) is dominant over white (r) flowers, and Thorny (T) is dominant over smooth (t) stems. The two genes are linked and have 30% recombination. The following cross is performed: RT/ rt X rt/rt If there are 500 offspring, how many plants are expected to have white flowers and Thorny stems? Hint: you are looking for an actual number of plants, not a percentage.

In bluebells, Blue (B) is dominant over white (b) flowers, and Hairy (H) leaves are dominant over smooth (h) leaves. The following cross is made:

BH/bh X bH/bh

What is the phenotypic ratio, if the genes show complete linkage?

Explanation / Answer

1. Given that the two genes (R and T) are linked and have 30% recombination, means 30% of offspring will be recombinant and remaining 70% are parental.

Now, cross between RT/ rt X rt/rt will have the offspring with the following genotypes.

RrTt ----------> Parental (Red flowers, thorny stems)

Rrtt ----------> recombinant (Red flowers, smooth stems)

rrTt -----------> recombinant (white flowers, Thorny stems)

rrtt -----------> Parental (white flowers, smooth stems)

Of these, rrTt and Rrtt comprises 30% of offspring, means 150 plants. Among these 150 plants, half are Rrtt and another half are rrTt genotype. Thus, the number of plants are expected to have white flowers and Thorny stems (rrTt) are 75.

2. If the genes show complete linkage, there will be no recombination to occur, all the offspring will be similar to parents.

Thus, the phenotypic ratio is, BH/bh (Blue flowers, hairy leaves): bH/bh (white flowers, hairy leaves) = 1 : 1

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