You are a plant geneticist crossing roses and you have a true-breeding line of r

Question

You are a plant geneticist crossing roses and you have a true-breeding line of red roses that have thorns and a truebreeding

line of white roses that don’t have thorns. From previous experiments you know that color is controlled by one gene and

that thorns are controlled by a single different gene. You cross the roses together and get all red roses with thorns. You then cross

these red and thorny roses back to the white and thornless roses and you get the following results. Explain completely what the

best interpretation of these results are.

Phenotype Number

Red and thorny 910

Red and thornless 110

White and thorny 90

White and thornless 890

Explanation / Answer

Let us consider the Genotype of true breeding red roses with thorns to be RRTT, and the genotype of true breeding white roses without thorns to be rrtt. According to the mendelian principles of genetics, their hybrid will have the phenotype of Red roses with thorns and the genotype of RrTt.

Now, if this RrTt plant is crossed with rrtt plant, then according to the typical mendelian ratio for test cross, the result should have been along the lines of 1:1:1:1 for each phenotype as can be seen from the punnet square given below:

However, the actual results obtained as follows:

These results present distorted mendelian ratios, with parental phenotypes being more prominent. Genetic linkage is the most prominent exception to Mendel's law of independant assortment. Genetic linkage is a connection between different genes, which are located on the same chromosome and influence each other. This relationship was discovered in 1912 by Thomas Hunt Morgan. It is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction. Two genetic markers that are physically near to each other are unlikely to be separated onto different chromatids during chromosomal crossover, and are therefore said to be more linked than markers that are far apart. In other words, the nearer two genes are on a chromosome, the lower the chance of recombination between them, and the more likely they are to be inherited together. The ratio given above suggest such a linkage relationship between the genes controlling the flower colour and thorns. This is the reason for distorted mendelian ratio.

RT Rt rT rt rt RrTt Rrtt rrTt rrtt rt RrTt Rrtt rrTt rrtt rt RrTt Rrtt rrTt rrtt rt RrTt Rrtt rrTt rrtt

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