Biology? ?

Question

Why does a ratio other than 9.3.3.1 indicate that two genes are on the same chromosome? How can you show this in a diagram?

Answer 1

This involves linkage and crossing over. When two genes are close to each other on the same chromosome they frequently get switched together during crossover during meiosis. This results in offspring with a higher percentage of genotypes that are exactly like one parent, and less which are a combination of the parents.

Answer 2

This only works if you are crossing to individuals who are heterozygous for both genes.  For example:  AaBb X AaBb.  If the genes segregate indpendently of each other, then you should get equal amounts of AB, Ab, aB, and ab gametes from each parent. When you create the punnett square, this results in a 9:3:3:1 expected phenotypic ratio.

However, if the genes don't assort independently through crossing over, then they are likely linked.  This will result in having more gametes that are AB and ab than Ab and aB (for example).  This will skew the observed phenotypic ratio.

To see examples of this, take a look at:  http://www.unity.edu/facultypages/aphill... and open up the Mendelian Inheritance Slides link.  Look especially at slides 7 & 12.