Question

SET 2

1. You are interested in determining the mode of inheritance for a particular trait (earwax color). You cross two
heterozygous individuals and observe the resulting progeny. For each of the following outcomes, what
conclusions should be drawn regarding the mode of inheritance?
a) You observed a 2 dark : 1 light phenotypic ratio
b) You observed a 1 dark : 2 moderate : 1 light phenotypic ratio
c) You observed a 9 dark : 7 light phenotypic ratio

Answer 1

Answer 1 :

a) You observed a 2 dark : 1 light phenotypic ratio

This ratio is typically observed when the trait shows lethal gene type of inheritance. In this inheritance, gene causes death of individual when it comes in homozygous condition.It can be homozygous dominant or recessive or both.
In this case, there are 2 dark colored earwax and 1 light color earwax . Hence, the dark colored earwax has the heterozygous genotype ; the light colored earwax is homozygous recessive and the homozygous dominant dark color is lethal and so that organism does not survive.

b) You observed a 1 dark : 2 moderate : 1 light phenotypic ratio

This type of inheritance is typical in blending type of inheritance or Incomplete dominance. In this , the traits blend with each other and in the heterozygous condition, there is a formation of an intermediate of the dominant and the recessive trait.
In this case the heterozygous moderate earwax is the blend of homozygous dominant dark color earwax and homozygous recessive light colored earwax.

c) You observed a 9 dark : 7 light phenotypic ratio

This type of inheritance is typical in complementary genes. In this, two pair of non-allelic genes are essential in dominant form to produce a particular character. Such genes act to together to produce an effect that neither of them can produce separately. Both types of genes must be present in dominant form to produce the desired effect.
In this case, the phenotypic ratio is modified such that when the dominant genes are together they produce the dark color earwax. And when there is any recessive combination present, the light color is expressed.