Inheritance Essentials

For most of my students, this will be a review of stuff you did in Freshman/Sophomore Biology. It is vital background which you will need to build on.  So, if you DON'T know this, you need to learn it.  If it is EASY for you, feel free to skip it.

Try these problems:
1. In mice, the allele for brown fur is dominant over white fur.  A pure breeding brown mouse is mated to a pure breeding white mouse.  What are the expected genotype and phenotype ratios?
2. A mouse from the offspring in problem 1 is mated to a white mouse.  What are the expected phenotype and genotype ratios?
3. Two mice from the offspring of the cross in problem 1 are mated.  What are the expected phenotype and genotype ratios?

[answers are at the end of this post.]

If these three problems were difficult for you to answer, watch the following video.  Otherwise, you can continue on to the next problem.

In some cases, two alleles mix to create a phenotype.  These cases are called incomplete dominance and codominance.  In incomplete dominance, the phenotype produced is intermediate.  In codominance, the phenotype produced is a mix of the two.  In reality, this is probably an unimportant distinction, but you can still find it in textbooks and on standardized tests.  Here's a sample problem:

4. Blood types A and B display codominance.  An individual homozygous for blood type A and an individual homozygous for blood type B have kids.  What are the odds that they will have a child with blood type A?  B?  AB?
5. Two individuals with blood type AB mate.  What are the odds that their children will have blood type A? B? AB?

If you had difficulty with problems 4 and 5, watch this video:


Some genes are located on the sex chromosomes.  Genes located on the X chromosome are particularly interesting since male humans only get one copy of the X chromosome (termed hemizygous) where as females get two copies.

6. The gene for red-green color vision is located on the X chromosome.  R = normal vision, r = red/green colorblind.  A man with normal vision marries a woman with normal vision whose father was colorblind.  What are the odds of having a color blind boy?  a color blind girl?

If you had difficulty with problem 6, watch this video:



You can get more practice with this type of problem here:
Arizona Monohybrid Crosses
It is vital that you have a good solid foundation with monohybrid crosses.

We will begin our investigation into inheritance with dihybrid crosses.






1. Genotype ratio: all Bb; Phenotype ratio:  all brown.
2. Genotype ratio: 1:1 Bb:bb; Phenotype ratio 1:1 brown:white
3. Genotpye ratio 1:2:1 BB:Bb:bb; Phenotype ratio 3:1 brown:white
4. A and B are impossible.  All of the kids will be AB.
5. 25% each for A and B, 50% for AB.
6. 25% of their offspring will be color blind males.  None of their female offspring will be colorblind (half of the girls are expected to be carriers)