HOW TO APPROACH GENETICS PROBLEMS

There is a three-step process I use to work all genetics problems in this course.

first, determine whether you are dealing with a monohybrid, dihybrid, or trihybrid cross,
determine whether it is a dominant-recessive pattern of inheritance or an incompletely dominant pattern,
if dominant-recessive, determine what trait is dominant and what is recessive, or if an incompletely dominant cross, what is homozygous and what is heterozygous.

To find out about Step 1, ask yourself "what am I looking at?" Are you looking at only color, or are you looking at color and size? Just looking at a single color, it would be a monohybrid cross. If you are looking at color and size, it is a dihybrid cross.

To find out about Step 2, if you end up with three of a kind, i.e., colors of red, white, and pink (which is a mix between red and white), then think incomplete dominance. If you end up with two of a kind, i.e., colors of red and white only, then you are probably dealing with a dominant-recessive trait.

To determine Step 3, ask yourself which one you have that is more of than the other. Are there more red flowers than white flowers? If there are three colors (incompletely dominant), then which one is the most in that group? In an incompletely dominant cross, there are twice as many heterozygous individuals as homozygous of each kind in the expected ratios.

Genetic Problem Example

Problem: The mating of two, black, short-haired guinea pigs produces a litter with some black, long-haired and some white, short-haired offspring. What are the genotypes of the parents?

Question 1: Is this a monohybrid, dihybrid, or trihybrid cross?
Answer: Dihybrid. Coat color (black and white) and hair length (long hair & short hair)

Question 2: Is this a dominant-recessive pattern of inheritance or an incompletely dominant pattern?
Answer: Dominant-recessive. Notice only two colors and two hair lengths are mentioned. If it were an incompletely dominant trait, there would be three colors (i.e., black, gray and white) or three hair lengths (i.e.,long, medium, and short). As it is, only black and white coat color and long and short hair length are mentioned in the problem.

Question 3:What is dominant and what is recessive?
Answer:Black is dominant over white and short is dominant over long. The reason you know this is by looking at the description of the parents. The parents were both black and short-haired. If these were recessive traits, then all the offspring would be black and short-haired. When two parents are recessive for both traits are crossed, they will always produce children exactly like themselves.

Since you now know black is dominant over white and short is dominant over long, the way you find the parent's genotype is to realize both parents must have a gene for black (dominant) and a gene for short (dominant). Why?

The below represent the parents as just mentioned above:

B_?S_? x B_?S_?

Note both parents have a gene for black and short, but we don't know what the other genes are.

Now look at the children. The parents produced both some long hair children (recessive) and some white children (recessive). Since the children must receive one gene each from their mother and one from their father, both parents must have passed on a recessive allele.

Therefore, the parent's genotypes must be: BbSs x BbSs.

To check your answer, make the cross above and see if you can get the children mentioned in the problem.

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