Daily Newsletter

October 17, 2012

Mendel's Second Law of Inheritance

Yesterday we talked about inheritance of one genetic trait (gene). But humans (and other organisms) are made up of more than one genetic trait. Every protein made by a cell is encoded in at least one gene (quaternary proteins would be encoded by multiple genes). While most genes are "house keeping" genes, i.e., needed for the organisms survival that don't have many variations among individuals(ribosomes, the genes for glycolysis, respiratory chains, etc...), some genes have greater variance. This is how we get the diversity of life, and more importantly, the uniqueness of individuals (this uniqueness is needed for evolution).

So, what happens if you are interested in more than one trait? This is where Mendel's second law comes into play. He was curious as to whether he could follow the inheritance probabilities of two traits, so he looked simultaneously at two traits in the pea.

NOTE: Going above two traits becomes mathematically more difficult, and we generally don't look at those problems at this level. When you take genetics you may see some of these higher order problems.

Mendel's experiments helped him propose what is now known as the Law of Independent Assortment. We now know that the traits Mendel looked at were found on different chromosomes (DNA molecules). The math would have been horrible if they had been on the same chromosome! As research into genetics progressed, and we realized that genes could be on the same chromosome, Mendel's second law (and the expected probabilities) became the model by which variations were assessed. Gene Mapping utilizes Mendel's probabilities for a dihybrid cross as the starting point.

With the Law of Segregation (first law), Mendel showed that for each individual trait, a pea plant (and by extension a human) has two possible allele that they can carry for a single trait (gene). When ovum and sperm (pollen) are produced, the parent donates only one allele to the ovum or sperm; the parent donates only one allele to the next generation. Therefore a new individual is composed of a set of genes (and alleles) from the mother, and another set from the father.

So, what happens when you look at two different traits (genes)? Ultimately, what Mendel discovered is that the two different traits do no interfere with each other. An allele from the first gene is donated independently, and is uninfluenced by, the allele from the second gene. So, you have a 50/50 chance of donating a given allele from the first gene, and a 50/50 chance of donating an allele from the second plant. [When looking at two distinct traits, we refer to the mating as a dihybrid, i.e., two trait, cross]

The math gets a little harder, but the idea is the same. The easiest way of showing what happens is to look at the Punnet Square for a visual interpretation of the probabilities. Below is a great picture of a Punnet square:

As you can see, on the top we put the Male Genetic Donation, and on the left side we put the Female Genetic Donation. There is a 50/50 chance the male will donate a given allele, same with the female. Look at how this is represented. Male donation is either B or b. Each has its own column. For the female, each possible donation has its own row. You then just cross-reference column and row to find out the possible offspring. The Punnet square also provides a rapid visual. 4 possible offspring, 3 of which are purple.

The Punnet square can be expanded to look at a dihybrid cross (two traits). Below is a good image of a dihybrid Punnet square. Note that the PHENOTYPIC ratio is 9:3:3:1. This is a critical ratio that will be seen repeatedly in biology.

Daily Challenge

Explain the concept of Independent Assortment. I made a point that this does not always occur when genes are on the same chromosome. So, what happens to Independent Assortment when genes (traits) are on the same chromosome? Why is it important?
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