Expert: John Locke - 7/23/2007

Question
Hi!  I am completely stumped.  I don't understand Mendel's inheritance
theories at all!  For instance, one of the questions I need to solve:  If 2
animals are heterozygous for a single pair of alleles, what percent of the
offspring would have the phenotype of the dominant allele?  Is there a good
source to turn to for understanding Mendel's theory.  I just don't get it no
matter how many times I try to read my textbook.  Thanks

Answer
Thanks for using AllExperts, Charlene.

Mendel's theories of inheritance are not always intuitive, to be sure, and to explain them all again here merely risks making you more confused. I'm not sure how much you understand, how much you don't, what parts of the theory you are comfortable with, and what parts you aren't. So here's what I'm going to do--I'll answer your question (at some length, I'm afraid) and give some commentary on how Mendel's laws apply in that case. Beyond that, I really do need your help in identifying what else you'd like explained--so please feel free to write back after you get this answer for a follow-up.

Now to the specific case mentioned here. The basis of Mendel's laws are the assumptions (now proven correct) that traits are carried in specific locations in the body; these are what we now call genes. Additionally, there are multiple copies of these genes in the body--Mendel postulated that everyone carries two copies, or alleles, for each gene. When the body forms the germ line cells (these are the reproductive cells that can combine to form offspring), all the alleles separate such that each germ cell receives only one allele. This is necessary because in sexual reproduction, an organism has two parents who each donate genetic material: if each donated a full copy, the offspring would wind up with twice the genetic material needed. This is avoided by having each parent contribute half of the total genetic material.

Just a bit more on alleles: Mendel postulated that traits are either dominant or recessive. Dominant traits require only a single copy of allele for the trait itself to show up in the organism, while recessive traits require that two copies of that allele be present for the trait to show up. By "trait," we could be talking about a physical feature, a disease, a variation in skin color--indeed, a "trait" is really anything that is controlled by a person's genetic makeup. That genetic makeup is termed a "genotype"; a genotype consists only of what alleles a person does or does not have.

A person's physical state is called their phenotype: whether someone has a disease, or a particular hair color, or a particular physical trait is the phenotype. The phenotype may depend entirely upon genotype, as in sickle cell anemia: you have the disease if and only if you have a particular genotype, while environmental factors play no role at all in determining whether you get the disease. A phenotype may also depend upon environmental conditions, as in Type II diabetes: your genotype may predispose you to developing the disease, but your dietary habits will also affect whether diabetes actually occurs.

To be heterozygous is to have one dominant and one recessive allele; to be homozygous is to have either two dominant or two recessive alleles (this ambiguity is usually resolved by identifying whether a homozygous individual has two dominant or two recessive alleles)). In the example given here, both parents are heterozygous: both parents have one dominant allele and one recessive allele. Because each has a dominant allele, the parents will show the phenotype that results from this dominant allele. Each parent contributes only one allele, but could contribute either allele with equal probability--another discovery by Mendel.

Let's represent the dominant allele by D and the recessive by d. Then each parent is heterozygous, or

Dd

Since there are two parents, we can represent their having children by "crossing" them:

Dd x Dd

I do not have access to extensive text formatting in this interface, but I will do my best to create a Punnett square here. Don't be worried by the slew of new terms--the Punnett square is merely a useful tool for representing the possible genetic combinations that could result from two parents having children. To create such a square, you write the two alleles of one parent horizontally across the top, and the two alleles of the other parent vertically down the left hand side. Four possible combinations will result, like this:

  D       d  <--These are the alleles of one parent
------------
D | DD  |   Dd<--This row shows two allele combinations
   |       |
   |       |
d | Dd  |   dd<--This row shows two more alleles combos
^
|
|
These are the alleles of the other parent, written down the left-hand side.

If this has confused you, please don't worry--a much better visual representation can be found on Wikipedia under the article "Punnett square." The important information is that three possible genetic combinations could result: dd, Dd, and DD. The question is asking about phenotypes, however--what percent of the offspring will have the dominant phenotype?

What phenotype results from each genotype? Let's have a look:

DD: Two dominant alleles. You need only one to show the dominant phenotype, so the phenotype is most certainly dominant.

Dd: One dominant and one recessive allele. The dominant phenotype, again, gets expressed if there is a single dominant allele present. Dominant phenotype.

dd: Two recessive alleles. The recessive trait only shows up if both alleles are recessive, and that's the case here. Recessive phenotype.

Two ways for the dominant phenotype to show up and one way for the recessive phenotype to show up. Please note, however, that the Punnett square shows you two Dd combinations: one where the D comes from mom and d from Dad, and the other where d comes from mom and D from dad. These are counted separately for mathematical reasons that I don't need to get into here; the upshot is that you really have three possible ways to have a dominant phenotype (DD and two Dd's) and one way to have the recessive phenotype (dd). The dominant phenotype shows up in 3/4 of the possible offspring, or 75%. That's equivalent to saying that there's a 75% chance that the offspring will have the dominant phenotype.

A good reference on this topic is Schaum's Outline Of Genetics by Susan Elrod. Please let me know if you have any further questions; I'd be happy to answer them. Good luck!