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Chapter 9: Inheritance - Complex Traits

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Crossing traits and Genes

Dihybrid crosses

BS bS bs Bs
bS BbSS BbSS BbSs bBsS
bs BbSs bbSs bbss Bbss
Bs BBSs BbSs Bbss BBss

Now we can consider what happens with two independent traits. Suppose your parents are carrying two genes, one for intelligence and one for beauty. We'll assume [purely for the sake of argument] that they each have a dominant smart and a recessive not-smart gene, and a dominant handsome/beautiful and recessive not-handsome/beautiful gene. At the outset, each of you and your siblings has a 1/4 chance of being not-smart, and a 1/4 chance of being not-gorgeous or whatever. Since the genes are not linked, there is a 1/4*1/4 = 1/16 chance that any one of you would be not-smart AND not-handsome/beautiful. There are three ways out of the sixteen possibilities that you would be either not-smart and handsome/beautiful, so we add those up to a 3/16 chance. Likewise, there is a 3/16 chance that you would be smart and not-handsome/beautiful. Note, however, that leaves whopping 9/16 chance that you would be both smart and handsome/beautiful.

Obviously, your parents took the chance, and if you have siblings, they took it more than once. I leave it to you to determine the phenotypes and possible genotypes of your siblings....

Linked genes

Mendel didn't have the mechanism to explain his results, but now we use the processes of meiosis and mitosis to explain all of these observations. In the example above, genes for beauty and intelligence would be on different chromosomes, so that inheritance of one trait does not influence the inheritance of another trait. When alleles for different traits are on the same chromosomes, the traits may be linked, and inheritance of one trait in the set may depend on the inheritance of the set of traits, and on how crossing over occurs.

Assume we have two traits again, beauty and smartness, and this time the genes which control them are next to one another on the same chromosome. During meiosis prophase I, synapses may occur on either side of the pair, but never occurs at the point where the two genes are connected, so that the pair is always linked. This time, we have homologous chromosomes, one of which contains a BS pair, as well as a gene for typing correctly "T", and the other of which contains a bs pair, and a gene for making mistakes while typing "t". [Again, purely hypothetical]. Thus in the original cell, we have two BST chromatids and two bst chromatids.

When crossing over occurs between two homologous chromosomes, each one produces an unchanged chromatid (same as it was in the parent, so a parental type) and a changed chromatid (recombinant type). The mix of traits in the recombinant chromatids is different from the mix of trait in the parental chromatids. The BS pairs are always passed together, but the T/t alleles are in a different part of the chromatid and in this particular meiosis prophase I, the typing alleles change places during crossing over. As a result, we have two parental chromatids, one BST and one bst, and two recombinant chromatids: one a BSt chromatid, and the other a bsT chromatid. Each chromatid goes to a different haploid cell in meiosis II, so instead of the two options we had to start with, we now have four.

Step 1

Step 2

Step 3

Sex genes

In humans, there are 22 genes that are the same form, regardless of sex. The 23rd pair differs between male and female. These are not numbered, rather they are the X (female) and Y (male) chromosomes. Everyone has at least one X chromosome; females have a second X (so XX) and males have a Y (so XY). Only males can donate a Y chromosome, so male genetic input determines the sex of any offspring.

Because we inherit pairs of genes, one of each from each parent, gene expression for genes 1-22 depends on the presence of dominants and recessives in the pairing: 22B from one and 22b from the other results in an expression of 22B. This is also true for the XX pair: XB will be expressed over Xb. However, because not all the genes on the X chromosome are expressed on the Y chromosome, in males, Xb will be expressed over Y0. This is why a number of recessive traits which are not expressed in the female chromosome X (which can be inherited from either parent in a daughter) will be expressed if that X is inherited by a son. Unfortunately, these include colorblindness and several forms of hemophilia.

Gene expression

Keep in mind that all these examples are simplified. Most of the time, you possess several alleles for a specific trait, and the pattern of dominance is not always clearcut. If, for example, there were 4 alleles for eye color, the dominance pattern could be something like Brown>Blue>Hazel>Green. Moreover, you may have multiple copies of the allele in different places on more than one chromosome. Usually, a cell deactivates all but one allele of a gene, so that the expression of the gene within the cell is consistent. But if the dominance pattern is complex so that some alleles are not clearly dominant, and cells deactivate different alleles, more than one allele expression may show up in the same individual. Add that to the possibility that the expression of one gene may control the expression of another, and suddenly predicting offspring traits becomes much more complex than our examples above.