Lesson 6
A scientist at Columbia University in the early 1900s named Thomas Hunt Morgan was studying flies. During his study he discovered some mutant flies that had white eyes versus the normal red color. Some other mutations were that the wings were short, and some had black or yellow bodies as opposed to the normal grey.
When he crossed a white-eyed mutant male with a normal red-eyed female fly the next generation contained only red-eyed flies, one male, one female. This led him to believe that the trait was recessive. The next generation exhibited the 3:1 ratio, the one thing that Morgan observed that was out of the ordinairy was the fact that only males had expressed the white-eye trait (not all males were white-eyed), so he began to think that this trait was related to the flies sex.
This cross can be represented in the following way. If XR stands for the X chromosome with the dominant red-eye allele, Xr stands for the recessive white-eye allele, and Y stands for the Y chromosome:
P1 Xr/Y (male, white-eyed) x XR/XR (female)
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gametes Xr[50%] Y[50%] XR[100%]
These gametes get crossed as follows: Xr x XR and Y x Xr, which yields:
F1 Xr/XR[50%] (f) XR/Y[50%] (m)
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Xr[50%] XR[50%] XR[25%] Y[50%]
These gametes are crossed: Xr x XR, Xr x Y, XR x Y, and XR x XR which gives us:
F2 Xr/XR[25%] (f) Xr/Y[25%] (white-eyed m) XR/XR[25%] XR/Y[25%]
Morgan continued to breed his flies and eventually got white-eyed females, he then crossed these with red-eyed males:
P1 XR/Y (red-eyed m) x Xr/Xr (white-eyed f)
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gametes XR[50%] Y[50%] Xr[100%]
These are crossed: XR x Xr and Y x Xr which means the offspring have:
F1 XR/Xr[50%] (f) Xr/Y[50%] (white-eyed m)
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gametes XR[50%] Xr[50%] Xr[25%] Y[50%]
Gametes cross: XR x Xr, XR x Y, Xr x Xr, and Xr x Y, yielding:
F2 XR/Xr[25%](f) XR/Y[25%](m) Xr/Xr[25%](w-e f) Xr/Y[25%](w-e m)
The pattern had changed from that of the first cross. Now all the males in F1 were white-eyed and all the females were red-eyed again. So Morgan put forth that the gene for eye-color was located on the X chromosome alone, not on the Y. This meant that a single recessive allele on a male's X chromosome would result in white eyes.
This is one example of X-linked traits, another is hemophelia, this one occurs in humans. Red-green color blindness is another.
A pedigree is a diagram that lists the phenotype that is governed by a particular genetic trait from one generation onward in a family. The symbols are kind of like those in a family tree, females are circles, males are squares, and those who express the recessive form of the trait are colored in black. Marraige between to people is represented by a line that runs between and in the middle of the two married people. If these people have children then a line runs down from the center of the marraige line and extends into a line which attaches to every child from the top. The figure below is a section of a pedigree.
Some of the other technical rules are that each generation be labelled with a Roman numeral, and each person in each generation is numbered. This is handy since it lets you locate a person rather quickly, and is much better than using their names since some people might have the same name.
Pedigrees allow us to investigate patterns of inheritance. Given just the right ammount of information, one can determine the genotype for every person shown on the pedigree (although sometimes you can't complete everyones genotype. In this case you write what you do know of the genotype followed by a question mark), and perhaps try to predict the future. For instance, lets say that the pedigree shown above is tracing the trait of having a hitchhikers thumb. If you flex your thumb back you can see if you have it, your thumb will either be straight or be arched back. Go ahead and give it a try, I'll wait. The dominant allele is that which does give you hitchikers thumb, so, according to the pedigree above, the parents both have hitchhikers thumb, and two of their children do, the other one has a straight thumb. Lets let H be the dominant hitchhikers thumb allele and h be the recessive form. First of all, the child that has a straight thumb has the genotype b/b. The rest must then have B/b, or B/B. Well, you know the other children have A big B in their genotype, but you aren't sure about the little b. In this case, under their genotype you'd write B/?. The parents must have B/b.
I'll show you how I worked this out in a second. First I have to show you a little secret of mine. Its called the Punnett square. If two people, male and female, with B/b, and B/b genotypes have children, the possible genotypes of the child can be determined as follows:
To go back to the pedigree, you are given the information that one of the children are homozygous recessive, meaning that you have one complete genotype. The other children do not exhibit the trait, and neither do the parents, but those recessive alleles must have come from the parents so they both must have little b's in their genotype. They don't exhibit the trait so they must have big B's in their genotypes as well, so their genotypes are B/b. The other children on the other hand could have either B/b or B/B as their genotypes, which means that we must put B/?.
We've really enjoyed teaching you these lessons on a subject as interesting as genetics. We hope that you've enjoyed it, too, and that you have learned a little something in the process. We can't fail to mention that genetics consists of a lot more than the basics that we've put down in these lessons. But, we hope, with a little patience and research, we'll expand these few lessons, and add some more.