Thomas Hunt Morgan discovered that genes lie on chromosomes through his research on the common fruit fly (Drosophila melanogaster) in his lab at Columbia University. He began breeding them in large quantities, wanting to see enough natural mutations lead to the creation of a new species. (At the time, he was skeptical of Darwin’s theory of evolution and Gregor Mendel’s ideas on heredity.) In 1910, Morgan found a male fruit fly with white eyes (all the others had red eyes). He mated it with a red-eyed female, and of their 1,237 offspring, only 3 had white eyes. These also were male. After mating these white-eyed offspring, white eyes became more common.
Morgan began to think that eye color was a sex-linked trait, present on the X chromosome. Using my past knowledge of Mendel’s laws of heredity, this is how I see things occurring. I’ll explain it in a moment.
The first fly to have white eyes was male, so he had XY chromosomes. If the eye color trait was on the X chromosome, his white-eye trait was on his X chromosome. He mated with a red-eyed female. As a female, she had XX chromosomes, both of which presumably had red as the setting for eye color.
So his XY chromosomes looked like WY (W being the X chromosome with the white-eye setting, and Y being his other Y chromosome), whereas her XX chromosomes looked like RR (R being an X chromosome with a red-eye setting).
When they had over 1,000 children, all those children had one of two possible sex chromosome pairs (WR or RY). Sex chromosomes are the chromosomes which allow a person to tell what sex an organism is. (A biological female’s sex chromosomes are XX, for example.) Some of those children had WR sex chromosomes, and other children had RY sex chromosomes. All the children that had RY chromosomes were male because of the Y. All the children (of this first generation) that had WR chromosomes were female, because W is an X chromosome (from dad) and so is R (from mom). So all the males in the first generation had a single X chromosome, and that was the R chromosome from mom. This makes sense because all the males received one of their sex chromosomes from mom, and the other from dad. Mom could only give R chromosomes, so they got an R chromosome, and dad could give them either W or Y, but if they had gotten W, they would have been female—since they were male, they must have gotten Y. This means that no males that belonged to the first generation had white eyes. (I know this doesn’t align with what I said before, which was that Morgan discovered 3 flies with white eyes in generation 1 which were male, but those cases are not ordinary, and I honestly don’t know how that occurred, though there’s no doubt an explanation that won’t prove wrong Mendelian laws of heredity.)
Haha, I kept saying children like they’re human—I meant offspring.
Anyway, you’re probably wondering if any females of generation 1 were white-eyed. After all, all the females of that generation were WR, and had an X chromosome coding for white eyes. Well, none of them did show up with white eyes. This was because W was recessive and R was dominant.
Generation 2 consists of the offspring of WR and RY. These red-eyed parents gave birth to four types of offspring: WR, WY, RR, and RY. (The W and R could have combined; the W and Y could have combined; the R and R could have combined; the R and Y could have combined.)
Just like last time, about half the offspring of generation 2 were female, and about half were male. WR and RR offspring were female (two X chromosomes present, or no Y chromosome present) and both had red eyes; WY and RY were male (Y chromosome present), and while RY males had red eyes, WY males had white eyes. ¾ of population 2 had red eyes; ¼ had white eyes and those flies were male, as you can see.
But things changed in generation 3. Suddenly, WW females appeared (with white eyes). From generation 2, there were 4 mating possibilities: WR & WY; WR & RY; RR & RY; RR & WY.
Parents WR and WY can have offspring with chromosomes: WW, WY, WR, and RY (you can see why a diagram is useful now—it makes keeping track of this easier, at least in my opinion).
WW: white-eyed females. WY: white-eyed males. WR: red-eyed females (R is dominant); RY: red-eyed males. Half of the offspring of WR and WY is white-eyed; the other half is red-eyed.
The same process can be used to figure out the nature of the other offspring that make up generation 3.
Morgan’s continued work allowed him to discover and study other mutations in the same time decade (1910). In The Mechanism of Mendelian Heredity, which he published with colleagues, it was stated that heritable factors were located on chromosomes, ones which Morgan would later call genes. The closer two genes on a chromosome, the lower the chance of the traits being separated; they would be associated unless somehow, through the processes of reproduction, they were physically separated. The published work also maintained that some “characteristics are sex-linked—that is, occur together because they arise on the same chromosome that determines gender.”
I feel really sleepy for whatever reason, so I may have sort of rushed the ending. Sorry about that, but thanks for reading!