Thomas Hunt Morgan was one of America's most famous geneticists. He worked on Fruit Flies. He used mutagens to change genes and came up with some pretty crazy mutants which he crossed with what he called the Wild-Type Fly, which was one with all the normal characteristics. His most famous mutant, the one he proved that Genes were, indeed, found on Chromosomes was in this cross:
P White-eyed Male (the mutant) X Red-Eyed Female (the wild type)
F1 All the flies were Red-eyed
F2 He got the classic 3:1 ratio, Red:White but something was different. ALL OF THE WHITE-EYED FLIES WERE MALE. So, he looked at the ratio differently using gender as well as eye color and, when he did, the ratio was 2:1:1; Red-eyed Females: Red-eyed Males: White-eyed Males. This was new. Before, gender did not make a difference. So, he had an idea. Fruit flies, like humans have differences in their chromosomes depending upon which gender they were:
GENDER DETERMINATION: In fruit-flies (and humans) males have an XY pair of chromosomes and females have an XX pair. This means that in females the last two chromosomes are the same ones with the same genes (but might have different alleles because they come from different parents). But, in males, the pair (XY) are totally different chromosomes with different genes. So, if a gene occurs on the X, it does NOT occur on the Y, and vice versa. The Father determines gender of the child because when he produces sperm cells the two chromosomes segregate so he produces X sperm and Y sperm. A female, on the other hand, only produces X eggs. So, one can do a punnet square for these chromsomes as follows:
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