What hypotheses can you make about inheritance of eye-color

Red is the ‘normal’ or ‘wild type’ color of the eyes in Drosophilamelanogaster.

Genetics of Drosophila melanogaster - Biology Junction

In particular, Morgan began to entertain the possibility that association of eye color and sex in fruit flies had a physical and mechanistic basis in the chromosomes. The shape of one of Drosophila's four chromosome pairs was thought to be distinctive for sex determination. Males invariably possess the XY chromosome pair (Morgan used a more cumbersome notation) while flies with the XX chromosome are female. If the factor for eye color was located exclusively on the X chromosome, Morgan realized, Mendelian rules for inheritance of dominant and recessive traits could apply.

Crossing white eyed Drosophila with sepia eye

In brief, Morgan had discovered that eye color in Drosophila expressed a sex-linked trait. All first-generation offspring of a mutant white-eyed male and a normal red-eyed female would have red eyes because every chromosome pair would contain at least one copy of the X chromosome with the dominant trait. But half the females from this union would now possess a copy of the white-eyed male's recessive X chromosome. This chromosome would be transmitted, on average, to one-half of second-generation offspring—one-half of which would be male. Thus, second-generation offspring would include one-quarter with white eyes—and all of these would be male.

These mutated, or changed genes produce differences that caught the interest of Thomas Hunt Morgan.Thomas Hunt Morgan (1866-1945), was the first to observe the white eye mutation in in 1910.


2 X linked Inheritance in drosophila white eye - YouTube

In Drosophila, normal flies have red eyes. Red eye color is dominant. Morgan discovered a recessive mutation (allele) that caused white eyes. When Morgan mated a red eyed female to a white eyed male, all the progeny had red eyes. This result makes perfect sense with a dominant/recessive inheritance pattern, and here is the Punnett square demonstrating that (x^w = recessive white eye mutant allele; x^W = dominant red eye wild-type allele):

wild typeâ color of the eyes in Drosophila melanogaster

These results support the chromosome theory of inheritance because the only way to explain them is if the eye color gene is on the X chromosome. This is sex-linkage, or inheritance of genes that are on the sex chromosomes (X and Y). Sex-linked traits show interesting inheritance patterns in part because females have two copies of each X chromosome, but males only have one. This inheritance pattern means that a male with the recessive allele will always show the recessive trait, because he only has one copy of the allele. In contrast, most genes are located on the autosomes, or non sex chromosomes, where both males and females have two copies of each gene. Recall that all the patterns of inheritance observed by Mendel, including the principle of segregation and the principle of independent assortment are explained by the behavior of chromosomes during meiosis. These principles are part of the chromosome theory of inheritance.

Chromosome theory of inheritance | Biology 1510 …

Thomas Hunt Morgan, an embryologist who had turned to research in heredity, in 1907 began to extensively breed the common fruit fly, Drosophila melanogaster. He hoped to discover large-scale mutations that would represent the emergence of new species. As it turned out, Morgan confirmed Mendelian laws of inheritance and the hypothesis that genes are located on chromosomes. He thereby inaugurated classical experimental genetics.

GNN - Genetics and Genomics Timeline

He found that there is a mutation which affects eye color, changing it from the normal “wild type” red found in nature, to unpigmented and therefore white eyes.