Biology Chapter 15: Chromosomal Theory of Inheritance Mendelian genes have specific loci along chromosomes, and it is the chromosomes that undergo segregation and independent assortment. Law of Segregation States that two alleles for each gene will separate during gamete formation, so that each gamete will be carrying 1 allele. Law of Independent Assortment Alleles of nonhomologous chromosomes align independently of another during gamete formation in metaphase 1. The first solid evidence associating a specific gene with a specific chromosome came early in the 20 century from the work of Thomas Hunt Morgan. Morgan chose to breed Drosophila melanogaster, a species of fruit flies. He chose these because a single mating produces hundreds of offspring, a new generation can be bred every two weeks, and they only have 4 chromosomes, 1 of which is a sex chromosome. Wild Type Phenotype for a character most commonly observed in in natural populations such as red eyes in Drosophila Mutant Types Phenotype for a character that is not commonly observed in nature such as white eyes in Drosophila. So basically, Morgan bred a ton of these flies, and he found some unique observations. He found that when he bred a white eyed male fly, with a red eyed female, he got all red eyed offspring, suggesting that red eyes were dominant. When he bred this generation, he got a 3:1 ratio, however all the females were red eyed, and half of the males were white eyed. *This showed that eye color was inherited on the x chromosome and not on the Y chromosome. Males only have one x chromosome and females have two so they need two of the recessive x chromosomes to inherit white eyes. **The SRY gene is what determines the sex of the offspring. Without this gene, the gonads will develop into ovaries. Females are XX and males are XY Each ovum contains an X chromosome while a sperm may contain either an X or a Y chromosome Other animals have different system for sex chromosomes. *Humans and most animals have an XY system.