During Metaphase I, homologous chromosome s will line up. A karyotype can be made by cutting and arranging photomicrographs of the homologous chromosomes thus revealed at Metaphase I. Two types of chromosome pairs occur. Autosomes resemble each other in size and placement of the centromere , for example pairs of chromosome 21 are the same size, while pairs of chromosome 9 are of a different size from pair 21. Sex chromosomes may differ in their size, depending on the species of the organism they are from. In humans and Drosophila , males have a smaller sex chromosome, termed the Y, and a larger one, termed the X. Males are thus XY, and are termed heterogametic .
Females are XX, and are termed homogametic.
In grasshoppers, which Sutton studied in discovering chromosomes, there is no Y, only the X chromosome in males. The male thus determines the sex of the offspring. Remember that in meiosis, each chromosome is replicated and one copy sent to each gamete. Morgan discovered a mutant eye color and attempted to use this mutant as a recessive to duplicate Mendel's results. Most mutations are usually recessive, thus the appearance of the white mutant presented Morgan a chance to test Mendel's ratios on animals.