Association of paternal and maternal chromosomes at meiosis

Physical Basis of Heredity 4. The Chromosome Theory of Inheritance
Association of paternal and maternal chromosomes at meiosis
Qualitative differences between chromosomes
Formulation of chromosome theory
Sex chromosomes and chromosome theory
As early as 1900, Von Winiwarter, discovered that in the overy of a half day old rabiit (Oryctolagus cuniculus) chromosomes had associated in pairs side by side for a part of their length. This association appeared at what we know call zygotene stage of meiosis, which was very common in rabbits 1½ and 2½ days old. Complete meiotic stages were traced out by Van Winiwarter in this rabbit. However, it was Montgomery (1901), who after a study of meiosis in 42 species of insects, came to the conclusion that each association of chromosomes in pairs at zygote involves one paternal and one maternal chromosome. This conclusion was based on two kinds of evidences. Firstly, whenever chromosomes differed in size a bigger chromosome never paired with smaller chromosome. Secondly, if diploid chromosome number was 14, only seven bivalents were observed. Contrary to this if paternal and maternal chromosome had to associate separately among themselves, seven paternal chromosomes would associate in three bivalents and one univalent and so will be the case with maternal chromosomes.

Sutton (1902) extended Montgomery's observations, while studying meiosis in grasshopper. The male grasshopper has 23 chromosomes in somatic cells, and these chromosomes differed very much in size, so that the largest chromosomes were 5-6 times as long as the smallest. Sutton observed that apart from one X-chromosome, 11 paired associations were present in each nucleus and showed same size differences as at mitosis. It was also shown that although due to degree of contraction, absolute lengths of paired chromosomes differed at different stages of meiosis, but their relative lengths were constant. This confirmed Montgomery's conclusion that associations were brought about only between paternal and maternal chromosomes.