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  Section: Genetics » Physical Basis of Heredity » The Chromosome Theory of Inheritance
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Sex chromosomes and chromosome theory

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

The first definite evidence for chromosome theory came from sex-determination. C.E. McClung in 1902 reported two types of sperms in grasshoppers, one containing an X-chromosome and the other lacking it. Since this was the only difference in sperms and since all eggs were similar with respect to X-chromosomes, it was concluded that X-chromosome must be responsible for determination of sex. Proof of parallelism between chromosomes and Mendelian factors with respect to independent assortment came from Elinors E. Carothers in 1913. She used a particular grasshopper strain, where homologous partners of three pairs of chromosomes could be morphologically distinguished. If these 3 pairs are designated as AA', BB' and CC', the possible arrangements at mctaphase I can be shown as in Figure 9.2. Eight types of combinations in gametes would then be possible in equal proportion from arrangements shown in Figure 9.2. When crosses of thus marked grasshoppers were made with normal grasshoppers, all eight combinations could be identified morphologically through cytological preparations, suggesting an assortment just like that discussed for Mendelian factors in a trihybrid cross in Mendel's Laws of Inheritance.
Eight possible arrangements of three pairs of homologous chromosomes.
Fig. 9.2. Eight possible arrangements of three pairs of homologous chromosomes.

A very strong cytological evidence for chromosomes theory of inheritance came later from C. Stern's experiment on crossing over (Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes)) and C.B. Bridge's experiment on the non-disjunction of X-chromosomes (Sex Linked, Sex Influenced and Sex Limited Traits and Sex Determination, Sex Differentiation, Dosage Compensation and Genetic Imprinting).

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