RRYY rryy

BY gametes ry

F, generation

Figure 1-3 How Mendel's second I am (independent assortment) operates.

!n this example, the inheritance of yellow {>') and green (y) seed coior is followed together with the inheritance of round (ft) and wrinkled (r) seed shapes. The R and Y alleles are dominant over r and y. The genotypes oi the various, parents and progeny are indicated by letter combinations, and (our different phenoiypes are distinguished by appropriate shading.

morphologically similar sets and that, during meiosis, every gamete receives only one chromosome of each homologous pair. He then used this fact to explain Mendel's results by assuming that genes are parts of the chromosome. He postulated that the yellow- and green-seed genes are carried on a certain pair of chromosomes and that the round- and wrinkled-seed genes are carried on a different pair. This hypothesis immediately explains the experimentally observed 9:3:3:1 segregation ratios. Although Sutton's paper did not prove the chromosomal theory of heredity, it was immensely important, for it brought together for the first time the independent disciplines of genetics (the study of breeding experiments) and cytology (the study of cell structure).

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