(3)

Linkage is the phenomenon of certain genes staying together during inheritance through generations without any change or separation due to their being present on the same chromosome. Linkage was first suggested by Sutton and Boveri (1902–1903) when they propounded the famous “chromosomal theory of inheritance.” Bateson and Punnett (1906) while working on sweet pea found that the factors for certain characters do not show independent assortment. However, it was Morgan (1910) who clearly proved and defined linkage on the basis of his breeding experiments in fruit fly (Drosophila melanogaster).

(1)

Translocation is a chromosomal abnormality caused by rearrangement of parts between non-homologous chromosomes. It may cause a gene to move from one linkage group to another.

(2)

A dominant epistatic allele suppresses the expression of a non-allelic gene whether the latter is dominant or recessive. The gene which suppresses the expression of a non-allelic gene is known as epistatic gene. The gene or locus which is suppressed by the presence of non-allelic gene is termed as hypostatic gene. In summer squash or Cucurbita pepo, there are three types of fruit colour — yellow, green and white. White colour is dominant over other colours while yellow is dominant over green. Yellow colour is formed only when the dominant epistatic gene is represented by its recessive allele (w). When the hypostatic gene is also recessive (y), the colour of the fruit is green, i.e.,

W_ Yy, W_ yy → White

wwY_ → Yellow

wwyy → Green

(3)

A classical dihybrid cross performed by Mendel involves.

A cross which was made between a pure round yellow seeded pea plant (RRYY) with wrinkled green seeded plant (rryy). Yellow colour is dominant over green and round seed shape over wrinkled seed shape. Phenotypic ratio in F2 generation

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