(4)

When the plant homozygous for yellow seeds and red flowers (YYRR) was crossed with a plant homozygous for green seeds and white flowers (yyrr), ${\mathrm{F}}_1$ plants produced with yellow seeds and pink flowers. This means inheritance of seed colour shows complete dominance while inheritance of flower colour shows incomplete dominance due to blending of two alleles. This can be shown as follows:

Phenotypic       Yellow seed : Yellow : Yellow : Green : Green : Green

Ratio                   Red flower        Pink        White      Red       Pink        White

                                      3            :            6           :           3          :         1        :         2         :       1

Therefore, six phenotypic categories are obtained in the ${\mathrm{F}}_2$ generation.

(1)

(1)

Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosomes as a measure of the distance between genes and ‘mapped’ their position on the chromosome.

(3)

The law of independent assortment is applicable to only those factors or genes which are either located distantly on the same chromosome or occur on different chromosomes.

(4)

(4)

Sutton and Boveri proposed the chromosomal theory of inheritance but its experimental verification was done by Thomas Hunt Morgan.

(4)

Genetic map is a linear graphic representation of the sequence and relative distance of various genes present in a chromosome. 1% crossing over between two linked genes is known as 1 map unit or centiMorgan (cM).

(4)

T.H. Morgan coined the term linkage to describe the physical association of genes on chromosome and term recombination to describe the generation of non-parental gene combinations. Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes.

(3)

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.

(1)

If in a dihybrid test cross more parental combinations appear as compared to the recombinants in ${\mathrm{F}}_2$ generation, then it is indicative of involvement of linkage. Linkage is the tendency of two different genes on the same chromosome to remain together during the separation of homologous chromosomes at meiosis. During complete linkage no recombinants are formed whereas in incomplete linkage few recombinants are produced along with parental combinations.

(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