Question

1. Why is a test cross usually used in preference to production of an F2 generation to measure linkage?

2. Why were the earliest studies of linkage done with genes on the X-chromosome?

Answer 1

1) Genes which are located on the same chromosome and are transmitted together in the offspring are called linked genes. The phenomenon of inheritance of linked genes together so as to retain parental combinations in the offspring is known as linkage.

Test cross is a cross between heterozygous F1 and the double recessive homozygous. To study linkage, test cross is usually used because here it is easy to study the chromosome segregation in meiosis in only one parent (heterozygous one). The other F1 parent will give rise to only one kind of gametes. In Mendelian dihybrid test cross the phenotypic ratio should be 1:1:1:1, i.e., the parental combination in F2 will be 50% and recombinant frequency will be 50%. But if it is seen that parental frequency in F2 is more than 50%, then the two genes are said to be linked.

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2) Thomas Hunt Morgan conducted experiments on heredity in Drosophila melanogaster and introduced the concept of sex linked inheritance.

After studying sex linked inheritance we come to know that female has XX chromosomes, while male has XY chromosome, i.e., only one X chromosome. So it is easy to study the linkage pattern in only one parent (female) as male is hemizygous; so there is no chance of cross over between two different chromosome X and Y. So, we can follow chromosome segregation in meiosis in only one parent.

Picture is provided below: