Biology · Chapter 11
Study notes aligned to the official NEB syllabus.
This chapter, built on the work of Thomas Hunt Morgan, extends Mendelian inheritance to genes that lie on the same chromosome: linkage, sex-linked inheritance, and crossing over.
Before introducing linkage, the notes recap Mendel's monohybrid and dihybrid crosses, which form the background against which linkage is understood.
Mendel allowed the F1 (hybrid) to self and obtained the F2 as tall and dwarf pea plants in the ratio of 3:1. This shows that the trait (dwarf) which did not appear in F1 reappeared in the F2 generation.
| Tall parent (TT) | Dwarf parent (tt) | |
|---|---|---|
| Gamete | T | t |
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
When a cross is made between two homozygous parents differing in two pairs of contrasting traits (two characters), it is called a dihybrid cross.
Mendel crossed a pea plant having yellow-coloured, round-shaped seeds with a pea plant having green-coloured, wrinkled-shaped seeds, and obtained all F1 as yellow-coloured, round-shaped seeds. This shows that yellow colour and round shape are dominant over green colour and wrinkled shape.
He allowed F1 to self and obtained F2 seeds as yellow round, yellow wrinkled, and green round (the source's own listing is cut off here by a page break before naming the fourth class, green wrinkled), in the ratio 9:3:3:1, which is called the dihybrid ratio.
Of the four phenotypes, yellow round and green wrinkled are the parental traits (matching the original parents), while yellow wrinkled and green round are the non-parental traits, also called recombinants. The ratio of parental to non-parental traits is given in the source as 70:6 (unclear/not a standard ratio; kept as stated in the source).
This can be represented as:
He (the source continues the discussion, considering two genes, one for seed colour and another for seed shape) crossed a maize plant having coloured, full seeds with a maize plant having colourless, shrunken seeds, and obtained all F1 as coloured, full seeds. This shows that coloured and full are dominant over colourless and shrunken.
He test-crossed the F1 and obtained offspring as coloured-full seeds, coloured-shrunken seeds, colourless-full seeds, and colourless-shrunken seeds. Of these four, coloured-full and colourless-shrunken are the parental traits, and the remaining two (coloured-shrunken and colourless-full) are the non-parental (recombinant) traits.