Monohybrid and Dihybrid Cross
The crossing between two plants, with respect to a single contrasting character is...
MONOHYBRID CROSS
The crossing between two plants, with respect to a single contrasting character is called monohybrid cross.
Mendel conducted hybridization experiment by crossing true-breed tall pea plant (TT) with true-breed dwarf pea plant (tt).
In F1 (First filial) generation only one of the parental traits (tall) was seen and trait of other plant (dwarf) was not seen.
Mendel self-pollinated the tall F1 plants and he found that in the F2 generation some of the offspring were ‘dwarf’; the character that was not seen in the F1 generation was now expressed.
The proportion of plants that were dwarf were 1/4th of the F2 plants while 3/4th of the F2 plants were tall.
F2 plants were identical to their parental type and did not show any blending.
DIHYBRID CROSS (INHERITANCE OF TWO GENES)
The cross between two parents differed in two pairs of contrasting traits; is called dihybrid cross.
Mendel also worked with and crossed pea plants that differed in two characters, as is seen in the cross between a pea plant that has seeds with yellow colour and round shape and one that had seeds of green colour and wrinkled shape. The genotype of the parents can then be written as RRYY and rryy.
The gametes RY and ry unite on fertilizations to produce the F1 hybrid RrYy.
Mendel found that the seeds resulting from the crossing of the parents, had yellow coloured and round shaped seeds.
When Mendel self hybridised the F1 plants he found that 3/4th of F2 plants had yellow seeds and 1/4th had green.
The yellow and green colour segregated in a 3:1 ratio. Round and wrinkled seed shape also segregated in a 3:1 ratio; just like in a monohybrid cross.
The phenotypes round, yellow; wrinkled, yellow; round, green and wrinkled, green appeared in the ratio 9:3:3:1.
This derivation can be written as follows: (3 Round : 9 Round, Yellow : 3 Wrinkled, Yellow: 3 Round, Green : 1 Wrinkled, Green.
Law of dominance:
It states that “In a dissimilar pair of factors one member of the pair dominates (dominant) the other one (recessive).
Law of Segregation:
This law states “A pair of alleles segregate from each other during gamete formation, such that a gamete receives only one of the two factors”.
Law of Independent assortment:
The law states that “When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters”.