Define Monohybrid and Dihybrid cross. Explain inheritance of two pairs of contrasting characters showing independent assortment of tall-dwarf stems v/s yellow Green seeds in Pea plant


MONOHYBRID CROSS
If only one pair of contrasting character is being studied the hybrids are called Monohybrids.

DIHYBRID CROSS
When two different pairs of contrasting characters are studied simultaneously, the hybrids are called 
Dihybrids.

Mendel performed numerous experiments with garden pea keeping in view the simultaneous inheritance of two different pairs of contrasting characters and always found that one of the two contrasts out of each pair was dominant in F1 generation; and it was always the same contrast which was dominant in single trait inheritance. He always obtained a ratio of 9: 3: 3: 1 in F2 generation of which 9 were of dominant parental types 3: 3 were of two different recombination types which did not exist previously and one was showing both the recessive contrasts. Mendel realised of his breeding experiments, two different pairs of alleles is completely at random (i.e. depends on chance). This principle of inheritance was called the law of independent assortment”. In its simplest forms it can be stated that the alleles for two different pairs of contrasting characters are transmitted independent of one another during their inheritance to the next generation.

Tall dwarf and yellow green cotyledons
The gene for the length of Pea plant has two contrasts i.e. modified forms (called alleles) one of which produces tallness (T) and the other produces dwarfness (t), the two characters called as a pair of contrasting characters. Similarly, the gene for cotyledons colour has two modified forms (alleles) one of which produces yellow cotyledons (Y) and the other produces green cotyledons (y); the two characters called as a pair of contrasting characters. If a pure tall plant (TT) with yellow cotyledons (YY) is crossed with a pure dwarf plant (tt) with green cotyledons (yy), all the F1 plants are tall and with yellow cotyledons. It indicates that tallness is dominant over dwarfness and yellow cotyledons are dominant over green. If F1 tall and yellow plants are self corssed 9: 3: 3: 1 ratio is obtained in F2 i.e. 9/16 are tall with yellow cotyledons, 3/16 are tall with green cotyledons, 3/16 dwarf with yellow cotyledons (3: 3 called recombinants, as these combination of characters did not exist before), 1/16 are dwarf with green cotyledons i.e. homozygous recessive for both traits.

During gamete formation T gene is free to go with y or Y. Similarly gene is independent of to go with either T or t. Thus the occurrence of a 9: 3: 3: 1 ration in F2 is truly evidence that the law of independent assortment is correct.

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