monohybrid cross and dihybrid cross | punnett square | NMDCAT 2021
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Dihybrid and Dihybrid Cross
After thoroughly studying each trait separately , Mendel decided to study the inheritance
of two simultaneously , e.g., seed shape and seed colour. Seed shape could be round
or wrinkled. Similarly , seed colour could be yellow or green. He crossed true breeding
round and yellow seed plants with true breeding wrinkled and green seed plants . All F1
dihybrid were round and yellow seeded due to dominance. Then he made a dihybrid
cross by allowing self-fertilization among F1 dihybrids. The results was quite surprising.
Seeds produced as F2 progeny were ot only in the two parental combination i.e.,
round yellow and wrinkled green, but also in two new phenotypic combination
i.e., round green and wrinkled yellow. A clear cut 9:3:3:1 phenotypic ratio was found
in F2. Appearance of these new recombinant phenotypes of F2 indicated that some
sort of shuling of alleles had occurred during gemete formation. Mendel inferred
the mechanism of this shuling as independent assortment of alleles into gametes.
He concluded that the alleles for seed shape and colour were not bound to remain in
parental combination forever, i.e., ‘R’ with ‘Y’ and ‘r’ with ‘y’ ;rather these were free to
assort independently . R could go with Y or y in any gamete with equal change.
Similarly, r could go with y or Y in any gamete with equal probability. Four types of
gametes, i.e., RY, Ry, rY and ry were formed in equal number in a perfect ratio of 1:1
: 1:1. When these gametes randomly fertilized each other, a 9:3:3:1 phenotypic ratio
was produced among F2 progeny (Fig 22.6).
Mendel formulated Law of Independent Assortment : “When two contrasting pairs of
traits are followed in the same cross, their alleles assort independently into gametes.”
Alleles of one pair inherit independently of alleles of the other pair. The distribution of
alleles of one trait into gametes has no inluence on the distribution of alleles of the
other trait. Thus the chance for a plant to be round or wrinkled is independent of its
chance of being yellow or green.
MENDEL’S LAWS OF INHERITANCE
Gregor Johann Mendel (1822 - 1884) laid the foundation of classical genetics by
formulating two laws of heredity; law of segregation and law of independent assortment.
He was a priest. He performed series of breeding experiments on garden pea, Pisum
sativum in his monastery garden for eleven years (1854 - 1865). Pisum sativum was
easy to cultivate and it grew well in his garden. Its lowers were hermaphrodite. It was
normally self-fertilizing, but could also be cross-fertilized. As the time gap between
generations was short, Mendel could raise many generations of pea within a short time.
Pea had many sharply distinct traits. Each trait had two clear cut alternative forms or
varieties; e.g., seed shape had a round or wrinkled phenotype, plant height was either
tall or short, seed colour could be yellow or green. Mendel called them contrasting pair
of a trait. He focussed on seven such pairs
After establishing 14 pure - breeding lines of seven characters, he cross-fertilized
plants that difered in one character only. The ofspring of such a cross were called
monohybrids. He cross-fertilized a true breeding round-seeded male plant with a true
breeding wrinkled-seeded female plant
He called it irst parental generation (Pi). Their ofspring were called Fi or irst ilial
generation. All Fi ofspring were round like one of the parents. Wrinkled phenotype did
not appear at all. Round dominated wrinkled. Its dominance was complete because no
ofspring intermediate between parents was found. He called the trait that appeared
in F, as dominant; while the trait,
which was masked, as recessive.
Then Mendel allowed self-
fertilization among F| monohybrids
to raise F2 progeny. As a result of
monohybrid cross 3A of F2 were
round and lA wrinkled.
breeding wrinkled-seeded female plant
Then Mendel allowed self-
fertilization among F| monohybrids
to raise F2 progeny. As a result of
monohybrid cross 3A of F2 were
round and lA wrinkled.
Mendel got similar results and
the same 3:1 ratio in ofspring of
monohybrid crosses for all the
seven contrasting pairs of traits.
Mendel proceeded a step ahead.
He self- fertilized F2 plants to raise
F3. He noted that 1/3 of F2 round
produced only round, while 2/3
of F2 round produced both round
and wrinkled in . 3:1 ratio; but F1
wrinkled produced only wrinkled.
He concluded that 1/3 of F2
rounds were true-breeding like Pi
round, and 2/3 of F2 rounds were
monohybrids like Fj round.
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Well explained
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genotypic ratio is 1:2:1:2:4:2:1:2 total 15 how? sir it's confusing 16 nh hona Chiaa
16 is right
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Sir, Monohybrid and Dihybrid cross are very difficult for me. Do you have any other videos on this topic?
Its easy
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Genotypic ratio of dihybrid cross is1:2:1:2:4:2:1:2:1
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