Mendel's Law of Independent Assortment
Mendel's law of independent assortment is the outcomes of is hybrid
cross. According to this law the segregation of two or more characters in the
same hybrid is independent of each other. Thus any allele of one gene is
equally likely to combine with any allele of the other gene and pass into the
same gamete. Independent segregation of 2 genes produces four different types
of gametes in equal proportion.A random
Union among these gametes give rise to 16 possible zygotes. These yieled
a 9:3:3:1 phenotype ratio which is known as the typical dihybrid ratio. The
principle of independent assortment is Mendel made a cross between pea plant
having yellow cotyledon colour and round seed surface and plants having green
cotyledon colour and rough or wrinkle seed surface. All the F1 hybrid of this
cross were phenotype with yellow and round seed surface when this F1 hybrid
self fertilized they produce 4 types of phenotypically different plans in the
following ratio.
Mendel's
dihybrid cross between pea plants with yellow round seed and green wrinkle seed
has been explain with the help of the cross as follows. The homologous pea
plant with yellow round seeds have the allele RRYY for yellow colour of seed or
one member pair of homozygous chromosome and alleles RR for round shape of seed
on the other member of a pair of homologous chromosome. Similarly green wrinkle
seed have the allele yy for green colour of seed on one member of a pair of
homologous chromosome and allele rr for wrinkle shape of seed on other member
of the pair of homologous chromosome. During gametogenesis the homozygous plans
having YYRR alleles produced gametes
with YP allele and the homozygous plants having YYRR allele produced
gametes with yr allele respectively. After fertilization between this game it's
a hybrid plan is produce with YyRr alleles showing complete dominance of Y
& R alleles over y and r allelel respectively. In the sale of F1 hybrid or
the one member of homologous pair of chromosome occur the alleles for yellow
and green cotyledon colour and on the other member of a pair of homologous
chromosomes are the alleles for round and wrinkle seed shape. in F1 hybrid with
YYRR alleles during gametogenesis produced four types of gametes with the
following composition.
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| Mandel's Dihybrid Cross Table |
These
4types of gametes i.e. YR,Yr,yR&yr of F1 hybrids unit randomly in the
process of fertilization & produced 16 type of individuals in F2 generation
in the ratio of 9:3:3:1. Then results hv prove the law of independent
assortment & show that each pair of contrasting characters behaves
independently & bears no parmanent association with allele R in parent but
it doesn't always remain associated with allele R in the hybrids. It also
remains associated with allele r.