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Inheritance Part 2

Inheritance:

Mendel’s Dihybridization Experiments:
Dihybridization experiment includes the inheritance of two characters simultaneously such as the shape of seeds and colour of seeds.

Mendel has selected Pea plants for dihybridization experiment with shape of seeds and color of seeds as a character. The gene for round shape of seeds (R) is dominant over the gene for wrinkled shape of seeds (r). Gene for yellow color of seeds (Y) is dominant over the gene for green color of seed (y).


Mendel has selected patent plants in P generation are homozygous yellow ad round seeded plants and homozygous green and wrinkled seeded plants. As yellow and round seeded genes are dominant, F1 generation are yellow and round seeded. Based on this experiment, Mendel has concluded the law of Independent assortment.

Law of Independent Assortment:
The segregation of gene controlling one character is independent of the segregation of genes controlling another character. In short, Inheritance of one character is independent of inheritance of another character.

Linkage:
Law of Independent Assortment is not true in all the cases. If two pairs of genes controlling two different characters are located on the same pair of homologous chromosomes, they cannot be segregated separately. Such genes are called Linked genes and their inheritance is called Linkage.

Batson and Punnett have performed experiment on Sweet Pea plants (Lathyrus odoratus. L.) to explain the Linkage. They used homozygous purple flower and homozygous long pollen plant as one parent and homozygous red flower and homozygous round pollen plants as another parent. In these plants purple color flower (R) is dominant over red flower color (r). Long shape of pollen grains (L) is dominant over round shape of pollen grains (l). As purple flower and long pollen grains are dominant, the F1 generation has purple flowers and long pollen. It was as per the expectation. Latter, F1 generation plants were allowed to self pollinate to obtained F2 generation. Result obtained was not as per expectation as per the mathematic law of probability.

Batson and Punnett concluded that as both the dominant genes are derived from one parent and hence they inherited together. Both recessive genes are also derived from one parent and hence they are also inherited together. They called such behavior as coupling.

Table: Result showing the Batson and Punnett’s experiment on Linkage


F2 generations Expected result Actual result
Purple flower
Long pollen
9 11
Purple flower
Round pollen
3 1
Red flower
Long pollen
3 1
Red flower
Round pollen
1 3


Crossing Over:
The process by which linkage between genes is broken and by which new combination of genes is developed is called crossing over.

Crossing over of genes can occur between homologous chromosomes during meiotic cell division. Chromatid undergoes crossing over forms recombinant genes which are responsible for variation.


Self study:
Example-1: Name the scientist who have described the Linkage.
a)  Robert hook
b)  Batson and Punnett
c)  Mendel
d)  Lamarch

Answer: Batson and Punnett have first described the concept of Linkage.


Example-2:  Crossing over of genes occurs during ……………cell division.
a)  Variation
b)  Meiosis I

c)  Meiotic
d)  Meiosis II

Answer: Crossing over of genes occurs during meiotic cell division.


Example-3: Batson and Punnett have selected ……………………..for Linkage.
a)  Sweet Pea Plant
b)  Yellow flower Pea Plant
c)  Round Seed Pea Plant
d)  Long pollen Pea Plant

Answer: Batson and Punnett have selected Sweet Pea Plant for Linkage.


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