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P1 RR X WW

LIVINGS

In

BIOLOGY

FOR

GENERAL SECONDARY CERTIFICATE

Unit (II)

Chapter (2)

The Inheritance in Livings

Gene Interaction

Lack of Dominance

In the previously mentioned Mendelian cases, it was noticed that if two homozygous individuals differing in one pair of allelomorphic characteristics are crossed, only one characteristic appears in F1 generation individuals, which is the dominant characteristic, while the second characteristic which is called the recessive characteristic disappears completely in F1 generation individuals. In F2 generation individuals both the dominant characteristic, and the recessive characteristic appear in the ratio 3 dominant :1 recessive. This Mendelian case is also called complete dominance. Examples for the Mendelian characteristics are colour of flowers, height of the stem, and colour of seeds in Pea plants, colour of skin in Mice, and colour of eyes and wideness of eyes in Humans.

It was noticed that some of the inherited characteristics do not follow the Mendelian laws and ratios. One of these inheritance mechanisms that contradict Mendelian laws is lack of dominance. In Antirrhinum plants, when a red-flowered plant was crossed with another white-flowered plant, the F1 individuals didn’t have red or white flowers, they had pink flowers. This means that neither the red colour of flowers dominates the white colour of flowers, nor the reverse. Each of the two genes that control the characteristic of the colour of flowers exerts its effect.

When the F1 generation individuals were self-pollinated, the F2 plants were red-flowered, pink-flowered, and white-flowered in the ratio 1 : 2 : 1

This case can be represented by symbols considering:

R : The gene for red flowers.

W : The gene for white flowers.

N.B. : Both the two genes are represented by capital letters because of the lack of dominance between the two genes.

Red-flowered White-flowered

Antirrhinum plant Antirrhinum plant

P1 RR X WW

       
   


G1

 
 



♂ ♀ R R
W RW RW
W RW RW

F1 RW


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