LIVINGS
In
BIOLOGY
FOR
GENERAL SECONDARY CERTIFICATE
Unit (II)
Chapter (2)
The Inheritance in Livings
Gene Interaction
1.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
Pink-flowered
Antirrhinum plants
100%
Pink-flowered Pink-flowered
P2 Antirrhinum plants X Antirrhinum plants
RW RW
G2
RR
RW RW WW
Red Pink White
F2 flowered flowered flowered
plants plants plants
1 : 2 : 1
♂ ♀
|
R |
W |
R |
RR |
RW |
W |
RW |
WW |
Such non-Mendelian cases can also be exampled by the inheritance of colour in the Andalusian fowl. When crossing take place between black-coloured and white-coloured, F1 generation will have blue colour. While F2 generation will have the three colours black, blue, and white in the ratio 1 : 2 : 1
B : represents the gene for black colour.
W : represents the gene for white colour.
BB : Is the genotype for black-coloured fowls.
WW : Is the genotype for white-coloured fowls.
BW : Is the genotype for Blule-coloured fowls.
Illustrate the production of f1 & f2 generations genetically.
2. Complementary Genes.
When two strains of white-flowered Pea plants were crossed, all the flowers of F1 plants appeared pink. In F2 the flowers produced were 9 pink : 7 white.
This indicates the presence of two pairs of genes that control the flower colour characteristic.
If the dominant gene in one pair is represented by a
And the recessive gene in that pair is represented a
While the dominant gene in the second pair is represented by B
And the recessive gene in that pair is represented by b
The genotypes of the white-flowered pea plants (P1) would be (AAbb) and (aaBB). Then the genotype of F1 plants will be (AaBb) as follows:
White-flowered pea plant White-flowered pea plant
Of the 1st. strain of the 2nd. strain
P1 AAbb X aaBB
G1
F1 AaBb
Pink-flowered
Pea plants
100%
♂ ♀
|
Ab |
Ab |
aB |
AaBb |
AaBb |
aB |
AaBb |
AaBb |
The individuals of the F2 will be as follows:
P2 Pink-flowered pea Pink-flowered pea
AaBb X AaBb
♂ ♀
|
AB |
Ab |
aB |
ab |
AB |
AABB pink |
AABb pink |
AaBB pink |
AaBb pink |
Ab |
AABb pink |
AAbb white |
AaBb pink |
Aabb white |
aB |
AaBB pink |
AaBb pink |
aaBB white |
aaBb white |
ab |
AaBb pink |
Aabb white |
aaBb white |
aabb white |
G2
F2 : Pink : White
9 7
The appearance of the ratio 9 pink : 7 white emphasizes that this characteristic is influenced by two pairs of genes. It indicates the deviation from the Mendelian ratio 9:3:3:1 because the last three genotypes have the same phenotype.
In order for the colour of flowers to appear, the two pairs of genes must be present in the dominant state (whether the homozygous or the heterozygous). If only one pair is found in the dominant state, and the other pair in the recessive state or both pairs are found in the recessive state, the colour of flowers will be white. Thus, both pairs of genes participate in the colour production. Since, each of them controls the production of a certain enzyme which affects part of the steps of the pigment production. If one of the two dominant genes (A or B) was absent, the steps of pigment production will not be completed, and the flowers appear white.
An experimental confirmation:
The validity of the above explanation was confirmed experimentally when extracts of flowers from both strains were mixed together in a test tube, the pink colour appeared as a result of the presence of all the enzymes, and the completion of the reaction that produces the pigment.