Pea plants

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The individuals of the F2 will be as follows:

P2 Pink-flowered pea Pink-flowered pea

AaBb X AaBb

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.

The Lethal Genes.

In some livings, some genes that control some characteristics when found in the homozygous state, lead to the retardation of growth and cause death to almost one fourth of the offsprings at different stages of life. These genes are referred to as lethal genes. Here is an example for these cases:

Sickle-cell Anaemia in Man:

Is a genetic disease that arises from genetic disorder in the gene responsible for Haemoglobin synthesis in the red blood cells. As a result, a change in the Haemoglobin molecule occurs. Consequently, the red blood cells become folded, and sickle-shaped. These abnormal red blood cells are unable to pick up Oxygen used in respiration. This leads in many cases to malfunctioning of the body, and finally to the death of the sick person.

If (S) represents the gene for normal Haemoglobin.