P1 tt X TT

G1

F1 Tt

Tall-stemed pea plants

100%

N.B.:

- The genotypes of the parents: tt and TT.

- The phenotypes of the parents: Short-stemed pea plant and tall-stemed pea plant.

- The genotype of all the progeny is Tt.

- The phenotype of all the progeny is tall-stemed pea plant.

Experiment 2:

When the individuals of F1 are left for self-pollination, fertilization takes place, and F2 plants are obtained. They are tall-stemed pea plants, and short-stemed pea plants in the ratio 3 tall-stemed : 1 short stemed as follows in the following 2 figures:

Tall-stemed pea plant Tall-stemed pea plant

P2 Tt X Tt

G2

TT Tt Tt tt

25 50 25

F2 Tall-stemed Short-stemed

Pea plants pea plants

3 : 1

Many experiments and researches continued the study of the inherited characteristics. Theses included animals, plants, and Man. It was found that Mendel’s first law is applicable to most of the inherited characteristics. The following represents the inheritance of some of these characteristic in animals and Man:

1. The inheritance of skin colour in Mice:

If a pure black-skinned male mouse is mated to a brown-skinned female mouse, the F1 generation will be all black-skinned. If the F1 individuals are left to mate freely among themselves, they produce the F2 which will be black-skinned, and brown-skinned in the ratio 3:1 (black colour of skin is dominant over the brown colour of skin in mice)

To express the previous results in symbols, let:

B : represents the factor for black colour of skin.

b : represents the factor for brown colour of skin.

The homozygous (pure) black mouse will have the genotype : BB

The homozygous (pure) brown mouse will have the genotype : bb

The heterozygous (hybrid) black mouse will have the genotype : Bb

2. The inheritance of some characteristics in Man:

Mendel’s first law can be applied to some Human characteristics such as hair (dark, and light), (curly, and straight), (dense, and light), the eye colour (brown, and blue), (wide, and narrow), eye-lashes (long, and short), skin colour (normal, and albino), ear lobules (free, and fused), and nose (pointed tip, and flat tip), (curved and straight).

Some abnormalities, and diseases such as deafness (recessive), flat-foot (dominant), high blood pressure (dominant), protrusion of the lower jaw (dominant), short fingers (dominant), memory loss (recessive), and idiocy (recessive).

3. The inheritance of eye-widness in Man:

If a homozygous wide-eyed man marries a narrow-eyed woman (pure because narrow-eyed characteristic is a recessive one), the individuals of the F1 generation will have wide eyes. If individuals having the same characteristic as those of F1 mate, some individuals of F2 will have wide eyes, while others will have narrow eyes in the ratio 3:1

To express the previous results in symbols, let:

W : represents the factor for wide eyes.

w : represents the factor for narrow eyes.

The homozygous wide-eyed person has the genotype : WW

The homozygous narrow-eyed person has the genotype : ww

The heterozygous wide-eyed person has the genotype : Ww

Mendel’s second law (law of independent assortement):

Mendel’s first law deals with the inheritance of one allelomorphic characteristic. Mendel in its second law studies the inheritance of two allelomorphic characteristics in the same time. This can be illustrated by studying the inheritance of colour of flowers, and height of the stem in pea plant as follows:

Mendel chose two homozygous pea plants one with pink flowers, and tall stems (two dominant characteristics), whereas the other with white flowers, and short stems (two recessive characteristics). When he crossed these two plants, he found that:

F1 generation : All the produced pea plants were pink-flowered and tall-stemed.

When he left the pea plants of the F1 generation to be self pollinated, he obtained:

F2 generation :

- Pink-flowered, and tall-stemed plants.

- Pink-flowered, and short-stemed plants.

- White-flowered, and tall-stemed plants.

- White-flowered, and short-stemed plants.

The ratio of the above mentioned individuals of F2 is : 9 : 3 : 3 : 1

Considering that:

- The characteristic of tall stems is symbolized as A and the short stems as a.

- The characteristic of pink flowers is symbolized as B and the white flowers as b

When these data were analyzed considering one characterisitic, it was found that:

1. In the first generation (F1):

All the plants were with pink flowers and tall stems, i.e. only the two dominant characteristics appeared.

2. In the second generation (F2):

a) The colour of flowers:

The number of pink-flowered plants was 12, whereas the number of white-flowered plants was 4, thus the ratio between them is 12:4 or 3:1

b) The height of the stem:

The number of the tall-stemed plants was 12, whereas the number of short-stemed plants was 4, thus the ratio between them is 12:4 or 3:1