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):

1. 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

2. 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

Several other studies and experiments carried out on the inheritance of other pairs of alleles of other characteristics indicated that they all behave the same way.

It is also true that if two homozygous individuals differing in more than two allelomorphic characteristics were crossed, each characteristic is inherited independently. Mendel deduced his second law which is known as (law of the independent assortment of the genetic factors) which states that:

When two homozygous individuals differing in two (or more) pairs of alleles are crossed, each pair of characteristics is assorted at random, and is inherited independently of the other, and will appear in the F2 generation in the ratio 3:1

The test cross:

In one of his experiments, Mendel crossed white-flowered Pea plants with others carrying the same characteristic (white flowers), the progeny were all with white flowers. This indicated that the cells of these flowers are homozygous (pure) for this characteristic. He arrived to the conclusion that: The individual that carries the recessive characteristic is surely pure for that characteristic:

White-flowered pea plant White-flowered pea plant

P1 rr X rr

G 1

♂ ♀	r	r
r	rr	rr
r	rr	rr

F1 rr

White-flowered pea plants

100%

When he crossed pink-flowered pea plants with white-flowered pea plants, the F1 generation resulted give one of the two following results: