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Normal-haired mother Bald-headed father

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  1. D. Describe on genetic basis the genotypes, and phenotypes of F1, and F2 generation on mating a bald man (homozygous) with normal-haired woman (homozygous).
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F1 Bald-headed male Bald-headed male and Healthy male

And hair fall female normal hair female and female

1 : 2 : 1

Inheritance of Twins

Studying of twins is very important to geneticists, as through that it is possible to study the effect of the environmental factors on livings. Some animals are capable of producing twins continuously in large numbers. Twins production is relatively rare in Man.

Twins may be identical (Monozygotic twins) when they originate from one ovum, in this case each individual carries the same genes and both are of the same sex. Twins may also be fraternal (Dizygotic twins) when they originate from the fertilization of two or more ova with more sperms. In this case, the embryos may differ in their sex and other traits. The resemblance between fraternal twins does not exceed the resemblance between brothers and sisters.

Identical twins are important in studying the effect of the environment on living organisms. In this case, the researcher is quite sure that the genetic factors in both individuals are exactly the same. If one of the identical twins is exposed to different environmental conditions, variable changes will occur between them in behavioural and physical characteristics such as weight and height. But the colour of the hair, skin, eyes, and other traits will not be affected. This was proved empirically by observing identical twins exposed to disasters such as wars and earth quakes that separated them during their childhood in different environments. When these twins met afterwards by chance, the for mentioned differences were seen.

Inheritance and the Environment

The environment has an effect on the genes and their characteristics. This was confirmed experimentally on both plants and animals:

When seeds germinate in the dark, no chlorophyll is formed in the seedlings. If these seedlings were transferred into the light, chlorophyll will be formed in a few days. Thus, the plastids which are already present in the seedlings cells need light in order that the gene responsible for chlorophyll production exerts its effect. Thus, the appearance of any characteristic in a living organism depends first on the presence of its gene, and secondly on environmental conditions that are suitable for the expression of this gene.

In the Himalayan strain of rabbits, the feet, tail, ear, and nose are always black, whereas the rest of the body is white. It was discovered that the permanent exposure of these parts to the coldness had led to this black colour. This was verified by an experiment, when the hair was removed from the back of the rabbit and a piece of ice was tied over it and kept for a long period, the hair underneath grew black in colour, while the rest of the hair remained white.

Another example to prove the effect of heat is found in Drosophila, when this insect was raised at a low temperature, by exposing both eggs, and larvae to 16◦ C, the wings (after metamorphosis of the insect from the pupa) grew in a strtaight manner. If these stages were exposed to higher temperatures, say 25◦C, the wings grew in a curved manner.


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