A gene is the section of DNA on a chromosome which codes for the formation of a protein controlling a specific characteristic (or trait) of the organism. Suppose a plant progeny has gene for the characteristic called ‘tallness’. Now, the gene for tallness will give instructions to the plant cells to make a lot of plant growth hormones.
And due to the formation of excess of plant growth hormones, the plant will grow much and hence become tall. On the other hand, if the plant has the gene set for dwarfness, then less plant growth hormones will be produced due to which the plant will grow less, remain short and hence become a dwarf plant.
Just like plants, the characteristics (or traits) in animals (including human beings) are also transmitted from the parents through genes by the process of sexual reproduction. We will now give an example of the transmission of colour of hair from the parents (father and mother) to the child. Before we do please keep in mind that black hair is a phenotype produced by the genotype HH or Hh. On the other hand, blonde hair (pale yellow hair) is a phenotype produced by the genotype hh. Let us give the example now.
A mother has black hair, the father has blonde hair (pale yellow hair), and the child has black hair. This can be explained on the basis of transmission of genes for ‘hair colour’ from the mother and father to the child as follows: Mother’s cell contains two genes HH for black hair. Both the genes HH are dominant genes, so the mother has black hair. Father’s cells contain two genes (hh) for blonde hair.
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The two genes hh are recessive genes, so the father has blonde hair (or pale yellow hair). Now, during the process of reproduction, the mother transmits one of the dominant genes H for black hair to the child and the father transmits one of his recessive genes h blonde hair to the child. Due to this, the child has the genes Hh for her hair.
Now, the gene H for black hail is the dominant gene but the gene h for blonde hair is the recessive gene. The dominant gene H for black hair shows its effect due to which the child has black hair.
The recessive gene h for blonde hair cannot show its effect in the presence of dominant gene H for black hair. Please note that the genes which dominate other genes are called dominant genes, and the genes which get dominated are called recessive genes.
We will now describe the inheritance of blood groups by the children from their parents. Please note that the gene which controls the blood groups is represented by the letter I. This gene has three different forms (called alleles) which are represented as IA, IB and I°.