The rules for inheritance traits in human beings are related to fact that both the father and the mother contribute practically equal amounts of genetic material to the child. This means that each trait can be influenced by both paternal and maternal DNA, for each trait, there will be two versions in each child.

What will, then, the trait is seen in the child be? Mendel worked out the main rules for inheritance of traits, and it is interesting to look at some of his experiments from more than a century ago. Mendel used a number of contrasting visible characters of garden peas � round/wrinkled seeds, tall/short, plants, white/violet flowers, and so on.

He took pea plants with different characteristics � a tall plant and a short plant, produced progeny from them, and calculated the percentages of tall or short progeny. In the first place, there were no halfway characteristics in this first generation, or F1 progeny � no �medium-height plants.

All plants were tall. This meant that only one of the parental traits was seen, not some mixture of the two. So the next question was, were the tall plants in the F1 generation exactly the same as the tall plants of the parent generation?

Mendelian experiments test this by getting both the parental plants and these F1 tall plants to reproduce by self-pollination. The progeny of the parental plants are, of course, all tall.

However, the second-generation, or F2, the progeny of the F1 tall plants are not all tall. Instead, one-quarter of them is short. This indicates that both the tallness and shortness traits were inherited in the F1 plants, but only the tallness trait was expressed. Thus, two copies of the trait are inherited in each sexually reproducing organism.

These two may be identical or may be different, depending on the parentage. The pattern of inheritance can be worked out with this assumption, which is as shown. In this explanation, both TT and Tt are tall plants, while the only tt is a short plant.

In other words, a single copy of �T� is enough to make the plant tall, while both copies have to be �it's for the plant to be short. Traits like �T� are called dominant traits, while those that behave like �t� are called recessive traits.

However, the question arises? What happens when pea plants showing two different characteristics, rather than just one, are bred with each other? What do the progeny of a tall plant with round seeds and a short plant with wrinkled seeds look like? They are all tall and have round seeds.

Tallness and round seeds are thus dominant traits. But what happens when these F1 progenies are used to generate F2 progeny by self-pollination? A Mendelian experiment will find that some F2 progeny are tall plants with round seeds, and some were short plants with wrinkled seeds. However, there would also be some F2 progeny that showed new mixtures.

Some of them would be tall, but have wrinkled seeds, while others would be short, but have round seeds. Thus, the tall/short trait and the round seed/wrinkled seed trait are independently inherited. How do these Traits get Expressed? How does the mechanism of heredity work? Cellular DNA is the information source for making proteins in the cell.

A section of DNA that provides information for one protein is called the gene for that protein. How do proteins control the characteristics that we are discussing here? Let us take the example of tallness as a characteristic.

We know that plants have hormones that can trigger growth. Plant height can thus depend on the amount of a particular plant hormone. The number of plant hormones made will depend on the efficiency of the process for making it. Consider now an enzyme that is important for this process. If this enzyme works efficiently, a lot of hormones will be made, and the plant will be tall.

If the gene for that enzyme has an alteration that makes the enzyme less efficient, the amount of hormone will be less, and the plant will be short. Thus, genes control characteristics or traits. If the interpretations of Mendelian experiments we have been discussing are correct, then both parents must be contributing equally to the DNA of the progeny during sexual reproduction.

We have discussed this issue in the previous chapter. If both parents can help determine the trait in the progeny, both parents must be contributing a copy of the same gene. This means that each pea plant must have two sets of all genes, one inherited from each parent. For this mechanism to work, each germ cell must have only one gene set.

However, the question arises? How do germ cells make a single set of genes from the normal two copies that all other cells in the body have? If progeny plants inherited a single whole gene set from each parent, then the previous experiment cannot work.

This is because the two characteristics �R� and �y� would then be linked to each other and cannot be independently inherited. This is explained by the fact that each gene set is present, not as a single long thread of DNA, but as separate independent pieces, each called a chromosome. Thus, each cell will have two copies of each chromosome, one each from the male and female parents.

Every germ cell will take one chromosome from each pair and these may be of either maternal or paternal origin. When two germ cells combine, they will restore the normal number of chromosomes in the progeny, ensuring the stability of the DNA of the species.

Such a mechanism of inheritance explains the results of the Mendel experiments and is used by all sexually reproducing organisms. But importantly asexually reproducing organisms also follow similar rules for inheritance of traits.

Read More: Evidence of Evolution: Morphological, Anatomical & Embryological