2.1 Dominant & Recessive Genes

We have so far learned some basic concepts and defined some genetic terms. There are more basic terms to be defined but we will skip them for now and move on to chapter 2 of this series. We will introduce other basic definitions as and when required.

We have introduced the DNA as the building plan for the living organism. We have defined the gene as a section of the DNA that controls a feature of the organism. We will now shift our focus to the genes and see how they are inherited by the offspring from the parents. We will also see how the genes from the father and the mother combine to form genes in the offspring.

For most genes there are two copies of the gene in the DNA. These two copies may be different or they may be the same. If they are the same we represent them with a repeated letter such as AA if they are different we represent them with two different letters AB. Let’s make this a little more realistic and talk about the two most common colors in budgerigars green and blue. We will represent the two copies of the color gene in the green budgerigar with letters GG. We represent the two copies of the color gene in the blue budgerigar with the letters BB. What happens when these two birds are bred together? Is the offspring Blue? Is it Green or somewhere in between? Let’s first look at what the genes tell us. The green parent has two copies of the gene represented by GG. We learned in inheritance that each parent contributes half its DNA in such a way that one half of each gene goes into the offspring. The green parent therefore contributes one copy of its gene G. Similarly the blue parent contributes one copy of its color gene B. Now we know that the offspring’s color genes look like GB. Now that we have figured what the genes look like we need one more piece of information to determine what the bird will look like. Which gene is dominant and which is recessive.

Dominant and recessive relationships
If two copies of the gene are different from each other one may act like a trump card and completely hide the effect of the other. In our example above, the offspring of the blue and green birds looks as green as its green parent. Visually, there is no way of telling if the pair of genes carried by a green bird is GG or GB. If a copy of the gene dominates in such a way over the other we say that it is dominant. The gene that gets hidden is called recessive.

We will discuss dominant and recessive relationships further with more examples in the next episode.

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