An introduction to Genetics
Who was the founder of genetics and how did he find out?
Gregor Mendel founded the ideas of modern genetics. He conducted many experiments with pea plants and realized that characteristics of parents were passed on to their offspring discretely through parts of DNA. He defined the unit of heredity as a gene and it is through this that traits are inherited.
Inside the nucleus of a cell, there is an acid known as deoxyribonucleic acid or DNA in short. This consists of some organic and inorganic compounds called bases. DNA is in fact like the English alphabet. It has 4 bases with the letters A, G, C and T. It is in the form of a spiraled ladder called a double helix. DNA contains all the genetic information about a creature. This had to be present in even the first cell. But yet in the higher organisms, the code is nearly intact. This code must have been passed from one cell to another till today. As organisms evolved, the code became more complex. This argument crushes the other theory and shows that Darwin was right.
What is a gene?
Small segments of DNA are known as genes. They occupy a specific position on the DNA strand called a locus.
Genes contain information about various parts of our body. Genes control traits of our body. For example, height, colour of our eyes, colour of hair, and innumerable others. Genes are also responsible for diseases like sickle cell anemia etc. This is because a gene does not contain the information necessary to do a certain function.
The two varieties of a single gene are called alleles. There are basically two alleles for every trait or characteristic. For example, as you saw in the animation, T and t are the two alleles that control the height of the plant. When a organism has two allele of the same kind, it is known as homozygous and when both the alleles are different, it is called heterozygous. From now on, we shall talk in terms of alleles.
The number of times a certain allele is present in a population or group of individuals is called its frequency. If allele A occurs in 40 people, its allele frequency is 40.
What are recessive and dominant alleles?
Suppose there are two alleles for height- 'T' and 't'. When a plant is tall, it has been found out that the 'T' allele is present. The presence of the other allele does not matter. This 'T' allele is then known as the dominant allele. The 't' allele's effect cannot be seen when the T allele is present. So 't' allele is called the recessive alleles. Only if both the alleles consist of 't', the plant will be short. To define both: A single allele whose effect can be seen is called a dominant allele. Alleles that can produce an effect when only in pairs are known as recessive alleles.
What are phenotypes and genotypes?
Observable characteristics of individuals are known as phenotypes. The shape of my nose is a phenotype. The genes carried by an individual, which govern a certain trait, are called genotypes.
The collection of all the genes of persons in a population is called a gene pool.
Mendel discovered the principles of inheritance during the late 1800's, but his results were forgotten until the early 1900's.Several observations were either made by Mendel or follow from his work.
1) The unit of inheritance is the gene.
2) Each individual has two alleles for each characteristic.
3) Each individual has a phenotype (what it looks like), and a genotype (what alleles it possesses).
4) Alleles can be either dominant or recessive.
5) Alleles segregate during mating. Each reproductive cell carries one allele of each of that parent's pair of alleles. The condition of having one allele for each characteristic is called haploid. A haploid sperm and a haploid ovum combine at reproduction toproduce a new diploid individual called a zygote.Each individual thus inherits one allele from mother and one allele from father.
6) Allele systems assort randomly. Whatever alleles you have for one characteristic should not influence what alleles you have for any other characteristic. For example, eye color has no connection with standing height, which is what Mendel's principle suggests.
