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Concept Building : Introduction to Genetics


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Before explaining the theories of genetics, few important terms and ideas must be known to us.

1. How different characteristics among members of a species varies?
2. How these characteristics transmitted from one generation to the next?
3. Why offspring of same parent are not exactly same but obviously have few similarities?
Such endless questions can be explained and solved by genetics. The search for answers is still going on.
These processes lead us to the following question :

What are Inheritance, Genetics and Gene ?

Inheritance : Biologically, inheritance is the mechanism by which chromosomes are transmitted from one generation to next generation. Each offspring acquire new genetic combinations (recombination) from both parents by the process of genetic inheritance .
As we can see, inheritance needs the understanding of the term genetics. Therefore, let us explore the meaning of Genetics.

Genetics : Scientific study, which deals with biological inheritance, genes, genetic regulation, genotypic and phenotypic relationships, genetic variation, hereditary characteristics etc. is termed as genetics.
In short, we can say it is the study of genes. So, the obvious question is :

Gene : A stretch of nucleotide sequences on DNA which codes for RNA transcript (m-RNA, t-RNA, r- RNA, sn- RNA or other RNAs) for expression and transmission of hereditary characteristics. When we dive into the discussion of DNAs sand RNAs then the acid we would like to talk about is Nucleic acid.

Nucleic Acid : These are biological macro-molecules consisting of monomeric nucleotides. Nucleic acids are of 2 major types- DNA and RNA (Detailed explanation about DNA and RNA will be on a another note).
In Eukaryotic organisms cellular DNA is present as chromatin fiber within nucleus and during cell division they become highly condensed structures called chromosomes. So,

What are chromosomes ?

DNA and protein made distinct structures are responsible for transmission of hereditary characteristics from one generation to next.
During cell division these structures are known as chromosome while during interphase they remain as relaxed structure known as chromatin fibre.

Types of chromosomes

1. Homologous chromosomes
2. Non-homologous chromosomes

A pair of chromosomes with similar structure, similar number and type of genes (obviously not the same alleles) at the same locus and each of them inherited from each parent is called homologous chromosome. Homologous chromosomes are in same shape, size, gene order.    

A pair of chromosomes of a diploid cell having different shape, size, gene number and type is called non- homologous chromosome of each other.

Homologous and Non-homologous chromosomes

Homozygous and heterozygous condition of chromosomes

At a particular locus of homologous chromosome pair, both have same number and type of genes but both the alleles of the same gene may be same or of alternative type.
If at the same locus of homologous chromosome pair, same kind of alleles are present, then such a condition is called homozygous condition
On the other hand, at the same locus of homologous chromosome pair having different alleles of the same gene is termed as heterozygous condition.

We can understand this with a simple example :

Let flower colour of a plant is expressed by gene B. Let us consider, ‘B’ allele is responsible for red flower whereas ‘b’ allele is responsible for white flower.
In such a condition BB and bb condition is referred as homozygous, on the other hand Bb and bB condition is known as heterozygous.

Photo Credit : Courtsey Byju’s – The Learning App

What is an allele ?

Alternative forms of a gene is termed as allele.
Let us consider height is a character which is regulated by the height gene. Let the height gene has 2 alternative phenotypic expressions namely tall and dwarf, if these phenotypes are expressed by T allele for tall height and t allele for dwarf then, we can say that height gene has two alleles T and t.

Phenotype Gene Homozygous Heterozygous
Tall Height TT Tt
Dwarf Height tt
Autosome, sex chromosome and hemizygous condition

Chromosomes related with somatic characteristics (except sexual characteristics) are called autosomes.
Chromosomes which bears genes for controlling sexual characteristics are called sex chromosomes.
In human beings, 23 pairs of chromosomes present in each cell among them 22 pairs are autosomes and 1 pair is sex chromosome.
In human males X and Y chromosomes are present as sex chromosome. Very limited part of these two chromosomes are homologous and maximum parts are non-homologous to each other. This non-homologous region bears different genes. This condition of sex chromosome pair is called hemizygous condition.

What is Locus ?

Any exact location in terms of nucleotide pair or centimorgan unit of a chromosome is called locus.

What is Crossing over ?

The exchange of genetic materials between non-sister chromatids of homologous pair of chromosomes are referred as crossing over.

Crossing Over | Photo Credit – https://www.genome.gov/genetics-glossary/Crossing-Over

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Published on April 6th, 2020 | by Prakash Nandi


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