HEREDITY

 

Higher Organisms  are 2n, means they are diploid. We have 23 pairs of chromosomes ie. 46 number of chromosomes. Out of 23 pairs or 46 number of chromosomes, 22 pairs or 44 number of chromosomes are called autosomes and one pair or 2 chromosomes are only sex chromosomes.

23 pairs of chromosomes an organism has 50% come from the mother side and other 50% come from the Father side.

Sexually reproducing organism produces gametes and the gametes are produced after Meiosis division or reduction division. Male gametes are called Sperms and female are called egg or ova.

Due to sexual reproduction both the both the gametes  ie. Sperm and an egg unite together to form Zygote. Zygote leads to the formation of an embryo or the future organism.

As the chromosomes come from two organisms to form zygote, therefore the organism that is formed shows variation .

Variation is a change due to which the species have equal chances of surviving in the environment they remain.

Chromosomes are the hereditary vehicles. It is long, slender thread like structure. Genes are present in the chromosome and it contains genetic materials. Chemical nature of gene is called DNA. Characters of father or mother that occurs in gametes are the DNA that is present there. Characters are also known by another name called Traits.

Gregor Johan Mendel is known as Father of Genetics .He did experiment on garden peas and blended his knowledge of science and mathematics to get the result of his experiment. He was the first person to give the information that the characters are transmitted from father to son. For doing his experiment  Mendel took 7 Pairs of contrasting characters. For example Height is a character which have two contrasting characters like Tall and Dwarf( Dwarf means short). When we say the person has height means he may be tall or short but it doesn’t mean that the height is tall.

With the garden peas Mendel conducted many experiment, which are known as  Monohybrid , Dihybrid  and Poly hybrid experiment. 

Monohybrid Experiment or Mono hybrid cross: For conducting the monohybrid experiment Mendel took one pair of contrasting character. One pair means mono and crossing them is the hybridization (or hybrid), the ratio of mono hybrid experiment is 3:1 .

Dihybrid Experiment or Dihybrid cross: For conducting the dihybrid Mendel took two pairs of contrasting characters. The ratio of Dihybrid cross is 9:3:3:1 .

From these above crosses Mendel formulated his famous laws, which are known as –

Law of Dominance, Law of segregation and Law of Independent assortment

Some Important Terminology:

Phenotype: Visible expression means can be seen, like tall or dwarf.

Genotype: Genetic constitution of individual, which cannot be seen. Like tall may be TT OR Tt. These  TT or Tt which are found through genetic study, known as Genotype.

Homozygous : The organism carrying similar alleles for a trait, eg TT or tt.

Heterozygous :  The organism carrying different  alleles for a trait eg Tt