short notes on "HEREDITY " by Nagraj Sir

short notes on "HEREDITY" by Nagraj Sir

Introduction

Heredity is the passing of traits from parents/ancestors to offspring. Heredity occurs through inheritance of genes. Biological inheritance is what is related to heredity.

Study of heredity is termed as Genetics.

Inheritance from a previous generation provides a common basic body design and few changes in it for the next generation. A trait that is genetically passed down from one generation to another is termed as ‘Inherited trait’.

Some of the common examples of inherited traits are hair colour (Black, brown, red etc.), eye colour(Black, brown, blue), height (tall, short), Ear lobes (free, attached)

 Variation : It means the differences in the characters or traits among the individuals of a species. Variations occur during reproduction both because of error in DNA copying and as a result of sexual reproduction. Variations contribute to evolution.

Causes of variations:

1.      Different combinations of genetic material.

2.      Some positive gene mutations.

3.      Interaction of genes with environmental changes (adaptations).

Importance of variations:

1.      It forms, the. basis of heredity.

2.      It causes adaptations due to which organism can easily adjust to its changing environment.

3.      Accumulation of variations forms the basis of evolution.

Mendel and His Work on Inheritance

• Gregor Johann Mendel (1822 & 1884) : Started his experiments on plant breeding and hybridisation. He proposed the laws of inheritance in living organisms. Mendel was known as Father of Genetics.

·         Plant selected by Mendel : Pisumsativum (garden pea). Mendel used a number of contrasting characters for garden pea.

MONOHYBRID CROSS

OR

MONOHYBRID INHERITANCE

 OR

 LAW OF SEGREGATION

Cross between two pea plants with one pair of contrasting characters is called a monohybrid cross. Example : Cross between a tall and a dwarf plant (short).

Observations of Monohybrid Cross

(i) All F1 progeny were tall, no medium height plant. (Half way characteristic)
(ii) F2 progeny 1⁄4 were short, 3⁄4 were tall.
(iii) Phenotypic ratio F2 – 3 : 1 (3 tall : 1 short)

      Genotypic ratio F2 – 1 : 2 : 1 – (TT : Tt : tt)

Conclusions

1. TT and Tt both are tall plants while tt is a short plant.
2. A single copy of T is enough to make the plant tall, while both copies have to be ‘t’ for the plant to be short.
3. Characters/traits like ‘T’ are called dominant trait (because it express itself) and ‘t’ are recessive trait (because it remains suppressed).

DIHYBRID CROSS

OR

DYHYBRID INHERITANCE

OR

LAW OF INDEPENDENT ASSORTMENT

A cross between two plants having two pairs of contrasting characters is called dihybrid cross.

Phenotypic Ratio

Round, yellow : 9
Round, green : 3
Wrinkled, yellow : 3
Wrinkled, green : 1

Observations

(i) When RRyy was crossed with rrYY in F1 generation all were Rr Yy round and yellow seeds.
(ii) Self pollination of F1 plants gave parental phenotype and two mixtures (recombinants round yellow and wrinkled green) seeds plants in the ratio of 9 : 3 : 3 : 1.(dihybrid ratio)

Conclusions

1. Round and yellow seeds are Dominant characters.
2. Occurrence of new phenotype combinations show that genes for round and yellow seeds are inherited independently of each other.

Sex Determination

Factors responsible for Sex Determination are:

• Environmental : In some animals, the temperature at which the fertilized eggs are kept decides the gender. E.g., in turtle
• Genetic :In some animals like humans gender or individual is determined by a pair of chromosomes called sex chromosome.

XX – Female
XY – Male

Sex Chromosomes : In human beings, there are 23 pairs of chromosome. Out of these 22 chromosomes pairs are called autosomes and the last pair of chromosome that help in deciding gender of that individual is called sex chromosome.

XX – Female
XY – Male