HEREDITY AND VARIATION

HEREDITY AND EVOLUTION

– Heredity – Variation – genetics

– Mendel’s factor – Chromosome – genes

– DNA

– Structure of Chromosome – Karyotype

– DNA

– Sex Determination

– Organic evolution–evidences – Theories of Evolution

•               In asexual reproduction, all the off springs are exactly identical to one another as well as to parents because of same genetic makeup.

•               In sexual reproduction the various off springs are similar to their parents but are not identical to items or to one another because of genetic various except IDENTICAL TWINS.

•               Identical Twins: grow from a single cell which splits after fertilization. Look alike

•               Non Identical twins: Look less like they grow from 2 different fertilized cells & hence don’t have exactly same genetic material.

•               Heredity: The transmission of characters (traits) from the parents to their off springs is called Heredity.

•               Variation: The differences in the characters among the individuals of a species is called variation is necessity for organic evolution.

Both Heredity & variation are fundamental factors in the process of Evolution of an organism.

•               Genetics: The branch of biology which studies heredity & variation is called genetic.

•               Terms gene was coined by scientist – Johanssen in 1909.

•               Chromosomes: Chromosomes are thread like structures present in the nucleus of a cell which contain hereditary information of the cell. They are made up of DNA & proteins.

		chromatids
		centromere
		Chromosome
2 chromatids	=	1 chromosome
One species	=	fixed no. of chromosome
Humans	=       46 chromosomes = 23 pairs of homologues chromosomes
			1

LECTURE NOTES – BIOLOGY                                                                         

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(Same size and Shape)

Types:

Metacentric	Sub–Metacentric	Acrocentric	Telocentric

•              Dipoid Cell: A cell which has the full no. of chromosomes with two of each pair, is called a diploid us.

•	Hiploid  Cell:  A  cell	c half  the  no.  of  chromosomes  with  one  of  each  pair  e.g.  gamete
	cell/sex cell.
•	Autosomes:  All   the   chromosomes   in   a   cell   (except   sex   chromosomes)   are   called
	autosomes.

•              Karyotype: During metaphase stage of cell division, the chromosomes become shorter, thicker & can be seen very clearly. In order to study the no. shape, size and type of chromosomes of an individual, the chromosomes during melaphase, are arranged acc to their length & position of centromere. Such an arrangement of chromosomes for the purpose of studying is called Karyotype. Identifying a chromosome thru microscope, special staming technique called “Banding Technique” is used.

Study of karyotype of an individual helps in the diagnosis of genetically disorders.

Down’s Syndrome: Any variation in the no. of chromosomes from the normal no or any change in the shape of chromosome during fertilization can lead to severe abnormality or disorder in the child after birth which are called Genetic / Heredity Disorder

e.g. Down’s syndrome which is caused by interitance of an extra 21^st  chromosome.

In this syndrome, full mental development does not take place.

Symptoms: Mental retardation, Flattened , nose, Defective ears, widely separated eyes, short & broad neck, protruding tongue, small & sturdy hands widely.

Gene: Gene is the unit of inheritance.

Various genes are located in the chromosomes at fixed position genes are responsible for our characteristic features.

Chemically gene is a segment of a large polynucleotide molecule called deoxyribonucleic acid

DNA: Dexyribo nucleic acid

DNA was first isolated by Frederick meisher from the nucleus of the pus cells. It is acidic in nature so the name is nucleic acid.

DNA is a macromolecule/polymer which is made up of a large no. of smaller units called Nucleotide.

So DNA is a polynucleotide. Nucleotide is the basic structure unit of DNA.

Components of a Nucleotide:		A
		Purines
1.	a nitrogen containg base
		G
2.	a pentose sugar
		C
3.	a phosphate gp
		Purimidiness
		T

Any one of nitrogen base pairs up only c a specific nitrogenous base of the opposite side. Such specific pairing is called COMPLIMENTARY PAIRING.

(Adenine) A pairs up  c T       (Thymine)

(Cytosome) C pairs up  c G   (Guanine)

DNA molecule is like that of a spiral staircase in which both the railing of the staircase are made of sugar & phosphate molecules alternative each other and held together by strong chemical bonds whereas the steps of the staircase are made of complimentary nitrogenous bases held together by weak bonds.

N.B.: DNA structure established by Wastson & Crick in 1953.

Each helical turn of the DNA molecule has a length of 3.4 nm in which 10 nucleotide units are present.

Pentose sugar

Phosphate gp

A – Adenine

T – Thymine

C – Cytosine

G –  Guanine

A	T	Complimentary parsing
B
	G

•               Genetic Engineering: The technique for altering an organisms genetic make up inserting genes from other organism its chromosomes is called Genetic Engineering.

•               Sex Determination: The process by which the sex of a per is determined is called sex determination.

The Chromosomes which determine the sex of a person called sex chromosomes which     are

of 2 types X & Y.

Males                            Female

X & Y                              X  X

XX                           XY      Zygote      (Male

Child)

(Female Child)

Q         XX combination is always found in females XY combination is always found in males.

So it is the sperm (carrying either X or Y) which determines the sex of a child

ORGANIC EVOLUTION

Evolution is the sequence of gradual changes which take place in the primitive organisms over millions of yrs. In which non species are provided.

Some imp. Sources which provide evidence for organic evolution are

•	Fossils	•	Homologus organs
•	Analogus organs	•	Vestigial organ

•              Embryology

•              Homologus organ: Organs having similar basic structure & origin but different functions e.g. porelnubs of a frog, birds & man.

•              Analogous organs: The organs with different basic structure but have similar appearance & performs similar functions.

e.g. wings of an insect & a bird.

•              Vestigial organs: Organs which are functionless useless but were functional in aucestors e.g. Vermiform appendix Nietuting membrame.

•              Embrgology: The study of the development of the embryo of an animal is called Embrgoloy.

•              Biogenetic law: It states chat “Ontogeny recapitulates Phylogeny”

It means that during the development of the embryo of any organism, its complete evolutionary history is repeated.

THEORIES OF EVOLUTION

1.      Lamarckism                 2.      Darwinism           3.      Synthetic Theory of Evolution

1.        Lamarckism: It states

(a)      The use & disuse of an organ by an organism leads to acquiring of variation in the features of that organ.

(b)      These variations (acquired characters) are inherited by the offsprings.

(c)      This leads to evolution

This theory was criticsed by August weismann who ejected the idea of inheritance of acquired characteristics.

DARWINISM / Theory of Natural Selection

(a)      All the species produce a large no of offsprings but population remains fairly constant due to struggle bt the members of same species & different species for food, space & mate

(b)      This struggle eliminates the unfit individuals. (Survival of the fittest)

(c)      This gives orgin to variations which pass into progeny & over a long period of time, leads to origin of new species.

Limitations: It could not explain how the variations arise. Synthetic Theory of Evolution:

The most accepted theory these a days which says that “origin of species is based on the interaction of genetic variation & natural selection’.