NCERT Notes of Ch 9 Heredity and Evolution (Class 10th Science)

Heredity

Transmission of charactersitics from parent to offsprings is called Heredity. Study of Heredity was first undertaken by Mendel.

Inheritance

Inheritance is the process by which characters are passed from parent to progeny.

Genetics

The branch of biology concerned with the study of heredity and variation.

Classical Genetics

It is based on Mendel's work when chemicalfact about chromosome, DNA and genes were not known.

Molecular Genetics

It deals with chemical nature of genes and the way they replicate and being transmitted.

Homogygous

Condition in which alleles of gene at a given locus on homologous chromosomes are similar. Ex. TT

Heterogygous

Condition in which alleles of gene at a given locus on homologous chromosome are dissimilar. Ex. Tt

Allele

Genes which code for a pair of contrasting character. Alleles are different form of same gene. Ex. T and t

Gene

Functional unit of DNA. These are unit of inheritance.

Factors

Mendel called characters as factors. Mendel's factors are now called genes.

Gamete

These are sex cells. These carry hereditary information. These constitute link between one generation to an other.

Variation

Difference in the characters or traits among the individuals of a species is called variation .Variations occurs due to inaccuracies in DNA copying .

Accumulation of Variation

Variation arises due to inaccurate copying of DNA during reproduction. In sexual reproduction chance of accumulation of variation is more as here two gametes fuse to form zygote. These variations increase generation after generation and ultimately result to formation of new species.

Dominant gene

Gene which decides the appearance of an organism even in the presence of an alternative gene. Ex. T

Recessive gene

The gene that decides the appearance of an organism only in the presence of another identical gene. Ex. t

Genotype

Description that show genetic constitution of an organism is called Genotype. Ex. TT, tt, Tt, etc.

Phenotype

The characteristics that are visible in organism is called phenotype. Ex. Tall, Dwarf, yellow.

Chromosome

Thread like structure present in Nucleus. These are made up of DNA and protein.

Rules for inheritance of Traits or Mendel's laws of inheritance

As we know in sexually reproducing organisms progeny gets equal amount of genetic material DNA will influence the traits in progenies. Mendel was first to work out rules of such inheritance. Conclusions drawn by him after observing inheritance of traits for many generation are called Mendel's laws of inheritance.

As Mendel has given basic rules in Genetics so he is considered as father of Genetics.

True breeding plant

A plant is true breeding that undergone continuing self pollination, shows stable traits inheritance.

Monohybrid Cross

A cross made to study one pair of contrasting character. Ex. Cross between tall and dwarf.

Dihybrid Cross

A cross made to study two pairs of contrasting characters. Ex. Yellow and wrinkled seeds.

G.J. Menel

Born in Austria in 1822. He studied science and Mathematics in University of Vienna. He conducted hybridization experiments on pea plant and by interpreting results proposed laws of inheritance.

Monohybrid Cross

In this hybridization he crossed True breeder Tall and Dwarf plant to study inheritance of one pair of contrasting character. When he grew seeds of first cross he found in F1 all plants were tall. Now Mendel let the F1 plant to self pollination. The seeds developed after this are F2. When they were grown it was found that 1/4 of the plants were dwarf while 3/4 were tall. Bussed on his observations on monohybrid cross Mendel proposed two general rules now hybrid cross Mendel proposed two general rules now called principles of inheritance.

(i) Law of Dominance

He used law to explain the expression of only one of the parental traits in F1. As all plants were tall. He called trait expressed in F1 as dominant while trait not expressed as recessive.

(ii) Law of Segregation or law of purity of gametes

Progenies found in F2 of the monohybrid cross result him to propose this law. He found F2 progenies in 3:1 between tall and dwarf indicated that traits were not blended but masked in presence of dominant allele but expressed when present in homozygous condition i.e. tt

Dihybrid Cross

When Mendel made cross between pea plants with two pairs of contrasting characters i.e. Round Yellow and Wrinkled Green. He observed that in F1 all plants found Round Yellow. On self cross between F1 progenies he found phenotypes Round Yellow, Wrinkled Yellow, Round Green and Wrinkled Green in ratio 9:3:3:1. This observation led him to propose the generalization now called Mendel's law of independent assortment. The law states that when two pairs of traits are combined in a hybrid segregation of one pair of traits are independent of other pair of traits.

Evolution

The gradual process by which the present diversity of plants and animals life arose from earliest primitive organisms. Before the theory of evolution by Natural selection and Darwin's origin of species it was thought that each species was divinely created. Theory by Darwin and Wallace was backed by evidence from fossils records and studies of comparative anatomy.

It is considered that evolution is the result of variation due to error during D.N.A copying and sexual reproduction as well as natural selection of these variations.

Genetic drift

The random change in the frequency of alleles in a population over successive generations due to loss some allele from the gene pool. It may be due to sampling error in gamete formation.

Gene flow

The exchange of genetic material by interbreeding between populations of same species or between individuals of a population.

Gene pool

All the genes and their different alleles that are present in a population of species of organism.

Acquired and Inherited traits

Acquired traits or characteristics are those that are developed during the lifetime of an individual e.g. low weight of beetle due to starvation. These traits cannot be inherited.

Inherited traits are those that are present in genetic material of the individuals in form of genes.

Species

A group of individuals that can interbreed and produce fertile offspring.

Speciation

The development one or more species from an existing species. It occurs when groups of similar population diverge so much from the parent population that they can no longer interbreed with them. This may occur due to physical, physiological or behavioral change. Geographical isolation and genetic drift along with natural selection may result into speciation.

Evolution and Classification

Classification is grouping of organisms on the basis of some characteristics. The more characteristics two species will have common the more closely they are related and more recently they will have common ancestors. This way the classification of organisms infact a reflection of their evolutionary relationship.

Homologous organ

Organs with similar basic structures but having different function. Ex. Wings of Bat and Arms of Human, limbs of birds, Reptiles frogs are homologous organ.

Analogous organ

Organs without similar basic structure but similar function are considered Analogous organ. Ex. Wings of Bats and birds or wings of bird and butterfly.

Dating Technique

Methods of estimating the age of rocks, palentological specimens, archaeological sites, etc.

Radio Carbon dating or Carbon dating

A method of estimating the age of archaeological specimens of biological origin. Living organisms contain 14C and 12C atoms in fix ratio. When they become dead this ratio falls due decay of radioactive carbon (14C). By measuring this ratio we can estimate the age of the specimen.

Fossils

The remains or traces of any organism that lived in the geological past. Generally hard part like bones, teeth are preserved but some time entire organism is preserved. Fossils provide direct evidence for evolution.

Age of Fossils

There are two ways through which we can know the age of a fossil—

(i) More deeper a fossil is found in the earth more older it will be .

Darwin theory of Evolution

Charles Robert Darwin gave theory of evolution in his book ‘ The origin of species ’. It is also called theory of Natural selection. According to him populations of organisms have naturally occurring variations which are selected by Nature due to their suitability and leads to evolution by origin of new species.

Synthetic theory of evolution

It states that variations which result into origin of species are result of genetic variation and these ultimately leads to natural selection.

Evolution by Artificial selection

By artificial selection cabbage, broccoli, cauliflower, kohlrabi, kale and are obtained by formers from wild cabbage.

Evolution and Progress

We can not equate evolution and progress. Evolution is simply generation of diversity and shaping of it by environmental selection. Progressive trend that found in evolution is formation of complex form. Although complex forms are formed but simpler forms are also present.