CBSE CLASS 10 CHAPTER 9 NOTES

 

CHAPTER 9

CBSE SCIENCE
CLASS 10

CHAPTER 9

HEREDITY & EVOLUTION

VARIATIONS

·        Variations arise during the process of reproduction

·        Variations – few in asexual reproduction

·        Many – in case of sexual reproduction

·        Minor variations – during asexual reproduction – due to inaccuracies in DNA copying

·        Major variations – during sexual reproduction – caused due to fusion of male and female gametes (DNA)

 

ACCUMULATION OF VARIATION DURING REPRODUCTION

·        Reproduction – produces offspring with similar body design of the parents

·        However, offspring – are not identical – show a great deal of variation from the parents – these variations get accumulated generation after generation

·        Depending on the nature of variations – different individuals would have different kinds of advantages

·        Beneficial variations – help species to survive better in the environment

·        Bacteria – withstand heat – will survive better in a heat wave

·        Nature selects – beneficial variations – leading to evolution

 

MENDEL’S EXPERIMENTS

·        Sexually reproducing organisms – humans – have two (or more) versions of genes for each trait – alleles

o   Eg: TT – homozygous dominant allele

o   Tt – heterozygous allele

o   tt – homozygous recessive allele

·        Gregor Johann Mendel – The father of genetics

·        Mendel carried out several experiments on pea plants

·        He used a number of contrasting visible characters of garden peas – round / wrinkled seeds, tall / short plants, white / violet flowers and so on.

 

MENDEL’S MONOHYBRID CROSS

·        Mendel selected pure bred tall & short plants

·        He cross-pollinated these plants

·        In F1 generation – he obtained only tall plants (tallness – dominant trait)

·        F2 generation was produced – by sending F1

·        3/4^th plants – Tall (75%) and 1/4^th plants – Dwarf (25%)

·        F2 generation – both tall and dwarf traits – present in the ratio 3:1

·        This shows – traits for tallness & dwarfness – present in F1 – but dwarfness (recessive) – does not express in the presence of tallness (dominant)

MENDEL’S DIHYBRID CROSS

·        Dihybrid cross – cross where two traits were taken into account – shape & colour of seeds

·        Cross between – round, green seeds & wrinkled yellow seeds

·        Yellow colour, round shape – dominant

·        Green colour, wrinkled shape – recessive

·        F1 generation – round & yellow seeds

·        Dominant traits (Round, yellow) –express itself – whereas recessive traits (wrinkled & green) – suppressed

·        On selfing of F1 generation – F2 generation shows phenotypic ratio 9:3:3:1

CONCLUSIONS GIVEN BY MENDEL

·        Mendel concluded – out of any pair of contrasting characters – one is dominant – the other recessive

·        Homozygous dominant trait – denoted by 2 capital letters – TT

·        Homozygous recessive trait – 2 small letters – tt

·        Genes controlling a particular trait – separate during gamete formation.

·        Each gamete will posses only one gene set

·        In crossing, if 2 or more traits are involved – their genes assort independently – irrespective of the combination present in the parent

 

HOW ARE PROTEINS CODED?

·        Genes in DNA – carry information for producing proteins

·        Proteins – control the various body characteristics

 

HOW DO GENES CONTROL THE CHARACTERISTICS?

·        DNA – source of information – making proteins in the cell

·        Section of DNA – gene

·        Example – the height of a plant – depends on the hormone

·        Amount of hormone – depends on the process of its formation – a protein is important for this process

 

HOW DO GENES CONTROL THE CHARACTERISTICS?

·        If this protein works efficiently – lot of hormone will be made

·        If gene responsible for that protein – has an alteration – it makes the protein less efficient

·        Amount of hormone is less – plant will be short

·        Genes control characteristics /traits

·        For a particular trait – offspring receives – one allele from father – one allele from mother

·        Combination of male & female germ cells – diploid zygote – thus, the normal diploid number of chromosomes – is restored in the offspring

 

SEX DETERMINATION

·        Different species use different strategies – for sex determination

·        It can be environmental cues or genetically determined

 

ENVIRONMENTAL CUES

·        Some animals – temperature at which the fertilized egg is kept – determines the sex

·        Snails – can change sex – Hermaphrodite

 

SEX DETERMINATION

GENETICAL CUE

·        Sex of human offspring – genetically determined

·        Humans – 22 pairs of autosome & 1 pair of sex chromosomes

·        Female – similar sex chromosome (XX)

·        Male – imperfect pair (XY)

·        X – normal size ; Y – short

·        Sex of the child – depends on whether egg fuses with sperm with X chromosome or sperm with Y chromosome

·        XX – Female child ; XY – Male child

EVOLUTION

·        Variations – occur during reproduction – both due to errors in DNA copy & as a result of sexual reproduction

·        Variations – gets accumulated generations after generations

·        Results in – a new species from already existing one with better survival advantages

·        This is called Evolution

 

VARIATIONS - ILLUSTRATION – CASE 1

CASE 1

·        Green beetles – got the survival advantage – they are naturally selected

·        This natural selection – exerted by crows – results in adaptation in the beetles – to be fit in the environment

 

NATURAL SELECTION

·        Selection of those variations in a population which give a survival advantage to fit their environment better

 

CASE 2

·        In a group of red beetles one beetle changes colour to blue

·        Blue beetle reproduces to form blue progeny

·        Crows can see both blue and red beetles & can eat them

·        As the population expands, there are a few blue beetles, but most are red

·        An elephant comes & stamps on the bushes

·        By chance, all the red beetles were killed & only few blue beetles survived

·        Beetle population slowly increased, but now the beetles in the population are blue

·        The colour change did not give any survival advantage

·        It is the accidental survival of one colour beetle (blue) that changed the common characteristic in a population

·        If the beetle population was large, the elephant would not have caused much damage

·        So, accidents in small population can change the frequency of some genes in the population. This is called genetic drift

 

·        GENETIC DRIFT is the flow of genes from one population to another by chance factor or randomly

·        Variation in the relative frequency of different genotypes in a small populations, owing to the chance disappearance of particular genes as individuals die or do not reproduce

·        We can conclude that accidents can change the frequency of some genes even if they do not get survival advantage. This is called as genetic drift and it leads to variation

 

CASE 3

·        Group of red beetles

·        Habitat of beetles suffer from plant disease (bushes)

·        Average weight of beetles decreases due to poor nourishment

·        Number of beetles kept on reducing

·        Later plant disease gets eliminated

·        Number and weight of beetle increases again

·        No genetic change has occurred in the population of beetle

·        The population of the beetle gets affected for a short period due to environmental change

·        This change in trait is called acquired trait (change that happen during one’s lifetime but not in their gene)

 

ACQUIRED & INHERITED TRAITS

 

EXAMPLE FOR ACQUIRED TRAITS

·        If we breed a group of mice, all the progeny will have tails

·        If the tails of these mice are removed by surgery in each generation, all their progeny will have tails

·        This is because removal of the tail cannot change the genes of the germ cells of the mice

 

SPECIATION

·      It is the evolution of reproductive isolation among once interbreeding population. i.e., the development of one or more species from an existing species

·        Speciation takes place when variation is combined with geographical isolation

·        MICRO EVOLUTION Evolution resulting from small specific genetic changes that can lead to a new sub species

FACTORS RESPONSIBLE FOR SPECIATION

·        Gene flow – must be stopped so that sub-population become genetically isolated

·        Gene flow – occurs between population that are partly or not completely separated

·        Mutations play a role in speciation

·        Genetic isolation – leads to formation of new species

·        Geographical barriers – mountain ranges, seas or rivers – produce barrier to gene flow – inability of organisms or their gametes to meet – leads to reproductive isolation – hence speciation

·        Sometimes crossing of two unrelated species can also lead to speciation

 

EVOLUTION & CLASSIFICATION

·          The fundamental characteristics used to classify organisms are

o      Presence of prokaryotic or eukaryotic cells

o      Whether organism is unicellular/ multicellular

o      Ability to perform photosynthesis (autotrophs)

o      Presence of endoskeleton / exoskeleton in heterotrophic organisms

 

HOW CLASSIFICATION IS RELATED TO EVOLUTION?

·          Classification of living organisms – is closely related to their evolution

·          The more characteristics two species have in common the more closely they are related

·          The more the number of characteristics shared by two organisms, more is the probability of them having common ancestors

·          Thus, classification of an organism is the reflection of its evolutionary path

·          As we go back in time – to trace common ancestors – we find – all organisms must have arisen & radiated from a single species – which in turn originated from non-living material – thus life arouse from non-living matter 

 

HOMOLOGOUS & ANALOGOUS ORGANS

 

FOSSILS

·        Fossils – preserved traces of living organisms – that got buried deep inside the earth – millions of years ago

·        For example: If a dead insect – get caught in hot mud – it will not decompose quickly – mud will eventually harden – retain the impression of the body parts of the insect – to form a fossil

 

DETERMINING THE AGE OF FOSSILS

·        Two methods – to determine the age of fossils

·        If we dig into the earth & start finding fossils – it can be assumed that the fossils closer to the surface are more recent – than those found in the deeper layers

·        By detecting the ratios of different isotopes of the same element in the fossil material (generally – carbon, called ‘Carbon Dating’)

IMPORTANCE OF FOSSILS

·        Fossils – very important in tracing the path of evolution

·        By studying characters of fossils – we can find their descendants

·        Thus, we can know what new variations have come

·        Fossils – provide evidence & missing links between the two classes

·        They are helpful in forming a sequence

 

EVOLUTION BY STAGES

·        Evolution is not a one step process, but a continuous process occurring in several stages

·        Complex organs – are formed slowly over many generations

 

EVOLUTION OF EYES

·        Complex organs like eye – has evolved from rudimentary organs – not by single DNA change – but created bit by bit over generation

·        Example: Rudimentary eye – Planaria – useful in detecting the light – to give only fitness advantage – comparatively human eye – form clear images

 

EVOLUTION OF WINGS

·         Sometimes the use of certain features gets modified with time

·        Example: Feathers provided insulation initially (protection from cold) – but later became associated with flight

·        Fossil archaeopteryx (bird-like dinosaur) – looks like a bird – with many features like reptiles – this gives a clue that birds have evolved from reptiles – later birds started using their wings for flight

 

ARTIFICIAL SELECTION FOR EVOLUTION

·        Wild variety of a plant – shows different variations

·        Humans have selected such variation and grown them for generations – during the course of time – they become totally different species

·        Example: Humans – carried out artificial selection for various features of wild cabbage to produce different vegetables

MOLECULAR PHYLOGENY

·        A method – used to trace evolutionary relationships

·        Help to compare the DNA of different species

·        If the differences in the DNA is greater – it implies the species are distantly related

 

EVOLUTION SHOULD NOT BE EQUATED WITH PROGRESS

·        Disappearance of the existing species is not a requirement for formation of new species

·        New species formed – better adapted to the environment – but need not be superior to the existing species

·        The common ancestor of humans & Chimpanzees – evolved in different ways to produce the present forms

·        Evolution – produces more diverse & complex body forms – over a period of time

·        But the newly formed species – are not more progressive than the already existing ones

·        So, it is wrong to say – evolution produces progressive higher forms from lower forms

·        Thus evolution should not be equated with progress

 

HUMAN EVOLUTION

·        All human beings, whether fair or dark skinned – belong to the same species – Homo sapiens – originated in Africa

·        Human ancestors – migrated from Africa to various parts of the world – Asia, Europe, Australia & America

·        They spread to different parts of the earth & adapted as best as they could to their environmental conditions