Heredity & Variation

Heredity & Variation

Evolution: is the orderly change of various forms through a slow but continuous process. It give rise to a new complex body design even while the simpler body designs continue to flourish.

Heredity: The transfer of characters or traits from the parents to their off springs is called heredity.

Variation: The differences between the characters or traits among the individuals of the same species are called variations.

Importance of variation:

1. They enable the organism to adapt them in changing environment.

2. Variation forms the basis of heredity

3. They form raw material for evolution and development of new species. Variations may or may not help organisms to survive:

a) Some variations help organisms to survive :-

Eg :- There are some beetles living in green bushes. They increase their numbers by reproduction. Crows can easily see the red beetles and they are eaten by the crows. During reproduction due to some variation some green beetles are produced instead of red beetles. The green beetles are not visible to crows and are not eaten by them. Then gradually the population of the red beetle’s decreases and the population of the green beetles increases. This variation has helped the organisms to survive.

b) Some variations do not help organisms to survive :-

Eg :- During sexual reproduction a colour variation occurs in red beetles and some blue beetles are produced instead of red beetles. Both the red and blue beetles are visible to crows and are eaten by them. Then the population of both red and blue beetles decreases. This variation has not helped the organisms to survive

c) Aquired traits cannot be passed from one generation to the next :-

Eg :- If the population of beetles increases and plants are affected by diseases, then the food available for the beetles decreases and their body weight also decreases. If after a few years the availability of food increases then the body weight of the beetles also increases. This acquired trait cannot be passed from one generation to the next because there is no change in their genetic composition.

Accumulation of variations during reproduction:

When organisms reproduce, the off springs show minor variations due to inaccuracies in DNA copying.

These variations are less in asexual reproduction and more in sexual reproduction.

Since DNA copying cannot be possible 100% always, DNA copying always involves some variations; thus it will bring a subtle change in it for the next generation. If this reproduces the newborn will have differences that they inherit from the first generation, as well as some newly created differences. Therefore variations gets accumulated generation after generation resulting in Speciation i.e. Evolution.Some variations are useful variations and they help the organism to adjust to the changes in the environment. Some variations do not help the organism to adjust to the changes in the environment and they may die and become extinct.Selection of variants by environmental factors forms the basis of evolutionary processes.

Heredity & variation in Asexual Reproduction: In asexual reproduction, organisms raised are the exact copies of their parents. They tend to preserve the similarities among all the individuals belonging to a given line of descent/species.They exhibit very little variations due to some environmental factors or mutations which are sudden change in genes. Out of these two factors only mutations are inheritable

Heredity & variation in Sexual Reproduction: In sexual reproduction two parents are involved and there is formation & fusion of gametes. The offspring show variations from their parents due to crossing over and exchange of gene segments. They are not carbon copy of their parents, due to recombination of parental genes; so variations which occur are inheritable.

Inherited traits: The Traits / Characters which transfers from parents to offspring generation after generation

Acquired Traits: It is a particular characteristic that is developed during lifetime of an individual.

such characteristics are not genetically controlled and cannot passed on to next generation.

Rules for inheritance of characters (traits) :-

Characters are transferred through genes present in the DNA molecules in the chromosomes present in the nucleus of the cell.

The inheritance of characters is due to the fact that both the father and mother contributes equal amount of genetic material to the child. So for each trait there are two factors one from the father and one from the mother.

Gregor Johann Mendel conducted experiments with garden pea plants and determined the rules for the inheritance of traits.

Gregor Johann Mendal: Was an Austrian Geneticist and regarded as ‘father of Genetics’.

He performed his experiment on Garden peas (Pisum sativum) during 1856 to 865 and published his results in a book ‘Experiments on plant hybridisation’. Mendal works remain unrecognised till his death in the year 1884.

Rediscovery of Mendal work: in 1900 by 3 scientist’s separately-

1) Hugo de vries (Dutch)- on Evening primerose (Oenothera lamarkiana)

2) Carl Correns(German)- on 4’O clock plant(Mirabilis jalapa )

3) Teschermack(Austrian)- on Garden pea. (Pisum sativum)

Mendal Experiment on Inheritance of Traits: Mendal used a number of contrasting visible

characters of garden pea like:

Sl.No.	Charecters	Dominant	Recessive
1.	Height	Tall	Dwarf / Short
2.	Colour of flower	Red	White
3.	Position of flower	Axial	Terminal
4.	Shape of pod	Inflated	constricted
5.	Colour of pod	Green	Grey
6.	Shape of seed	Round	Wrinkled
7.	Colour of cotyledons	yellow	Green

IMPORTANT TERMS:

^ØMendal called ‘gene’(by Johensen) - Factor

^ØAllele: Two alternate forms of a factor which control the contrast in character.

eg- ‘T’ and ‘t’ are allelers of factor controlling height.

^ØF1 generation: First filial generation.

^ØF2 generation: Second filial generation

^ØDominant Trait: The allele which is expressed in the hybrid is called dominant.(T)

^ØRecessive Trait: The allele which remain supressed is called recessive. (t)

A single copy of dominant allele(T) is enough to show dominant character, while both copy must present to express the recessive character.

^ØGenotype: the genetic make-up is called genotype.

^ØPhenotype: The external morphology is called phenotype.

^ØGenotypic ratio: the ratio of all the genotype in a population.

^ØPhenotypic ratio: the ratio of all the phenotype in a population.

Monohybrid cross:- The cross between the plants having only one pair of contrasting character (Eg:- tall and short plant) is called monohybrid cross.

Mendel selected pea plants having one pair of character – a tall pea plant and a short pea plant. He selected pure tall (TT) and pure short (tt) pea plants and cross pollinated them. He obtained all tall plants (Tt) in the first generation (F₁ ). When the first generation plants were self pollinated, he obtained tall and dwarf plants in the ratio 3:1 (Phenotypic Ratio) in the second generation. (F₂)

The ratio of pure tall (TT), hybrid tall (Tt) and pure dwarf (tt) / Genotypic Ratio was in the ratio 1:2:1

Dihybrid cross:- The cross between the plants having two pairs of contrasting characters (Eg:- shape and colour of seeds) is called Dihybrid cross. Mendel selected pea plants having two pairs of characters – shape and colour of seed. He selected plants having round yellow seeds (RRYY) and wrinkled green seeds (rryy) and cross pollinated them.

He obtained all plants with round yellow seeds (RrYy) in the F₁ generation. When these plants were self pollinated in the F₂ generation out of 16 plants, 9 had round yellow (RrYy), 3 had round green (Rryy), 3 had wrinkled yellow (rrYy) and 1 had wrinkled green (rryy) seed. In the ratio 9:3:3:1 (Phenotypic ratio).

Sex determination in human beings : Human beings have 23 pairs of chromosomes in the nucleus of the cell. Out of this two chromosomes are sex chromosomes X and Y. The female has two X chromosomes

(XX) and male has one X and one Y chromosome (XY). The sperms and eggs have one set of sex chromosomes. Some sperms have X chromosome and some have Y chromosome. All eggs have X chromosome.If a sperm having X chromosome fuses with an egg having X chromosome the child will be a girl. If a sperm having Y chromosome fuses with an egg having X chromosome the child will be a boy.

Speciation :- Formation of new species from existing species

The formation of new species from existing species is mainly due to one or more of the following factors.

They are: - Accumulation of variations, Physical barriers, Genetic drift, Natural selection and migration.

i) Accumulation of variations :- The differences between the individuals of the same species is called variations. The accumulation of variations over several generations produces new species.

ii) Physical barriers :- Populations may get separated by physical barriers like mountains, rivers, lakes etc. These isolated groups produce variations which can produce new species.

iii) Genetic drift :- Natural calamities or introduction of new members of the same species in an area can produce changes in the gene pool of the population and new variations are produced which can produce new species.

iv) Natural selection: - Only those individuals of a species which have useful variations and can adapt to the changes in the environment survive and the others die. These organisms can produce variations and new species.

v) Migration :- Some individuals of a species may migrate to a new geographical area and adjust to the changes in the environment there and develop new variations and produce new species.

Evolution

The gradual changes taking place in living organisms giving rise to new organisms due to changes in their genetic composition is called evolution.

Inherited traits :- Inherited traits are traits in an organism due to changes in the genetic composition and it can be passed from one generation to the next and it results in evolution..

Acquired traits :- Acquired traits are traits which are acquired by an organism during its lifetime and it cannot be passed from one generation to the next and it does not result in evolution.

Evidences of evolution :-There are a number of common features in different organisms which provide evidence to show evolutionary relationship. The main evidences of evolution are from the study of :- Homologous organs, Analogous organs and Fossils.

Homologous organs :- are organs which are similar in structure but different in functions.

Eg :- The fore limbs of amphibians, reptiles, birds, and mammals have similar structures but different functions. Frog (amphibian) uses its fore limb to raise the front of the body. Lizard (reptile) uses its fore limb for walking and running. Birds fore limbs are modified as wings for flying. Mammals use the fore limbs for grasping, walking, running, swimming, flying etc. This shows evolutionary relationship.

ANALOGOUS ORGAN: are those organs which are dissimilar in origin i.e. have different basic structural organ but have similar appearance and perform similar functions.

e.g.- the wings of birds and bats look similar but have different designs in their structure. Wings of bats are skin folds stretched between elongated fingers but wings of birds are covered by feathers all aling the arm.

Tracing evolutionary relationship: - fossil provides best evolutionary relationship among various organisms.

Fossils (L. Fadio= to dig out) :- are either the remains or impressions of organisms which lived long ago.

Study of fossil is called Paleontology. Leonardo de Vinci is called “father of Paleontology” George Cuvier is known as “father of modern Paleontology”

From the study of fossils we can know their structures and the time period in which they lived. The fossils of complex and recent organisms are found closer to the surface of the earth and the fossils of simpler organisms are found deeper inside the earth. The age of fossils can be determined by Radio Carbon Dating. The study of fossils shows evolution of simpler forms into complex forms and their evolutionary relationship.

Types of fossils:

(A) Unaltered fossils:-

(B) Altered fossils: represent the impression of the past organisms not remains of the past organisms.

i)Petrified fossils: are those fossils which are formed on the principal of replacement of molecules of past organisms. The replacing molecule can be eitherIron pyrites or Silica or Calcium Carbonate. It gives the idea of the past organisms even at molecular level.

ii)Moulds: Represents impression of past organisms on harden mud or soil.

iii)Compression: Represent thin carbon film of the past organisms giving outline of the external features.

iv)Tracks: are foot impression of past organisms on wet sand or soil.

v)Trails: are impression of foot of crawling organisms.

vi)Caprolites: are fossil preservation of undigested and excretory wastes of past organisms.

Formation of fossil: is called Fossilisation. Formed layer by layer in the earths crust.

FOSSIL DATING: The age of fossils can be estimated by two ways:-

1. If we dig into the earth and start finding fossils, it can be assumed that fossil closer to the surface are more recent to those found in deeper layers.

2. By detecting the ratio of different isotopes of the same element in the fossil material. e.g.- Carbon-14 dating technique.

Evolution by stages: Complex organisms and its organs developed from simpler organisms gradually over generations.

i) Evolution of eyes :- The eyes of planaria are just eye spots to detect light. It developed gradually into a complex organ in higher animals.

ii) Evolution of feathers :- Feathers were first developed in dinosaurs and used for protection from cold. Later birds used them for flying.

iii) Evolution by artificial selection :- Humans cultivated wild cabbage for over 2000 years and produced different vegetables from it by artificial selection.

Cabbage – by selecting short distance between the leaves.
Cauliflower – by selecting sterile flowers.
Kale – by selecting large leaves
Kholrabi – by selecting the swollen stem
Broccoli – by arresting flower growth

Evolution should not be equated with progress :-

Evolution has not resulted in progress. Evolution has resulted in the formation of several complex species from simpler species due to variations, genetic drift and natural selection. This does not mean that one species gets eliminated when new species are formed or that the new species are better than the older species. Species get eliminated only if they are not able to adapt to the changes in the environment. Several species which could adapt to the changes in the environment still continue to survive for example bacteria.

Human beings have not evolved from chimpanzees. They had a common ancestor from which they evolved separately. Human beings are not the pinnacle of evolution but they are only one species among the several evolving species.

Human evolution :- (Homo sapiens)

There is a great diversity among human beings in their form and features around the world. Human beings evolved in Africa. Some of them stayed there and others migrated to different parts of the world. Then due to genetic variations and the environmental changes in different geographical regions they developed changes in their forms and features.

Following are the important theories which are given to explain evolution:

1. Lamark’s theory: A/c to Lamark

^ØOrganisms and their organs tend to increase their size continuously due to some unknown forces of life.

^ØNew organs in the organisms are found due to new needs which occur due to change in the environment.

^ØTheory of use and disuse of organs.

^ØTheory of inheritance of acquired character.

2. Darwin’s theory: Charles Robert Darwin was a British naturalist who formulated his hypothesis that

evolution took place due to natural selection. Darwin’s theory of evolution tells us how life evolved from simple to more complex forms.

A/c to Darwin:-

^ØOver production.

^ØAlmost constant population.

^ØStruggle for existence- Intraspecific, Interspecific, Environmental Struggle.

^ØVariation: Appearance of variation in organisms during struggle for existence.

^ØNatural selection / Survival of the fittest

^ØInheritance of useful variation

3. Mutation Theory: by Hugo de Vries

“A new species is originating from pre-existing species slowly in single step due to genetic variation called ‘mutation’’

Mutation: is sudden inheritable change. It occurs commonly in naturally breading population. Mutation is direction less occurring in any direction. It is a subject for natural selection & most of the mutation is lethal or fatal.

Mutation can be brought either by numerical change in chromosome. It can be:-

(A)Ploidy mutation:-

(i) Euploidy: changes involving complete set of chromosome.

(ii) Aneuploidy: changes due to loss or gain of one or two chromosomes from a diploid set.

(B) Structural change in chromosome: -

(i)Chromosomal abberation:

(a) intra- chromosomal: changes within a chromosome brought through deletion, duplication or Inversion(paracentric & pericentric)

(b) Inter-chromosomal inversionTranslocation: which may be Non-reciprocal translocation or by gene mutation(any alteration in normal base sequence of a gene/DNA either by deletion or addition which affect phenotype is called gene mutation)

4. Neo-Darwinism: is explanation & elaboration of natural selection of Darwin in the light of modern genetics- Genetic variation, Natural selection & isolation leads to evolution/speciation

Origin of life on earth: J.B.S. Haldane, a British scientist(who later become Indian) suggested that life must have developed from the simple inorganic molecules which were present on the earth at that time( Ammonia, Methane & Hydrogen sulphide; but no oxygen) to more complex organic molecules. Thus the first primitive organisms would arise from further chemical synthesis.

Mendel’s Experiments

Monohybrid Cross

It is the cross between two plants which have one pair of contrasting characters. For Example, a cross between a tall pea plant and a short (dwarf) plant. The following diagram explains this in detail.

Observations & Conclusion

In the first generation (F1), the progeny were tall. There was no medium height plant.
In the second generation (F2), 1/4th of the offspring were short and 3⁄4 were tall.
The Phenotypic ratio in F2 – 3: 1 (3 tall: 1 short)
The Genotypic ratio in F2 – 1: 2: 1 – (TT: Tt: tt)
For a plant to be tall, a single copy of “T” is enough. But if a plant has to be short, both the copies should be “t”
In Tt, ‘T’ is expressed and ‘t’ is suppressed. Hence, the characters ‘T’ is the dominant trait and ‘t’ is the recessive trait.

Dihybrid Cross

It is the cross between two plants which have two pairs of contrasting characters. This takes into consideration alternative traits of two different characters. For example, a cross between one pea plant with round and green seeds and the other pea plant having wrinkled and yellow seeds. The following diagram explains the dihybrid cross in detail.

Observations & Conclusion

The F1 generation contains 100% hybrid plants. Because, when RRyy crosses with rrYY, all offspring plants are Rr Yy with round and yellow seeds in the first generation. Therefore, the Round and Yellow seeds are the dominant characters.
In F2, the phenotype ratio is 9:3:3:1 and the genotype ratio is a very complex one.
This shows that the genes independently inherited.

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Heredity & Variation