Rusty's Bio-zone: HEREDITY AND EVOLUTION

(CHAPTER – 9):-HEREDITY AND EVOLUTION

BASIC TERMINOLOGY TO UNDERSTAND GENETICS

GENE:

A segment of DNA capable of self expression eg. Gene for height of plant

ALLELE:

Alternate forms of a gene eg Gene expressing differently as Tall or Dwarf

CHARACTER:

Unique feature regulated by a gene eg. Eye colour

TRAIT:

Various expressible forms of a character eg. Blue/black/green eye colour

PHENOTYPE:

Observable or visible features eg. Height, colour, shape

GENOTYPE:

Genetic makeup of phenotype eg. Pure Tall (TT), Hybrid Tall (Tt)

HOMOZYGOUS:

Both the alleles of a gene, are same in the cell eg. TT or tt for height of plant

HETEROZYGOUS:

Both the alleles of a gene, are different in a cell eg. Tt

SELFING:

Reproduction between organisms with similar genetic trait eg. TT/TT, or Tt/Tt

CROSSING:

Reproduction between organisms with different genetic trait eg. TT/tt, Tt/TT

PURE/TRUE BREED:

A condition of homozygosity giving 100% same offspring, on selfing eg TT, tt

HYBRID:

A condition of heterozygosity giving different ratio of offsprings, on selfing eg. tt

FIRST FILIAL (F₁ -Generation):

The probability ratio/nature of offsprings for the cross between Parent generation. For a Mendelian cross, it is always a Hybrid (100%)

SECOND FILIAL (F₂ -Generation):

The probability ratio/Nature of offsprings for the cross between F1 – generation (selfing). For a Mendelian nature, it is always 3:1 in monohybrid cross.

MONO-HYBRID CROSS:

A genetic cross, in which inheritance of only one character is studied.

DI-HYBRID CROSS:

A genetic cross, in which inheritance of two characters are studied simultaneously.

MONOHYBRID RATIO:

PHENOTYPIC (3:1) GENOTYPIC (1:2:1)

DIHYBRID RATIO:

PHENOTYPIC (9:3:3:1) GENOTYPIC (1:2:1:2:4:2:1:2:1)

DOMINANT:

Out of the two different alleles/Traits in cell, only one expresses phenotypically, it is called Dominant allele/ Trait.

RECESSIVE:

Out of the two different alleles/traits, in the cell, the one which is masked or hidden in the presence of other is called Recessive allele/Trait. It expresses only in Homozygous condition

BACK CROSS:

A cross involving a progeny/offspring and any of the parent.

TEST CROSS:

A cross involving F1 – generation and Recessive parent only ( A Type of Back Cross) where zygosity of the offspring is identified. 100% result means Homozygous and 50% : 50% or 1:1 result means Heterozygous

1a) Heredity :-

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

b) Variations :-

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

2) 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.

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.

3) 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.

To know about genes watch video by clicking [HERE ]

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.

Father of Genetics: Sir Gregor Johann Mendel (1822-1884)

a) When plants having one pair of character (Eg:- tall and short plant) was crossed (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 in the second generation. (F₂)

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

The trait that is expressed in the F₁generation is called the dominant trait and the trait that is supressed in the F₁ is called the recessive trait.

(Mendelian Mono-Hybrid Cross)

b) When plants having two pairs of characters (Eg:- shape and colour of seeds) were crossed (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 F₁generation. 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.

To understand more on Mendel's Mono & Di-Hybrid Crosses watch video by clicking [ HERE ]

4) 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.

[ sex determination- video ]

5. Variations may or may not help organisms to survive :-

Let us study with the help of an example. A colony of green beetle living on grassy ground or green branches of a plant. Due to error in DNA copying slight variations accumulate in nature. These variations give different directions to organism in terms of evolution and speciation, like:

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 beetles 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.

sometimes such variations helped by calamity or disaster, may results in formation of new species by chance. (case of Elephant Foot in beetle example)

c) Acquired 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.
[ beetle example- video]

6. 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, Mutation 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 produce new species.

ii) Physical barriers (Isolation):- 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.

Iv) Mutation: A sudden inheritable change in the genotype of an organism. It help in quich speciation.

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 & speciation - video]

7. Evolution :-

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

a. 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.

b. 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.

9. 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.

a) 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. Bird’s forelimbs are modified as wings for flying. Mammals use the fore limbs for grasping, walking, running, swimming, flying etc. This shows evolutionary relationship.

b) Analogous organs :- are organs which are different in structures but similar in functions.

Eg :- The wings of butterfly, bird and bat have different structures but similar functions. This shows evolutionary relationship.

c) Fossils :- are the remains of organisms which lived long ago.

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 (Palaeontology) shows evolution of simpler forms into complex forms and their evolutionary relationship.

( Fossil Stegosaurus from Jurassic period)

( Fossil Plant log from Cretaceous Period)

(Frozen Wooly Mammoth from Pleistocene Deposits)

(Fossil of Upper Himalayan Fish from Oligocene)

(Invertebrate fossil Shell)

(Fossil Trilobit from Early Cambrian)

10. 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.

Eg :- 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

Embryology: Study of structural, developmental and comparative aspects of embryos of organisms. It help in establishing evidences of evolution by stages.

(Comparative Embryology in Evolution)

[ Evolution by stages video lecture ]

11. 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.

12. 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.