There are different life forms which may be similar at cellular level of organization but at organismal level, a lot of diversity can be seen. Members of a particular species resemble closely & have a set of similar characters by which they are identified. Genetics deals with the study of these characters how these are passed on from one generation to another (ie, parents to offsprings)
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Inheritance: Heredity and Variation
Genetics (GK. 'genesis'-descent) is the branch of biology that deals with the study of heredity and variations. The term 'Genetics' was coined by William Bateson in 1906.
Offspring resemble their parents. This resemblances might not be 100% which means that difference among the members of a species can also be seen.
These differences shown by individual of a species and also by offspring of same parent are called 'VARIATIONS'.
The transmission of characters, resemblances and variations, from one generation to the next is called Heredity.
Heredity:-
So there appears to be a continuity of life which can be summed up in the phrase 'Like begets like'. Heredity & variation are also associated with the mode of reproduction. Higher forms reproduce sexually, there are two patents & character are received from both whereas in lower forms which reproduce asexually, characters are received from one parent only.
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Pre- Mendelian Ideas about inheritance
Many ideas were put forward before Mendel to explain the transmission of characters from parents to offspring's. They are often called as 'THEORIES OF BLENDING INHERITANCE' as they believed that characters of parents blended (got mixed) during their transmission to the offspring's.
(i)
Vapour Theory:- Pythagoras (500 B.C) believed that each organ of body produces vapours which combine to form body part of embryo.
(ii)
Fluid Theory:- Empedocles (504-433 B.C.) said that each body part produces a fluid, fluid from both parents mixes up 2 forms embryo. Any defect in mixing results in missing of characters.
(iii)
Particulate theory:- maupertius (1689-1759) proposed that each animal produces minute particles for reproduction and a new individual is formed by union of particles of two parents.
(iv)
Theory of Pangenesis:- Proposed by Charles Darwin in 1868. According to it, every part of the body produces a representative 'gemmule' which gets collected in semen by the help of blood circulation. These 'gem mules' theta transmitted the characteristics to the offspring's.
In 1892, August Wiseman rejected this concept and said that lining organisms consist of 2 types of materials- somatoplasm and germplasm. Somatoplasm makes the whole body, grows & develops & ultimately dies. Germplasm is immortal & is passed to next generation.
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Gregor Mendel and his theory of Inheritance:-
Gregor Jonann Mendel (1822-1884) is known as 'Father of Genetics'. In 1856, he observed the occurrence of two types of seeds in pea plant growing in his monastry garden at Brno. He started experimenting on these & gave some results in 1865 which went unnoticed. Meadel died in 1884 & his work was renoticed in 1900 independently by twice geneticists. They were Hugo de vries (Holland), carl correns of Germany & Erich von Tschermark of Austria.
MENDEL'S EXPERIMENT:-
Experimental Material:-
Mendel close garden pea (Pisum satervum) for his experiments. He chose this plant because-
1 - It had number of contrasting characters.
2 - It is small & easy to grow.
3 - It reproduces a large number of offspring's and completes its life cycle in one season.
4 - A large number of true breeding varieties are available
5 - It can be cross -bred manually
6 - It is bisexual & self fertilizing in nature.
Experimental Method:-
Mendel's experiment involved cross- breading of two pea plants with different contrasting traits. He noted 7 contrasting characters in pea plant which are-
Mendel's experiment had 3 types:-
(i)
Selection of pure parents:- All T characters noticed by Mendel were true-breeding i.e, were passed from parents to offspring's eg. Tall parent will give rise to a tall offspring. So, plants with these true characters formed pure parents (p) for next step.
(ii)
Hybridization:- In this step, pure parents with contrasting characters like Tall & Dwarf, Green &yellow pod colour were crossed. Pollen grains from one type of plant. Were used to fertilize the egg of other. Offspring's in the form of seeds were collected. These seeds constitute next generation termed as
'FIRST FILIAL' orF1 generation.
Monohybrid Cross:- the cross in which two contrasting forms of a single character are taken into consideration eg. Height of plant (character). Tall or Dwarf (contrasting characters).
Dihybrid Cross:- the cross involving contrasting forms of two characters
Eg. Height and seed colour.
Trihybrid Cross:- cross involving contrasting forms of three characters.
(iii)
Self- Breeding:- the plants of F
1 generation were allowed to self- pollinate. New seeds were produced which formed new plants. These were
'SECOND FILIAL' or F
2 Generation. Further self pollination produced
'THIRD FILIAL' or F
3
Results of the Experiments:-
Mendel first performed Monohybrid crosses and then Dihybrid. He considered single character-stem height contrasting pairs were Tall (T) & Dwarf (t). When two parents (Tall & Dwarf) were crossed, the F1 generation produced all tall plants.
When these F1 tall plants were self-crossed then out of total 1064 F2 plants, 787 were tall and 277 were dwarf. Many tall & dwarf plants were crossed and every time same results were seen. The ratio was 787:277 ie, 2.84 :1 or approx,. 3:1 (3 tall and 1 dwarf)
The results were same even when reciprocal crosses were performed. A Reciprocal cross involves the same trait but carried by sexes opposite to those in the original cross. Eg. In original cross, pollens from tall plants × stigmas from dwarf plant. In reciprocal cross:-
Pollens from dwarf plant × stigmas from tall plants
Mendel's Monohybride cross
The consist result obtained from different non hybrid crosses led Mendel to propose four postulates which are called
'PRINCIPLES OF INHERITANCE'.
POSTULATE 1. LAW OF UNIT OR PAIRED FACTORS:-
According to this law, the various characters are controlled by a pair of factors'. An individual receives one factor from each parent and thus has 2f. These factors are called 'Alleles' or allelomorphs'.
Eg, these are 2 unit factors for Tallness and 2 unit factors for dwarfness. So, these combinations are possible when these are crossed, either there are 2 factors for tall (TT) or two factors for dwarf (tt) or 2 factors- one from tall & one from dwarf (Tt).
POSULATE 2: LAW OF DOMINANCE:-
Each character has two contrasting factors which are called alleles. According to this law. Out of 2 alleles, one factor is able to express itself and the other is not. The one, which expresses itself is called 'Dominant unit factor' & the other which fails to express is called 'Recessive unit factor' eg, Tall is dominant & dwarf is recessive. These 'unit factors' are now called as 'Genes'.
POSTULATE 3: LAW OF SEGGRE GATION:-
According to this law, the factors of each character segregate (separate) during gamete formation so that each gamete receives only one factor for each character.
When tall & dwarf parents are crossed factors get segregated. Each gamete hill receive one factor, either tall or dwarf. When these gametes fertilize, F1 plants will have one factor for tall & one for dwarf but as Tall is dominant, so all F1 plants will be tall.
When F1 plants are self crossed, then again gametes will be formed. Factors will segregate and each ganete will receive factors rand only (either tall or dwarf).
On fertilization, which is a chance event, F2 generation will have 4 combinations of factors.
Tall/Tall. , Tall/ Dwarf, dwarf/tall, and dwarf/dwarf
Phenotypically, three will look tall (TT, Tt, Tt) and one will look dwarf (t). Ratio :3:1. Genotopically, one will be homozygous tall (TT), two will be heterozygous tall (Tt) and one will be homozygous dwarf (tt). Ratio 1:2:1
POSTULATE 4: PRINCIPLE(LAW) OF INDEPENDENT ASSORTMENT
According to this principle, the two factors of each character assort or separate' independently of the factors of other characters at the time of gamete formation and get randomly rearranged in the offspring.
The above can be explained by a dihybrid cross conducted by Mendel. He crossed plant with round &yellow seeds (RRYY) with plants producing wrinkled & green seeds (rryy). The F1 plants were all round & yellow (RrYy) according to law of Dominance (Round is dominant over wrinkled and yellow is dominant over green).
When F1 plants were self - crossed, Mendel noticed that one pair of unit factors segregates independently of the other pair of unit factors. This also means that these assort independently. During fertilization, factors distributed randomly & four combinations were produced.
9 yellow-round, 3 yellow-wrinkled, 3 green-round and 1 green wrinkled. Ratio :9:3:3:1
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GENETIC TERMS & SYMBOLS:-
F1 generation:- 'First filial. It is the generation of hybrids produced in a cross between two genetically different but pure parents.
Eg. F1 generation produced by crossing TT and tt parent plant has Tt individuals.
F2 Generation:- It is the 'second filial ' generation of young ones produced in a genetic cross between two F1 organisms
Reciprocal Cross:- It is a cross involving two types of individuals where the male of one type is crossed with the female of second type and vice-versa.
Back Cross:- A cross performed between hybrid and one of its parent.
Test Cross:- A cross between an individual and its recessive parent. This is done to find out whether individual is homozygous or heterozygous for dominant character
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METHODS OF ANALYSIS:-
PUNNETT SQUARE:-
It is a checker board type square devised by Regthald.c. Punnett (1875-1967) to show the results of a cross between two organisms. It tells both, genotype and phenotype, of a progeny.
In Punnett square, each gamete is placed in individual column, vertical column represents female & horizontal represents male parent. Possible combinations are made and entered in boxes.
TEST CROSS:-
It is a cross between F1 individual and a recessive parent to fine out whether as individual (F1) is homozygous or heterozygous for dominant trait.
In monohybrid cross, when F1 is test crossed, the ratio come out to be 1:1. In Dihybrid cross, when F1 is test crossed, ratio comes out to be 1:1:1:1. (It offspring is 100% dominant then it means F1 was homozygous dominant . If offspring are 50% dominant and 50% recessive, then F1 was heterozygous dominant)
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REDISCOVERY OF MENDEL'S WORK:-
Mendel started his work in 1856, presented for first time in 1865. But his work was not noticed for about 35 years because
- His ideas on heredity appeared too simple
-His algebraic (mathematical ) method of calculating combinations & was alien for other scientists.
&Wallace's continuous variations postulates
-At that time lot of emphases was on organic evolution.
In early twentieth centaury, hybridization experiments were performed independently by Hugo devoies, carl correns &Erich Tschermak. These were similar to Mendel's experiments. The results led to rediscovery of Mendel's work &science of Genetics.
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POST -MENDELIAN ERA:-
After rediscovery of Mendel's laws in 1900, geneticists started performing Mendel's experiments on other organisms. Many important principles & discoveries were made. These were described as Post-Mendelian Genetics.
Genes control the phenotype but sometimes the influence or interaction of other genes can alter the phenotype. Gene interactions can be of two types:-
(i)
Intragenic:- Intractions between 2 alleles of same gene. It produces phenotype different from normal phenotypes. Eg. Incomplete dominance, co dominance, Multiple alleles.
(ii)
Intergenic:- Interaction between alleles of different genes present on same or different chromosomes. These also after the phenotype. Eg, Epistasis, pleiothropy, Polygenes, complementary & Supplyementary genes, lethal genes
INCOMPLETE DOMINANCE:-
In this phenomenon, the F1 hybrid produced is intermediate or mixture of the two factors of parents. Dominant allele is not able to suppress recessive allele completely.
Eg. Incomplete dominance can be seen in Mirabilis jalapa (4'o'clock plant) and Antirrhinum majus (snapdragon).
In 4 'o' clock plant, there are plants with red flowers (RR) and with white flowers (rr). When the two are crossed, F1 produced are all pink (rr) flowered generation are of these types-Red, pink, white in the ratio of 1:2:1. pink colour appears due to incomplete dominance of red colour or mixing of red& white coloures.
CO- DOMINANCE:-
it is a phenomenon in which alleles do not show dominant- recessive relationship but express themselves independently when present together. Symbols used for co- dominant genes are different. These are capital letters with superscripts.
Eg. Alleles for blood group A (IA) and blood group B(IB) are co- dominant so that when they come together they produce blood group AB.
MULATIPLE ALLELES:-
Two alleles govern a character but three or more alleles can also be found. So, more than two alternate forms of a gene present on same touce are called 'Multiple alleles'.
These are formed when mutation occurs in same gene but different directions. Each allele produces different phenotype & different combinations of alleles also produce different phenotype.
Eg: 4 Blood groups in humans -A, B, AB, O. These letters represent antigens. 3 alleles -I
A, I
B, I
O - produce the above 4 phenotypes
Eg. Eye colour in Drosophila is also governd by multiple alleles.
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TWO GENE INTERACTIONS:-
These interactions take place between alleles of 2 different genes. Two gene interactions produce-
-Different phenotypes
-Homozygotes & heterozygoters governed by dominant allele.
-F1 indiciduals which are heterozygous for 2 gene pairs
-F2 genetypes in the ratio of 9:3:3:1.
Epistasis is the best example of gene interactions
EPISTASIS:-
Epistasis is the phenomenon of masking or suppressing the action of one gene by another non-allelic gene
The gene which masks the other gene is called 'Epistatic Gene' and the gene which gets masked is called 'Hypostatic Gene'.
Dominant or Recessive , any gene can mask the other.
Eg. Coat colour in Mice:- In mice agouti (grey) colour is dominant over black. Coat colour is determined by A/a pair (A-gives agouti, aa gives black phenotype). In B/b allele pair, recessive allele 'b' is epistatic over A/a. This means when bb is present, the effect of A- and aa gets masked and both produce the same phenotype with no colour ie, albino.
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) POLYGENIC TRAITS:-
These are also called 'Quantitative traits'. These don't exhibit just 2 distant contrasting characters but show a wide range of phenotypes, usually controlled by more than one pair of gene(polygenes) and environmental factors.
The inheritance of polygenic traits is called 'polygenic inheritance'. In this kind of inheritance , F1 individuals are very similar to one another and intermediate between the 2 parents. On selfing F1, a wide variety of F2 is produced in which few individuals look like one grand parent few look like parent, few are intermediates.
Eg. Skin colour in Humans. It is controlled by three genes-A, B, C. A cross between pure Negro i.e, very dark (AABBCC) and a pure white cevery light (aabbcc) produces F1 with genotype AaBbCc with mulatto skin colour. On selfing F1 from 8 gametes in each sex thus producing 64 combinations in F2 generation.
Ratio is
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