In heredity, genes are transmitted from parents to the off springs. Certain principles of genetics are followed in this. Genes are located on chromosomes. So, the genetic material is transmitted from one generation to the other by chromosomes. This is inheritance on the basis of chromosomes.
Sometimes genes present on same chromosome do not assort independently but remain linked together and pass on to next generation together. This is linkage.
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PARALLELISM BETWEEN GENES AND CHROMOSOMES:-
There is a perfect parallelism between genes & chromosomes which can be indicated by following facts:-
1 - Each species has a specific number of chromosomes in its somatic cell. In diploid cell, chromosomes occur in pairs (homologous pairs), one member of pair comes from paternal and other comes from maternal parent. Similarly genes also occur in pairs (Allelomorphic pair); genes in pair are contributed by both parents.
2 - During mitosis, the two chromatids of a chromosome separate into 2 daughter nuclei which later form 2 daughter cells. Similarly, each gene of allelomorphic pair goes into each daughter cell
3 - AT meiosis, chromosomes of each kind (homologous chromosome) separate. Similarly, genes of each kind also separate in meiosis
4 - Chromosomes unite & form a diploid pair after fertilization. Similarly, genes also form a pair after fertilization.
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CHROMOSOMAL THEORY OF INHERITANCE:-
According to this theory, chromosomes carry the hereditary information, possess Mendelian factors/ genes, seggregate & assort independently during transmission from one generation to next. So chromosomes are the vehicles of heredity.
Chromosome theory was proposed by Sutton and Boveri in 1902. It was later confirmed and expanded by Thomas H. Morgan, Alfred Sturtevant and Calvin bridges on Drosophila.
Chromosomes were seen in nuclei of Salamander cells by William Flemming in 1879.Salient features of chromosomal theory are:-
1 - Sperms & ovum carry all hereditary characters.
2 - Both, sperms and ovum contribute equally in heredity of offspring. Sperm provides only nuclear part to egg. This means, nucleus governs hereditary characters
3 - Nucleus contains chromosomes. There fore, chromosomes must carry hereditary traits
4 - Every chromosomes has a definite role in development of an individual. Loss of complete chromosome or a part of it produces deficiency in an organism.
5 - Both, chromosomes & genes occur in pairs in somatic or diploid cells.
6 - A gamete contains only one chromosome of a type and only one out of two alleles of a trait
7 - Paired condition of chromosomes & mendelian factors get restored during fertilization.
8 - Homologous chromosomes synapse during meiosis and then separate or seggregate independently into different cells.
9 - In many organisms, sex of an individual is determined by specific chromosomes called 'sex- chromosomes'.
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INDEPENDENT ASSORTMENT Vs. LINKAGE:-
Linkage is the phenomenon of certain genes staying together during inheritance through generations without any change or separation due to their being present on the same chromosome.
Thus Mendel's law of Independent Assortment is not universal and holds good for only those genes which are present on different chromosomes.
Linkage was first suggested by Sutton and Boveri(1902-1903). The genes present on same chromosome are called 'Linked Genes' and the genes present on different chromosomes are called Unlinked Genes' Morgan gave chromosomal theory of Linkage which states that
(a) Linked genes occur in same chromosome
(b) Genes lie in linear sequence in the chromosome
(c) There is tendency to maintain the parental combination of genes except for occasional crossovers.
(d) Strength of the linkage between two genes is inversely proportional to the distance between the two.
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COMPLETE AND INCOMPLETE LINKAGE:-
Linkage is of two types:- Complete and Incomplete
(a) Complete Linkage:- The genes located in the same chromosome do not separate and are inherited together to next generation because of no crossing over. This is complete linkage and parental traits are inherited as such. This is rare
Eg, A red eyed- normal winged (wild type) RRWW female Drosophila is crossed to recessive purple eyed- vestigial winged male, rrww
The progeny of F
1 generation is all heterozygous Red Eyed- normal winged (RrWw). When F
1 males are test crossed to recessive female (rrww), only two types of individuals are produced- Red eyed normal winged and purple eyed- vestigial in the ratio of 1:1 Recombinants are absent.
(b) Incomplete Linkage:- Crossing over occur in genes present in same chromosome. The genes remain incompletely linked, separate by crossing over and go to different gametes and off springs.
Eg, A red eyed normal winged dominant homozygous female Drosophila (RRWW) is crossed to recessive purple eyed vestigial winged male (rrww). F1 produced are heterozygous red eyed normal winged (RrWw). F1 females are test crossed with homozygous recessive males. It does not yield ratio of 1:1:1:1 but ratio comes out to be 9:1:1:8. Recombinants are produced.
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CROSSING OVER:-
Definition:- The phenomenon of exchange of genetic material between two homologous chromosomes is called as Crossing-over
The chromatids resulting from, exchange of segments are known as 'Cross- overs' or 'Recombinants'
Chromatids in which no crossing -over occurs are called 'Non-Cross overs' or Parental chromatids.
Janssens (1909) was the first person to discover chiasma formation and related process of Crossing over. Morgan used fruit fly Drosophila for his monohybrid and dihybrid crosses to deduce genetic information.
e.g. He performed a dihybrid cross between yellow body white eyed mutant female and grey body red eyed normal male. F1 females were all wild (normal) types and males had mutant characters. On crossing these females & males, F2 consisted of
98.7% yellow bodied white eyed files and wild files (Parental type ) 1.3% were yellow boded red eyed, and grey bodied white eyed (Recombinant type)
Recombinants were produced as a result of crossing over. Janssen observed chiasma formation during prophase I of meiosis I. Morgan proposed that chiasma are the points of crossing over.
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) RECOMBINATION:- It is a meiotic process that results in a haploid product which is different in genetic constitution from both the parents. The product of recombination is called a 'Recombinant' Recombinants are formed by crossing over in Meiosis I. This can be explained by Chiasma -type hypothesis as follows.
1 - Synapsis :- During Zygotere of Prophase I, homologous chromosomes pair (synapsis). Each chromosome consists of two chromatids. The paired homologous chromosomes are called Bivalents'.
2 - During pachytene, two non- sister chromatids break & then rejoin.
3 - The rejoining occurs between non- sister chromatids thus leading to exchange of segments.
4 - Exchange causes reshuffling of genes and thus recombination occurs.
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CHROMOSOME MAPPING:-
Definition:- A chromosome map is a linear graphic representation of the sequence of genes on a chromosome and relative distances of various genes present on a chromosome. It is also called 'Genetic Map' or 'Linkage map'
Basis;- Based on 2 genetic principles
* The genes are arranged in linear order in the chromosome
* The frequency of crossing over between two genes is directly proportional to the distance between them on a chromosome. Closer the genes, lesser the crossing over. Farther the genes, more is the crossing over Relative distance between genes is indicated by percentage of their recombination or crossing over.
Map Units:- 1% crossing over between two linked genes is equal to 1 map units or 1 centimorgan (CM) in honor of T.H Morgan.
Preparation of a chromosome Map:-
e.g, Frequencies of crossing over or recombination between three genes- yellow body (y), white eye (w) and miniature wing (m) are:-
Yellow body- white eye: (y-w) 1.5%
White eye-miniature wing (w-m) 34.5%
Yellow body - miniature wing (y-m) 36.1%
Now what can be the order of genes? As genes lie in a linear order, so there distances should be additive let us put maximum frequency/ distance first.
(Distance between w-m automatically becomes 34.5)
Importance:-
(i) Gives exact location of genes on a chromosome
(ii) Indicates strength of linkage between two genes
(iii)Tells chances of crossing over between two genes
(iv) Predicts number of cross-overs
(v)Confirms the linear arrangement of genes on a chromosome
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SEX- LINKED INHERITANCE:-
In sexually differentiated organisms, a specific pair of chromosome determines the sex of an individual. These are called 'sex-chromosomes' or 'Allosomes' or 'Heterosomes'. All other chromosomes are called 'Autosomes'
In humans, males have XY and females have XX as sex chromosomes. Inheritance of a trait linked to sex- chromosomes is called sex- linked Inheritance.
e.g. Morgan studied a mutant allele which was located on X- chromosome in Drosophila. This mutant allele gave white eye phenotype (w) which was recessive to its wild type red eye(w
+) allele. His results showed that inheritance of white eye was related to sex of parent carrying mutant allele. It can be explained by following cross.
(a) First Cross
(b) Second Cross:- (Reciprocal of first),
According to Mendel, both crosses should show similar results in F
1& F
2. But in the above two , results are different. Mrogan suggested that X- linked recessive allele shows criss- cross pattern of inheritance ie, recessive genes are transmitted from homozygous mothers to all male off springs.
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BASIS OF SEX DETERMINATION:-
Establishment of male or female individuals is called sex determination.
(i) Humans have autosomes (2A) and sex chromosomes. Females have 2A +XX, Males have 2A+XY.
Studies have shown that Y chromosome carries a gene 'sry' (sex-determining region) which codes for a product 'TDF' (Testis- determining factor). TDF is required for development of male sex. Its absence results in female sex. So Y chromosome determines maleness.
(ii) In Drosophila, work of Calvin Bridges tells that sex determining factor is number of X chromosomes /set of autosomes. Thus
-In 2A+2X or 3A+3X, ratio is 2/2 &3/3 which equal to 1.0. If ratio is 1.0, the sex is female and is fertile
-If ratio is more than 1.0 (3X+2A), sex is metafemale which is weak and infertile.
- 2A+XY and 2A +XO are males. 2A+XY are fertile. 2A+XO are sterile.
-When ratio decreases (e.g.3A+XY, ratio is 0.33), infertile metamales are produced.
- Flies with X/A ratio between 0.5 and 1.0 were 'intersexes' & had lots of abnormalities.
(iii) In birds & butterflies, male is ZZ and female is ZY. Female produces 2 different gametes instead of male as in above cases. So, egg, determines sex.
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GENETIC VARIATION:-
Whole set of characters are transmitted from parents to off springs. Still off springs look different from each other and from parents also. These are variations which occur due to recombinations, gene mutations, gene- environment interaction and chromosomal aberrations.
1 -Recombinations:- New combinations occur during independent assortment, crossing over and random fertilization of gametes. This results in reshuffling of genes & genetic variations.
2 - Gene Mutations:- Term 'mutation' was first introduced by Hugo de Varies in 1901 in plant Evening Primrose (oenthera lamarckiana)
Gene Mutations can be defined as sudden, random and discrete change in genetic material which is heritable. Most of the existing variations are due to gene mutations. Mutations occurring is germ cells are called 'Germinal Mutations' and those occurring in somatic cells are 'somatic Mutations'.
Mutations arising suddenly are called 'spontaneous Mutations'. When mutations are induced by certain agents then these are known as Induced Mutations. Agents inducing mutations are called 'Mutagens' e.g. radiations, chemicals.
Mutations cause variations which help in natural selection and thus lead to biological evolution. Geneticists study the modified genes & their transmission. Thus mutations provide 'Markers' which help in genetic analysis.
(a) Chromosomal Aberrations:- Alternation in chromosomes takes place in this. The structure or number of chromosome gets altered. So these are also called 'chromosomal Mutations'.
(i) Change in Chromosome structure:- Every living cell has a specific ser of chromosome with specific size & structure. Change in structure can occur by 4 ways- Deletion, Duplication, Inversion & Translocation.
Deletion:- A part of chromosome is lost either from end (terminal) or between ends (Interstitial ).
e.g. Cri-du- chat syndrome (cry of infant resembles sound of cat)- half of short arm of chromosome 5 is lost in humans.
Duplication- A portion of chromosome is repeated. It can be in tandem sequence or reverse order.
e.g. Development of bar eye in Drosophila.
Inversion:- In this, a segment of chromosome breaks, rotates at 180˚ and rejoins. When Centromere is included in rotated (inverted) segment, it is known as
pericentric inversion . When only arms are there in inverted segment & Centromere is not induded, it is called Paracentric inversion.
Translocation:- A segment of chromosome breaks off and rejoins a non- homologous chromosome. Chromosome suffering breakage becomes shorter and the other non-homologous chromosome to which the broken segment joins becomes longer than normal. This is simple translocation. In Reciprocal translocation, two non-homologous chromosomes exchange segments. So the chromosomes remain same in length but have rearranged gene blocks.e.g., In chronic myloid leukemia (CML), malignant cells have chromosome 22 shortened due to translocation of piece of its long arm.
(ii)
Changes in Chromosomes Number :-
Every organism has a specific number of chromosome. Basic set of chromosomes for any individuals is called haploid (X). When 2 gametes unite, it produces an individual with double set of chromosomes called Diploid (2X). When change in chromosome number occurs, more than usual two sets of chromosomes are formed. This is called 'Polyploid'. Polyploids can be Triploids (3X), Tetraploids (4X), pentaploids (5X), hexaploids (6X) etc.
Polyploids are of two types- Autopolyploids (increase in number of same set of chromosome) and Allopolyploids (increase in number of same chromosomes of hybrids of different species).
e.g. of Autopolyploids - Maize, Rice, Gram.
e. g. of Allopolyploids- Wheat, cotton, Triticale(Man-Made)
When changes occur in an individual chromosome instead of whole set of chromosome, it is called 'Aneuploidy' and organism as 'Aneuploid'.
Aneupoploidy can be Hyperploidy (addition of chromosome) or Hypoploidy (loss of chromosome). Hyperploidy:- Trisomic (2n+1)-: It has 1 extra chromosome.
e. g. Down's syndrome (chromosome 21 is extra)
Tetrasomic (2n+2):– It has 2 extra chromosomes.
Hypoploidy:- Monosomic (2n-1) -:It has 1 less chromosome
Eg, Turner's syndrome
Nullisomic (2n-2)-: complete pair of homologous chromosome is absent. These don't survive.
Applications of Polyploids & Aneuploids:-
1 - Polyploids are used in crop breeding and horticulture.
2 - Allopolyploidy helps to pool genetic information from 2 different species to create a new species.
3 - Aneuploidy is used in crop engineering
4 - Aneuploidy is used to find roles of certain chromosomes in human development
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PROKARYOTIC CHROMOSOME:-
Chromosome of prokaryotes is long (approx 1.2 mm), lies in nucleoid which is a compact structure (as prokaryotes lack cell membrane), is made up of DNA and proteins.
Size of chromosome is long as compared to size of cell, so it lies coiled and super coiled.
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EUKARYOTIC CHROMOSOME:-
Chromosome of Eukaryotes is complex because of more number of chromosomes (46 in humans), more DNA/chromosome and large number of histone proteins. Length of 46 human chromosomes together is 2 metres. These lie in nucleus (size of nucleus-: upto 5 metres in diameter)
DNA associates with histone proteins to form Nucleosomes. (DNA is wrapped around histones , H1 histone lies outside DNA)
Chromatin fibre (10nm thick) condenses to form solenoid (30nm diameter) which coils to form chromatin fibre of 200 nm diameter and then a chromatid of 700 nm diameter.
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HUMAN GENETICS:-
The branch of genetics which deals with the study of inheritance of characters in man is called as 'Human Genetics'. Sir Archibald Garrod is regarded as 'Father of human Genetics'.
Human Genetics cannot be studied by breeding experiments as families are small & generation time is long. Various techniques are used to study it Pedigree analysis is one of the most important techniques. In this, a diagrammatic record of inheritance of a particular trait over two or more generations is maintained by using standard symbols. Then this record is analyzed. Transmission of a particular disease like albinism, Dwarfism, colour- blindness from parents to off springs can be found out by studying a pedigree
GENETIC DISORDERS:- After the rediscovery of Mendel's laws of Inheritance, human genetic analysis were done more extensively. Many genetic disorders were found in humans. In many cases, there was an error in a metabolic step which caused these disorders. Some disorders are :-
(i) Sickle- Cell Anaemia:- In this diseases, RBCs of the affected individuals become sickle shaped under low oxygen situation (Normal RBCs are biconcave). These individuals suffer attacks as RBCs aggregate in capillary system of venous side. Tissues also get damaged. This is called Sickle- cell crisis. Single pair of alleles Hb
A &Hb
s control it. Three genotypes are seen Hb
A/ Hb
A, Hb
sHb
s, Hb
AHb
s exhibiting sickle cell trait. Heterozygotes (Hb
A Hb
s) are carriers of defective gene.
These disease occurs due to substitution of value for glutamine at 6
th position is b-chain of globin molecule of Haemoglobin.
(ii) Phenylketonuria:- It is inborn metabolic disorder in which individuals lack enzyme to convert phenylalanine to tyrosine. So there is overproduction of phenylalanine which then accumulates in cerebrospinal fluid causing mental retardation.(some phenyl alanine changes to phenylpyruvic acid which comes out in urine).
(iii) Down's Syndrome:- (Mongolism or Trisomy-21).
First reported in 1866 by Langdon Down. It is due to imbalance is chromosome number or chromosome rearrangement. In it chromosome 21 is extra (Both the chromosomes of pair number 21 pass into single egg). Thus egg has 24 chromosomes instead of 23. Offspring then has 47 chromosomes instead of 46 (45+XY in males, 45 +XX in females)
Symptoms:- Small round head, protruding furrowed tongue, partially opened mouth, short broad hands, mentally retarded.
Note:- 1 chromosome is extra as paired homologous fail to separate at anaphase.
(iv)Alzheimer's Disease:- In this, memory and judgment ability is lost. There is physical impairment. Amyloid protein plaques accumulate in brain and degenerate the neurons causing this disease.
(v) Sex- Chromosomes linked genetic disorders:- Alteration in sex chromosomes is seen in these disorders which are:-
Turner's Syndrome:- (XO). Individual has 45 chromosome instead of 46. Formed by union of X sperm & abnormal O egg. or X egg and abnormal O sperm.
They are sterile females with rudimentary ovaries, undeveloped breasts, small uterus & abnormal intelligence.
Klinfelter's Syndrome:- (XXY). Individual has 47 chromosomes formed by union of XX egg & normal Y sperm OR normal X egg & abnormal XY sperm. They are sterile males with undeveloped testes, sparse body hair, long limbs & some female characteristics.
Super females:- Have 47(44+XXX), 48(44+XXXX), 49(44+XXXXX) chromosomes. Such females have abnormal sexual & mental development. More the number of 'X' chromosome, severe are the symptoms.
Super males:- Have 47 chromosomes with genotype XYY. There is overproduction of male hormones. These people are more aggressive, have unusual height, are mentally retarded with criminal bent of mind.
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