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Heredity may be defined as the transmission of characteristics through generations. The characteristics include all physical, physiological and psychological and mental characteristics in organisms. All these characteristics are called the traits.
If we look around us, we see different kinds of organisms. The organisms are of different species. Even among the species, there are differences. For example, one cat is different from the other, one lion is different from the other and also human beings are different from each other. Except for identical twins, even offspring born to same parents are different. All these differences are classified as variations. Both heredity and variation play important roles in evolution of organisms.
In man traits such as height, colour of the eyes, intelligence, etc. are all inheritable.
The study of the mechanism of transmission of characteristics from parents to the next generations is called genetics.
The study of genetics was started by Gregor Mendel an Austrian monk during 1850s. However, his studies were not recognised by the world till after his death. Mendel is known as the Father of Genetics.
Before Mendel, although people knew that parents with blue eyes would get blue eyed children, baldness in the family, etc., they did not know the exact mechanism of transfer of these traits from the parents to the offspring. They did not know that there is a physical basis for heredity. This was identified by Mendel through his pioneering work.
Introduction to accumulation of variation during reproduction
Although the off springs inherit the characters of the parents and resemble them very closely, but the resemblance is not complete in all respects. The off springs are never a true copy of the parents. In fact, no two individuals are exactly alike and the members of any one species differ from one another in some characters or the other.
These differences are known as variations. So, from the biological point of view, variation is the occurrence of differences among the individuals of a species. For example, people have different heights. Their complexion, type of hair, color of eyes, shape of nose and chin also show differences.Â The differences in the characters among the individuals of a species are called variation. For example, human height is a trait which shows variation.Â This is because some people are very tall, some are less tall, some have medium height, and some have short height whereas others are very short.
Description of Accumulation of Variation
Some amount of variations is produced even during asexual reproduction but it is very small. The number of variations produced during sexual reproduction is, however, very large. For example, the sugarcane plants reproduce by the process f asexual reproduction, so if we observe a field of sugarcane, we will find very little variations in various sugarcane plants. All the sugarcane plants almost look alike.
But in animals which reproduce by the process of sexual reproduction, a large number of variations are produced. It is due to these variations that no two individuals are alike. From the above discussion we conclude that the number of successful variations is maximized by the process of sexual reproduction. The variation is a necessity for organic evolution.
Conclusion to Accumulation of Variation
The reproduction of organisms produces variations. The variations produced in organisms during successive generations get accumulated in the organisms. The significance of a variation shows up only if it continues to be inherited by the offspring for several generations. The great advantage of variation to a species is that it increases the chance of its survival in a changing environment.
You tube video: Heredity And Evolution
Heredity and Effect of Variations or Effect of Variations on Evolution
We find variations in all populations. Within a single species no two organisms are similar. Only asexual reproduction will produce offspring which are genetically identical. Even clones and identical twins might acquire new external characteristics when placed in two different environments.
Sexual reproduction produces variations due to genetic recombination during meiosis. Â
Variations may be defined as differences among individuals of a species.
Hence we can say that these variations can come up due to genetic as well as environmental factors like food, temperature etc.
Mutations can also cause alteration in genetic information and cause variations. Â
Variations are important as they enable the organisms to adapt itself to the changing environment.
It is of common knowledge that "Like produces like" - human beings give birth to babies, a cat gives birth to kittens and a hen produces chicks. Although offspring's resembles their parents, they are not identical to them. They exhibit a departure in some of the qualities from the previous generations.
These differences shown by the individuals of a species are termed as variations. Thus variation means, the differences that exist in the individuals of a species. It represents the differences that permits distinction between two individuals of the same race, or between the offspring's of the same parents. The scientific study of the transmission of hereditary character from one generation to the other is known as genetics.
An important result of the reproductive cycle is production of similar featured individuals for generations. The transmission of characteristics through successive generations refers to heredity. These characteristics include all physical, physiological and psychological characteristics in organisms. We refer to them as traits. The study of the mechanism of transmission of characteristics from parents to the next generations is called genetics.
A man's characteristics, such as height, colour of the eyes, intelligence, etc. are all inheritable. Often we see that the children of parents with blue eyes would get blue eyes; baldness passes on in the family from one generation to another; even disorders like diabetes and heart diseases are often inherited. One should know how these traits pass on and the rules that govern the inheritance of traits.
Rules for the Inheritance of Traits - Mendel's Contribution
Physical basis of Heredity
According to Mendel, the genotype is made of certain structures called factors.
The factors controlled the inheritance of all traits.
These factors are present in pairs. The factors are now called genes.
Thus the physical basis of heredity are the genes or factors.
The different expressions of the same genes are called alleles.
Each trait may be represented with an alphabet. If the alphabet T represents height, T represents tallness and t represents shortness. If the letter R represents the colour of the flower, R represents red and r represents white.
General convention is to represent the dominant character or the character expressed in F, generation by capital letter.
Further, each individual for sexual reproduction produces gametes.
Gametes are haploid. Each gamete will have only one allele for each trait. Thus the gametes will be of two kinds - having allele T or t.
Since each individual is formed by the fusion of gametes, they are represented by writing two letters - TT, Tt or tt depending on their origin.
If the plants are pure breeds producing only one type of offspring, they are called homozygous (TT or tt).
If the plants produce both tall and short plants among the offspring, then they are heterozygous (Tt).
Mendle's Monohybrid Ratio
In case of height, the tall plants of P generation are represented as TT and the short plants as tt. Each parent produces only one type of gamete. Tall plants produce gametes having allele 'T' and short plants produce allele having 't' gametes. When these two types of gametes fuse, the resultant plants are heterozygous (Tt). However, only tallness is expressed, as all plants of this generation are tall. Thus, tallness is the dominant allele and shortness is the recessive allele. Thus the recessive allele remains hidden in the first generation and becomes expressed on self-pollination in the next generation.
The F1Â generation plants are then self-pollinated. This results in tall and short plants in the ratio 3:1. This is called the phenotypic ratio. Of the tall plants, 1/3rdÂ of the plants is homozygous for tallness and the remaining 2/3rdÂ are heterozygous. The heterozygous plants on self-fertilization again result in offspring that show 3:1 ratio for tallness to dwarfness. Thus the genotypic ratio in the F2Â generation is 1:2:1. This means that 1/3rdÂ are tall and homozygous, 2/3rdÂ are tall and heterozygous and 1/3rdÂ are dwarf and homozygous. Note that recessive traits are phenotypically expressed only in the homozygous genotype.
Similar crossing experiments were also carried out with more than one trait. However, it is not possible to follow all the traits at the same time. The other traits studies by Mendel in garden pea were colour of the seed, the shape of the seed, colour of the seed coat, the flower colour, flower position, pod colour and pod shape.
The crosses that study two traits together are called the dihybrid crosses. In the dihybrid crosses, it was found that the two traits were inherited independent of each other. The dominant alleles for each of the two traits asserted their dominance independent of the other.
You tube video: Mendelian Genetics
How do these Traits get Expressed?
The genetic material that controls a characteristics or a trait is a nucleoprotein called chromatin.
The chromatin material just before cell division forms into chromosomes. Each chromosome is made up of two longitudinal strands called the chromatids.
The chromosomes are present in pairs. The pairs are called the homologous pairs. A species will always have the same number of chromosomes. This is called the chromosome number and it will always be an even number. This number is called the diploid number. During gamete formation, the homologous chromosomes separate and the gametes will have only half the number of chromosomes. This number is called the haploid number. Thus the somatic or the vegetative cells of all organisms are diploid and the gametes are haploid.
The chromosome numbers of some of the plants and animals are given below:
Somatic cell(2n) Chromosome No.
Somatic (2n) Chromosome No.
Whatever may be the number of chromosomes, it is possible to keep track of the behaviour of these chromosomes as they are all different in some respects. They are of a specific shape and carry specific genes at specific locations.
The pictorial representation of the entire set of chromosomes is called the karyogram.
Autosomes and Sex Chromosomes
As can be seen from the human karyogram, the chromosomes are classified into two types, autosomes and sex chromosomes. The autosomes have homologous chromosomes as pairs whereas the sex chromosomes are of two different types - X and Y. A female has two X-chromosomes and a male has an X and a Y chromosome.
Each chromosome has a double helical DNA molecule.
Introduction to Types of Chromosomes
What are chromosomes?
The word 'chromosome 'Â has the literal meaning of 'coloured body'Â as "chroma"Â means colour and "Â soma"Â refers to body.Basically these can be defined as the condensedÂ nuclear reticulum which become visible during the late prophase or early metaphasre of mitotic /meiotic cell divisionsÂ IN the eukaryotic nucleus, tjhe DNA remains associated with the histone (rich in amino acids like arginine; lysine etc) proteins H2A, H2B, H3and H4 to form the nucleosome .This nucleosome solenoid undergoes first ,second and third order packaging to form the chromatin fiber whose aggregation forms the chromosomes. The chromosomes underÂ a microscope are visible when stained with dyes likeÂ acetocarmine. A chromosome typically consists of the chromatids, centromere, primary and secondary constrictionÂ regions (with satellite), pellicle, matrix and chromonemata, kinetochore
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Relationship between DNA, histones, Chromatin,Chromosome
Types of Chromosomes:
Â Â Â Â Â The chromosomes that are not directly concerned with reproduction and sex determination are called autosomes.These are Â Â Â Â Â Â Â identical in both the two sexes in manÂ and have loci occupied by autosomal genes.In man, outÂ of 46 (2n) chromosomes,Â Â Â Â Â Â Â Â Â 44Â or 22 pairsare te autosomes and carry genes that control expression ofÂ phenotypic characters.The term "autosome " w Â Â Â Â Â Â was Â coined by T.H. Montogomery in 1904.
B)Â Â AllosomesÂ /Â Heterosomes
Â Â Â Â TheseÂ chromosomes are directlyÂ associatedÂ witjh reproduction and differ from allosomes in size, form and behaviour> Usually there area single pair of allosomes in mammals termed as 'X"Â and "Y"Â chromosomes. In birds however,they are namesd as WÂ and Z respectivelyIn bugs of HeteropteraÂ like locusts, the female has two X chromosomes while the male has one X.The Y chromosome is absent in these species.
Normal human Karyotype
Tyes on Basis of Position of Centromere
On the basis of the position of theÂ centromere, the chromosomes can be of theÂ followngÂ types:
Acroentric:They are also calledÂ submetacentricÂ and containÂ centromereÂ at almost one endÂ giving t the rod shape.One arm is extremely short in this type.it looksÂ like 'J "Â inÂ Â anaphase.it can be observed in locust.
TelocentricÂ : TheÂ centromereÂ is present at one end hence one arm is almost equal to the length of the chromosome while the other isÂ iikeÂ Â Â Â Â aÂ dot.they look like "I"Â duringÂ anaphaseÂ and are rare .
SubmetacentricÂ : theÂ centromereÂ is present almost at the centre. I tÂ appearsÂ like "L" duringanaphase.
MetacentricÂ :Â Â Â Â Â TheÂ centromereÂ is placed a tÂ theÂ middle of theÂ lengthÂ of the chromosome giving it "V" shape at anaphase. Â Classification of chromosomes on position ofcentromereÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
Homologous and Non Homologous Chromosomes
Â The chromosomes that can pair during meiosis forming a sypapse and have the genes for the same traitsÂ at the same lociÂ Â Â Â Â Â Â Â ( alleles may be homo or hetero) are called homologousÂ chromosomes.These are similar in length (other than theÂ sex chromosomes in certain taxa.The holomologous chromosomesÂ have a pair of sister chromatids that synapse and crossing over occurs between interchangeable segments.
Non Homologous chromosomes: I
In non homologous chromosomes ,eachÂ Â chromosome of the pairÂ Â is obtainred from theÂ each parent in diploidsÂ and contains all theÂ genepool of that organism .In polyploids, there may be more than two pairs in a set formingÂ polyploidy
Â from a set .
DNA or Deoxyribonucleic Acid
You tube video: DNA Structure
DNA is a double stranded molecule (double helix) with each strand being made up of many nucleotide units. Each nucleotide is made up of a nitrogenous base, a sugar molecule and a phosphoric acid molecule.
Each DNA molecule has two such polynucleotide chains joined to each other by hydrogen bonds. The hydrogen bonds are between the nitrogenous bases of the opposite nucleotides.
If the polynucleotide chains are the sides of the ladder, the bonds can be compared to the rungs of the ladder. The whole molecule can be likened to a rope ladder twisted around to from a helical structure. The DNA molecule is twisted around a core of proteins.
Short chains of the DNA form the genes. Each gene is, therefore, a series of nucleotides in a particular sequence. The genes are arranged in a linear manner on the chromosomes. The position of a gene on the chromosome is called the locus. It always remains the same for a gene. The alleles of a gene are present on the homologous chromosomes at the same loci. Each gene codes for a particular trait. The proteins part of the DNA control the characteristics of a trait.
Just when Mendel was making his observations it was the American scientist William Sutton who noticed the similarities between the behaviour of the Mendelian factors or genes and that of the chromosomes during meiosis. He noticed the following similarities:
Both occurred in pairs - genes as alleles and chromosomes as homologous chromosomes.
Both separated from the pairs and entered the gametes independently - each gamete received only one of the alleles of a pair and also received only one of the homologous chromosomes during meiosis.
The pairs of both are restored during fertilization - the zygotes receive one allele each from the two parents through the gametes and the zygotes also receive the two homologous chromosomes, one from each parent, during fertilization. The above observations led Sutton and Boveri to formulate the Chromosomal theory of inheritance, independently in 1902. According to this theory,
Each adult organism grows from a zygote. The zygote is a diploid cell having two sets of chromosomes - maternal and paternal. The zygote by mitotic divisions results in the formation of an adult and thus, all the vegetative or somatic cells of an organism are diploid with two sets of chromosomes.
The chromosomes maintain their individuality through out the life cycle of the organism.
The chromosomes contain the Mendelian factors or genes, which determine the various characteristics of an individual.
In asexual reproduction similar rules of inheritance are followed. Here the DNA material is distributed equally to their progeny. Here, the paternal and maternal difference is not present as the chromosomes involve a single parent.
From the above observations of production of different traits, it is clear that in sexual reproduction, the genetic materials from the two sexes participating are different from each other. All human chromosomes are not paired similarly, the sex chromosomes in men is odd in not always being a perfect pair. It is a mismatched pair in which one is a normal-sized X while the other is a short one called Y. Women have a perfect pair of sex chromosomes, both called X. So women are XX, while men are XY.
The sex of organisms is dependent on different factors of environment, temperature or choice. However, in human beings, the sex of the individual is largely genetically determined. Parental genes decide whether the offspring will be boy or girl. How is the sex of a newborn human determined by genetic inheritance?
During the formation of gametes, the females will have only one type of gametes, all with one X chromosome. However, the males will produce two types of gametes or sperms, half with X chromosome and half with Y chromosome. Thus the sex of a zygote is determined by which male gamete fuses with the female gamete.
If the X gamete fuses with the female gamete (also X) the zygote will be a female. If the Y gamete fused with the female gamete, the zygote will be a male. This type of sex determination is called XX-XY type.