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Principles of Inheritance and Variation





Introduction

The science primarily concerned with precise understanding of biological properties (genes), which are transmitted from parents to offspring’s is called genetics.
The process of transmission of characters from one generation to another is known as Inheritance or heredity.

MENDEL & HIS EXPERIMENTS

Mendel conducted experiments on garden pea (Pisum satirum) for seven years (1856-1863), before proposing the laws of inheritance.
He selected garden pea because of two reasons:
(1) Many varieties were available with observable alternate forms for a trait/ character.
(2) The pea plant was easy to cultivate & from one generation  to next took only a single grown season.
(3) Pea had many sharply defined inherited characters. Thus, they possess many desirable features.
(4) The cross pollination  & self-pollination can also be achieved easily.
(5) The flowers are sexual & hermaphrodite.
(6) The selected varieties that differed with respect to seven traits with easily distinguishable contrasting forms.
He selected fourteen varieties as shown in the table below:
S. No.
Characters
Dominant form
Recessive form
1
Stem height
Tall (T)
Dwarf (t)
2
Seed shape
Round (R)
Wrinkled (r)
3
Seed colour
Yellow (Y)
Green (y)
4
Pod shape
Inflated (I)
Constricted (i)
5
Pod colour
Green (G)
Yellow (g)
6
Flower colour
Violet (V) or Purple (P)
White (v) or (p)
7
Flower position
Axial (A)
Terminal (a)
 

MENDEL’S APPROACH
  • Mendel’s methodology during the investigation of the inheritance pattern included mathematical logic and statistical analysis of the data.
  • His experiments had  a large  sampling size, which gave greater credibility to the data that he collected.
  • Mendel first made sure that each of the fourteen varieties seven pairs of contrasting forms was true breeding, by allowing successive generations to self-pollinate and eliminating any offspring that was not true-breeding.
  • He studied the inheritance of each of the seven characters individually by conducting monohybrid crosses and then in combinations by conducting dihybrid and trihybrid crosses.
  • He hybridised/cross pollinated plants with alternate forms of a trait and used the seeds to generate the First filial (F1) or First hybrid generation.
  • He allowed each F1 offspring to self-pollinate to produce the second filial (F2) generation.
  • He also conducted test crosses.
 
MENDEL'S  OBSERVATIONS
(1) The F1 hybrids always showed one of the parental forms of the trait.
(2) Both the parental forms of the trait (contrasting forms of the trail) appeared without any change in the F2 generation.
(3) The two contrasting forms did not show any blending either in the F1 generation or F2 generation.
(4) The form of the trait that appeared in the F1 hybrids is called dominant form & it appeared in the F2 generation about three times in frequency as its alternate (recessive) form.
MENDEL’S  INFERENCES
  • The characters are controlled by some “factors” that are stable passed down without any change from the parents to the offspring through the gametes.
  • The factors occur in pairs.
  • In a pair of dissimilar factors of a trait, one of them is dominating & the other is recessive.
 
 

THE INTERPRETATION OF MENDEL’S RESULTS

The following principles of inheritance were given by Mendel-
(1) Principle of dominance.
(2) Principle of segregation or Purity of gametes.
(3) Principle of independent assortment.
 
LAW OF DOMINANCE(First Law)
The law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 & the expression of both in the F2.
This law states that “When two individuals of a species, differing in a pair of contrasting forms of a trait are crossed, the form of the trait that expresses itself is called dominant while the other which has not shown its effect is termed as recessive.”
  • When a cross was made between a true breeding Tall pea plant (TT) and a true-breeding dwarf pea plant (tt), all the plants in F1 generation were tall.
  • When the F1 individuals were  allowed to self-pollinate and F2 generation was raised, it was found that the tall plants & the dwarf plants were in the ratio of 3:1.
  • When the dwarf plants were further self-pollinated, they produced only dwarf plants in successive generations, showing that they are homozygous/ true breeding.
  • When the tall plants were self-pollinated, some of them produced only tall plants in the successive generation, while others produced both tall and dwarf plants, showing that they are heterozygous.

Following conclusion can be made for laws of dominance:
(1)Characters are controlled by discrete units called factors.
(2) Factors occur in pairs.
(3) In a dissimilar pair of factors, one member of the pair is dominant while the other fails to appear is called recessive.
 
 
LAW OF SEGREGATION (PURITY OF GAMETES- Second Law)
This  principle states that “The member of the allelic pair that remained together in the parent, segregate/ separate during gamete formation & only one of the factors enters into a gamete.”
As a result, the gametes are pure for a character (have only one of the alleles).
A homozygous individual produces only one type of gametes, while a heterozygous individual/ hybrid produces two types of gametes in equal frequencies.
To understand the idea of law of segregation monohybrid cross is taken:
  • For e.g. When a cross was made between a true breeding tall pea plant (TT) and a true breeding dwarf pea plant (tt), all the plants in F1 generation were tall.
  • When the F1 individuals were allowed to self-pollinate and an F2 generation was raised, it was found that the tall plants & the dwarf plants were in the ratio of 3:1.
  • In this case tallness is dominant  over dwarfness & it has appeared in F1 generation.
  • The recessive character dwarfness has remained hidden in the F1 generation but appeared again in the F2 generation.
  • The two forms of the trait, height of stem, have appeared in the ratio of 3:1, which is called Mendel’s monohybrid phenotypic ratio.
  • It is because the two factors T & t remained together in the F1 hybrid but segregated from each other and entered into different gamete. The paired condition is restored on random fertilisation.
  • From Punnett Square it can be seen that one of the tall plants is homozygous(TT) while the other two are heterozygous(Tt).
  • Hence the monohybrid genotypic ratio is 1:2:1 ( 1 is true breeding dominant :2 hybrid or heterozygous dominant : 1 true breeding recessive)
 
LAW OF INDEPENDENT ASSORTMENT(Third Law)
The law states that “The genes of different characters located in different pairs of chromosomes are independent of one another in their segregation during gamete formation (meiosis).”
The principle of independent assortment can also be defined as “If we consider the inheritance of two or more genes at a time, their distribution in the gametes and in progeny of subsequent generations is independent of each other.”
This states that the different factors or allelomorphic pairs in gametes & zygotes assort themselves and segregate independently of one another.
After considering the character pair singly, Mendel now began his experiments with two pairs of characters simultaneously & thus obtained the dihybrid ratio.
The following cross between a pure breeding plant with yellow, round seeds (RRYY) & another pure breeding plant with green, wrinkled seeds (rryy) can be taken as an example of this law.

In the cross the factors for colour seeds & those for shape of seeds have segregated independently & each gamete has one factor for each of these two traits.
 

IMPORTANCE OF MENDELISM

  1. Improvement of Plants:- Hybridization is used for obtaining improved  varieties of plants. This process results in combinations of desirable characters of two or more species or varieties. 
  2. Improvement of Animals:- Mendelism  has enabled the plant breeders to improve the races of domestic animals.
  3. Improvement of Human race:- Laws of heredity postulated by Mendel are equally applicable to mankind.
  4. Disputed parentage:- Study of inheritance of the blood group can solve the disputed parentage of a child.
  5. Genetic Counselling:- With the knowledge of Mendelism, genetic counselors can predict the possibility of hereditary defect in a future (unconceived child) and even detect genetic disorder in early Foetus.

REASONS FOR MENDEL’S  SUCCESS

  • His choice of plant as pea plant (Pisum sativum) for his breeding experiments was excellent.
  • Mendel kept a complete record of every cross.
  • He also used statistical methods & law of probability for finalizing his results.
  • Mendel was fortunate also that the characters which by chance he selected for his breeding experiments did not show linkage, incomplete dominance, gene interaction etc.

TEST CROSS

It is a cross derived by Mendel where the offspring or individual with dominant phenotype whose genotype is not known is crossed with an individual homozygous recessive for the trait.
A monohybrid test cross is as follows:-
The  F1 hybrid of a cross between heterozygous tall plant and a pure dwarf plant i.e., Tt X tt
Diagram of monohybrid test cross  

The progeny consists of tall and dwarf plants in the ratio of 1:1
The monohybrid test cross ratio is 1:1
A dihybrid test cross is shown below:   
The F1 hybrid of a cross between heterozygous round and yellow seed and a pure wrinkled and green seed i.e., RrYy X rryy
Diagram of dihybrid test cross  

The dihybrid test cross ratio is 1:1:1:1
Since this cross is used to determine the genotype of an individual, it is called a test cross.

INCOMPLETE DOMINANCE

In incomplete dominance the genes of an allelomorphic pair are not expressed as dominant and recessive but express themselves partially when present together in the hybrid. As a result F1 hybrid shows character intermediate to the effect of two genes of the parents.
The inheritance of flower colour in dog flower/ snapdragon (Antirrhinum majus) is an example of this phenomenon.
When a cross was made between a red flowered plant and a white flowered plant, the F1 hybrid was pink.
When the F1 individual was self-pollinated/ self-hybridised, the F2 generation consisted of red, pink and white flowered plants given below: 
monohybrid-cross-snapdragon.PNG
The phenotypic and genotypic ratio are the same, i.e., 1: 2: 1 [1 red(RR), 2 pink (Rr): 1 white (rr)].

CO- DOMINANCE

When the dominant character is not able to suppress even incompletely the recessive character and both the characters appear side by side in F1 hybrid, the phenomenon is called co- dominance.
For e.g. In cattles, if a cattle with black coat is crossed to a cattle with white  color, the F1 hybrid possesses a roan coat. In a roan coat both black & white patches appear separately.
The alleles which are able to express themselves independently when present together are called co- dominant alleles.
 Another example of co-dominance is AB blood group in which both alleles are co- dominant.

MULTIPLE ALLELES

When more than two allelic forms of wild type are located on the same locus in a given pair of chromosomes, they are known to compose the series of multiple alleles.
Multiple alleles possess the following characteristics
(a) multiples alleles are  located at the same locus in the chromosome.
(b) multiple alleles regulate a particular character.
(c) the process  of crossing over is not exhibited by multiple alleles among themselves due to their location on the same locus.
(d) multiple alleles may show the dominant or intermediate phenotypes, while wild type of series is usually dominant.
A well- known example of this phenomenon is the inheritance of ABO blood group in humans.
  • The gene for blood group exists in three allelic forms , IA, IB and i (IO).
  • Any individual carries two of these alleles.
  • The alleles IA produces glycoprotein A, found on the surface/ membrane  of red blood cells.
  • The allele IB produces glycoprotein B, found on the surface/ membrane of red cells.
  • The allele i (IO) does not produce any glycoprotein.
  • The allele IA and IB is dominant over i.
  • The blood group of the person is determined by the presence or absence of one or both the glycoprotein, i.e., group A has glycoprotein A, group B has glycoprotein B, group AB has both the glycoproteins while group O has neither of them.
The following table shows the blood groups & their possible genotypes:

  The inheritance of blood group character follows the Mendelian pattern of inheritance.

PLEIOTROPY

 It is a phenomenon in which A single gene may produce more than one effect.
Even in garden peas, such  phenomena has been observed in the following character
(a)Starch synthesis/ size of starch gains & the shape of seeds are controlled by one gene BB/bb.
(b) Flower color & seed coat color are found to be controlled by the same gene.
 

COMPLEMENTARY GENES

When two (non-allelic) genes complement the effect of each other to produce a phenotype, they are called complementary genes.
Flower color in Lathyrus odoratus (sweet pea) is due to complementary genes, where one gene complements the expression of another gene.
The dominant allele ‘P’  determines the formation of purple color; but PP or Pp does not express the color unless another dominant allele of a gene CC or Cc is present along with them.
A cross between a pure purple (PPCC) and white (ppcc) flowered plants produces an F2, where the ratio between purple flowered and white flowered is 9:7 (instead of the normal Mendelian ratio of 9:3:3:1).

 

REDISCOVERY OF MENDEL’S LAW

Though Mendel published his work and the laws of inheritance in 1865, they remained unrecognised till 1900 because of following reasons:
(1) His work could not be widely publicised as communication was not easy.
(2) His concept of ‘factors’ as stable & discrete units that controlled the expression of traits  and that of pair of alleles which did not blend with each other, were not accepted by his  contemporaries as the explanation for variation.
(3) Though Mendel’s work suggested that factors were discrete units, he could not provide any physical proof for the existence of factors or prove what they are made of.
(4) In 1900, Hogo de Vries, Correns and Tschermak independently rediscovered Mendel’s results on the inheritance of characters.
(5) By then, there had been advancements in microscopy & scientists were able to observe cell division, nucleus, chromosomes, etc.
(6)By 1902, chromosome movements during cell division had been worked out.
 

CHROMOSOMAL THEORY OF INHERITANCE

Walter Sutton and Theodor Boveri independently postulated  this theory in 1902.
They found that the behavior of chromosomes was parallel to the behavior of mendelian factors (genes) & used the chromosomes movements to explain Mendel’s Law.
The  similarities are as follows:
  • Both genes & chromosomes occur in pairs in normal diploid cells.
  • Both of these segregate during gamete formation and only one member of each pair enters a gamete.
  • Members of each pair segregate independently of the members of the other pair (s).
Sutton & Boveri argued that the pairing & separation of the homologous pair of chromosomes would lead to the segregation of a pair of factors they carried.


THOMAS HUNT MORGAN AND GENETICS

  • Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance.
  • Experimental verification of the chromosomal theory of inheritance by THOMAS HUNT MORGAN and his colleagues led to discovering the basis for the variation that sexual reproduction produces.
Morgan worked with the tiny fruit flies, Drosophila melanogaster which were found very suitable for such studies.
  1. They could be grown on simple synthetic medium in the laboratory.
  2. They complete their life cycle in about two weeks & a single mating could produce a large no. of progeny files.
  3. There were a clear differentiation of the sexes the male & female files are easily distinguish
Morgan carried out many dihybrid crosses in Drosphila, with the genes that were sex linked , i.e., the genes are present on the X- Chromosome.
He observed that the two genes under consideration in his experiments did not segregate independently as in the case of characters studied by Mendel.

LINKAGE AND RECOMBINATION

The inheritance of genes of the same chromosome together and capacity of these genes to retain their parental combination in subsequent generations is known as linkage.
  • Morgan et al observed that when the two genes in a dihybrid cross are located on the same chromosome show more linkage than the non-parental or new combination (also called recombination) of genes.
  • They also found that the proportion of recombinants varies, even if the two genes are present on the same chromosome.
  • If the linkage is stronger between two genes the frequency of recombination is low & vice-versa.
He hybridized yellow- bodies and white-eyed females with brown- bodies & red- eyed males (wide type) (cross 1) & intercrossed their f1 progeny.
The f2 generation contained the following:


The parental combinations were 98.7% & the recombinations were 1.3%.
In another cross between white bodied female fly with miniature wing & a male fly with yellow- body and normal wing (cross 2).
The progeny contained the following
The parental combination were 62.8% while the recombinant were 37.2%.
  • It is evident that the linkage between genes for white body and yellow eyes is stronger than that between genes for white body and miniature wings.
  • Sturtevant used the frequency of recombination between the genes pair on the same chromosome as a measure of the distance of the genes and mapped their position on the chromosome.
  • Today genetic chromosome maps are used in the sequencing of genes of organisms.
SIGNIFICANCE OF LINKAGE
The linkage does not permit the breeders to bring the desirable characters.
2- Linked characters are maintained for generations because linkage prevents the incidence of recombination.
CROSSING OVER
Crossing over is the process of exchange of genetic material or segments between non- sister chromatids  of two homologous chromosomes. Crossing over occurs due to interchange of sections of homologous chromosome.

SEX DETERMINATION

(1) The concept of genetic/ chromosomal basis of sex determination came from the cytological observation made in a no. of insects.
(2) H. Henking (1891) could trace a specific nuclear structure. All through spermatogenesis in a few insects.
(3) He observed that 50% of the sperms received this structure , while the remaining 50% did not receive it.
(4) Henking named the structure as ‘X’ body, but could not explain its significance.
(5) Later it was found to be a chromosome & it was named as X-chromosome.

XY type of sex determination
  • In many insects like Drosophila melanogaster & in human beings this type of sex determination is seen.
  • The males have an X-chromosome and another small but characteristically shaped Y-chromosome i.e. males have XY chromosomes along with autosomes.
  • The females have two X- chromosomes along with the other autosomes.
  • Females are homogametic whereas males are heterogametic.
  • Sex of an individual is decided at the time of fertilization by the type of sperm.
It is as given below:


XO type of Sex  determination
  • A large no. of insects loke grasshopper show XO type of sex determination.
  • All the ova/ eggs bear an X-chromosome, while some of the sperms bear an X-chromosome & some sperms bear no X chromosome (nil).
  • When an ovum is fertilized by a sperm having X-chromosome, the zygote develops into a female.
  • When the ovum is fertilized by a sperm having no X chromosome, the zygote develops into a male.
ZW- type of sex determination
  • This type of sex determination is seen in certain birds.
  • The females have ZW chromosomes along with the autosomes & the males have ZZ chromosomes.
  • In this case, the sex of the individual is determined by the type of ovum that is fertilized to produce the offspring.
  • In the cases of XO type & XY type of sex determination, the males are heterogametic, while the females are homogametic.
  • In the case of ZW type of sex determination the males are homogametic, while the females are heterganetic.


RECOMBINATION

The organisms which express characters of both the parents are known as recombinants & the events responsible for mixing maternal & paternal characters in sexually reproducing organisms are called recombinations.
New combinations appear in three  ways:
(a) By independent assortment of chromosomes at the time of formation of gametes.
(b) By random fertilization.
(c) By reciprocal recombination of linked genes in chromosomes by crossing over in Prophase-1 of meiosis.

MUTATION

The mutation can be defined as sudden, stable discontinuous and inheritable variations which appear in organisms due to permanent change in their genotype.
The physical & chemical agents/ factors that bring about mutation are called mutagens.
Mutations are also responsible for variations.
Mutations are of the following type:

GENE MUTATION
Gene Mutation or point mutations are stable changes in genes i.e. DNA chain.
Gene mutations result in alteration in the sequences of bases of DNA or a change in the base and there by change in the genotype & phenotype of an organism.
POINT MUTATION
A mutation that involves  alteration only single base change is called point mutation; A classical example of point mutation is sickle- cell anaemia.

FRAME SHIFT MUTATION
Deletion/ insertion/ duplication/ addition of one or two bases in the DNA results in a change in the reading frame, thereby resulting in a polypeptide with a different set of amino acids.

CHROMOSOMAL MUTATIONS
Chromosomal mutation only alters the no. or position of existing genes. They may involve modification in the morphology of chromosomes or a change in no. of chromosomes.
1. STRUCTURAL VARIATIONS
Structural alteration of the chromosomes results due to loss or gain of a large segment of DNA as DNA /genes are located on the chromosomes.
Deletion (deficiency):
  • Sometimes a segment of chromosome breaks off & gets lost.
  • If a terminal segment of a chromosome is lost, it is called deficiency and if intercalary segment is lost it is termed deletion.
Duplication:
  • Here a gene or many genes are repeated twice or more times in the same chromosome.
Translocation:
  • If the broken end of a chromosome joins another non- homologous chromosome, it is referred as translocation.
Inversion:
  • Sometimes as a result of a doubly broken chromosome, the centre piece is inverted.

2. NUMERICAL VARIATION
Aneuploidy
  • When the members of a homologous pair of chromosomes fail to segregate during meiosis, aneuploidy results; there is loss or gain of one or more chromosomes.
Polyploidy
  • Failure of separation of the duplicated chromosomes into daughter nuclei results in polyploidy, a phenomena in which the cell has three, four or more sets of chromosomes.

PEDIGREE ANALYSIS

Inheritance pattern of traits in human beings cannot be studied by crosses as in other organisms, for the following reasons:-
  • Controlled crosses cannot be performed.
  • The progeny produced is very small (usually one) & takes a long time.
Pedigree analysis is the important method to study  human genetics.
Symbols used in Pedigree charts

A record of the occurrence of a trait in several generations of a human family is called pedigree analysis. The person from whom the case history of a pedigree starts is called proband.
Autosomal Dominant
  • The trait never skips a generation & the marriage between normal & affected individuals leads to 1:1 ratio of normal to affected child.
  • There is no carrier stage.

Autosomal Recessive
  • Great majority of affected individuals have normal parents.
  • All  the children of two affected parents are affected.
  •  Normal  offspring from marriage of a normal & affected person.
  • 3:1 ratio of normal to affected children of heterozygous parents (carriers).

X-Linked Dominant
Pedigree shows affected males & normal wife transmitting the character to daughters only.
X-Linked Recessive
Affected male do not come from the affected father but from the carrier mother.
 

GENETIC DISORDERS

Genetic disorder can be grouped into two categories:
1-Mendelian disorder
2- Chromosomal disorder

MENDELIAN DISORDERS

  • These are mainly due to alteration or mutation in a single gene.
  • These disorders may be dominant or recessive.
  • The disorders are transmitted from one generation to the next following Mendel’s principles of heredity.
These disorder may be:
Autosomal as in cystic fibrosis, sickle cell anaemia & phenylketonuria or,
Sex-linked as in haemophilia, colour blindness & myotomic dystrophy
Some Mendelian disorder are discussed below:
HAEMOPHILIA
  • It is a sex linked recessive disorder.
  • The defective allele produces a defective protein, which is part of the cascade of proteins involved in the clotting of blood.
  • Clotting of blood is abnormally delayed that even a single/ small cut will result in non- stop bleeding in the affected individual.
  • Heterozygous female is a carrier & passes on the disease to some of her sons. E.g., Queen Victoria was a carrier of this disease & produced haemophilic descendents.
CYSTIC FIBROSIS
  • It is caused by a recessive mutant allele on an autosome (Chromosome 7)
  • The gene produces a unique glycoprotein that leads to the formation of mucus of abnormally high viscosity.
  • This type of mucus interferes with the functioning of many exocrine glands like sweat glands, liver, pancreas & lungs.
SICKLE CELL ANAEMIA (Autosomal recessive)
It is caused by a mutant recessive allele on chromosome 11.
The mutant gene causes the substitution of glutamic acid ( glu) by valine (val) at the 6th position of the beta globin chain haemoglobin.
The defective haemoglobin undergoes polymerization under low oxygen tension & changes the shape of RBC from biconcave cells to sickle shaped elongated cells.
The disease is controlled by a single pair of alleles, HbS and HbA of the three possible genotypes. Only individual homozygous for HbS (HbSHbS) show the disease, though heterozygous individuals (HbS HbA) are carriers.
PHENYLKETONURIA
  • This inborn error of metabolism is also inherited as the autosomal recessive trait.
  • It is caused by a recessive mutant allele on chromosome 12.
  • The affected individuals lack an enzyme that catalyses the conversion of the amino acid phenylalanine into tyrosine.
  • Consequently phenylalanine is metabolized into phenyl pyruvate & other derivatives.
  • Accumulation of these chemicals in the brain results in mental retardation.
  •  These are also excreted in the urine as they are not absorbed by the kidney.

CHROMOSOMAL DISORDERS

These are caused due to absence or excess or abnormal arrangement (str.) of one or more chromosomes. Such a situation leads to serious consequences in the individual.
Some of the chromosomal disorder are discussed below:
Down Syndrome
  • It is caused by the presence of an extra copy of the chromosome 21, i.e. trisomy of the 21st chromosome.
  • This disorder was first discovered by Langdown  (1866).
The affected person shows the following symptoms:
  • Mental retardation.
  • Broad flat face with slanting eyes.
  • Congenital heart diseases.
  •  Partially opened mouth with furrowed tongue.
  • Short stature & small round head with a flat back.
Klinefelter's Syndrome
  • It is caused by the presence of an extra X-chromosome in the male i.e. , XXY; the individual has 47 chromosomes.
  • Though the individual is a male; he shows a no. of feminized characters.
Following are the characteristic symptoms of  the disorder.
  • Poor beard growth.
  •  Sterility
  • Tall stature with feminized physique.
  • Breast development.
  • Female type of pubic  hair pattern.
Turner’s Syndrome
  • This disorder is caused due to the absence of one of the X-Chromosome in a female, the karyotype has only 45 chromosomes & they are called as XO females.
The following are the symptoms of an affected individual.
  • Webbed neck.
  • Rudimentary ovaries.
  • Poor development of breaks.
  • Sterility.
  • Shield- shaped thorax.
 



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