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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.
 
 
 



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