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class12 genetic basis of inheritance Mendel’s Experiment NEET

Mendel’s Experiments

  • Mendel selected a common garden pea (Pisum sativum) for his experiments because
  • Plants are annual with a short life cycle
  • Plants have bisexual autogamous flowers
  • Normally self-pollinated but can be artificially crossed
  • Produced a large number of progenies
  • Possessed a number of contrasting characters
  • Mendel’s experiments were performed in three stages :
  • Selection of pure or true-breeding parents.
  • Hybridisation and obtaining F1 generation of plants.
  • Self-pollination of hybrid plants and raising of subsequent generations like F2, F3, F4 etc.
  • Finally,Mendel selected seven pairs of contrasting characters :
  • Each character has two contrasting faces. Mendel suggested that there are certain elements or factors (named as genes by Johanssen in 1909), which control the two faces of a character.
  • Tall plant: 6 – 7 ft and dwarf plants 3/4 – 1ft.
  • Out of 14 chromosomes in Pisum sativum, characters are located on only 4 chromosomes (1, 4, 5, and 7).
  • Mendel obtained wrinkled seeds due to the absence of starch branching enzyme (SBE)
  • Flower colour and seed colour are related characters governed by the same gene (pleiotropic gene). Hence a plant with a white flower produces white seeds.

Reasons for Mendel’s Success

  • Mendel studied the inheritance of one character at a time, unlike his predecessors who had considered many characters at a time.
  • He maintained the statistical record of all the experiments.
  • He analysed the number of qualitative characters.
  • A combination of luck, foresight, mathematical ground and scientific aptitude contributed to the success of Mendel’s experiments.

Terminology

  • Character: The feature of the individual is called
  • character, e.g., stem height, flower colour, pod shape, seed shape, etc.
  • Trait:  An inherited character and its detectable variant is called trait, e.g., tall or dwarf, etc.
  • Allele or Allelomorph: The two contrasting aspects of the same character located at the locus in the homologous chromosomes are called alleles (Gk. word, meaning ‘belonging to one another’). The term was given by Bateson (1905). The allele indicates alternative forms of the same gene. However, presently the term is employed for any two forms of a gene present in the same locus in the two homologous chromosomes, viz., T-T, t-t or T-t.
  • Gene: A unit of DNA responsible for the appearance and inheritance of a character is called a gene. Gene is also defined as a unit of inheritance which consists of linear chromosome segment that can be assigned to the expression of a particular character.  The terms, gene and allele are interchangeable, but while genes can be used for any factor, the allele is used with reference to another allele.
  • Parents: Individuals having contrasting characters are called parents (P1 and P2)
  • First Filial (L. progeny) Generation (F1): The immediate hybrid obtained by crossing of P1 and P2 forms F1 and the subsequent generation produced by the self-fertilization of hybrids are called F2, F3, F4, etc.
  • Monohybrids: Hybrids for one pair of contrasting character (n = 1) are called Monohybrids.
  • Dihybrids: Hybrids for two pair of contrasting characters (n = 2) are called Dihybrids.
  • Trihybrids: Hybrids for three pairs  of contrasting
  • characters (n = 3) are called Trihybrids.         
  • Reciprocal crosses: This involves two crosses concerning the same characteristics, but with reversed sexes
  • Dominant: The character, which appears in F1 (e.g. P1) is called dominant.
  • Recessive: The character which does not appear in F1 but remains hidden (e.g. P2) is called recessive.
  • Phenotype (Gk. phainein = to appear; typos = image) ; The physical appearance of an individual (e.g. tall, dwarf, etc.) is called the phenotype.
  • Genotype (Gk. genos = race; typos =  image) : The genetic makeup of an individual (e.g. TT, Tt, tt, etc.) is called genotype.
  • The terms phenotype-genotype and pure line were coined by Johannsen (1911).
  • Homozygous: Organisms having similar alleles for a particular trait (e.g. TT, tt) are called homozygous. The homozygote is pure for the character and breeds truth on self-fertilization.
  • Heterozygous: Organisms possessing dissimilar alleles     (e.g. Tt) for a specific trait are called heterozygous. The heterozygote is not pure and is called hybrid for that character. It does not breed truth on self-fertilization.
  • Pure line: Generations of homogeneous individuals who produce offspring of only one type form a pure line.
  • Hemizygous: A condition, where one of the genes of a pair lost (T or t)  is called hemizygous.
  • Pseudoallele: in heterozygous condition, where a dominant allele is lost (becoming hemizygous), the recessive allele expresses its phenotype. This expression allele is called pseudoallele and the expression is called pseudodominace.       
  • W. Johannsen gave the concept of the line (1900), genotype and phenotype (1911).
  • Dominant Factor or Allele: It is one of a pair of alleles which can express itself whether present in homozygous or heterozygous state.

Recessive Factor or Allele: The factor of an allelic or allelomorphic pair which is unable to express its effect in the presence of its contrasting factor in a heterozygote.

Monohybrid Cross: Results

In monohybrid cross, one pair of contrasting aspects of the same character is considered. Mendel took pure tall pea plants and a pure dwarf pea plant and crossed them.

Each parental cross (P1 x P2) in homozygous conditions produces F1 hybrids, which are impure (heterozygous) genotypically but phenotypically they resemble the dominant parent (P1). From this result, Mendel concluded his first principle called Principle of dominance which states, that of the two factors, one that expresses in F1 is called dominant and the other which remains hidden is called recessive. in F1 hybrid (Tt) only ‘T’ expresses itself, so dominant and ‘t’ is masked, so recessive.

       Now, when such F1 hybrids are selfed, the following results are obtained in F2:       

       Phenotypic  ratio = 3 : 1 (Tall : dwarf)         Genotypic ratio =

From the results of F2 generation, Mendel concluded another principle called the Principle of segregation. This was converted by Correns as the first law. This law states that the two contrasting factors remain together in the hybrid (Tt) without being contaminated but whenever they get chance (during gametogenesis), they get segregated into pure forms. Mendel called this separating or segregation process the “splitting of hybrids”. The concept of segregation, identified as Mendel’s principle, can be phrased in this way: The paired genes (allelic pairs) separate from one another and are distributed to different sex cells.

Thus, on repeated selfing, 100% Tt will segregate into almost 50% TT and tt. This also suggests that hybrids (T t) are impure but their gametes (T or t) remain always pure.  So this law is also called the Law of Purification of Gametes.  

     In haploid organisms like bacteria and most fungi the characters are governed by only one member of an allelic pair,  hence the principles of dominance and segregation do not apply there.

  The process of segregation takes place in spore mother cells at the time of spore formation in plants. The activities of anaphase – I of meiosis provides a basis for segregation.

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