PRINCIPLES OF INHERITANCE AND VARIATION

 

PRINCIPLES OF INHERITANCE AND VARIATION

• Genetics: deals with the inheritance, as well as the variation of characters from parents to offsprings.
• Inheritance: is the process by which characters are passed on from parent to progeny.
• Variation: is the degree by which progeny differ from their parents.

MENDEL’S LAWS OF INHERITANCE:

• Gregor Mendel. Conducted hybridization experiments on garden peas for seven years (1856 – 1863) and proposed laws of inheritance.
• Mendel conducted artificial pollination/cross pollination experiments using several true-breeding pea lines.
• A true breeding line is one that, having undergone continuous self-pollination for several generations.
• Mendel selected 14 true-breeding peas’ plant varieties, as pair’s which were similar except for one character with contrasting traits.
  • True breed selected by Mendel
  • Stem height- Tall / dwarf
  • Flower color- Violet/white
  • Flower position – Axial / terminal
  • Pod shape- Inflated / beaded or constricted
  • Pod color- Green / yellow
  • Seed  color- Yellow/ green
  • Seed shape – round / wrinkled

INHERITANCE OF ONE GENE:

• Mendel crossed tall and dwarf pea plants to study the inheritance of one gene.
• He collected the seeds produced as a result of this cross and grew them to generate plants of the first hybrid generation. This generation is called filial progeny or the F₁.
• Mendel observed that all the F₁ progeny plants ere tall, like one of its parents; none were dwarf.
• He made similar observations for the other pairs of traits – he found that the F₁ always resembled either one of the parents, and that the trait of the other parent was not seen in them.
• Mendel then self – pollinated the tall F₁ plants and to his surprise found that in the F₂ generation some of the offsprings were ‘dwarf; the character that was not seen in the F₁ generation was now expressed.
• The proportion of plants that were dwarf was 1/4^th of the F₂ plants while 3/4th of the F₂ plants were tall.
• The tall and dwarf traits were identical to their parental type and did not show any blending, that is all the offsprings were either tall or dwarf, none were of in between height.
• Similar results were obtained with the other traits that he studied: only one of the parental traits was expressed in the F₁ generation while at the F₂ stage both the traits were expressed in the proportion of 3:1.
• The contrasting traits did not show any blending at either F₁ or F₂ stage.

 Mendel’s proposition:

• Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as ‘factors’.
• Now a day we call them as genes.
• Gene is therefore are the units of inheritance.
• Genes which codes of a pair of contrasting traits are known as alleles, i.e. they are slightly different forms of the same gene.

Alphabets used:

• Capital letters used for the trait expressed at the F₁ stage.
• Small alphabet for the other trait.
• ‘T’ is used for Tall and‘t’ is used for dwarf.
• ‘T’ and‘t’ are alleles of each other.
• Hence in plants the pair of alleles for height would be TT. Tt. or tt.
• In a true breeding tall or dwarf pea variety the allelic pair of genes for height are identical or homozygous, TT and tt respectively.
• TT and tt are called the genotype.
• Tt plant is heterozygous for genes controlling one character (height).
• Descriptive terms tall and dwarf are the phenotype.

Test cross:

• When F₁ hybrid is crossed back with the recessive parent, it is known as test cross.
• It is used to know the genotype of the given plant/animal.

Law of Dominance:

• Characters are controlled by discrete units called factors.
• Factors occur in pairs.
• In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive).

Law of Segregation:

• The alleles do not show any blending and that both the characters are recovered as such in the F₂ generation though one of these is not seen at the F₁ stage.
• The parents contain two alleles during gamete formation; the factors or alleles of a pair segregate or separate from each other such that a gamete receives only one of the two factors.
• Homozygous parent produces all gametes that are similar i.e contain same type of allele.
• Heterozygous parents’ produces two kinds of gametes each having one allele with equal proportion.

CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
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