Mendelian Inheritance

What is genetics?

 ð  Transmission of genetically based characteristics from parents to off 

springs is called heredity.

 ð  Study of heredity is called Genetics.

Heredity and Variation:

 ð  Difference in characteristics between the individuals in a progeny is variation. It can be between any two people.

 ð  It has survival value for a population

 ð  Arises due to mutation or sudden change. Also when genes get shifted and exchanged during meiosis.

 ð  Fertilization is the random mixing of parental chromosomes with gene combinations. The source of variation which is usually common is called recombination.

Mendel the father of genetics:

 ð  Was an Australian monk. Used garden pea plant and submitted his results in 1865.

 ð  First to suggest principles of inheritance and is known as the father of genetics.

Genetics in a garden:

 ð  Two plants were cross bred and the new plant which arises was called a hybrid plant.

 ð  Pea plants were chose for the following reasons:-

a) Small and easy to grow                               b) True breeding varieties of pea plant were available

c) Large number of off springs were produced and it could complete its life cycle in one season.

d) Could be cross pollinated experimentally.

e) Plant has a number of contrasting characters

Hybridization experiment of Mendel:

 ð  Monohybrid is the hybrid plant with reference to one pair of contrasting characters.

 ð  Pure tall bred plant was crossed with Pure dwarf plant.

 ð  Emasculation process was carried out.

 ð  F1 generation plants were achieved.

 ð  F1 generation plants were self-pollinated.

 ð  F2 generation plants were achieved with tall to dwarf ratio as 3:1

Homozygous condition:

       A condition where the Genotype has two same alleles for a trait. The plant produces only one type of gamete and is usually pure bred.

Heterozygous condition:

       A condition where the genotype has 2 different and distinct alleles for a trait. One character is dominant and one character is recessive.

Phenotype:

      Physical or observable characteristics of an organism. Describes the external expressions. Phenotype ratio of F1 generation is 3:1

Genotype:

      Describes the actual genetic composition or genetic make-up. Genotypic ratio of F1 generation is 1:2:1

Gene:

      Unit of genetic information occupies a specific position on a chromosome and also comes in alleles.

Allele:

      It is a group of genes.

Dominant:

      Alleles which have the same phenotypic effect when present in both homozygous and heterozygous conditions.

Recessive:

      Alleles which have same phenotypic effect only in homozygous conditions.

The 7 characteristics of pea plants:

 ð   Purple and white flower

 ð  Axillary and terminal flower

 ð  Yellow and green seeds

 ð  Round and wrinkled seeds 

 ð  Green pod and yellow pod

 ð  Inflated pod and constricted pod

 ð  Tall and dwarf plant

Mendelian concepts of heredity:

   a)      Concept of Unit character:

 ð  Factors responsible for expression of characters are called unit characters or alleles.

   b)      Concept of purity of gametes:

 ð  It suggests a gamete carries only one gene which regards to one character

   c)       Concept of dominance:

 ð  The traits which appear in F1 generation are dominant and the traits which don’t appear in F1 generation are recessive, when two pure bred plants are cross pollinated.

   d)      Law of segregation:

 ð  In a cross involving a single character, the factors responsible for the two contrasting traits stay together in F1 generation but segregate during the formation of gametes.

e  e)      Law of independent assortment:

 ð  In a di hybrid cross where two contrasting characters are brought together, factors responsible for 2 pairs of contrasting characters stay in F1 generation but assort independently in during the formation of gametes.

Back cross:

 ð  Cross between the F1 hybrid with one of its parents or with an individual genetically identical to the parent

Test cross:

 ð  Cross between the F1 hybrid and recessive plant is known as test cross.

 ð  Significance of test cross:-

a)      It is conducted to make sure of the genotype of F1.

b)      Determines if dominant character is because of a homozygous or heterozygous condition.

c)       50%  show dominance and 50% show recessive.

d)      Both monohybrid and di hybrid test cross are conducted.

Mono hybrid test cross.

Di hybrid test cross.

Incomplete dominance:

 ð  Expression of dominant gene is incomplete or partial.

 ð  E.g.:- Color of a flower.

Multiple alleles:

 ð  More than two genes occupying the same locus on some pair of homologous chromosomes.

 ð  E.g.:- Inheritance of blood groups in human beings has 4 groups i.e. A, AB, O and  B which are formed with the presence of 3 genes.