Gene • Genes act as units of heredity, transferring the same instructions from

parent to offspring.

• Genes are functional units of DNA that contain the instructions for making

proteins or RNA.

• Each somatic cell has two copies of a gene, while gametes have only one

copy

• The name "gene" was coined

by Johannsen in 1909

Allele

• Different forms of the same gene

• Occur at loci on chromosomes

• Alleles govern the contrasting forms of a character

• The genes are particulate. the two alleles of a gene do not modify each

other when they stay together in the same cell

• They separate and pass into different gametes of the hybrid

Locus – fixed position of a gene on a chromosome

Note in the diagram that the genetic sequence of the two alleles is different. 

Note that the gene will likely function the same, even though the genetic sequences are slightly different

Character/ trait = a feature that can be inherited

Quantitative traits

Qualitative traits

S.No. Quantitative traits Qualitative traits

1 Governed by polygenes By oligogenes

2 Effect of each gene is not detectable

Detectable

3 Variation is continuous Discontinuous

4 Separation into different classes is possible

Separation into different classes is not possible

5 Highly influenced by environmental factors

Little influenced by environmental factors

Homozygous and Heterozygous

A certain terminology is used to describe how each individual has inherited the alleles of specific genes.

Homozygous - having an identical pair of alleles for a specific character or feature

Heterozygous - having different alleles for a specific character or feature

Hemizygous

A condition in which only one of the alternate allele of a gene pair is present as in sex linkage or as a result of deletion

Dominant • One allele express itself at the expense of the other in the heterozygote

• Only one allele at a locus is expressed. The other (recessive gene) is suppressed

Recessive

• An allele, which is incapable expressing itself phenotypically

• An allele that is suppressed in the presence of a dominant gene. If the two recessive alleles occur at a locus, then the trait is expressed.

Codominant

• Both alleles at a locus are expressed with equal force

Genotype

The genetic constitution of an organism

Not very easily influenced by envt.

Predicted based on phenotypic ratios

Phenotype

The observable characteristics of an organism

Can be altered by envt.

Can be measured or grouped in to distinct classes

Generations

• P generation - represents parents

• F1 generation (first filial) - offspring of these (P) parents

• F2 generation (second filial) - offspring of these (F1)

parents

Monohybrid cross

Hybridization between two individuals or strains, which differ for a single character

Dihybrid cross

Hybridization between two individuals or strains, which differ for two characters

Polyhybrid

The offspring of parents differing from each other in more than two characters.

BackcrossThe cross of an F1 hybrid to one of its parents

TestcrossThe cross of an F1 hybrid with an individual or strain having the recessive phenotype

Purpose: to verify the ratio in which the different gametes of the F1 hybrid are produced

It crosses a dominant phenotype (P_) with a recessive phenotype (pp). 

If the cross creates:

PP x pp -> Pp 100% purple offspring, or

Pp x pp -> Pp 50% purple offspring and

pp 50% white offspring

so it proves the existence of a  recessive allele in the dominant parent.

Punnett Squares - These are the tools that we use to determine the genetic possibilities of various combinations of parents

BB Bb Bb bb¼ 2/4 ¼

3 : 1

MENDEL’S EXPERIMENTS

• Born in 1822 in Brunn (Austria), now Brno in Czechoslovakia

• Joined Monastery in 1843

• In 1847, became priest

• In 1854, appointed as substitute science teacher

• In 1857, began his famous experiments on pea

• In 1865, presented his paper before Natural Science Society of Brunn

• In 1866, his paper ‘Experiments on plant hybridization” published in the proceedings of the society

• Died in 1884 at an age of 62 years

Gregor Johann Mendel

Mendel's Monastery Garden

Selection of pea as an experimental material

• Self pollinated

• Large flowers

• Seeds are large; Easy to grow

• Needs small space

• Reproduces in one season

• Varieties had contrasting forms of several characters

Seven pairs of contrasting traits

TraitDominant

ExpressionRecessive Expression

Form of ripe seed (R) Smooth (Round) Wrinkled

Color of seed coat (Y) Yellow Green

Color of flower (P) Purple White

Form of ripe pods (I) Inflated Constricted

Color of unripe pods (G) Green Yellow

Position of flowers (A) Axial Terminal

Length of stem (T) Tall Dwarf

Several characters had two contrasting forms

Purple x White White x Purple

Crossing or hybridization

S.No. Structure Character Dominant Recessive Ratio in F2

1 Seed Form 5,474 (R) 1,850 (W) 2.96 : 1

2 Cotyledon Colour 6,022 (Y) 2,001 (G) 3.01 : 1

3 Seed coat Form 882 (I) 299 (C) 2.95 : 1

4 Seed coat Colour 705 (G) 224 (W) 3.15 : 1

5 Unripe pods Colour 428 (G) 152 (Y) 2.82 : 1

6 Flowers Position 651 (A) 207 (T) 3.14 : 1

7 Stem Length 787 (L) 277 (S) 2.84 : 1

Results of Mendel’s original crosses for seven pairs of characters

Hybrid

Aa They do not mix or blend

Hybrid

Aa

Hybrid

Aa

Hybrid

Aa

Remain together in a pure form without affecting each other (Law of purity of gametes)

Separate into different gametes in equal numbers Separation occurs during Anaphase I

When there is complete dominance, segregation leads to phenotypic ratio 3:1

Mendel's First Law The law of segregation

Round Yellow seeds x Wrinkled Green seeds

Mendel's Second Law The law of Independent Assortment/

Law of free recombination

During gamete formation the segregation of the alleles of one gene is independent of the segregation

of the alleles of another gene

Main features of SECOND LAW

• Explains simultaneous inheritance of two plant characters

• In F1 two gene controlling two different characters, come together, each gene exhibits independent dominant behaviour without affecting or modifying the effect of other gene

• Segregation occurs during gamete formation – independently

• Allele of one gene can freely combine with the allele of another gene – equal chance

The two laws of Mendel, especially the law of segregation, are the basis of the science of genetics, and Mendel is aptly known as the

‘Father of Genetics’

Rediscovery of Mendel's work

• Mendel presented his paper in 1865 but the scientific world failed to

recognize its importance for nearly thirty- five years.

• Hugo de Vries in Holland working on Oenothera; Carl Correns in

Germany working on peas and maize and Eric Tschermak in Austria

obtained results similar to those obtained by Mendel, but none of them

had seen Mendel's paper before beginning their own experiments.

• The results were published by

Hugo de Vires in March 1900

Carl Correns in April 1900

Eric Tschermak in June 1900

• The rediscovery initiated a spurt of research activity in the field of genetics

• Thus the ‘Science of Genetics’ was truly borne