Mendel’s Laws

Of

Inheritance

Vasanthan V09MTBI29

GREGOR JOHANN MENDEL

•1822 - 1884•Austrian monk•Experimented with pea plants•He thought that ‘heritable factors’ (genes) retained their individuality generation after generation•Principles of genetics were developed in the mid 19th century•Experimented with pea plants, by crossing various strains and observing the characteristics of their offspring•Gregor mendel is the father of modern genetics.

•Gregor Mendel–studied inheritance of seven traits in pea plants and first used the term dominance and recessiveness•Proposed similar but separate inheritable characters, one from each parent, later to be called genes.

INHERITANCE Parents and offspring often share observable

traits.

Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent organisms to their offspring; it underlies much of genetics.

They were initially derived from the work of Gregor Johann Mendel published in 1865 and 1866 which was "re-discovered" in 1900, and were initially very controversial..

THE UNITS OF INHERITANCE ARE ALLELES OF GENES

Traits are controlled by alleles - alternate forms of a gene Found on homologous chromosomes at a

particular gene locus The dominant allele masks the

expression of the other allele - the recessive allele

Genotype refers to the alleles an individual receives at fertilization Homozygous - an organism has two

identical alleles at a gene locus Heterozygous - an organism has two

different alleles at a gene locus Phenotype - the physical appearance of

the individual.9-5

DOMINANT AND RECESSIVE

A dominant allele is represented by a CAPITAL letter. It is always expressed when present. (BB or Bb)

A recessive allele is represented by lower case letter. It is only expressed when an individual has 2, one from mom and one from dad. (bb only)

GENOTYPE AND PHENOTYPE

A genotype is the genetic make-up of an individual, expressed in letters. (BB, Bb, bb)

A phenotype is the physical appearance of an individual, determined by his or her genotype. (black, brown, short, tall, etc)

HOMOZYGOUS AND HETEROZYGOUS

Homozygous: when both alleles of a genotype are the same (either both dominant, BB, or both recessive, bb)

Heterozygous: when one allele is dominant and one is recessive (Bb only)

PUNNETT SQUARES

Punnett Squares are used to show the mating of two parents and the possible offspring they can produce.

MENDEL OBSERVED SEVEN PEA PLANT TRAITS THAT ARE EASILY RECOGNIZED IN ONE OF TWO FORMS: 

1.        Flower color : purple or white 2.        Flower position : axial or terminal 3.        Stem length : Tall or Dwarf 4.        Seed shape : round or wrinkled 5.        Seed color : yellow or green 6.        Pod shape : inflated or constricted 7.        Pod color : green or yellow

True-breeding plants make offspring that are the same as the parents every time

From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic One characteristic

comes from each parent!!!

FLOWER COLOR

FLOWER POSITION

SEED COLOR

SEED SHAPE

POD SHAPE

POD COLOR

STEM LENGTH

Purple White

Axial Terminal

Yellow Green

Round Wrinkled

Inflated Constricted

Green Yellow

Tall Dwarf

Mendel’s seven pea characteristics

MENDEL DEDUCED THE UNDERLYING PRINCIPLES OF GENETICS FROM THESE PATTERNS

1. Dominance2. Segregation3. Independent

assortment

MENDEL’S DATA

1.MENDEL’S LAW OF DOMINANCE

If your two alleles are different (heterozygous, e.g. Bb), the trait associated with only one of these will be visible (dominant) while the other will be hidden (recessive). E.g. B is dominant, b is recessive.

B b

B BB Bb

b Bb bbEggs

Sperm

CONTD..

Law of dominance : In a hybrid union, the allele which expresses itself phenotypically is the dominant allele while the other allele which fails to express itself phenotypically is the recessive allele. The hybrid individual shows phenotypically only the dominant character.

The law of dominance is often described as Mendel’s first law of inheritance.

2.MENDEL’S LAW OF SEGREGATION

A normal (somatic) cell has two variants (alleles) for a Mendelian trait.

A gamete (sperm, egg, pollen, ovule) contains one allele, randomly chosen from the two somatic alleles.

E.g. if you have one allele for brown eyes (B) and one for blue eyes (b), somatic cells have Bb and each gamete will carry one of B or b chosen randomly.

Law of segregation – the separation of alleles into separate gametes. B b

B BB Bb

b Bb bbEggs

Sperm

CONTD..

The law of segregation states: Each individual has two factors for each

trait The factors segregate (separate) during

the formation of the gametes Each gamete contains only one factor

from each pair of factors Fertilization gives each new individual two

factors for each trait

MONOHYBRID CROSS What happens

when true breeding plants with two distinct forms of a trait are crossed?Progeny show only one form

of the trait.

The observed trait is called dominant.The masked trait is called recessive.

Progeny- A genetic descendant or offspring

CONTD..

When we are looking at one trait (such as eye color or height), we do a monohybrid cross.

Parents are called P(Parental Generation) Children are called F1(First Fililal Generation) Grandchildren are called F2(Second Filial

Generation)

CONTD..

Two heterozygous parents produce gametes

with T or t allele equally frequently.

Offspring genotypes 1/4 TT : 1/2 Tt : 1/4 tt

Offspring phenotypes 3/4 tall : 1/4 short

3.LAW OF INDEPENDENT ASSORTMENT

"When a dihybrid (or a polyhybrid ) forms gametes, (i) each gamete receives one allele from each allelic

pair and (ii) the assortment of the alleles of different traits

during the gamete formation is totally independent of their original combinations in the parents.

 In other words, each allele of any one pair is free to combine with any allele from each of the remaining pairs during the formation for the gametes

This is known as the Law of Independent Assortment of characters.

 It is also referred to as Mendel’s third law of heredity.

EXPLANATION OF THE LAW OF INDEPENDENT ASSORTMENT:  The principle of independent assortment was

explained by Mendel with the help of a dihybrid cross involving characters of cotyledon color (yellow / round) and seed shape (round / wrinkled).

Mendel crossed a true breeding variety of pea having yellow cotyledons (YY) and round seeds (RR) with another true breeding variety having green cotyledons (yy) and wrinkled seeds (rr).

Thus, the yellow round parent has the genotype (YYRR) and the green wrinkled parent (yyrr). Since each parent is homozygous for both characters (color and shape), each will produce only one type of gametes.

The (YYRR) parent will produce all (YR) type gametes and the (yyrr) will produce all (yr) type gametes.

CONTD..

All F1 dihybrids resulting from the fusion of these gametes would be double heterozygous with (YyRr) genotype and appear yellow round.

This indicated that in the dihybrid cross also in each pair, the alleles behaved exactly in the same way as in the monohybrid cross.

Both the dominants (Y and R) expressed themselves in F1while both the recessive alleles (y and r ) remained hidden.

TYPES OF GAMETES FORMED BY F1 DIHYBRID:

 According to Mendel, during gamete formation by the F1 dihybrid, the alleles in both pairs Y-y and R-r first segregate from each other (Law of segregation).

Each pair segregates independently of the pair. Then the alleles enter the gametes. A gamete can receive only one allele from each pair, i.e. Y or y and R or r.

CONTD.,

Similarly, a gamete that receives a factor (gene) for color must also receive factor for shape (a factor for every character must be present in each gamete).

Thus, a gamete that receives Y for color may receive R or r for shape. This would result in (YR) and (Yr) types of gametes. Similarly, a gamete that receives y for color may receive R or r for shape.

CONTD..

This would give (yR) and (yr) types of gametes. In other words, the F1 dihybrid would produce four types of gametes (YR), (Yr), (yR) and (yr) in equal proportions. This is the principle of independent assortment of characters.

There will be four types of male gametes and four types of female gametes formed by the F1 dihybrid.

9:3:3:1

CONTD..

During self-fertilization or inbreeding of the F1 dihybrids to produce an F2generation, these male and female gametes can form maximum to dihybrid unions as shown in the Punnet’s Checker-board .

These can be grouped into four kinds on the basis of phenotypic appearance. i.e. yellow round, yellow wrinkled, green round and green wrinkled in the ratio of 9:3:3:1 respectively.

This is called the Phenotypic dihybrid ratio.

CONTD..

The 16 squares of the checker board are serially numbered for convenience.

The squares represent the 16 possible combinations of the gametes which might result. The genotypes and the phenotypes of the F2 offspring are shown in the sixteen squares.

A count of these squares shows the four kinds of phenotypes and their ratio in the F2 generation.

MENDEL CAME TO FOUR IMPORTANT CONCLUSIONS FROM THESE EXPERIMENTAL RESULTS:  

1.   The inheritance of each trait is determined by "units ÂÂ" or "factors" (now called genes) that � �are passed on to descendents unchanged.

2.   An individual inherits one such unit from each parent for each trait.

3.   A trait may not show up in an individual but can still be passed on to the next generation.

4.   The genes for each trait segregate themselves during gamete production.

Vasanthan V

09MTBI29