MENDELIAN GENETICS

Gregor MendelStudied inheritance in peas

Mendel’s Garden

He artificially crossed

plants

He became known as the“Father of Genetics”

Genetics : the study of the

inheritance of traits

1. Traits are controlled by genes on chromosomes.

2. Genes exist in different forms called alleles.

3. There can be many alleles for a particular trait.

4. All the alleles an organism has is its genotype.

5. The physical expression of alleles is a phenotype.

6. Each parent provides one or more alleles for a trait.

Mendel’s Principle of Dominance:

If an allele from one parent is expressed while the matching allele from the other parent is not expressed, that allele is said to be dominant and the allele not expressed is said to be recessive.

Capital letters stand for dominant

Lower case letters mean recessive

In peas, tall ( T ) is dominant to short ( t )

Homozygous : allele from each

parent is the same ( pure )

TT = tall tt = short

Heterozygous : different allele from each parent ( hybrid )

Tt = tall ( tall dominates )

Mendel’s Law of Segregation :

The alleles from each parent are separated during the

formation of gametes ( sex cells ).

The formation of sex cells ( sperm or egg ) is called meiosis.

Meiosis is also known as reduction division

During division of cells to form gametes, chromosomes inherited from each parent are separated ( segregated )

The number of chromosomes per cell has been reduced and the chromosomes, ( and therefore the alleles ), from each parent have been segregated.

For particular traits, the results of a cross between two parents can be predicted using the laws of probability.

A monohybrid ( one factor ) cross:

Female = Tt, so ½ of eggs = T and ½ of eggs = t

Male = Tt, so ½ sperm = T and ½ sperm = t

Reginald Punnett developed the Punnett Square to predict outcomes of crosses

For each possibility, determine the chance for occurring and then multiply the chances.Cross: Tt x Tt

Possible eggs: T or tPossible sperm: T or

t

Prediction : chances for offspring

TT = ¼Tt = ½tt = ¼

From Punnett square:

Genotypic ratio

= 1:2:1

Phenotypic ratio =

3:1

Mendel’s Law of Independent Assortment

Different traits are inherited independently of each other

A dihybrid ( two-factor ) cross :

Cross: TtBb x TtBb

Possible eggs: TB,Tb,tB,tb

Possible sperm : TB,Tb,tB,tb

Prediction from Punnett square:

Phenotypic ratio = 9:3:3:1

Genotypic ration=1:2:1:2:4:2:1:2:1

Test Cross:A cross to determine if an

individual is heterozygous or homozygous for a given trait.

Example: NN and Nn look the same due to dominance

crossing each with the recessive trait will show the difference:

NN x nn = all NnNn x nn = ½ Nn and ½ nn

The test determines which is pure and which is hybrid.Non-Mendelian Inheritance

1. Incomplete dominanceNeither allele is dominant

and the hybrid is a blend

2. Codominance

Both alleles are expressed equally

3. Multiple alleles

More than two alleles for a trait exist in the

population

Human blood type is controlled by three alleles

IA (type A)IB (type B) i (type O)

IAIA and IAi = type AIBIB and IBi = type BIAIB = type ABii = type O

4. Polygenic Inheritance

Several alleles control a trait at the same time.

Polygenic phenotypes have a continuous range of expressions and show a typical bell-shaped curve of values in a population