Question

Describe the tenets of Mendelian genetics. Are these tenants held true for all genes?What are some things that can cause exceptions to Mendelian genetics. Why do these cause complications?

Answer 1

The three Mendelian principles governing genetics are:

a)Law of Dominance:

This law states that when the ptrue breeding individuals of a trait are crossed,then the offspring only one of the trait will be seen,The trait the will appear in the progeny is termed as dominant, and the trait that remains measked and does not show itself in the progeny is called the recessive trait.

For example:

Cross of pure breeding tall and short pea plants

Tall x Short

TT x tt

	T	T
t	Tt(Tall)	Tt(Tall)
t	Tt(Tall)	Tt(Tall)

Thus from the cross it is clear that the progeny of the true breeding parents are heterozygous indviduals and that the trait / allele for tallness is dominant over short.

2.Law of Segregation:

This law explains that during the process of gamete formation of a diploid organisms, segregation happens to the allele pairs such that the offspring will have copies of an allele which is inherited from both the parents.

For example, a heterozygous parents for height in pea plants is expressed 'Tt'

During the formation of gametes these allles separate such the the gametes contains either allele 'T' or 't'.

And during fertilisation these alleles will combine randomly and result in diploid organism with two alleles of that trait.

3.Law of Independent assortment:

This law explains that when two characters or traits are combined in a cross ; the segreagtion of the alleles of one character is independent of the other.

This can be understood by a dihybrid cross where two traits are considered:

Round yellow seeds of pea x Green wrinkled seeds

RRYY x rryy

F1 : RrYy

F2:

	RY	rY	Ry	ry
RY	RRYY	RrYY	RRYy	RrYy
rY	RrYY	rrYY	RrYy	rrYy
Ry	RRYy	RrYy	RRyy	Rryy
ry	RrYy	rrYy	Rryy	rryy

Here, the characters from the parents green and wrinkled do not segreagte together rather they separate independently.

EXCEPTIONS OF MEDELIAN PRINCIPLES:

Not every genes strictly follows the mendelian principle.There exist some exceptions which are as mentioned:

a)Incomplete Dominance:

Here , in some species as in Mirabilis jalapa, when parents with red and white flowers are crossed . The offspring shows a blended inheritance of pink flowers.

Thus the red is not completely dominant or white completely recessive .We say that the red is incompletely dominant over white.

b)Co dominance

In ABO blood grouping in humans, there are three different alleles A,B and O

The allele for A and B are dominant over alleles over O and when expressed in an individual will result in blood group AB where A and B are together expressed exhibiting co-dominance.

c)Pleiotrpophic effects:

Here the traits show a wide range of expressions because in involvement in a variety of pathways.

d)Polygenic Inheritance:

Here a character is governed by more than a one gene.

Example :Skin color in humans controlled by three genes.

e)Epistasis:
Here a gene hinders the expression of a functional gene

Example :Coat color in rabbits.

These cause complication because in case of these exceptions the movement and alleles are hard to understand.Sometimes they also result in disorders.