This presentation is an introduction to the various types of non-Mendelian inheritance (genetic inheritance that does not follow Mendel's laws)

1. Non-Mendelian Inheritance
By
Sanjana Hemdev

2. Mendelian Inheritance
The process of genetic transmission of traits from parents to offspring is called
inheritance. It is also defined as the sum of the characteristics genetically
transmitted from parents to offspring. Mendelian Inheritance is that which follows
Mendel’s laws.
Mendel’s laws are as follows:-
1. Principle of segregation: A given pair of alleles segregate randomly when
gametes are formed.
2. Law of independent assortment: Each pair of alleles will assort independently
and randomly from every other pair of alleles.

3. Non-Mendelian Inheritance
While most pairings led to offspring with geotypic and phenotypic ratios that
matched those expected from Mendel’s laws, some deviations were also found.
These natural deviations are examples of non-Mendelian inheritance. There are two
types of natural deviations:-
1. Mendel’s laws do not apply at all: In this type, both the genotypic and the
phenotypic ratios vary from those expected from Mendel’s laws.
2. Genotypic ratio is that expected from mendel’s laws but the phenotypic ratio is
not: In this type, the underlying genotypic ratios are somehow hidden.

4. 1.Incomplete Dominance
In this type of non-Mendelian inheritance,
the genotypic ratios are those expected
from Mendel’s laws but the phenotypic
ratios are not. The heterozygotes have
a different phenotype are compared to
the homozygotes. There appears to be
blending of the parental phenotypes in
the heterozygotes.

5. 2. Codominance
Codominance is the phenomenon in which both alleles of a gene pair are
expressed in the phenotype of the organism.
Codominance is best explained using an example. The most common instance of
codominance is the genetic coding of human blood groups. Human blood group
phenotypes are controlled by genes that have three different allelic forms: IA, IB and
i. The IA and IB alleles are codominant while the i allele is recessive. The possible
genotype/phenotype combinations are: IAIA/type A, IBIB/type B, IAIB/type AB and
ii/type O. This is because the IA allele codes for A antigens on the surfaces of the
red blood cells, IB codes for B antigens on the surfaces of the red blood cells and i
does not code for any antigens.

6. Another common example of codominance is that of the coloration
of Rhododendron flowers. The homozygous dominant gene pair
gives rise to the red phenotype, the homozygous recessive gene
pair gives rise to the white phenotype and the heterozygous pairing
gives rise to the pink phenotype, which is a clear case of blending
of the parental traits.

7. 3. Multiple alleles/Polygenic Inheritance
This is the phenomenon by which a single trait is controlled by multiple alleles and
has many possible phenotypes. Traits that are passed down in this manner are also
under the influence of environmental factors such as photoperiod and duration of
day etc.
Some common examples of this type of inheritance are human blood groups (which
has been outlined earlier), height and eye color in humans and fur color in animals.

9. 4. Linked traits
Traits passed down by this method of inheritance are those coded by genes which
are located close to each other on the same chromosome. This is due to genetic
linkage.
Genetic linkage is defined as the tendency of DNA sequences that are close
together on a chromosome to be inherited together during the meiosis phase of
sexual reproduction.
A very common instance of linked traits is the combination of red hair, fair skin and
freckles in humans.

10. Sex-linked traits
This is a type of linked trait inheritance in which the linked traits have genes that
have their loci on the same sex (X or Y) chromosome.

11. Hemophilia
The inheritance of hemophilia is a good example of sex-linked inheritance.
Hemophilia is a recessive sex-linked trait that affects only males while females in
the family tree are generally carriers of the disease or of the normal phenotype.
This is because the gene that codes for hemophilia is present only on the X
chromosome. In heterozygous females, the disorder is not expressed as it is a
recessive disorder and the so, the other normal X chromosome will be expressed.
However, in males with a XC chromosome, the disorder will be expressed as the
gene pair for the affected X chromosome will be a Y chromosome which does not
have the normal gene to dominate over the affected X chromosome. In this way,
hemophilia is passed down by the females of a bloodline and expressed usually in
males.

14. References
1. Khan Academy (khanacademy.org)
2. http://www.powershow.com/view/3b5e0a-NTQwZ/Non-
Mendelian_Genetics_powerpoint_ppt_presentation
3. www.montgomery.k12.ky.us