Chapter 13 : 

All the patterns of inheritance below mentioned are exceptions to Mendel’s  two laws and law of dominancy. So they are also called non- Mendelian inheritance. They are an outcome of gene interaction which can be  broadly, classified into two types:

1.    Allelic gene interaction

2.    Non –Allelic gene interaction

1.Allelic gene interaction:

Alleles of same gene only  interact and play role in phenotype of an organism.

For instance :

1.  Incomplete dominance .

The  interaction making law of dominance  fail  where   despite a dominant gene , recessive gene also express as a result , intermediate phenotype is seen  rather of either one’s.

 Example: 

 cross done by Carl correns  in Mirabilis jalapa  (4 0’clock ) plant .

taking ,  red flowered plant : RR

                White flowered plant : rr

 Here , instead of having red flower as red gene  being dominant , we get pink that is intermediate to red and white.

 It  must not be confused that the genes have blended to give pink because  upon selfing  Rr. we can retain red and pink  in progeny RR and rr respectively, where they are genotypically homozygous and heterozygous Rr is still  pink .

Thus, this leads to deviation from typical Mendelian ratio of 3:1 to 1:2:1 .

2.  Co dominance

The allelic interaction where all the  traits  get equally expressed  regardless of dominancy  without change in actual trait they carried.

 Observed examples:

·        Blood group of human

·        Roan coat colour of cattle

·        Sickle cell anaemia

 So here also upon selfing RW, we get non Mendelian phenotypic ratio of 1:2:1 for monohybrid cross like incomplete dominance. But the difference is here , the individuality of gene don’t  get lost rather is maintained completely intact during expression in progeny.

3.   Lethal genes

The interaction  where presence of  same lethal alleles  with no  alternative of it , can be  fatal to progeny. These genes are mutant or defective. Cuenot 1^st observed yellow coat genes as lethal  in  cross of agouti and yellow coat coloured mice .

Examples :

·       Yellow coat colour of mice

·       Sickle cell anaemia

·       Epilepsy

·       Retinoblastoma

Lethal genes make the progeny impossible to live so this kind of allelic interaction modifies Mendelian monohybrid phenotypic ratio from 3:1 to 2:1.

2. Non-allelic gene interaction

 Alleles of different gene interact to express hereditary characters.

They get independently assort but they are exception to Mendel’s dihybrid cross’ results.

 Sub types :

A.   Epistatic

B.   Non epistatic

A.Epistatic gene interaction

 The masking of expression of one allele by other of the different gene in different locus.

Gene being suppressed: hypostatic

Gene  suppressing: epistatic

1.Dominant epistasis

The epistasis where dominant gene suppresses expression of  other dominant gene or recessive ones.

 Observed example :  of Cucurbita pepo cross  between

white fruited  plant : AABB

 green fruited plant : aabb

when crossed and progeny was  done selfing  we get f2 generation progeny as follows:

(Let us say B gene is the dominant epistatic gene and A is the

hypostatic gene)

Here  phenotypic ratio:

white : yellow : green = 12 : 3: 1

that deviated from Mendelian dihybrid ratio 9: 3: 3: 1.

2.Recessive epistasis  / supplementary gene interaction

Here, recessive gene in  there homozygous  state are able to mask the other dominant and recessive gene’s expression.

 For example :

Cross between agouti coat coloured mice (AABB)  With albino coat coloured mice (aabb) . Then their cross  progeny is selfed to get following result:

(Here, A gene is the recessive epistatic gene and B is hypostatic gene) 

Observed phenotypic ratio:

          Agouti: albino: black = 9: 3: 4

This interaction is  also called supplementary because  here albino gene supplements agouti’s expression in progeny. Without albino i.e dominant (B), agouti was suppressed and wasn’t expressed.

B.Non epistatic gene interaction.

Interaction between  alleles of different genes without masking one another.

1.Pleiotropism

Interaction due to which   one gene end up regulating two independent characters.  Eg: sickle cell anaemia gene, yellow coat determining gene

 Like in the below cross example,  the lethal gene (YY) here determined the coat colour and the survivability of the progeny.

2.Polygenic inheritance

When more than one gene regulates to express a  single character is termed as a polygenic inheritance.

Eg: kernel colour of wheat‘s  gene, height, IQ determining genes, the skin colour of human

 Below mentioned is an illustration of human skin colour:

    They are also called quantitative inheritance as in the aforementioned example, the darkness of skin was directly proportional to the number of dominant genes to determine dark skin colour by aiding melanin production.

3.Multiple allelism

The occurrence of more than two alleles in the homologous chromosome for the same character is multiple allelism.

 Eg: eye colour of Drosophila, coat colour of rabbit,  human blood type, self sterility in  Nicotiana tobacco.

Illustration for human blood type as multiple alleles:

Characters of these  type of alleles :

1.   In population there may be many alleles but a genome posses any one of them only.

2.   They show dominancy or co- dominancy.

3.   They don’t undergo crossing over.

4.   They provide many alternates to a single character.

5.   They are at the same locus.

6.   Number of genotype for  multiple alleles  is  

·       n(n-1) ÷2  where n is the number of alleles.

YOU CAN REFER TO THIS FOR FURTHER CLARIFICATION :