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.
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.
· 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 1st observed yellow coat genes as lethal in cross of agouti and yellow coat coloured mice .
· Yellow coat colour of mice
· Sickle cell anaemia
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 :
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
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
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.
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.
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.
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.
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