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 Page 1


Extension of Mendelian genetics 
 
                        Institute of Lifelong Learning, University of Delhi                                   1 
 
 
 
 
 
 
 
 
 
 
Genetics 
Lesson :Extension of Mendelian genetics 
Lesson Developer: Ms. Jyotsna Singh 
2
 Dr. Anjana Singha Naorem 
3 
Ms. Anna Senrung 
Lesson Reviewer: Dr. Neera Mehra 
 
College/Dept:
 1
Deparment of Zoology, University of 
Delhi, Delhi-110007 
2
Miranda House, University of Delhi, Delhi-110007 
3
Daula Ram College, University of Delhi, Delhi-110007 
 
 
 
 
 
 
Page 2


Extension of Mendelian genetics 
 
                        Institute of Lifelong Learning, University of Delhi                                   1 
 
 
 
 
 
 
 
 
 
 
Genetics 
Lesson :Extension of Mendelian genetics 
Lesson Developer: Ms. Jyotsna Singh 
2
 Dr. Anjana Singha Naorem 
3 
Ms. Anna Senrung 
Lesson Reviewer: Dr. Neera Mehra 
 
College/Dept:
 1
Deparment of Zoology, University of 
Delhi, Delhi-110007 
2
Miranda House, University of Delhi, Delhi-110007 
3
Daula Ram College, University of Delhi, Delhi-110007 
 
 
 
 
 
 
Extension of Mendelian genetics 
 
                        Institute of Lifelong Learning, University of Delhi                                   2 
 
 
 
 
 
 
 
 
 
Table of Contents       
 
Chapter: Extension of Mendelian Genetics 
? Introduction  
? Incomplete dominance 
? Codominance 
? Multiple alleles 
? ABO blood group 
? White eye locus Drosophila 
? Lethal alleles 
? Lethal recessive genes 
? Lethal dominant genes 
? Epistasis 
? Complementary epistasis 
? Recessive epistasis 
? Dominant epistasis 
? Pleiotropy 
? Environmental effects on Phenotypic Expression 
? Temperature influence 
? Nutritional influence 
? Summary  
? Exercise 
? Glossary 
? References 
 
Page 3


Extension of Mendelian genetics 
 
                        Institute of Lifelong Learning, University of Delhi                                   1 
 
 
 
 
 
 
 
 
 
 
Genetics 
Lesson :Extension of Mendelian genetics 
Lesson Developer: Ms. Jyotsna Singh 
2
 Dr. Anjana Singha Naorem 
3 
Ms. Anna Senrung 
Lesson Reviewer: Dr. Neera Mehra 
 
College/Dept:
 1
Deparment of Zoology, University of 
Delhi, Delhi-110007 
2
Miranda House, University of Delhi, Delhi-110007 
3
Daula Ram College, University of Delhi, Delhi-110007 
 
 
 
 
 
 
Extension of Mendelian genetics 
 
                        Institute of Lifelong Learning, University of Delhi                                   2 
 
 
 
 
 
 
 
 
 
Table of Contents       
 
Chapter: Extension of Mendelian Genetics 
? Introduction  
? Incomplete dominance 
? Codominance 
? Multiple alleles 
? ABO blood group 
? White eye locus Drosophila 
? Lethal alleles 
? Lethal recessive genes 
? Lethal dominant genes 
? Epistasis 
? Complementary epistasis 
? Recessive epistasis 
? Dominant epistasis 
? Pleiotropy 
? Environmental effects on Phenotypic Expression 
? Temperature influence 
? Nutritional influence 
? Summary  
? Exercise 
? Glossary 
? References 
 
Extension of Mendelian genetics 
 
                        Institute of Lifelong Learning, University of Delhi                                   3 
 
 
 
 
 
 
 
Introduction 
The work of Gregor Mendel paved way for the widely acclaimed principles and patterns 
of inheritance in modern genetics. As already described in the previous chapters the 
process of gene transmission can be summarized by Mendelian laws according to which 
genes are located on homologous chromosomes which segregate from each other and 
assort independently from the other segregating chromosomes during gamete formation. 
The expression of these sets of genes in an offspring determines the phenotype of that 
individual. 
However, with more advances in the field of genetics in early 20
th
 century, it came to 
light that not all inheritance patterns strictly follow Mendelian laws. The monohybrid ratio 
as proposed by Mendel is found to be altered in case of co-dominance, incomplete 
dominance and lethal alleles. Further the novel phenotypes observed due to epistasis are 
a clear modification of Mendel’s dihybrid ratio. 
This chapter highlights these complex modes of inheritance as an extension of Mendelian 
genetics and also the role of environment on the genetic expression of the organism.  
Incomplete dominance 
We have discussed, in detail, in the previous chapter about the concept of dominance 
proposed by Mendel. Soon many works followed to understand the concept of gene 
expression in several other organisms. Biologists realised that not all the characteristics 
of the organisms can be explained by this concept. Mendel himself observed the same 
anomaly in the flowering time of the pea plants. He observed that the plant 
heterozygous for long and short flowering times had a flowering time that was 
intermediate unlike the one anticipated in accordance with the concept of dominance. 
This kind of gene action in the heterozygote that exhibits intermediate phenotype 
between those of its homozygous parents is termed incomplete dominance. 
Page 4


Extension of Mendelian genetics 
 
                        Institute of Lifelong Learning, University of Delhi                                   1 
 
 
 
 
 
 
 
 
 
 
Genetics 
Lesson :Extension of Mendelian genetics 
Lesson Developer: Ms. Jyotsna Singh 
2
 Dr. Anjana Singha Naorem 
3 
Ms. Anna Senrung 
Lesson Reviewer: Dr. Neera Mehra 
 
College/Dept:
 1
Deparment of Zoology, University of 
Delhi, Delhi-110007 
2
Miranda House, University of Delhi, Delhi-110007 
3
Daula Ram College, University of Delhi, Delhi-110007 
 
 
 
 
 
 
Extension of Mendelian genetics 
 
                        Institute of Lifelong Learning, University of Delhi                                   2 
 
 
 
 
 
 
 
 
 
Table of Contents       
 
Chapter: Extension of Mendelian Genetics 
? Introduction  
? Incomplete dominance 
? Codominance 
? Multiple alleles 
? ABO blood group 
? White eye locus Drosophila 
? Lethal alleles 
? Lethal recessive genes 
? Lethal dominant genes 
? Epistasis 
? Complementary epistasis 
? Recessive epistasis 
? Dominant epistasis 
? Pleiotropy 
? Environmental effects on Phenotypic Expression 
? Temperature influence 
? Nutritional influence 
? Summary  
? Exercise 
? Glossary 
? References 
 
Extension of Mendelian genetics 
 
                        Institute of Lifelong Learning, University of Delhi                                   3 
 
 
 
 
 
 
 
Introduction 
The work of Gregor Mendel paved way for the widely acclaimed principles and patterns 
of inheritance in modern genetics. As already described in the previous chapters the 
process of gene transmission can be summarized by Mendelian laws according to which 
genes are located on homologous chromosomes which segregate from each other and 
assort independently from the other segregating chromosomes during gamete formation. 
The expression of these sets of genes in an offspring determines the phenotype of that 
individual. 
However, with more advances in the field of genetics in early 20
th
 century, it came to 
light that not all inheritance patterns strictly follow Mendelian laws. The monohybrid ratio 
as proposed by Mendel is found to be altered in case of co-dominance, incomplete 
dominance and lethal alleles. Further the novel phenotypes observed due to epistasis are 
a clear modification of Mendel’s dihybrid ratio. 
This chapter highlights these complex modes of inheritance as an extension of Mendelian 
genetics and also the role of environment on the genetic expression of the organism.  
Incomplete dominance 
We have discussed, in detail, in the previous chapter about the concept of dominance 
proposed by Mendel. Soon many works followed to understand the concept of gene 
expression in several other organisms. Biologists realised that not all the characteristics 
of the organisms can be explained by this concept. Mendel himself observed the same 
anomaly in the flowering time of the pea plants. He observed that the plant 
heterozygous for long and short flowering times had a flowering time that was 
intermediate unlike the one anticipated in accordance with the concept of dominance. 
This kind of gene action in the heterozygote that exhibits intermediate phenotype 
between those of its homozygous parents is termed incomplete dominance. 
Extension of Mendelian genetics 
 
                        Institute of Lifelong Learning, University of Delhi                                   4 
 
 
In incomplete dominance, neither allele is dominant, for example flower colour in 
Mirabilis jalapa (4 O’clock plant) or snapdragon plant. When red flowering 4 O’clock plant 
is crossed to white flowering plant, all F1 plants have pink colour flowers, exhibiting 
neither red nor white colour of parental plants, but instead an intermediate colour (pink) 
(Figure 1). 
 
Figure 1. Incomplete dominance in Mirabilis jalapa flower colour 
Source: Author 
As seen in the figure both the phenotypic and genotypic ratios are same (1:2:1), neither 
of the allele is fully dominant or fully recessive.  As neither of the alleles is dominant so 
the use of capital and small letter symbols is to be avoided. There are two ways of using 
symbols in such situation- either by denoting the red and white alleles as R1R1 and 
R2R2 (or W1W1 and W2 W2)  or using letters such  as C
W 
and C
R
, where C 
indicates “colour ” and the W  and R superscripts indicate white and red,  respectively.   
Codominance 
As the name suggests, this type of dominance involves equal and complete expression of 
both the distinct alleles which co-exist in the phenotype. If two alleles are codominant, 
then the heterozygote will have complete expression of both the alleles. For example, 
MN blood types in humans. At the MN locus, there are two alleles: the L
M
 allele coding 
for M antigen and the L
N
 allele coding for N antigen. Individuals homozygous for L
M
L
M
 
express the M antigen on their red blood cells and have the M blood type. Similarly 
individuals homozygous for L
N
L
N
 have the N blood type. Heterozygous individuals with 
Page 5


Extension of Mendelian genetics 
 
                        Institute of Lifelong Learning, University of Delhi                                   1 
 
 
 
 
 
 
 
 
 
 
Genetics 
Lesson :Extension of Mendelian genetics 
Lesson Developer: Ms. Jyotsna Singh 
2
 Dr. Anjana Singha Naorem 
3 
Ms. Anna Senrung 
Lesson Reviewer: Dr. Neera Mehra 
 
College/Dept:
 1
Deparment of Zoology, University of 
Delhi, Delhi-110007 
2
Miranda House, University of Delhi, Delhi-110007 
3
Daula Ram College, University of Delhi, Delhi-110007 
 
 
 
 
 
 
Extension of Mendelian genetics 
 
                        Institute of Lifelong Learning, University of Delhi                                   2 
 
 
 
 
 
 
 
 
 
Table of Contents       
 
Chapter: Extension of Mendelian Genetics 
? Introduction  
? Incomplete dominance 
? Codominance 
? Multiple alleles 
? ABO blood group 
? White eye locus Drosophila 
? Lethal alleles 
? Lethal recessive genes 
? Lethal dominant genes 
? Epistasis 
? Complementary epistasis 
? Recessive epistasis 
? Dominant epistasis 
? Pleiotropy 
? Environmental effects on Phenotypic Expression 
? Temperature influence 
? Nutritional influence 
? Summary  
? Exercise 
? Glossary 
? References 
 
Extension of Mendelian genetics 
 
                        Institute of Lifelong Learning, University of Delhi                                   3 
 
 
 
 
 
 
 
Introduction 
The work of Gregor Mendel paved way for the widely acclaimed principles and patterns 
of inheritance in modern genetics. As already described in the previous chapters the 
process of gene transmission can be summarized by Mendelian laws according to which 
genes are located on homologous chromosomes which segregate from each other and 
assort independently from the other segregating chromosomes during gamete formation. 
The expression of these sets of genes in an offspring determines the phenotype of that 
individual. 
However, with more advances in the field of genetics in early 20
th
 century, it came to 
light that not all inheritance patterns strictly follow Mendelian laws. The monohybrid ratio 
as proposed by Mendel is found to be altered in case of co-dominance, incomplete 
dominance and lethal alleles. Further the novel phenotypes observed due to epistasis are 
a clear modification of Mendel’s dihybrid ratio. 
This chapter highlights these complex modes of inheritance as an extension of Mendelian 
genetics and also the role of environment on the genetic expression of the organism.  
Incomplete dominance 
We have discussed, in detail, in the previous chapter about the concept of dominance 
proposed by Mendel. Soon many works followed to understand the concept of gene 
expression in several other organisms. Biologists realised that not all the characteristics 
of the organisms can be explained by this concept. Mendel himself observed the same 
anomaly in the flowering time of the pea plants. He observed that the plant 
heterozygous for long and short flowering times had a flowering time that was 
intermediate unlike the one anticipated in accordance with the concept of dominance. 
This kind of gene action in the heterozygote that exhibits intermediate phenotype 
between those of its homozygous parents is termed incomplete dominance. 
Extension of Mendelian genetics 
 
                        Institute of Lifelong Learning, University of Delhi                                   4 
 
 
In incomplete dominance, neither allele is dominant, for example flower colour in 
Mirabilis jalapa (4 O’clock plant) or snapdragon plant. When red flowering 4 O’clock plant 
is crossed to white flowering plant, all F1 plants have pink colour flowers, exhibiting 
neither red nor white colour of parental plants, but instead an intermediate colour (pink) 
(Figure 1). 
 
Figure 1. Incomplete dominance in Mirabilis jalapa flower colour 
Source: Author 
As seen in the figure both the phenotypic and genotypic ratios are same (1:2:1), neither 
of the allele is fully dominant or fully recessive.  As neither of the alleles is dominant so 
the use of capital and small letter symbols is to be avoided. There are two ways of using 
symbols in such situation- either by denoting the red and white alleles as R1R1 and 
R2R2 (or W1W1 and W2 W2)  or using letters such  as C
W 
and C
R
, where C 
indicates “colour ” and the W  and R superscripts indicate white and red,  respectively.   
Codominance 
As the name suggests, this type of dominance involves equal and complete expression of 
both the distinct alleles which co-exist in the phenotype. If two alleles are codominant, 
then the heterozygote will have complete expression of both the alleles. For example, 
MN blood types in humans. At the MN locus, there are two alleles: the L
M
 allele coding 
for M antigen and the L
N
 allele coding for N antigen. Individuals homozygous for L
M
L
M
 
express the M antigen on their red blood cells and have the M blood type. Similarly 
individuals homozygous for L
N
L
N
 have the N blood type. Heterozygous individuals with 
Extension of Mendelian genetics 
 
                        Institute of Lifelong Learning, University of Delhi                                   5 
 
 
genotype L
M
L
N
 will exhibit codominance expressing both the M and the N antigen and are 
said to have blood type MN. 
The ABO blood group also represents an apt example of co-dominance. When both I
A
 
and I
B 
alleles are present in a heterozygote, neither of them is dominant or recessive to 
each other, instead there is joint expression of both alleles leading to the formation of a 
distinct blood group denoted as AB (presence of both antigens). 
  
Multiple alleles 
Three or more alleles that can occupy a given gene locus are referred to as multiple 
alleles. The multiple alleles exist in a population. However, in a diploid individual only 
two alleles for a particular gene can exist as they are present on the homologous 
chromosomes. So, an individual would have two alleles for a single gene but different 
individuals will have different combinations of alleles for that gene. They always occupy 
the same locus and influence the same trait. Also, they are conspicuous only during 
population studies. 
 
ABO blood group 
The ABO blood group system in humans is the simplest example depicting multiple 
alleles. This system was discovered by Karl Landsteiner in the early 1900, who was 
awarded Nobel Prize in Physiology or Medicine in 1930 for this discovery.  These alleles 
give rise to four possible phenotypes: A, B, AB, and O blood groups. These blood groups 
are distinguished by the presence or absence of A and B antigens on the surface of the 
red blood cells (RBCs) (Figure 2). The synthesis of these antigens is controlled by a 
gene located on chromosome 9 having three variant alleles (represented as I
A
, I
B
 
and i).    
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