Extensions to Mendelian Analysis Notes 2023

Extensions to Mendelian Analysis Notes 2023.Mendelian) inheritance Definition .Variation on dominance Notes 2023.Extensions to Mendelian Analysis Notes 2023.

1 – Extensions to Mendelian Analysis

Mendelian) inheritance Definition :

Simple (or Mendelian) inheritance refers to the inheritance of traits controlled by a single gene with two alleles, one of which may be completely dominant to the other.

Extensions to Mendelian Analysis

In Principle

Mendel’s rules assume two alleles per locus
                                           complete dominance
                                           independent loci
                                           independent phenotypes

But there are multiple alleles per locus
intermediate dominance relationships among alleles
interactions among two (or more) loci (Epistasis)
complex phenotypes with contributions from several loci (Quantitative genetics)

Pleiotropy :

In genetics, Pleiotropy is define as the expression of multiple traits by a single gene. Pleiotropy is deriv from a Greek word meaning more ways. A simple example of a Pleiotropy is phenylketonuria is a disease. It is a genetic disorder caus by the low metabolism of the amino acid phenylalanine in the body cells.

Multiple allelesMultiple alleles is the term used to describe cases where the population has more than two alleles of the same gene,
Semi dominance Incomplete dominance (biology definition): a lack of dominance in either of two different alleles in heterozygotes, so that the phenotype is intermediate between that of homozygotes for either of the two alleles. 
 EpistasisEpistasis is a circumstance where the expression of one gene is modified (e.g., masked, inhibited or suppressed) by the expression of one or more other genes.
 Dominant epistasisDominant epistasis happens when the dominant allele of one gene masks the expression of all alleles of another gene
Pleiotropytwo loci function interchangeably to affect one trait
                              Ex.: occurs in “diploidization” of a polyploid
Co-dominancecodominance, in genetics, phenomenon in which two alleles (different versions of the same gene) are expressed to an equal degree within an organism.example Flower colors.
AllelesAlternative form of gene is called Alleles
GenesGenes are Located on Chromosomes .They are made of DNA.They are transfer of hereditary from parent to offspring is called genes
GenotypeMake up of a gene or individual is called Genotype
PhenotypeAppearance character of individual are called Phenotype

2 – Variation on dominance

A variation on incomplete dominance is codominance, in which both alleles for the same characteristic are simultaneously expressed in the heterozygote. An example of codominance is the MN blood groups of humans.

There are different types of dominance:

  •  incomplete dominance
  • co-dominance
  • complete dominance

 incomplete dominance:

Incomplete dominance is a form of Gene interaction in which both alleles of a gene at a locus are partially expressed, often resulting in an intermediate or different phenotype.


The Carnation plant (which is an example of incomplete dominance) has true-breeding white flowers and true-breeding red flowers. A cross between white- and red-flowering carnation plants may result in offspring with a phenotype of pink flowers. Four o’clock flowering plants are an example of incomplete dominance.

Co-dominance :

codominance, in genetics, phenomenon in which two alleles (different versions of the same gene) are expressed to an equal degree within an organism.

Complete dominance:

In complete dominance relationships, one allele is dominant and the other is recessive. The dominant allele for a trait completely masks the recessive allele for that trait. The phenotype is determine by the dominant allele.

3 – Multiple alleles and Lethal Alleles

Lethal Alleles : Lethal alleles (also referred to as lethal genes or lethals) are alleles that cause the death of the organism that carries them. They are usually a result of mutations in genes that are essential for growth or development.

Multiple alleles: Multiple alleles refer to the occurrence of three or more than three alleles for a particular gene.

 For example, for the gene encoding for height, one allele can be for tallness, whereas the other can be for dwarfness.

4 – What are reduce penetrance and variable expressivity?

Reduced penetrance and variable expressivity are factors that influence the effects of particular genetic changes. These factors usually affect disorders that have an autosomal pattern of inheritance, although they are occasionally in disorders with an autosomal recessive inheritance pattern.


Penetrance refers to the proportion of people with a particular genetic variant (or gene mutation) who exhibit signs and symptoms of a genetic disorder. If some people with the variant do not develop features of the disorder, the condition is to have reduce (or incomplete) penetrance.

Reduced penetrance

Reduced penetrance probably results from a combination of genetic, environmental, and lifestyle factors, many of which are unknown. This phenomenon can make it challenging for genetics professionals to interpret a person’s family medical history and predict the risk of passing a genetic condition to future generations.

Variable expressivity

Although some genetic disorders exhibit little variation, most have signs and symptoms that differ among affected individuals. Variable expressivity refers to the range of signs and symptoms that can occur in different people with the same genetic condition. For example, the features of Marfan syndrome vary widely— some people have only mild symptoms (such as being tall and thin with long, slender fingers), while others also experience life-threatening complications involving the heart and blood vessels. 

5 – Several Genes affecting the same character

A character is an observable feature of a particular group of organisms. Characters are inherite as genes by organisms from parents. One gene can determine several characters while several genes are responsible for the determination of a particular character.