Non-Mendelian inheritance refers to any trait or genetic pattern that is not governed by dominant and recessive alleles (Mendelian Inheritance). When patterns are governed by Mendel's principles and involve genes that directly influence the result of an organism's attributes.
- In eukaryotic organisms, most genes follow a Mendelian pattern of inheritance. Few, on the other hand, do not.
- The term "non-mendelian inheritance" refers to any pattern of heredity in which features do not segregate according to Mendel's laws.
- These principles describe how features associated with single genes on chromosomes in the nucleus are passed down through generations.
- Some discoveries in humans imply that, in addition to this form of genetic inheritance, other environmental factors such as a lack of vitamin D and adolescent obesity have a role in certain types of genetic illnesses.
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Key Words: Inheritance, Trait, Heterozygote, Codominance, Incomplete dominance, Non-mendelian, Genetic, Polygenic
Types of Non-Mendelian Inheritance
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Non-mendelian inheritance includes various types.
Codominance Inheritance
Codominance is a form of incomplete dominance in which both alleles for the same feature are expressed in the heterozygote at the same time. The MN blood types of humans are an example of codominance.
- The M and N alleles are expressed on the surface of red blood cells in the form of an M or N antigen.
- Homozygote’s (LM LM and LNLN) can have either the M or N allele, but heterozygote’s (LMLN) can have both.
- The three probable child genotypes in a self-cross between heterozygotes expressing a codominant trait are phenotypically diverse.
- The 1:2:1 genotypic ratio of a Mendelian monohybrid cross, however, still applies.
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Human blood types show codominance as well; the AB blood type is the result of both the lA and lB alleles being codominant.
There are also subcategories of codominance, such as the ones listed below:
Multiple Alleles
Some populations have multiple alleles for the same gene. In rabbits, for example, there is a C gene that determines the breed's coat colour.
- This gene has four common alleles.
- CC, which produces black or brown fur.
- CchCch, which produces grey fur, often known as chinchilla colouration.
- ChCh, which produces white body fur with dark ears, face, paws, and tail.
- cc, which produces pure white fur with reddish eyes due to albinism.
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Pleiotropy
Pleiotropy occurs when a single gene impacts numerous aspects of an individual's personality.
- Phenylketonuria is an example of a disease.
- It is caused by a single gene deficiency on chromosome 12 and it affects several systems, including the skin's integumentary system and the neurological system.
- Albinism is another example of a single gene altering the colour of the skin, eyes, and hair.
Lethality Alleles
At times, the combination of many alleles might make an individual's survival difficult to impossible.
- For example, a hybrid between two heterozygous yellow mice produces yellow and brown mice in a 2:1 ratio.
- Mice with homozygous alleles die during pregnancy, particularly during embryonic development.
- These fatal alleles can be found in dominant or recessive forms, and they can manifest features in homozygous or heterozygous individuals.
Extra-Nuclear Inheritance
Though nuclear chromosome genes play a large and critical role in the transmission of practically all traits from one generation to the next, they cannot be considered the exclusive means of inheritance.
- Many prokaryotic and eukaryotic cells have extranuclear genes or DNA molecules in their cytoplasm.
- Mitochondria and chloroplasts, for example, have their own DNA and replicate independently within each cell.
- Because the mitochondria in sperm cells do not make it into the egg, mitochondrial DNA is passed down from a mother to her children.
- As a result, the offspring's complete mitochondrial DNA, whether male or female, originates from the female parent alone.
Incomplete Dominance
In a heterozygote, the dominant allele does not always entirely cover the phenotypic expression of the recessive gene, resulting in an intermediate phenotype. This is referred to as "incomplete dominance”.
- Both qualities are expressed at the same time when co-dominant alleles are present.
- A similar thing happens with partial dominance, but the features are mixed together rather than happening in isolated patches.
- The condition in heterozygote where the phenotypic trait is intermediate between the two homozygotes is referred to as this.
- Pink tulips, carnations, and roses are examples of incomplete dominance; any pink blossoms in these flowers are owing to the mixing of red and white alleles.
- In some animals, such as rabbits, incomplete dominance can be observed.
- The offspring of a long-furred Angora and a short-furred Rex have medium-length fur.
Polygenic Inheritance
Height, IQ, skin colour, eye colour, and other features are influenced by a large number of genes.
- When two or more genes control the same trait, it is known as polygenic inheritance.
- Genes are frequently large in number but little in effect.
- For example, more than 400 genes regulate our height.
- Several genes play a role in skin colouring.
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Gene Linkage
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There are several genes on each chromosome. If the genes are on the same chromosome and are close to one another, they are more likely to be inherited together. Linkage refers to the coexistence of two or more genes on the same chromosome.
- Some combinations of wing shape and body colour, for example, are inherited jointly.
- Fruit flies with brown bodies have normal wings, but fruit flies with black bodies have vestigial wings.
Polygenic Inheritance
Also Read: Chromosomal Theory of Inheritance
Things to Remember
- Non-menedlian inheritance is the inherited trait that is not governed by mendelian inheritance.
- Codominance is a form of incomplete dominance in which both alleles for the same feature are expressed in the heterozygote at the same time.
- The MN blood types of humans are an example of codominance.
- Some populations have multiple alleles for the same gene.
- Pleiotropy occurs when a single gene impacts numerous aspects of an individual's personality.
- Though nuclear chromosome genes play a large and critical role in the transmission of all traits, they are not the exclusive means of inheritance.
- Many prokaryotic and eukaryotic cells have extranuclear genes or DNA molecules in their cytoplasm.
- Polygenic inheritance means some traits governed by many genes.
- Gentic Linkage is the term that denotes the closeness of two or more genes.
- Linked genes are more likely to be inherited together in nexte generation.
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Sample Questions
Ques: Distinguish between dominance, co-domination, and partial dominance. (3 marks)
Ans: The phenomenon of dominance occurs when one variant of a gene covers the action of another version of the same gene.
The link between two alleles of a gene is known as co-dominance. In this case, none of the alleles is recessive, and all of the alleles' phenotypes are expressed.
Incomplete dominance is a type of intermediate inheritance in which one allele for a particular trait does not totally dominate its matched allele.
Ques: When alleles have incomplete dominance, what happens? (2 marks)
Ans: When incomplete dominance exists, the dominant allele does not entirely cover the effect of the recessive gene, and the heterozygous F1 individual exhibits an intermediate phenotype that differs from neither of the parents. When true-breeding snapdragon plants with red and white flowers are crossed, the F1 generation produces all pink blooms.
Ques: Even if a character has multiple alleles, why does an individual have only two alleles? (2 marks)
Ans: Multiple alleles are different variants of an allele that occur on the same gene locus. However, each person has only two alleles. The union of haploid sperm and egg results in the formation of a zygote. For each attribute, they only have one allele. Each trait has two alleles when the zygote becomes diploid.
Ques: What are the patterns that don't follow Mendelian Inheritance's simple rules? (2 marks)
Ans: Non-Mendelian inheritance is characterised by traits that do not segregate according to Mendel's law of segregation or obey the law of dominance. This could be due to trait mixings, such as incomplete dominance, linkage during crossing over, codominance, multiple alleles, pleiotropy, epistasis, polygenic inheritance, and sex-linked inheritance, among other things.
Ques: What Effect Does Having Multiple Alleles Have on Inheritance? (3 marks)
Ans: Multiple alleles are non-Mendelian inheritance patterns in which more than two alleles code for a trait in a species. Based on the recessive or dominant alleles available in the characteristic and the dominance pattern the individual alleles follow when placed together, multiple alleles can have more than two phenotypes.
When many alleles are functional for a feature, there are a variety of dominance patterns that occur. In other circumstances, one of the alleles is entirely recessive to the others and is disguised by any of the dominant alleles. Other alleles can be co-dominant and exhibit the same phenotypic traits.
Ques: What is Non-Mendelian inheritance, and how does it apply to the ABO blood group system? (2 marks)
Ans: When two alleles for a gene are expressed equally in the phenotypic of heterozygotes, it is called codominance. IAIB heterozygotes have a unique phenotype in the ABO blood type, as they have both A and B antigens in their blood (type AB blood).
Ques: What is an example of pleiotropy? (2 marks)
Ans: Phenylketonuria is one of the most well-known examples of pleiotropy in humans (PKU). A lack of the enzyme phenylalanine hydroxylase, which is required to convert the essential amino acid phenylalanine to tyrosine, causes this condition.
Ques: What's the difference between pleiotropy and polygenic? (2 marks)
Ans: Pleiotropy is the ability of a gene to impact several qualities, whereas polygenic inheritance is controlled by a number of distinct genes, each having a tiny and similar effect. Several genes have cumulative effects.
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