Introduction
Mendel’s laws of heredity—Law of Dominance, Law of Segregation, and Law of Independent Assortment—form the foundation of classical genetics. However, later studies revealed several deviations from these laws due to more complex genetic interactions. These deviations help explain traits that do not follow simple dominant-recessive inheritance patterns.
In this article, we will explore key deviations from Mendel’s principles, including incomplete dominance, codominance, multiple alleles, polygenic inheritance, pleiotropy, gene interaction, and linkage.
1. Incomplete Dominance
In incomplete dominance, neither allele is completely dominant over the other. As a result, the heterozygous offspring display an intermediate phenotype between the two parental traits.
Example: Snapdragon Flower (Antirrhinum majus)
When a red-flowered plant (RR) is crossed with a white-flowered plant (WW), the heterozygous offspring (RW) exhibit pink flowers instead of red or white.
| Parental Genotype | Offspring Genotype | Offspring Phenotype |
| RR (Red) x WW (White) | RW | Pink |
This deviates from Mendel’s principle of dominance, as the heterozygous trait is a blend rather than a dominant or recessive trait.
2. Codominance
In codominance, both alleles in a heterozygous condition are fully expressed, leading to a phenotype that displays both traits simultaneously.
Example: ABO Blood Group System
The ABO blood group in humans is controlled by the IA, IB, and i alleles. Here, IA and IB are codominant, meaning that an individual with genotype IAIB will have AB blood group, expressing both A and B antigens.
| Genotype | Phenotype (Blood Group) |
| IAIA or IAi | A |
| IBIB or IBi | B |
| IAIB | AB (both A and B antigens are expressed) |
| ii | O |
This deviates from Mendel’s concept of dominant-recessive traits.
3. Multiple Alleles
Mendel’s experiments focused on genes with only two alleles. However, some traits are controlled by more than two alleles, known as multiple alleles.
Example: Rabbit Fur Color
The coat color of rabbits is controlled by four alleles:
| Allele | Phenotype |
| C | Full color (dominant) |
| cch | Chinchilla (grayish coat) |
| ch | Himalayan (white with dark extremities) |
| c | Albino (recessive) |
Since an organism can only have two alleles at a time, different combinations produce varying phenotypes.
4. Polygenic Inheritance
Unlike Mendelian traits controlled by a single gene, polygenic inheritance involves multiple genes influencing a single trait. These traits show a wide range of variations rather than distinct categories.
Example: Human Skin Color
Skin color is determined by multiple genes controlling melanin production. Individuals inherit a combination of dominant and recessive alleles, leading to a spectrum of skin tones.
This contradicts Mendel’s concept of distinct dominant and recessive traits.
5. Pleiotropy
In pleiotropy, a single gene affects multiple traits in an organism.
Example: Sickle Cell Anemia
A mutation in the HBB gene results in abnormal hemoglobin production, leading to:
- Misshapen (sickle-shaped) red blood cells
- Reduced oxygen transport
- Increased susceptibility to infections
This deviates from Mendel’s one-gene-one-trait assumption.
6. Gene Interaction (Epistasis)
In epistasis, one gene affects or masks the expression of another gene.
Example: Coat Color in Labrador Retrievers
Two genes control coat color:
- B gene (Black or Brown)
- E gene (Expression of pigment)
| Genotype | Coat Color |
| BBEE, BbEE | Black |
| bbEE, bbEe | Brown |
| BBee, Bbee, bbee | Yellow (E gene prevents pigment production) |
Here, the E gene is epistatic to the B gene, influencing the final coat color.
7. Linkage and Crossing Over
Mendel’s Law of Independent Assortment states that genes assort independently. However, linked genes (located close together on the same chromosome) tend to be inherited together.
Example: Eye Color and Wing Shape in Drosophila (Fruit Fly)
Genes for eye color and wing shape in fruit flies are linked. They are often inherited together unless crossing over (genetic recombination) separates them.
This shows that Mendel’s law does not always apply to genes located on the same chromosome.
Mendel’s laws laid the foundation for genetics, but further studies revealed complexities that do not always align with his principles. Incomplete dominance, codominance, multiple alleles, polygenic inheritance, pleiotropy, gene interactions, and linkage demonstrate how inheritance patterns can deviate from Mendel’s original laws.
Understanding these deviations helps explain the diversity of genetic traits observed in nature, highlighting the complexity of heredity beyond Mendelian genetics.