The Law Of Dominance: When It Fails To Rule

Gregor Mendel's Law of Dominance states that when parents with pure, contrasting traits are crossed, only one trait appears in the first generation. However, this law does not always hold true, and there are cases when it does not apply. For example, it is only valid for diploid organisms and those that undergo sexual reproduction. It also does not account for multiple alleles, where there are more than two alleles coding for a trait, as seen in human blood types.

The law is not applicable to all living organisms

Gregor Mendel, the father and founder of genetics, formulated the Law of Dominance through experiments on pea plants. Mendel's Law of Dominance states that when parents with pure, contrasting traits are crossed, only one trait appears in the next generation. The trait that appears in the offspring is called the dominant trait, and the trait that does not express is called the recessive trait.

However, it is important to note that the Law of Dominance is not applicable to all living organisms. Specifically, the law only applies to diploid organisms and those that undergo sexual reproduction. Diploid organisms are those that have two sets of chromosomes, with one set inherited from each parent. During sexual reproduction, the maternal and paternal gametes fuse during fertilization, each contributing one allele for a particular trait.

In the case of organisms that do not reproduce sexually, such as those that reproduce asexually through methods like binary fission or budding, the Law of Dominance does not apply. Additionally, the law is not relevant to organisms that do not have two sets of chromosomes, deviating from the diploid state.

Furthermore, the Law of Dominance is based on the concept of dominance and recessiveness, which is not observed in all forms of inheritance. Other patterns of inheritance, such as blending inheritance, co-dominance, and incomplete dominance, have been discovered and studied. These alternative modes of inheritance demonstrate that dominance is not the only mechanism at play in the transmission of traits from parents to offspring.

It does not account for co-dominance

Gregor Mendel's Law of Dominance states that when two alleles of an inherited pair are heterozygous, one allele is expressed (dominant) and the other is not (recessive). Mendel's laws of inheritance were formulated based on experiments on pea plants, observing their pattern of inheritance from one generation to the next.

However, the Law of Dominance does not account for co-dominance, a phenomenon in genetics where two alleles (different versions of the same gene) are expressed to an equal degree within an organism. In such cases, the traits associated with each allele are displayed simultaneously, rather than one trait being dominant over the other. For example, in humans, the ABO blood group system demonstrates co-dominance. People with type AB blood have one allele for A and one for B, and both alleles are expressed at the same time, resulting in an AB blood type.

Another example of co-dominance is seen in speckled chickens, which have alleles for both black and white feathers, resulting in a combination of both colours. Similarly, roan cattle express alleles for both red and white hair, leading to a mixture of red and white hair. In plants, rhododendrons can display co-dominance by exhibiting flowers with both red and white petals, due to the simultaneous expression of red and white genes for flower colour.

Thus, the Law of Dominance has limitations and does not apply to co-dominant alleles, where both alleles are expressed instead of one being dominant over the other.

It does not account for incomplete dominance

The Law of Dominance, as discovered by Gregor Mendel, states that "when parents with pure, contrasting traits are crossed together, only one form of the trait appears in the next generation". This trait is known as the dominant trait, and the trait that does not express is called the recessive trait. Mendel's law of dominance can be explained by his experiments on pea plants.

However, the Law of Dominance does not account for incomplete dominance, which is a form of gene interaction in which both alleles of a gene at a locus are partially expressed, resulting in an intermediate or different phenotype. In other words, incomplete dominance is a phenomenon where neither of the two alleles is completely dominant over the other, resulting in a phenotype that is a combination of both.

For example, in roses, the allele for red colour is dominant over the allele for white colour. However, heterozygous flowers with both alleles are pink in colour. This is because the red allele is not completely dominant over the white allele, resulting in a pink colour that is a blend of the two.

Another example of incomplete dominance is seen in snapdragons. When true-breeding red (RR) and white (rr) snapdragons are crossed, the F1 generation produces pink-coloured flowers with the Rr pair of alleles. This is because the red allele is only partially dominant, and the white allele is not completely recessive, leading to an intermediate phenotype.

Incomplete dominance can also be observed in humans, such as in hair type. The child of parents, each with curly hair and straight hair, will always have wavy hair. This is because the alleles for curly and straight hair are not completely dominant or recessive, resulting in an intermediate trait.

It does not account for blending inheritance

The Law of Dominance, also known as Mendel's First Law of Inheritance, states that when parents with pure, contrasting traits are crossed, only one form of the trait appears in the subsequent generation. This trait is referred to as the dominant trait, while the unexpressed trait is called the recessive trait. Mendel's Law of Dominance was formulated based on experiments conducted on pea plants, observing their pattern of inheritance across generations.

However, the Law of Dominance does not account for blending inheritance, an obsolete 19th-century theory in biology. Blending inheritance proposes that offspring inherit characteristics as an average of their parents' traits. For example, crossing a red flower variety with a white variety of the same species would result in pink-flowered offspring. This concept contradicts Mendel's Law of Dominance, which asserts that only one trait, either dominant or recessive, is expressed in the offspring.

Charles Darwin's theory of inheritance by pangenesis implied blending inheritance. According to pangenesis, each part of a parent's body emits gemmules, tiny particles that migrate and contribute to the formation of gametes. This theory suggested that characteristics of all body parts, such as the shape of the nose or length of legs, are inherited from both parents. However, it faced criticisms, including the fact that many characteristics can change during an individual's lifetime and are influenced by environmental factors. Additionally, the blending of gemmules upon fertilisation implied that offspring would be intermediate between the two parents in every characteristic, which contradicted the observed facts of inheritance, such as the distinct presence of male and female offspring.

Despite being refuted by Mendel's experiments, the concept of blending inheritance is not entirely irrelevant. Incomplete dominance, a form of non-Mendelian inheritance, may superficially resemble blending inheritance. In cases of incomplete dominance, alleles do not exhibit complete dominance, and the offspring display a mixture of the two phenotypes. For instance, in snapdragon flowers, the alleles for red and white flower colours both express, resulting in pink flowers in the heterozygous genotype. While this may appear similar to blending inheritance, it can still be explained using Mendel's laws with modifications.

It does not account for multifactorial traits

The Law of Dominance, also known as Mendel's Law of Dominance, states that "when parents with pure, contrasting traits are crossed together, only one form of the trait appears in the next generation. The hybrid offspring will exhibit only the dominant trait in the phenotype."

However, this law does not account for multifactorial traits, which are caused by a combination of hereditary, environmental, and unpredictable influences. In other words, multifactorial traits are influenced by various genes and environmental factors, whereas Mendelian traits are influenced by a single gene.

Multifactorial traits, such as height, hair texture, and skin colour, do not follow Mendelian analysis. While some traits obey Mendel's laws, many others do not. In these cases, there are no definite recessive or dominant traits observed, and the environment also plays a role in producing varying phenotypes.

For example, type 2 diabetes is a multifactorial disease caused by the inheritance of susceptibility genes and environmental factors such as obesity. Obesity itself is also a multifactorial trait, influenced by both genetic and environmental factors.

Multifactorial traits within families segregate, but there is no uniform or recognised inheritance pattern. While polygenic traits are always continuous, multifactorial traits can be continuous or discontinuous.

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