Incomplete Dominance

Gregor Mendel introduced the theory that in all living organisms, the hereditary traits are passed down from parents to offspring through a set of genes, which constitutes its genetic makeup.

He was also the first one to propose based on his experiments with pea plants that genes which are generally in pairs do not unite; instead, they segregate when sex cells fuse. During segregation, each member from a pair attaches itself to one sex cell each.

Mendel states in his law of segregation that for a particular trait, every organism has two alleles, one each from a male and female organism. If these two alleles are identical, then the organism is referred to as homozygous. If they are dissimilar, they are called heterozygous.

In the case of heterozygous organisms, the trait of the dominant allele hides the trait carried by the repressive allele. Thus, an organism’s genotype is the sum of all alleles that it receives from its parents. 

Do You Know? 

The word gene was coined only in the 29th century!

What Is the Law of Complete Dominance?

Mendel’s law of dominance based on his experiments with pea pod states that the dominant allele trait will completely mask the character of a recessive allele, which will also influence an organism’s phenotype. His pea plant experiment serves as a complete dominance example.

Let us look at it – 

Mendel took a purebred white, and purple flower and crossbred them. The resulting hybrid was a purple coloured flower and not a combination of the two colors. Thus, the purple flower was a dominant trait which completely hid the traits of the white flower. Nonetheless, when he allowed the hybrid plant to self-fertilize, the recessive traits appeared.

However, the study of genes in recent times has shown that gene alleles do not behave in such a straightforward way. It results in several other patterns of dominance, such as incomplete dominance. 

Let us Examine What is Incomplete Dominance

Incomplete Dominance

For instance, if a homozygous red(R1R1) and homozygous white-flowered plant (W1 W1) is crossbred, then the first generation will produce pink coloured flowers of the variety (R1W1). It shows that alleles of red and white coloured flowers were unable to dominate the other, thus resulting in incomplete dominance. Thus, the law of incomplete dominance says that when none of the two alleles exerts complete dominance over the other, the offspring will be a mixture of parents’ phenotypes. 

Aside from flowering plants, incomplete dominance takes place in human beings and animals as well. One such incomplete dominance example in human beings is the growth of wavy hair. In this case, one parent has straight hair, and the other has curly hair, but none of the traits is entirely dominant, so it results in a new hair type – wavy hair.

Did You Know? 

The gene for six fingers is a dominant trait.

Complete Dominance vs Incomplete Dominance

Codominance

Another pattern of dominance that does not follow Mendel’s law is codominance. In this type, the gene alleles of both parents are dominant, and together they form an entirely new phenotype for their offspring. Let us consider the incomplete dominance example of the red and white flower again.

In case of incomplete dominance, we saw that the color of a hybrid flower turned pink as neither of the traits was dominant enough. However, in coherence, the bulb will show both red and white colors. Another instance of codominance is blood group in human beings. Individuals with AB blood group inherited one allele for A blood group from one parent, and another for B blood group from another parent, where both alleles are equally dominant.

Incomplete Dominance vs Codominance

The main difference between Incomplete Dominance and Codominance have been summarized in the Table below - 

Dominance is a relationship between alleles of a single gene in genetics. He was the first to provide a scientific explanation, but he was not the first to carry out these experiments. He tweaked the trials to suit his needs, focusing on two to three genes at a time. Genes are a hereditary unit in organisms that exist as a pair of alleles in diploid organisms, as far as we know. These alleles may or may not be related. 

For example, a heterozygous gene has two sets of alleles that are distinct, whereas homozygous genes contain identical alleles. Heterozygous alleles have different information on attributes than homozygous alleles. There are two possible explanations for why one characteristic is dominant over the other:

• either it is non-functional, or

• is less active than the normal allele

Mechanism of Incomplete Dominance

Incomplete dominance occurs because none of the two alleles is completely dominant over the other. As a result, the phenotype is a hybrid of the two. As we've seen, it doesn't always happen with flower color; roses and tulips, for example, display partial dominance, while Mendel's pea plants show total dominance. Gregor Mendel experimented on pea plants. He analyzed seven characters with disparate qualities, and they all had a similar pattern of inheritance. On this basis, he generalized the law of inheritance. Mendel's experiment was later replicated on other plants by academics. Surprisingly, scientists discovered that the F1 Generation deviated from the regular pattern of inheritance. The monohybrid cross produced F1 progeny that had no resemblance to either of the parents, but rather an intermediate progeny.

The Snapdragon plant's red and white blooms were crossed in a monohybrid cross. Consider that the pure breed of the red flower has an RR pair of alleles, whereas the pure breed of the white flower has an RR pair of alleles. First, true-breeding red (RR) and white (rr) snapdragon flowers were crossed. The Rr pair of alleles resulted in a pink blossom in the F1 generation. The F1 offspring were then self-pollinated. This resulted in a 1:2:1 ratio of red (RR), pink (Rr), and white (rr) flowers. Remember that Mendel's genotypic ratio of F2 generation in the monohybrid cross generated the same ratio of 1:2:1. The phenotypic ratio, however, has shifted from 3:1 to 1:2:1. The cause of this variation is the allele R's partial dominance over the allele r. This resulted in color mixing in flowers.

Examples of Incomplete Dominance in Human

Due to the expression of both the curly and straight alleles, a kid born to a parent with straight hair and a parent with curly hair will often have wavy or somewhat curly hair. Many other physical traits, such as skin color, height, hand size, and voice tone, show incomplete dominance. 

Tay-Sachs disease patients lack an enzyme that breaks down lipids, causing an excess of lipids to build in the brain and other regions of the nervous system. This causes nerve damage and a loss of physical and mental capacities. Tay-Sachs disease affects persons who have two recessive alleles for the illness, whereas people who just have one allele are carriers but do not have symptoms. They do, however, generate half the usual amount of the enzyme, exhibiting an intermediate phenotype between individuals with the illness and those without any recessive Tay-Sachs genes.

Examples of Incomplete Dominance in Animal

The Andalusian chicken, a kind of chicken native to Spain's Andalusia region, has a slight advantage in feather colour. Offspring with blue-tinged feathers are frequently produced by a white male and a black mother. This is caused by a dilution gene, which dilutes the pigment melanin and lightens the feathers. 

When long and short-furred rabbits are bred, their offspring will have medium-length fur. The length of a dog's tail is another example of this phenomenon. In addition, if a spotted animal is mated with a non-spotted species, the child will get a few spots. This is commonly observed in dogs, cats, and horses.

Additionally, you can avail detailed study guides on genetics at Vedantu’s online portal along with charts and diagrams which will help to gain a better understanding of the concept. Alternatively, you can download a pdf version as well directly from your phone’s Vedantu app.