Tag: inheritance

Even though Mendel did not try to explain traits that did blend, other scientists eventually did. The discovery of incomplete dominance is usually credited to German Botanist Carl Correns who studied four o'clock plants. Instead of having only two colors of petals on the flowers, there was an intermediate colour that would show up that corresponded to Mendel's genotype ratio of 1:2:1 instead of his pheontype ratio of 3:1. This showed that each genotype corresponded to its own unique phenotype and that the heterozygotes were showing a blend of both alleles instead of one being completely dominant over the other.
Incomplete dominance was actually seen and recorded long before Carl Correns published his works and even before Gregor Mendel worked with his pea plants and published his findings. Since there was no discipline known as Genetics at that time, however, it was not fully explored. Scientists dating back to ancient times discussed the blending of traits in their writings which could be attributed to incomplete dominance. However, it wasn't until after Mendel and Correns that the term "incomplete dominance" came into common usage and the mechanism for how it worked was known.

Incomplete Dominance is the blending of alleles to create a phenotype that is a combination of both traits the alleles code for. Incomplete dominance is a type of non-Mendelian genetics.

As we know that trait is represented by two contrasting factors of a gene in a heterozygous individual; the allele/factor that can express itself in the heterozygous individual is called as the dominant trait. The other factor, whose effect is masked by the presence of dominant factor, is called recessive factor. In the case of complete dominance, the heterozygous individual shows dominant phenotype. Law of incomplete dominance tells that where none of the factors of a gene is dominant, the phenotype of the heterozygous dominant individual is the blend of dominant and recessive traits. For example, flower colour in Mirabilis jalapa and sweet peas (Lathyrus odoratus) shows incomplete dominance and the monohybrid cross between two pure varieties gives 1:2:1 phenotypic ratio in F$ _2$ generation which is 3:1 in dominant traits. Monohybrid cross in Oenothera lamarckiana does not give typical 3:1 ratio, hence does not follow complete dominance. Pisum sativum was the experimental plant of Mendel and show complete dominance. 

Partial dominance is also known as incomplete dominance. It occurs when the dominant allele is not able to mask the effect of the recessive allele completely. It is commonly seen in Snapdragon and Mirabilis jalapa. In F$ _2$ generation, the red allele cannot completely mask the effect of white allele resulting in a new phenotype i.e. pink.

Incomplete dominance is the condition when none of the factors of a gene is dominant and the phenotype of a heterozygous dominant individual is a blend of dominant and recessive traits. For example, flower colour in Antirrhinum majus shows incomplete dominance and the monohybrid cross between two pure varieties gives 1 red : 2 pink : 1 white phenotypic ratio in F$ _2$ generation. Here, the heterozygous genotypes show the blending of dominant and recessive phenotypes. Epistasis is the non-allelic gene interaction where the presence of one gene masks the expression of another gene; it is an attribute of polygenic trait while flower colour in Antirrhinum majus is controlled by a single gene; option B is wrong. As we know that, a trait is represented by two contrasting factors of a gene in a heterozygous individual; the allele/factor that can express itself in a heterozygous individual is called as a dominant trait. The other factor whose effect is masked by the presence of dominant factor is called as the recessive factor. A cross between pure breeding dominant and recessive varieties produces all dominant varieties which are not the case in flower colour in Antirrhinum majus; option A is incorrect. When both recessive and dominant traits are expressed in a heterozygous genotype; it is codominance; but in Antirrhinum majus, a new blending trait is expressed in heterozygous genotype, option C is incorrect. 

Incomplete dominance is the condition when the dominant allele is not able to mask the effect of recessive allele completely. This results in an organism showing a physical appearance with a blend of both the alleles. It is also called partial dominance. 

Incomplete dominance means that one allele for a trait is not completely dominant over the other one and thus the heterozygous genotype shows a combination of dominant and recessive phenotypes. Mirabilis plant shows incomplete dominance of flower colour. Multiple allelism is a condition when more than two alleles of a gene govern the trait. Human blood group inheritance is the example of multiple alleles. It is governed by three alleles namely I$^{A}$, I$^{B}$ and I$^{A}$ and I$^{B}$ show codominance while allele i is recessive to both "I$^{A}$" and "I$^{B}$". This gives total six genotypes and four phenotypes- A (I$^{A}$ I$^{A}$ and I$^{A}$i), B (I$^{B}$ I$^{B}$ and I$^{B}$i), AB (I$^{A}$ I$^{B}$) and O (ii). 

• Gregor Mendel was the founder of modern genetics. He performed experiments of on pea plant and established many rules of heredity, now referred to as the laws of Mendelian inheritance.
  
• Hugo de Vries gave the concept of genes, rediscovered laws of heredity, introduced the term mutation and gave the mutation theory of evolution.
  
• Carl Correns is known for his independent discovery of the principles of heredity and also discovered non-mendelian inheritance like cytoplasmic inheritance or cytoplasmic inheritance.
  
• Tschermark developed several new disease-resistant crops. Along with Vries and Correns, he discovered Gregor's work.
  

Incomplete dominance is the condition when none of the factors of a gene is dominant; the phenotype of a heterozygous dominant individual is a blend of dominant and recessive traits. A monohybrid cross between two pure varieties of Mirabilis jalapa with red (RR) and white (rr) flowers gives red, pink and white colored flowers in 1:2:1 phenotypic ratio in $F _2$ generation. This is because the red color exhibits incomplete dominance over white color and the heterozygous dominant genotype (Rr) produces different phenotype from homozygous dominant (RR).  Flower color in Mirabilis jalapa is governed by a single gene. Thus, the correct answer is option B.