Tag: classical genetics

Karl Correns discovered incomplete dominance in which none of the factors of a gene is dominant, the phenotype of the heterozygous dominant individual is the blend of dominant and recessive traits. He studied flower colour in Mirabilis jalapa and showed that a monohybrid cross between two pure varieties one with red flower petals another with white flower petals gives pink-flowered F$ _1$ plants. If these pink-flowered F$ _1$ plants are crossed, the F$ _2$ plants appear in a ratio of 1 : 2 : 1, having red, pink, or white flower petals, respectively. Pink-flowered plants are heterozygotes with a petal colour intermediate between the red and white colours of the homozygotes. Here, dominant allele (R) specifies red pigment colour, and recessive allele specifies no colour (r) and the flower petals have a white background colour. The pink heterozygotes (Rr) have about half the red pigment of the flowers in red homozygotes (RR) because the heterozygotes carry only one copy of “R” allele that produces colour, while the homozygotes have two copies.

Genes are units of hereditary information that are located on segments of chromosomes. The genes for a specific trait may exist in different forms known as alleles. An example of an allele or a gene is flower color. A flower may have alleles that make it red or pink and so on. Alleles that always show up when they are present are known as dominant alleles. Alleles that are masked or hidden by dominant alleles are known as recessive alleles. In some situations both alleles are expressed equally. A genetic scenario where neither allele is dominant or recessive and both get expressed is known as 'codominance'.
Blood types are excellent examples of codominance. 

Incomplete dominance is the condition when the dominant allele is not completely dominant over the recessive resulting in a mixture of dominant-recessive phenotype in heterozygous condition. When a single trait is governed by more than one gene, it is said to be under control of multiple genes. Human blood group inheritance is the example of codominance and multiple alleles. It is governed by three alleles of gene "I", namely I$^{A}$, I$^{B}$ and I$^{O}$. I$^{A}$ and I$^{B}$ show codominance while allele "I$^{O}$" 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$^{O}$), B (I$^{B}$ I$^{B}$ and I$^{B}$ I$^{O}$) , AB (I$^{A}$ I$^{B}$) and O (I$^{O}$ I$^{O}$). Correct option is "E".

When a red flowered plant is crossed with a white flowered plant, ideally the offsprings should bear either red and white flower. However, in the case where the trait  flower color exhibits incomplete dominance, all the offsprings will give forth pink colored flowers due to the blending of traits.

Incomplete dominance is the condition when none of factors of a gene is dominant, the phenotype of heterozygous dominant individual is blend of dominant and recessive traits. For example flower colour in Mirabilis jalapa 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 otherwise dominant traits. This is because the heterozygous genotypes do not show the dominant trait but they are blend of dominant and recessive phenotype. A cross between two pure breeding genotypes that differ in two pairs of trait (dihybrid cross) followed by selfing of F$ _1$ hybrid gives 9 : 3 : 3 : 1 phenotypic ratio and 1 : 2 : 1 : 4 : 1 : 1 : 2 : 1 : 2 : 1 genotypic ratio. Option B and C show both dihybrid (9 : 6), a dihybrid cross gives total 16 types which make these two options wrong.

Heterozygous tall plant with pink flower is test crossed with homozygous recessive partner, as a result of which different phenotypes are produced in the F1 offspring. The cross is between TtPp and ttpp which is shown in the Punnett Square.

Codominance occurs when two alleles are equally dominant, and both appear in a phenotype. The classic example is human blood type, in which the A and B alleles are codominant, so anyone with both will have the phenotype AB.

In a cross of PpBb and PpBb,

Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. In the early 1900s, German botanist Carl Correns (1864–1933) would conduct similar research on four o'clock plants. While Mendel's work laid a foundation, it isCorrens who is credited with the actual discovery of incomplete dominance. In his work, Correns observed a blend of colors in flower petals. So, the correct option is 'Law of incomplete dominance'.

Incomplete dominance is a type of intermediate inheritance in which one allele is not completely dominant over its paired allele. In this, the third phenotype is observed in heterozygous condition. In monohybrid cross with incomplete dominance, the phenotypic and genotypic ratio is 1:2:1, where one part is with dominant phenotype, 2 parts are with intermediate phenotype and 1 part is with recessive phenotype. For example, a snapdragon flower shows red (dominant), white (recessive) and pink (intermediate) flowers.