In Sorghum Vulgare (Pearl maize), the gene located for the sterility pollens are located in the mitochondrial DNA. This phenomenon is called as cytoplasmic male sterility

Mirabilis Jalapa (4 0' clock plant). Incomplete dominance ratio is 1 : 2 : 1

When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent to the other pair of characters. Genes that are located in different chromosomes assort independently during meiosis.

Seed shape, flower color, flower position & pod color.

(i) Back cross is a cross of F, hybrid with any one of the parental genotypes. 
(ii) They are of two types - Dominant back cross recessive back cross.

The German Botanist Carl Correns's (1905) Experiment - In 4 0' clock plant, Mirabilis jalapa when the pure breeding homozygous red (R¹ R¹) parent is crossed with homozygous white (R ²R²), the phenotype of the F₁ hybrid is heterozygous pink (R¹R²). The F₁ heterozygous phenotype differs from both the parental homozygous phenotype. This cross did not exhibit the character of the dominant parent but an intermediate colour pink. When one allele is not completely dominant to another allele it shows incomplete dominance. Such allelic interaction is known as incomplete dominance. F₁ generation produces intermediate phenotype pink coloured flower.
When pink coloured plants of F ₁ generation were interbred in F₂ both phenotypic and genotypic ratios were found to be identical as 1 : 2 : 1(1 red: 2 pink: 1 white). Genotypic ratio is 1 R¹R¹ : 2 R¹R² : 1 R²R². From this we conclude that the alleles themselves remain discrete and unaltered proving the Mendel's Law of Segregation. The phenotypic and genotypic ratios are the same. There is no blending of genes. In the F₂ generation R¹ and R² genes segregate and recombine to produce red, pink and white in the ratio of 1 : 2 : 1. R ¹ allele codes for an enzyme responsible for the formation of red pigment. R² allele codes for defective enzyme. R¹ and R² genotypes produce only enough red pigments to make the flower pink. Two R¹R¹ are needed for producing red flowers. Two R²R² genes are needed for white flowers. If blending had taken place, the original pure traits would not have appeared and all F₂ plants would have pink flowers. It is very clear that Mendel's particulate inheritance takes place in this cross which is confirmed by the reappearance of original phenotype in F₂.

(i) The Pleiotropic gene-influences a number of characters simultaneously & such genes are called pleiotropic gene.
(ii) Mendel noticed pleiotropy while performing breeding experiment with peas (Pisum sativum).
(iii) Peas with purple flowers were cross with white flowers.
(iv) Brown seeds were crossed with light coloured seeds.
(v) Dark spot on the axils of the leaves were crossed with no spot on the axils of the leaves.
(vi) The above characters all were inherited together as a single unit.
(vii) This is due to the pattern on inheritance where the 3 traits were controlled by a single gene with dominant & recessive alleles.
(viii) E.g. Sickle cell anemia.

Law of Dominance: The characters were controlled by discrete units called factors which occur in pairs. In a dissimilar pair of factors one member of the pair is dominant & the other is recessive.
(i) This law gives an explanation to the monohybrid cross.
(a) the expression of only one of the parental characters in F₁ generation.
(b) the expression of both in the F₂ generation
(ii) It also explains the proportion of 3:1 obtained at the F₂

Monohybrid Cross 
(i) Monohybrid inheritance is the inheritance of a single character i. e, plant height.
(ii) It involves the inheritance of two alleles of a single gene.
(iii) When the F₁ generation was selfed Mendel noticed that 7847 of 1064 F₂ plants were tall, while 277of 1064 were dwarf.
(iv) When tall pea plants were pollinated with the pollens from a true breeding dwarf plants, the result was all tall plants.
(v) Dwarf trait appears in the F₂ generation.
(vi) When the parental types were reversed, the result was the same - All tall plants.
(vii) Tall (♀) \(\times\) Dwarf (♂) and Tall (♂) \(\times\) Dwarf (♀) mating's are done in both ways.
(viii) It was concluded that the trait is not sex dependent.
(xi) The gene for plant height has two alleles: Tall (T) \(\times\) Dwarf (t). (TT, Tt, Tt: tt)

a) Dominance
b) Incomplete dominance

Pleiotropy - A single gene affecting many traits. Here the single gene AA controls the traits for flower colour, seed colour and leaf axil spot.

Four gametes - RY, Ry, rY, ry