1. It is found in 40'ciock plant (Mirabilis jalapa).
2. Two types of variegated leaves.
(i) Dark green leaved plants
(ii) Pale green leaved plants
3. When the pollen of dark green leaved plant (male) is transferred to the stigma of pale green leaved plant (female) and pollen of pale green leaved plant is transferred to the stigma of dark green leaved plant, the F₁ generation of both the crosses must be identical as, per mendelian inheritance.
4. But in the reciprocal cross the F₁ plant differs from each other.
5. In each cross, the F₁ plant resembles female plant.
6. This inheritance is not due to nuclear gene.
7. This is due to the chloroplast gene found in the ovum of the female plant which contributes the cytoplasm.
8. During fertilization male gamete contribute only the nucleus but not cytoplasm.

(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.

Continuous variation Discontinuous variation Inheritance of phenotype is determined by the combined effects of many genes. The characteristics are controlled by one or two major genes which may have two or more allelic forms. Most of the characteristics exhibit a complete gradation, from one extreme to the other. Individuals produced by this variation show differences without any intermediate form between them. The variation may be due to the combining effects of environmental & genetic factors. They are unaffected by environmental conditions. This is also known as quantitative inheritance. This is also known as qualitative inheritance, E.g. Human height & skin colour E.g. Style length in Primula, plant height of garden pea.

(i) Polygenic inheritance - Several genes combine to affect a single trait.
(ii) A group of genes that together determine (contribute) a characteristic of an organism is called polygenic inheritance. It gives explanations to the inheritance of continuous traits which are compatible with Mendel's Law.
(iii) The first experiment was demonstrated by H. Nilsson - Ehle (1909) in wheat kernels.
(iv) He crossed the two pure breeding wheat varieties dark red & white.
(v) Dark red genotypes R₁R₁R₂R₂ and
(vi) white genotypes are r₁r₁r₂r₂ 
(vii) In the F₁ generation medium red were obtained  R₁r₁R₂r_2.
(viii) F₁ wheat plant produces four types of gametes R₁R_2, R₁r_2, r₁R_2, r₁r_2.
(xi) The intensity of the red colour is determined by the number of R genes in the F₂ generations.
(x) Four R genes -  Dark red kernel.
(xi) Three R genes - Medium dark red kernel 
(xii) Two R genes - Medium red kernel
(xiii) One R gene - Light red kernel.
(xiv) Absence of R gene - White kernel.


F₂ 

1	6	15	20	15	6	1
Dark red	Moderate red	Red	Intermediate red	light red	very light red	white

(xv) In F₂ generation plants have kernels with wide range of colour variation. 
(xvi) This is due to that the genes are segregating and recombination takes place.
(xvii) The cumulative effect of several pairs of gene i interaction gives rise to many shades of kernel colour.

Interlocus interactions take place between the alleles at different loci i.e. between alleles of different genes
Dominant Epistasis: It is a gene interaction in which 2 alleles of a gene at one locus interfere & suppress or mask the phenotypic expression or mask the phenotypic expression of a different pair of alleles of another gene at another locus.
The gene that suppresses or masks the phenotypic expression of a gene at another locus is known as epistatic. The gene whose expression is interfered by non-allelic genes & prevents from exhibiting its character is known as hypostatic.
When both the genes are present together, the phenotype is determined by the epistatic gene and not by the hypostatic gene.
In the summer squash, the fruit colour locus has a dominant allele 'W' for white colour & a recessive allele 'w' for coloured fruit.
'W' allele is donminant that masks the expression of any colour. In another locus hypostatic allele 'G' is for yellow fruit and its recessive allele 'g' for green fruit.
In the first locus the white is dominant to colour where as in the second locus yellow is dominant to green.
When the white fruit with genotype WWgg is crossed with yellow fruit with genotype wwGG, the F1 plants have white fruit and are heterozygous (WwGg).
When F1 heterozygous plants are crossed they give rise to F2 with the phenotypic ratio of 12 white:3 yellow: 1 green.

Since W is epistatic to the alleles 'G' and 'g'. the white which is dominant, masks the effect of yellow or green.
Homozygous recessive ww genotypes only can give the coloured fruits (4/16).
Double recessive 'wwgg' will give green fruit (1/16).
The Plants having only 'G' in its genotype (wwGg or wwGG) will give the yellow fruit(3/16).