Plasmogenes are independent, self-replicating, extra-chromosomal units located in cytoplasmic organelles, chloroplast, and mitochondrion.

An allele which has the potential to cause the death of an organism is called a Lethal Allele. 

(a) Incomplete dominance
(b) Codominance
(c) Multiple alleles
(d) Pleiotropic genes are common examples for intragenic interaction.

Four different gametes are produced by a dihybrid plant and on selfing, it yield 16 offsprings.

i) The Law of Dominance,
ii) The Law of Segregation

Emasculation: Removal of anthers from the flower
Alleles: Alternate forms of a gene

(a) Classical genetics
(b) Molecular genetics
(c) Population genetic
(d) Quantitative genetics

A group of genes that together determine a characteristic of an organism.

"Genetics" is the branch of biological science which deals with the mechanism of transmission of characters from parents to off springs.

(i) Hugo de Vries of Holland
(ii) Carl Correns of Germany
(iii) Erich von Tschermak of Austria

A true breeding lines means it has undergone continuous self pollination having stable trait inheritance from parent to off spring. 

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.

The protein called starch branching enzyme (SBEI) is encoded by the wild-type allele of  the gene (RR) which is dominant. When the seed matures, this enzyme SBEI catalyzes the formation of highly branched starch molecules. Normal gene (R) has become interrupted by the insertion of extra piece of DNA (0.8 kb) into the gene, resulting in r allele. In the homozygous mutant form of the gene (rr) which is recessive, the activity of the enzyme SBEI is lost resulting in wrinkled peas. The wrinkled seed accumulates more sucrose and high water content. Hence the osmotic pressure inside the seed rises. As a result, the seed absorbs more water and when it matures it loses water as it dries. So it becomes wrinkled at maturation. When the seed has at least one copy of normal dominant gene heterozygous, the dominant allele helps to synthesize starch, amylopectin an insoluble carbohydrate, with the osmotic balance which minimises the loss of water resulting in smooth structured round seed.

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).

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)

Atavism is a modification of a biological structure whereby an ancestral trait reappears after having been lost through evolutionary changes in the previous generations. Evolutionary traits that have disappeared phenotypically do not necessarily disappear from an organism's DNA. The gene sequence often remains, but is inactive. Such an unused gene may remain in the genome for many generations. As long as the gene remains intact, a fault in the genetic control suppressing the gene can lead to the reappearance of that character again. Reemergence of sexual reproduction in the flowering plant Hieracium pilosella is the best example for Atavism in plants.

Male sterility found in pearl maize (Sorgum vulgare) is the best example for mitochondrial cytoplasmic inheritance. So it is called cytoplasmic male sterility. In this, male sterility is inherited maternally. The gene for cytoplasmic male sterility is found in the mitochondrial DNA.

The phenomenon in which two alleles are both expressed in the heterozygous individual is known as codominance. The codominance was demonstrated in plants with the help of electrophoresis or chromatography for protein or flavonoid substance. Example: Gossypium hirsutum and Gossypium sturtianum, their F ₁ hybrid (amphiploid) was tested for seed proteins by electrophoresis. Both the parents have different banding patterns for their seed proteins. In hybrids, additive banding pattern was noticed. Their hybrid shows the presence of both the types of proteins similar to their parents.

A cross between homozygous parents that differ in three gene pairs (i.e. producing trihybrids) is called trihybrid cross.
F₂ Phenotypic ratio - 27: 9 : 9 : 9 : 3 : 3 : 3 : 1

The Law of Segregation (Law of Purity of gametes): Alleles do not show any blending. During the formation of gametes, the factors or alleles of a pair separate and segregate from each other such that each gamete receives only one of the two factors. A homozygous parent produces similar gametes and a heterozygous parent produces two kinds of gametes each having one allele with equal proportion. Gametes are never hybrid.

He chooses pea plant because,
(i) It is an annual plant & bears contrasting characters controlled by a single gene separately.
(ii) Self-fertilization occurred in garden pea plants.
(iii) Mendel used both self-fertilization and cross-fertilization.   
(iv) The flowers are large hence emasculation & pollination are very easy for hybridization.

It is based on independent, self-replicating extra chromosomal unit called plasmogene located in the cytoplasmic organelles, chloroplast & Mitochondrion.

Incomplete Dominance	Co-dominance
When one allele is not completely dominant to another allele.	lt occurs due to simultaneous expression of both alleles in the heterozygote.
E.g: Flowers colour of Mirabilis Jalapa	E.g:  ABO blood group system in human beings.