Revision: Genetics and Evolution >> Principles of Inheritance and Variation Biology Science (English Medium) Class 12 CBSE

Heredity (heirship or inheritance) is the transmission of genetically based characters from parents to their offsprings.

Mendelism refers to the principles of inheritance proposed by Gregor Mendel based on his experiments with pea plants.​ These principles explain that traits are inherited in a predictable manner through discrete hereditary units.

The trait expressed in a heterozygous condition is called dominant.

An organism having unlike alleles for a character, such as Tt, is called heterozygous.

An organism having identical alleles for a character, such as TT or tt, is called homozygous.

The trait that remains unexpressed in a heterozygous condition but appears in homozygous form is called recessive.

Alternative forms of the same gene controlling a pair of contrasting traits are called alleles.

Mendel's first experiments were with the varieties of garden pea that differed in only one visible character. These are known as monohybrid experiments.

A Punnett square is a graphical diagram developed by Reginald C. Punnett to represent genetic crosses. It is used to predict all possible genotypes and their probabilities in the offspring by arranging the gametes of parents along the top row and left column and showing their combinations in a square format.

A cross between parents differing in two heritable traits is called a dihybrid cross. e.g., a cross of a pure, tall, round seeded plant with a dwarf, wrinkled-seeded plant.

A back cross is defined as a genetic cross between an F₁ hybrid and either of its parental forms (dominant or recessive) to study inheritance of traits.

A test cross is defined as a genetic cross between an individual showing a dominant phenotype with unknown genotype and a homozygous recessive individual to determine the genotype of the dominant individual.

Incomplete dominance is the inheritance pattern in which neither allele of a gene is completely dominant over the other, so the heterozygous individual shows an intermediate phenotype between the two parental traits.

Co-dominance is the pattern of inheritance in which both alleles of a gene express themselves equally and simultaneously in the heterozygous condition, so both parental traits appear side by side in the phenotype.

• Mendel investigated not only those crosses in which the parent differed in single pair of characters, but also others in which the parent differed in two pairs. Such a cross which involves two pairs of contrasting characters simultaneously is called dihybrid cross.
• A genetic cross involving two pairs of contrasting characters simultaneously is called a dihybrid cross.

Homologous chromosomes are chromosome pairs that are similar in length, gene position and centromere location.

The physical association of two or more genes located on the same chromosome, due to which they tend to be inherited together and do not assort independently, is called linkage.

The process by which new (non-parental) combinations of genes are produced due to exchange of genetic material between homologous chromosomes during meiosis, is called recombination.

• The inheritance of a trait that is controlled by two or more genes, where each gene contributes additively to the phenotype and the trait shows continuous variation rather than distinct categories, is called polygenic inheritance.
• When two or more genes control the traits having distinct alternate forms, it is called polygenic inheritance.

The biological mechanism by which the sex (male or female) of an individual is established based on genetic or chromosomal factors, is called sex determination.

Mutation is a sudden change in one or more genes, or in the number or in the structure of chromosomes.

or

Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism.

The law of dominance states that when a pair of alleles or allelomorphs are combined in an F₁ hybrid, only one of them expresses itself, hiding the expression of the other. A monohybrid cross was used to investigate the simultaneous inheritance of a single pair of Mendelian components. A monohybrid cross is one that considers only different versions of a single character. The feature that occurred in the F₁ generation was referred to as dominant, whereas the trait that did not appear in the F₁ population was known as recessive.

Thus, when a pair of alleles come together in an F₁ hybrid, only one of them expresses itself, hiding the expression of the other entirely. In the above example, in the Tt - F₁ hybrid (tall), only ‘T’ expresses itself as dominant, while ‘t’ remains hidden as recessive. This instance illustrates and indicates the law of dominance.

The law of dominance states that, out of a pair of allelomorphic characters, one is dominant and the other recessive.

1. In a pair of contrasting traits, only one trait is expressed—this is the dominant trait.
2. The trait that remains unexpressed is called recessive.
3. The recessive trait can express itself only when both alleles are recessive (homozygous recessive).

Or

When two homozygous individuals with one or more sets of contrasting characters are crossed, the alleles (characters) that appear in F₁ are dominant and those which do not appear in F₁ are recessive.

Law of segregation states that, when a pair of allelomorphs are brought together in the hybrid (F₁), they remain together in the hybrid without blending but separate complete and pure during gamete formation. 

1. Each pair of alleles separates during gamete formation, with one going into each gamete.
2. No blending occurs; alleles remain pure and distinct.
3. Gametes fuse randomly during fertilisation to form a zygote.

or

When hybrid (F₁) forms gametes, the alleles segregate from each other and enter in different gametes.

Mendel’s Law of Independent Assortment states that, when two pairs of independent alleles are brought together in the hybrid F₁ they show independent dominant effects. In the formation of gametes, the law of segregation operates, but the factors assort themselves independently at random and freely. 

1. When two pairs of traits are considered, alleles of each trait assort independently during gamete formation.
2. The inheritance of one trait does not affect the inheritance of the other.
3. This law is clearly demonstrated in the F₁ generation of a dihybrid cross.

or

When a hybrid possessing two (or more) pairs of contrasting factors (alleles) forms gametes, the factors in each pair segregate independently of the other pair.

• Genetics is the study of inheritance and variation in living organisms.
• Inheritance means passing traits from parents to offspring.
• Variation refers to differences between offspring and their parents.
• Early humans knew that sexual reproduction causes variation (around 8000–1000 B.C.).
• Humans used selective breeding to develop desirable traits (e.g., Sahiwal cows).

• Gregor Johann Mendel (1822–1884), an Austrian monk, is known as the Father of Genetics for his pioneering work on heredity.
• He studied science and mathematics at the University of Vienna, which helped him apply a quantitative approach to biological problems.
• Mendel conducted systematic hybridization experiments on garden pea (Pisum sativum) from 1856 to 1863.
• From these experiments, he formulated the fundamental Laws of Inheritance, explaining how traits are transmitted across generations.
• Although his work was ignored during his lifetime, it was rediscovered in 1900, leading to widespread recognition and the foundation of modern genetics.

• Gregor Mendel is known as the Father of Genetics; he worked on pea plants (1856–1863).
• He used true-breeding pea plants and studied inheritance using cross-pollination experiments.
• Mendel selected 7 pairs of contrasting traits (e.g., tall/dwarf, round/wrinkled, yellow/green).
• He introduced the concepts of dominant and recessive traits.
• His experiments had a large sample size and statistical analysis, making the results reliable.
• Mendel’s work formed the basic laws of inheritance, explaining how traits pass from parents to offspring.
• His findings were confirmed by repeated experiments across generations.

• Punnett Square was developed by British geneticist Reginald C. Punnett; it is a diagram used to show all possible genotypes of offspring in a genetic cross.
• In a Monohybrid Cross, Tall (TT) × Dwarf (tt) gives F₁ plants that are all Tt (tall) because T (tall) is dominant over t (dwarf).
• When F₁ (Tt) is self-pollinated, F₂ offspring are TT, Tt, Tt, tt, giving phenotypic ratio = 3:1 (tall: dwarf) and genotypic ratio = 1:2:1 (TT:Tt:tt).
• TT and Tt plants are phenotypically identical (both tall) but genotypically different because T allele dominates over t allele.
• Test Cross involves crossing an organism of dominant but unknown genotype with a homozygous recessive (tt) parent to determine its genotype.
• If all offspring are tall, the unknown parent is TT (homozygous dominant); if half tall and half dwarf, the unknown parent is Tt (heterozygous).
• The ratio 1/4 TT: 2/4 Tt: 1/4 tt can also be expressed using the binomial expansion (aT + bt)² formula.

• Back cross is the cross between the F₁ hybrid and either of its parents (dominant or recessive).
• A test cross is a special type of backcross where the F₁ hybrid is crossed with a homozygous recessive parent.
• Backcross is used to obtain desirable traits and may produce all dominant offspring when crossed with a dominant parent.
• A test cross is used to determine the genotype (homozygous or heterozygous) of an organism showing a dominant trait.
• In a test cross, a 1:1 ratio of dominant and recessive traits indicates a heterozygous condition.
• If all offspring show dominant traits in a test cross, the parent is homozygous dominant.
• Test cross is simple, reliable, and widely used in plant breeding and crop improvement.

• Incomplete Dominance - Exception to law of dominance; neither allele is completely dominant; F₁ hybrid shows an intermediate expression of both characters.
• Example - Red (RR) × White (rr) in Mirabilis jalapa → F₁ offspring are Pink (Rr); neither red nor white dominates completely.
• F₂ Generation - Selfing of F₁ (Rr × Rr) gives:
  Genotypic ratio - 1RR : 2Rr : 1rr
  Phenotypic ratio - 1 Red : 2 Pink : 1 White
• Both phenotypic and genotypic ratios are 1:2:1 (unlike Mendel's 3:1 phenotypic ratio), which is the key difference from complete dominance.

• Co-dominance - Both alleles of an allelomorphic pair express themselves equally in F₁ hybrids; neither allele is dominant or recessive over the other.
• Example - Red cattle (RR) × White cattle (WW) → F₁ hybrids are Roan (RW); roan coat has a mixture of red and white hair - both traits expressed equally.
• F₂ Generation - Selfing of F₁ (RW × RW) gives:
  Genotypic ratio - 1RR : 2RW : 1WW
  Phenotypic ratio - 1 Red : 2 Roan : 1 White
• In co-dominance, genotypic and phenotypic ratios are identical (1:2:1); key difference from incomplete dominance is that both alleles are fully expressed, not partially.

• Mendel proposed the Law of Independent Assortment based on dihybrid crosses; it states that segregation of one pair of characters is independent of the other pair.
• In dihybrid crosses, the F₂ phenotypic ratio is 9:3:3:1, derived as a combination of 3 yellow:1 green with 3 round:1 wrinkled.
• During meiosis in F₁ (RrYy), each gene pair segregates independently, producing 4 types of gametes - RY, Ry, rY, ry, each with a frequency of 25%.
• A Punnett square of F₁ × F₁ produces 16 combinations, giving 9 different genotypes and 4 different phenotypes in F₂.
• F₂ phenotypic distribution - 4 genotypes give Round yellow, 2 genotypes give Round green, 2 genotypes give Wrinkled yellow, 1 genotype gives Wrinkled green (rryy).
• The genotypic ratio at the F₂ stage is 1:2:1:2:4:2:1:2:1, while the phenotypic ratio is 9:3:3:1.

• Mendel's work (1866) was unrecognised until 1900, when Hugo de Vries, Correns, and von Tschermak independently rediscovered it.
• Sutton and Boveri (1903) proposed the Chromosomal Theory of Inheritance; chromosomes are carriers of genetic material.
• Homologous chromosomes pair, segregate, and assort independently during meiosis; each gamete gets only one chromosome from a pair.
• Male and female gametes carry hereditary traits and are the link between parents and offspring; their fusion restores the diploid number.
• Genes and chromosomes always occur in pairs in diploid organisms; alleles segregate along with chromosomes during gamete formation.

• Linkage - physical association of two or more genes on the same chromosome; linked genes do not assort independently.
• Recombination - generation of non-parental gene combinations due to crossing over between linked genes.
• Morgan's dihybrid crosses in Drosophila showed the F₂ ratio deviated from 9:3:3:1; genes were located on the X chromosome.
• Genes on the same chromosome show more parental combinations than non-parental (recombinant) types.
• Tightly linked = low recombination (white & yellow = 1.3%); loosely linked = high recombination (white & miniature wing = 37.2%).
• Alfred Sturtevant (Morgan's student) used recombination frequency to measure gene distance and create chromosome maps, now used in Human Genome Sequencing.

1. Polygenic traits are controlled by two or more independent genes, with each contributing allele adding an equal, additive increment to the final phenotype.
2. Instead of falling into distinct categories, these traits form a continuous spectrum that characteristically results in a bell-shaped, normal distribution curve within a population.
3. This type of inheritance lacks simple dominant-recessive relationships and gene masking (epistasis); alleles are simply contributing or non-contributing, and the genes assort independently.
4. The final physical expression of these traits is not strictly dictated by genetics, as it is heavily modified by environmental conditions, such as nutrition, affecting a person's height.
5. Prominent examples include human skin colour, human height (involving roughly 400 genes), human eye colour, and wheat kernel colour.

• Pleiotropy - A single gene controls two or more different non-related traits; such a gene is called a pleiotropic gene; e.g. sickle-cell anaemia gene (Hb^S).
• Example - Normal gene Hb^A is dominant; heterozygous carriers (Hbᴬ/Hbˢ) show mild anaemia with sickle-shaped RBCs under low O₂; homozygous recessive (Hb^S/Hb^A) die of total anaemia.
• Ratio - Cross between two carriers gives 1 Normal: 2 Carriers: 1 Sickle-cell anaemic; since anaemics die, the surviving ratio becomes 2:1 (carriers: normal) instead of the usual 3:1.
• The gene for sickle-cell anaemia is lethal in a homozygous condition but produces sickle-cell trait (mild anaemia) in a heterozygous condition - two different expressions from a single gene.

• Sex determination: It is the mechanism by which an organism develops into a male or a female based on genetic factors.
• Types of organisms: Organisms may be bisexual (hermaphrodite), having both sex organs, or unisexual (dioecious), like humans, with separate sexes.
• Discovery: Henking (1891) discovered the X-body, later identified as the X chromosome involved in sex determination.
• XX–XY system: Females are XX (homogametic) and males are XY (heterogametic), seen in humans and Drosophila.
• ZW–ZZ system: Females are ZW (heterogametic) and males are ZZ (homogametic), seen in birds and some reptiles.
• Haplodiploidy: In honeybees, unfertilized eggs develop into haploid males and fertilised eggs into diploid females.

• Type of system: Honey bees show haplodiploid sex determination, where sex depends on the number of chromosome sets.
• Chromosome number: Females are diploid (2n = 32), and males are haploid (n = 16).
• Formation of gametes: The female produces haploid eggs by meiosis, while the male produces sperm by mitosis.
• Fertilisation: Fertilised eggs develop into diploid females (queen or worker), while unfertilised eggs develop into haploid males (drones) by parthenogenesis.
• Caste differentiation: Female larvae fed royal jelly develop into queens, while others develop into worker bees.

• A mutation is a sudden heritable change in DNA sequences that leads to changes in the genotype and phenotype of an organism.
• Loss of DNA segment = deletion; gain of DNA segment = insertion/duplication; both cause chromosomal aberrations, commonly seen in cancer cells.
• Frame-shift mutation - caused by loss or gain of a DNA segment; Point mutation - change in a single base pair (e.g., sickle cell anaemia).
• Physical mutagens that cause mutation include UV radiation, X-rays, alpha, beta and gamma rays; Chemical mutagens include mustard gas, phenol and formalin.
• Mutation is an important source of genetic variation in organisms, alongside recombination.

• Pedigree Analysis is the study of inheritance patterns of traits across several generations of a family; the chart representing this is called a family tree (pedigree).
• Since controlled crosses (like in pea plants) cannot be done in humans, family history analysis is used as an alternative to study inheritance.
• Pedigree analysis is a powerful tool in human genetics used to trace the inheritance of a specific trait, abnormality or disease.
• Standard symbols used - Square = normal male, Circle = normal female, Diamond = sex unspecified; filled/shaded shapes = affected individuals.
• A horizontal line between two symbols = mating; a double horizontal line = consanguineous mating (mating between relatives); children are shown below parents in order of birth from left to right.