Revision: Class 12 >> Principles of Inheritance and Variation NEET (UG) Principles of Inheritance and Variation

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

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.

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.

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.

Non-disjunction occurs when chromosomes fail to split during cell division, resulting in aberrant chromosomal combinations.

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.

• 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'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.

• 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 (Hbˢ).
• Example - Normal gene Hbᴬ is dominant; heterozygous carriers (Hbᴬ/Hbˢ) show mild anaemia with sickle-shaped RBCs under low O₂; homozygous recessive (Hbˢ/Hbˢ) 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.

• Definition: Genetic disorders are diseases caused by abnormalities in genes or chromosomes.
• Types: They are broadly classified into Mendelian disorders and chromosomal disorders.
• Mendelian Disorders: Caused by a mutation in a single gene; examples include thalassemia, sickle-cell anaemia, colour blindness, haemophilia, and phenylketonuria.
• Chromosomal Disorders: Caused by the absence or excess of chromosomes or structural abnormalities; examples include Down’s syndrome, Turner’s syndrome, and Klinefelter’s syndrome.
• Examples of Effects: Down’s syndrome causes mental retardation; Turner’s syndrome leads to sterile females; Klinefelter’s syndrome causes sterility in males; thalassemia affects haemoglobin production.