Revision: Genetics and Evolution >> Principles of Inheritance and Variation Biology (Theory) ISC (Science) ISC Class 12 CISCE

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

The process of crossing two genetically different individuals to produce hybrids is called hybridization.

An offspring produced by crossing two individuals differing in one or more characters is called a hybrid.

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

• 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.

An F₁ hybrid produced from a cross involving more than three pairs of contrasting characters is called a polyhybrid.

A cross between an F₁ hybrid and a homozygous recessive parent to determine the genotype of the hybrid is called a test cross.

Two crosses in which the same parental genotypes are used but their sex roles are reversed are called reciprocal crosses.

An F₁ hybrid produced from a cross involving three pairs of contrasting characters is called a trihybrid.

Alternative forms of the same gene present at the same locus on homologous chromosomes and controlling the expression of a character are called alleles or allelomorphs.

An individual possessing identical alleles for a particular gene (e.g., RR or rr) is called homozygous.

An individual possessing dissimilar alleles for a particular gene (e.g., Rr) is called heterozygous.

The genetic constitution or allelic composition of an organism for a particular character is called its genotype.

The observable external expression of a character resulting from the interaction of genotype and environment is called the phenotype.

The numerical proportion of different genotypes obtained in a cross is called the genotypic ratio.

The numerical proportion of different phenotypes expressed in the offspring of a cross is called the phenotypic ratio.

A genetic cross between two individuals differing in a single pair of contrasting characters is called a monohybrid cross.

A cross between an F₁ hybrid and either of its parental forms is called a back cross.

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.

Multiple alleles are the three or more alternative forms of the same gene that occupy the same locus on homologous chromosomes and control the same character in a population, though only two alleles occur together in an individual.

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

The degree or strength with which two genes remain associated on the same chromosome during inheritance is called linkage value.

The tendency of two or more genes located on the same chromosome to be inherited together and not assort independently during inheritance is called linkage.

All the genes present on a single chromosome that are inherited together as a unit are called a linkage group.

The condition in which genes located very close together on the same chromosome are inherited together without separation due to absence of crossing over is called complete linkage.

Genes that are located on the same chromosome and tend to be inherited together as a unit are called linked genes.

Genes that are located on different chromosomes and assort independently during meiosis are called unlinked genes.

Mutual exchange of blocks of homologous genes between a pair of homologous chromosomes is known as crossing over.

The representation of the relative positions of genes on a chromosome is called chromosomal mapping.

Sex chromosomes (also called allosomes) are the kind of chromosomes that determine the sex of an organism. Every human has only l pair of sex chromosomes.

Autosomes are the kind of chromosomes which determine general body features like complexion, height, seed colour, etc. Humans have 22 pairs of autosomes.

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.

Sex-linked inheritance is the appearance of a trait which is due to the presence of an allele exclusively either on the X chromosome or on the Y chromosome.

Inheritance of X-linked genes as in colour blindness and haemophilia is also called criss-cross inheritance.

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

Failure of separation of homologous chromosomes or sister chromatids during meiosis, resulting in gametes with an abnormal number of chromosomes is called non-disjunction.

Variations are the term used to describe the minor differences between members of the same species. Minor variations among individuals within a single species, including those within the same breed, can be observed.

Any inheritable feature of an organism is a character. 

The alternative forms of a character are called traits.

Even within the same race or tribe, the individual members in the population show differences. Further, within a family, members show differences in body features. These small differences among the individuals of the same species are called variations. 

Organisms that exhibit a combination of characters derived from both parents are called recombinants.

The phenomenon by which maternal and paternal genes are reshuffled to produce new combinations of characters in sexually reproducing organisms is called recombination.

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.

A mutation that occurs at a specific fixed position (locus) of a gene on a chromosome due to change in one or few nucleotides is called a point mutation.

Gene mutations that occur during DNA replication due to errors in copying the DNA sequence are called copy-error mutations.

A frame shift mutation is a type of gene mutation caused by the insertion or deletion of one or more nucleotides in a DNA sequence, which shifts the reading frame of codons and alters the entire amino acid sequence of the protein.

The structural changes in chromosomes which appear phenotypically are known as chromosomal mutations or aberrations.

The presence of a part of a chromosome double of the normal complement is known as duplication

Variations that involve entire sets of chromosomes are known as euploidy.

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.

• 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.

• Experiments were carefully planned with large samples; results were expressed as ratios.
• Chose pea plant (Pisum sativum) with easily recognisable contrasting characters.
• All 7 characters were each controlled by a single factor, making patterns simple to study.
• The 7 factors are on separate chromosomes and transmitted from generation to generation without mixing.
• Introduced key concepts of dominance and recessiveness — the foundation of genetics.

• 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.

• 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.

• Multiple Alleles - More than two alternative forms of a gene in a population occupying the same locus on homologous chromosomes; e.g. ABO blood group in humans.
• Origin - Multiple alleles arise by repeated mutations of the wild-type gene; the wild type is dominant over all other mutant alleles.
• Dominance - Different alleles in a series may show dominant-recessive relationship, co-dominance, or incomplete dominance among themselves.
• Example in Drosophila - Wing size is controlled by multiple alleles; Normal wings (vg⁺) → Nicked (vgⁿⁱ) → Notched (vgⁿº) → Strap (vgˢᵗ) → Vestigial (vg); normal wing is wild type (dominant), vestigial is recessive.

• 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.

• 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.

1. Sex determination in organisms occurs by three main mechanisms: environmental, genetic (genic), and chromosomal methods.
2. Environmental sex determination depends on external factors like temperature; for example, in reptiles such as crocodiles and turtles, incubation temperature decides sex.
3. Genetic (genic) sex determination is controlled by specific genes rather than chromosomes, as seen in bacteria (fertility plasmids) and algae like Chlamydomonas.
4. Chromosomal sex determination is based on differences in sex chromosomes; organisms possess autosomes and one or more sex chromosomes (X, Y, Z, W).
5. Chromosomal systems include:

• Female homogametic (XX–XY, XX–XO) as in humans and insects
• Male homogametic (ZW–ZZ, ZO–ZZ) as in birds, butterflies, and moths

• 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.

• Sex-linked inheritance: It is the inheritance of genes located on sex chromosomes (X and Y) from parents to offspring.
• X-linked genes: These genes are present on the X chromosome and usually do not have corresponding alleles on the Y chromosome.
• Expression in males and females: X-linked recessive traits appear more in males (one X chromosome), while females need two recessive alleles; females with one allele are carriers.
• Examples of X-linked traits: Haemophilia, colour blindness, muscular dystrophy, and night blindness.
• Y-linked genes: These genes are present on the Y chromosome and are passed directly from father to son (e.g. hypertrichosis).

• Meaning: Colour blindness is an X-linked recessive disorder in which a person cannot distinguish between red and green colours.
• Cause: It is caused by a recessive gene (Xᶜ) that prevents the formation of colour-sensitive cone cells in the retina.
• Expression in Sexes: Males (XᶜY) are more commonly affected, while females are affected only if both X chromosomes carry the recessive gene (XᶜXᶜ); otherwise, they act as carriers (XᴄXᶜ).
• Inheritance Pattern: It shows criss-cross inheritance, where the gene passes from father to daughter (carrier) and then to grandson.
• Example Crosses: Colour blind male × normal female produces carrier daughters; carrier female × normal male produces 50% colour blind sons.

• 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.

• Down’s syndrome is a chromosomal disorder caused by trisomy of chromosome 21.
• Affected individuals have 47 chromosomes instead of the normal 46 due to non-disjunction.
• It is characterised by mental retardation, distinctive facial features, and short stature.
• Congenital heart defects and low muscle tone are commonly associated features.
• The risk increases with advanced maternal age (above 35 years) and can be detected by amniocentesis.