Revision: Inheritance and Variation Biology HSC Science (General) 12th Standard Board Exam Maharashtra State Board

The transmission of characters from the parents to their offsprings is called heredity. 

The term heredity may be defined as "transmission of genetically based characteristics from parents to offspring".

or

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.

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.

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

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.

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.

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.

One vertical half of a duplicated chromosome is called a chromatid.

Each chromosome in its condensed form as visible during the start of cell division, consists of two sister chromatids joined at some point along the length. This point of attachment is called centromere, and it appears as a small constricted region.

Two identical chromatids that are joined by a centromere are called sister chromatids which eventually get separated during anaphase. 

After duplication, a chromosome has two identical sections. A chromosome is formed during cell division by the union of two chromatids.

Aneuploidy is the addition or deletion of one or two chromosomes in a diploid chromosomal pair.

Aneuploidy refers to the chromosomal variation due to a loss or a gain of one or more chromosomes deviating from the normal genome number for that species due to nondisjunction of the homologous chromosome.

The nucleus contains most of the cell's DNA, which is organised into discrete units called chromosomes.

The nucleus contains most of the cell's DNA which is organized into discrete units called chromosomes.

or

Chromosomes are highly coiled, ribbon-like structures formed by the condensation of chromatin fibres during cell division. 

Each chromosome contains one long DNA molecule associated with many proteins. This complex of DNA and proteins is called the chromatin.

A pair of corresponding chromosomes of the same shape and size, one obtained from each parent.

The tendency of the genes on the same chromosome to inherit together is called linkage.

Test cross is a cross between an organism with an unknown genotype and a recessive parent. It is used to determine whether the individual is homozygous or heterozygous for a trait.

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.

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

The transmission of characters from parents to offspring through genes present on autosomes is called autosomal inheritance.

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.

• Mendel is the father of modern genetics; he discovered the basic principles of heredity.
• Hugo de Vries (1901) proposed the mutation theory explaining sudden genetic changes.
• Walter Sutton (1902) linked chromosomes with heredity through his study on grasshoppers.
• Avery, McCarty & McLeod (1944) proved DNA is the genetic material in living organisms.
• Jacob & Monod (1961) developed a model of protein synthesis, leading to recombinant DNA technology.

• 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.

• 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.

• Mendel’s laws were based on simple assumptions, like one gene controlling one trait with two alleles showing complete dominance.
• Later studies showed deviations from Mendelian inheritance, known as Neo-Mendelism.
• These deviations include incomplete dominance, codominance, multiple alleles, polygenic inheritance, and pleiotropy.
• Intragenic interactions occur between alleles of the same gene (e.g. incomplete dominance, codominance).
• Intergenic interactions occur between different genes (e.g. epistasis, polygenes, pleiotropy), affecting phenotypic expression.

• 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.

• 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.

• 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.

• 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.

• Chromosomes are thread-like (filamentous) structures present in the nucleus, made of DNA and proteins, and carry genetic information (genes). They were first observed by Walther Flemming (1882).
• During cell division, chromatin condenses to form visible chromosomes, ensuring equal distribution of genetic material to daughter cells.
• Each chromosome consists of two sister chromatids joined at a centromere, which has a kinetochore for attachment to spindle fibres.
• Important parts include telomeres (protect chromosome ends), chromonemata (coiled DNA fibres), and satellites (small segments).
• Chromosome number is constant for each species and is best observed at the metaphase stage of cell division.
• Ploidy refers to the number of chromosome sets: euploidy (exact multiples like haploid, diploid, polyploid) and aneuploidy (abnormal numbers like monosomy and trisomy).
• Chromosomes play a key role in heredity, gene expression, and maintaining genetic stability across generations.

• Chromosomes are classified into four types based on centromere position: metacentric, sub-metacentric, acrocentric, and telocentric.
• Metacentric chromosomes have a centromere in the middle, forming a V-shape with equal arms.
• Sub-metacentric chromosomes have a centromere slightly off-centre, forming an L-shape with unequal arms.
• Acrocentric chromosomes have a centromere near one end, forming a J-shape with one very short arm.
• Telocentric chromosomes have a centromere at the end, forming an I-shape with only one arm.
• Chromosomes occur in pairs: homologous (similar) and heterologous (dissimilar); sex chromosomes determine sex, while others are autosomes.

Linkage:

• Linkage is the tendency of genes on the same chromosome to be inherited together.
• Linked genes form linkage groups, equal to the haploid chromosome number.
• It is of two types: complete linkage (no crossing over) and incomplete linkage (with crossing over).
• Linked genes do not assort independently and show more parental combinations.

Crossing Over:

• Crossing over is the exchange of genetic material between non-sister chromatids during prophase I (pachytene stage) of meiosis, producing recombination.
• Crossing over increases genetic variation and leads to the formation of new gene combinations in offspring.
• Sex linkage is the inheritance of genes located on sex chromosomes (X and Y), including X-linked, Y-linked, and XY-linked traits.

• Autosomal inheritance is the transmission of traits through autosomes (22 pairs of chromosomes other than sex chromosomes).
• Autosomal traits affect both males and females equally, as they are not related to sex chromosomes.
• Autosomal traits can be dominant or recessive depending on the gene involved.
• Autosomal dominant traits (e.g. widow’s peak, Huntington’s disease) appear even in a heterozygous condition.
• Autosomal recessive traits (e.g. phenylketonuria, cystic fibrosis, sickle cell anaemia) appear only in a homozygous condition and may skip generations.
• Phenylketonuria (PKU) is a genetic disorder caused by a lack of phenylalanine hydroxylase, leading to the accumulation of phenylalanine and affecting brain development.

• 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.

• 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.

• 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.

• Thalassemia is an autosomal recessive inherited blood disorder caused by defective synthesis of haemoglobin chains.
• It occurs due to mutation or deletion of genes controlling globin chains—alpha chains (HBA1, HBA2 on chromosome 16) or beta chain (HBB on chromosome 11).
• Based on the affected chain, thalassemia is classified into alpha-thalassemia and beta-thalassemia.
• It leads to symptoms like anaemia, pale skin, abnormal RBCs, slow growth, and often requires repeated blood transfusions for management.

• Turner’s syndrome is a sex chromosomal disorder caused due to non-disjunction, resulting in XO karyotype (44 autosomes + XO).
• Affected individuals are phenotypically female but show poor development of ovaries and breasts, leading to sterility.
• Common features include short stature, webbed neck, broad chest, low posterior hairline, and reduced intelligence.

• Klinefelter’s syndrome is a sex chromosomal disorder caused by the presence of an extra X chromosome, giving the genotype 44 + XXY due to non-disjunction during meiosis.
• Affected individuals are phenotypically male but show feminized features, including gynaecomastia, underdeveloped testes, and absence of spermatogenesis, leading to sterility.
• Common characteristics include tall stature with long arms, harsh-pitched voice, and overall masculine appearance with some feminine traits.