Biology Chapter 4 Notes of CBSE Class 12

This note covers the fundamental principles of inheritance established by Gregor Mendel, deviations from Mendelian inheritance patterns, the chromosomal theory of inheritance, and the concepts of linkage, recombination, mutation, and genetic disorders. It explains how traits are passed from parents to offspring through genes (formerly called factors) and alleles, which are located on chromosomes. Key concepts include dominance , segregation , and independent assortment . We also explore how interactions between alleles (like incomplete dominance and co-dominance ) and multiple genes ( polygenic inheritance , pleiotropy ) affect phenotypes. The note touches upon the role of mutation in generating variation, the chromosomal basis of sex determination , and the analysis of inheritance patterns in humans through pedigree analysis , which helps in understanding Mendelian and chromosomal disorders .

1. Foundations of Inheritance

• Genetics: The branch of biology that scientifically deals with inheritance and variation of characters from parents to offspring.
• Inheritance: The process by which characters are passed from parent to progeny; it is the basis of heredity .
• Variation: The degree by which progeny differ from their parents.
• Early humans used artificial selection and domestication, recognizing variation naturally present in wild populations. They bred organisms with desirable characters (e.g., Sahiwal cows from wild cows). However, they had little idea about the scientific basis of these phenomena.
• Mendel's work gave an idea of inheritance patterns, though the nature of the 'factors' determining phenotype was unclear initially. Understanding the structure of genetic material and the structural basis of genotype and phenotype conversion became a major focus in biology.

2. Mendel's Laws of Inheritance

• Gregor Mendel conducted hybridisation experiments on garden peas from 1856-1863.
• His investigations were the first time statistical analysis and mathematical logic were applied to biology problems.
• Key aspects of Mendel's experiments:
• Large sampling size, providing greater credibility to data.
• Confirmation of inferences from experiments on successive generations.
• Investigated characters manifested as two opposing traits (e.g., tall or dwarf plants, yellow or green seeds).
• Used true-breeding pea lines (show stable trait inheritance over generations through continuous self-pollination).
• Selected 14 true-breeding varieties differing in one character with contrasting traits.
• Contrasting traits studied included: Stem height (Tall/dwarf), Flower colour (Violet/white), Flower position (Axial/terminal), Pod shape (Inflated/constricted), Pod colour (Green/yellow), Seed shape (Round/wrinkled), Seed colour (Yellow/green).

2.1 Monohybrid Cross (Inheritance of One Gene)

• Example: Crossing tall and dwarf pea plants.
• F1 Generation (First Filial progeny): Produced by crossing parents. Mendel observed all F1 plants were tall, resembling one parent; the dwarf trait was not seen. Similar results for other traits, F1 always resembled one parent.
• F2 Generation (Second Filial generation): Produced by self-pollinating F1 plants.
• Mendel was surprised to find some F2 offspring were dwarf; the trait not seen in F1 was expressed.
• Proportion in F2: 1/4th dwarf, 3/4th tall.
• Traits were parental types (tall or dwarf), no blending.
• Phenotypic ratio in F2 for one trait: 3 (dominant) : 1 (recessive).
• Key Concepts from Monohybrid Cross:
• Factors (now Genes): Stably passed down, unchanged, from parent to offspring via gametes. Genes are units of inheritance containing information for a trait.
• Alleles: Genes coding for a pair of contrasting traits; slightly different forms of the same gene. Example: T (Tall) and t (dwarf) are alleles for height.
• Genotype: The allelic pair of genes for a character. Example: TT, Tt, tt for height.
• Phenotype: The observable, descriptive trait. Example: Tall, dwarf.
• Homozygous: Allelic pair is identical (e.g., TT, tt).
• Heterozygous: Allelic pair is dissimilar (e.g., Tt). Also called a monohybrid when heterozygous for one character.
• Dominant Factor (Allele): In a dissimilar pair, one factor dominates the other, expressing its trait in F1 (e.g., T for tallness). Represented by a capital letter.
• Recessive Factor (Allele): The factor that is not expressed in the heterozygote (e.g., t for dwarfness). Represented by a small letter.
• Genotypic Ratio in F2: 1 (homozygous dominant TT) : 2 (heterozygous Tt) : 1 (homozygous recessive tt). This ratio can be expressed as (1/2T + 1/2t)² = 1/4 TT + 1/2 Tt + 1/4 tt.
• Punnett Square: A graphical representation developed by Reginald C. Punnett to calculate the probability of all possible genotypes of offspring in a genetic cross. Possible gametes from each parent are written on two sides, and combinations are shown in boxes. Shows F1 (Tt) from TT x tt cross, and F2 (TT, Tt, tt) from Tt x Tt self-pollination. Resulting genotypes are 1/4 TT, 1/2 Tt, 1/4 tt.
• Test Cross: A cross to determine the genotype of an organism showing a dominant phenotype. The organism with the dominant phenotype is crossed with the recessive parent instead of self-crossing. Progeny analysis predicts the test organism's genotype.

2.2 Mendel's Laws (Principles of Inheritance)

1. First Law / Law of Dominance:
• Characters are controlled by discrete units called factors.
• Factors occur in pairs.
• In a dissimilar pair, one member dominates (dominant) the other (recessive).
• Explains why only one parental character is seen in F1 of a monohybrid cross and both in F2.
• Explains the 3:1 ratio in F2.
2. Second Law / Law of Segregation:
• Alleles do not show any blending; both characters are recovered in F2.
• During gamete formation, the factors (alleles) of a pair segregate from each other so each gamete receives only one allele.
• Segregation is a random process; a gamete has a 50% chance of containing either allele.
• Homozygous parent produces similar gametes; heterozygous parent produces two kinds of gametes with equal proportion.

3. Deviations from Mendelian Dominance

• Incomplete Dominance:
• Sometimes, the F1 phenotype is intermediate between the two parents.
• Example: Flower colour in the dog flower (snapdragon or Antirrhinum sp.).
• Cross between true-breeding red-flowered (RR) and white-flowered (rr) results in pink F1 (Rr).
• Self-pollination of F1 produces F2 with ratio 1 (RR) Red : 2 (Rr) Pink : 1 (rr) White.
• Genotype ratios in F2 are Mendelian (1:2:1), but phenotype ratios change from 3:1 to 1:2:1 because the heterozygote has a distinct phenotype. R is not completely dominant over r.
• Co-dominance:
• The F1 generation resembles both parents.
• Example: ABO blood grouping in human beings.
• Controlled by gene I, with three alleles: Iᴬ, Iᴮ, and i.
• Iᴬ and Iᴮ produce slightly different sugars; i produces no sugar.
• Iᴬ and Iᴮ are completely dominant over i.
• When Iᴬ and Iᴮ are present together (heterozygote IᴬIᴮ), both express their types of sugars. Red blood cells have both A and B types of sugars.
• This simultaneous expression of both alleles in the heterozygote is co-dominance.
• Three alleles (Iᴬ, Iᴮ, i) result in six possible genotypes (IᴬIᴬ, IᴬIᴮ, Iᴬi, IᴮIᴬ, IᴮIᴮ, Iᴮi, ii) and four phenotypes (A, B, AB, O). Note IᴬIᴮ and IᴮIᴬ are the same genotype. The source lists 7 genotype possibilities in the table including IᴬIᴮ and IᴮIᴬ as separate rows, resulting in 6 unique genotypes: IᴬIᴬ, IᴬIᴮ, Iᴬi, IᴮIᴮ, Iᴮi, ii with phenotypes A, AB, A, AB, B, B, O respectively.
• Multiple Alleles: When more than two alleles govern the same character in a population (e.g., the three alleles Iᴬ, Iᴮ, i for ABO blood groups). An individual can only have two alleles.
• Explanation of Dominance: Dominance is not an inherent feature of a gene or its product.
• A gene contains information to express a trait, often by producing an enzyme.
• Diploid organisms have two alleles.
• Modified alleles might produce: normal/less efficient enzyme, non-functional enzyme, or no enzyme.
• If the modified allele produces a functional product (even less efficient), it might be equivalent to the unmodified allele, resulting in the same phenotype.
• If the modified allele produces a non-functional or no enzyme, the phenotype depends on the unmodified allele.
• The unmodified (functioning) allele is generally dominant; the modified allele is generally recessive. Recessive traits are often due to non-functional enzyme or lack of enzyme.
• Dominance depends on the gene product, phenotype production from the product, and the specific phenotype being examined if the gene influences multiple phenotypes. Example: Starch synthesis in peas. Allele B for large grains, b for small. BB (large grains, round seed), bb (small grains, wrinkled seed). Bb (intermediate grains, round seed). Seed shape shows dominance (B dominant over b), but starch grain size shows incomplete dominance (Bb is intermediate).

4. Inheritance of Two Genes (Dihybrid Cross)

• Mendel crossed pea plants differing in two characters, e.g., yellow/round seeds (RRYY) with green/wrinkled seeds (rryy).
• F1 Generation: All seeds were yellow and round.
• Yellow is dominant over green; Round is dominant over wrinkled.
• Genotype of F1 hybrid: RrYy (from gametes RY and ry).
• F2 Generation: Produced by self-hybridising F1 plants.
• Mendel observed 3/4 yellow and 1/4 green seeds; yellow/green segregated in 3:1 ratio.
• Round/wrinkled seeds also segregated in 3:1 ratio.
• Phenotypes observed in F2 were: round, yellow; wrinkled, yellow; round, green; wrinkled, green.
• Phenotypic ratio in F2: 9 : 3 : 3 : 1. This ratio is a combination of two independent 3:1 ratios: (3 Round : 1 Wrinkled) x (3 Yellow : 1 Green) = 9 Round, Yellow : 3 Wrinkled, Yellow : 3 Round, Green : 1 Wrinkled, Green.

4.1 Law of Independent Assortment

• Based on observations from dihybrid crosses.
• States that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.
• Punnett square demonstrates independent segregation of alleles for two genes during gamete formation. For RrYy, alleles R and r segregate independently of alleles Y and y.
• This results in four types of gametes (RY, Ry, rY, ry), each with a frequency of 25% (1/4th).

5. Chromosomal Theory of Inheritance

• Mendel's work was unrecognised until 1900 due to: poor communication, concepts of discrete factors/non-blending alleles were not accepted due to apparent continuous variation, mathematical approach was new and unacceptable to many biologists, and no physical proof for factors was provided.
• In 1900, Mendel's results were independently rediscovered by de Vries, Correns, and von Tschermak.
• Advancements in microscopy allowed scientists to observe cell division and chromosomes (colored bodies) that double and divide. Chromosome movement during meiosis was worked out by 1902.
• Walter Sutton and Theodore Boveri noted the parallel behaviour of chromosomes and genes.
• Both chromosomes and genes occur in pairs.
• The two alleles of a gene pair are located on homologous sites on homologous chromosomes.
• During gamete formation (meiosis), chromosome pairs segregate such that only one of each pair is transmitted to a gamete. Similarly, gene pairs (alleles) segregate.
• Independent pairs of chromosomes segregate independently. Similarly, one pair of genes segregates independently of another pair.
• Sutton united chromosomal segregation knowledge with Mendelian principles, calling it the chromosomal theory of inheritance .

6. Linkage and Recombination

• Experimental verification of the chromosomal theory by Thomas Hunt Morgan and colleagues using Drosophila melanogaster (fruit flies). Drosophila was suitable because: easy to grow, short life cycle, large progeny, clear sex differentiation, many hereditary variations visible with low power microscope.
• Morgan conducted dihybrid crosses in Drosophila that deviated significantly from the 9:3:3:1 ratio.
• Example: Yellow-bodied, white-eyed females crossed with brown-bodied, red-eyed males. Genes for body colour (yellow/brown) and eye colour (white/red).
• Observation: When two genes were on the same chromosome (e.g., on the X chromosome), the proportion of parental gene combinations in progeny was much higher than non-parental types.
• Linkage: The physical association of genes on a chromosome. Linked genes tend to be inherited together.
• Recombination: The generation of non-parental gene combinations. Occurs due to crossing over during meiosis.
• Morgan found genes on the same chromosome can be:
• Tightly linked: Show very low recombination (e.g., white and yellow genes showed 1.3% recombination).
• Loosely linked: Show higher recombination (e.g., white and miniature wing genes showed 37.2% recombination).
• Morgan's student, Alfred Sturtevant, used recombination frequency between gene pairs on the same chromosome as a measure of the distance between genes to 'map' their position on the chromosome. Genetic maps show the arrangement of genes on a chromosome.

7. Other Inheritance Patterns

Polygenic Inheritance:

• Traits controlled by three or more genes.
• Phenotypes show a gradient, not distinct alternate forms (e.g., human height).
• Also influenced by environment.
• Effect of each allele is additive; phenotype reflects contribution of each allele.
• Example: Human skin colour controlled by multiple genes (A, B, C for dark; a, b, c for light). Genotype AABBCC gives darkest skin; aabbcc gives lightest. Intermediate genotypes have intermediate skin colour based on the number of dominant alleles.

Pleiotropy:

• A single gene exhibits multiple phenotypic expressions.
• Underlying mechanism is often the gene's effect on metabolic pathways that contribute to different phenotypes.
• Example: Phenylketonuria in humans. Caused by mutation in a single gene coding for phenyl alanine hydroxylase. Results in mental retardation and reduced hair/skin pigmentation (multiple effects from one gene).

8. Mutation

• Mutation: A phenomenon resulting in alteration of DNA sequences, consequently changing the genotype and phenotype.
• A source of variation, along with recombination.
• Can involve alterations in chromosomes (chromosomal aberrations) like loss (deletions) or gain (insertion/duplication) of a segment of DNA.
• Point mutation: Change in a single base pair of DNA.
• Classical example: Sickle cell anemia.
• Deletions and insertions of base pairs can cause frame-shift mutations.
• Factors that induce mutations are called mutagens (e.g., UV radiations).

9. Genetic Disorders

• Pedigree Analysis:
• Analysis of inheritance pattern of a trait in several generations of a family.
• Used in human genetics to trace inheritance of specific traits, abnormalities, or diseases, as control crosses are not possible in humans.
• Uses standard symbols to represent the family tree and inheritance.
• Disorders can be associated with inherited changed or altered genes or chromosomes.
• Mendelian Disorders:
• Mainly determined by alteration or mutation in a single gene .
• Transmitted according to Mendelian principles.
• Inheritance pattern traced by pedigree analysis.
• Can be dominant or recessive.
• Can be autosomal or sex-linked.
• Examples: Haemophilia, Cystic fibrosis, Sickle-cell anaemia, Colour blindness, Phenylketonuria, Thalassemia.
• Colour Blindness:
• Sex-linked recessive disorder due to defect in red or green cones.
• Mutation in genes on the X chromosome.
• Occurs in ~8% males, ~0.4% females.
• Males (one X) express the recessive trait if they inherit the allele. Females (two X) are usually carriers (heterozygous), with the normal dominant allele suppressing the recessive one. A daughter would typically only be colour blind if her mother is a carrier and her father is colour blind.
• Haemophilia:
• Sex-linked recessive disease.
• Transmission from unaffected carrier female to male progeny.
• Defect in a single protein needed for blood clotting; simple cuts result in non-stop bleeding.
• Heterozygous females are carriers and can transmit the disease to sons.
• Female haemophilic is extremely rare (mother carrier, father haemophilic, but father being haemophilic is often unviable). Queen Victoria's family pedigree showed haemophilic descendants as she was a carrier.
• Sickle-cell Anaemia:
• Autosome linked recessive trait.
• Transmitted when both parents are carriers (heterozygous).
• Controlled by alleles Hbᴬ and Hbˢ.
• Genotype HbˢHbˢ shows the diseased phenotype.
• Heterozygous individuals (HbᴬHbˢ) are apparently unaffected but are carriers (sickle-cell trait) with a 50% chance of transmitting the mutant gene.
• Caused by substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of haemoglobin.
• Amino acid substitution results from a single base substitution in the beta globin gene, changing the sixth codon from GAG to GUG.
• Mutant haemoglobin polymerises under low oxygen tension, causing RBCs to change from biconcave discs to elongated sickle shapes.
• Phenylketonuria:
• Autosomal recessive trait, an inborn error of metabolism.
• Affected individual lacks enzyme converting phenylalanine into tyrosine.
• Phenylalanine accumulates and converts to phenylpyruvic acid and other derivatives.
• Accumulation in brain causes mental retardation. Excreted in urine due to poor kidney absorption.
• Thalassemia:
• Autosome-linked recessive blood disease.
• Transmitted when both parents are unaffected carriers.
• Defect (mutation or deletion) results in reduced synthesis rate of globin chains (alpha or beta) of haemoglobin.
• Causes formation of abnormal haemoglobin molecules, leading to anaemia.
• Alpha Thalassemia: Affected alpha globin chain production, controlled by HBA1 and HBA2 genes on chromosome 16 (four genes total from parents). More genes affected mean less alpha globin.
• Beta Thalassemia: Affected beta globin chain production, controlled by HBB gene on chromosome 11 (two genes total from parents). Occurs due to mutation in one or both genes.
• Differs from sickle-cell anaemia: Thalassemia is a quantitative problem (too few globin molecules); sickle-cell is a qualitative problem (incorrectly functioning globin).
• Chromosomal Disorders:
• Caused by absence, excess, or abnormal arrangement of one or more chromosomes.
• Aneuploidy: Gain or loss of chromosome(s) due to failure of chromatid segregation during cell division.
• Example: Down's syndrome (gain of extra copy of chromosome 21 - trisomy 21). Turner's syndrome (loss of an X chromosome in females - 45, X0).
• Polyploidy: Increase in a whole set of chromosomes due to failure of cytokinesis after telophase. Often seen in plants.
• Normal human cells have 46 chromosomes (23 pairs: 22 pairs autosomes, 1 pair sex chromosomes).
• Common examples: Down’s syndrome, Turner’s syndrome, Klinefelter’s syndrome.
• Down’s Syndrome:
• Caused by presence of an additional copy of chromosome 21 (trisomy of 21). Total chromosomes: 47.
• First described by Langdon Down (1866).
• Symptoms: Short stature, small round head, furrowed tongue, partially open mouth, broad palm with characteristic crease, retarded physical, psychomotor, and mental development.
• Klinefelter’s Syndrome:
• Caused by presence of an additional copy of X-chromosome. Karyotype: 47, XXY.
• Symptoms: Overall masculine development, but also feminine development (gynaecomastia - breast development), sterile.
• Turner’s Syndrome:
• Caused by absence of one of the X chromosomes. Karyotype: 45, X0.
• Symptoms: Sterile females (rudimentary ovaries), lack of other secondary sexual characters, short stature.

10. Sex Determination

• Mechanism was a puzzle; initial clues from insect studies (Henking's X body).
• X body identified as a chromosome (X-chromosome).
• Sex Chromosomes: Chromosomes that determine sex (e.g., X, Y, Z, W).
• Autosomes: Remaining chromosomes common to both sexes.
• Mechanisms:
• XO Type (e.g., grasshopper): Females have pair of X chromosomes (+ autosomes); males have only one X chromosome (+ autosomes). Males produce gametes with or without X.
• XY Type (e.g., humans, Drosophila ): Females have pair of X chromosomes (+ autosomes); males have X and Y chromosome (+ autosomes). Males produce gametes with X or Y. This is male heterogamety (males produce two different gamete types regarding sex chromosomes).
• ZW Type (e.g., birds): Females have Z and W chromosomes (+ autosomes); males have pair of Z chromosomes (+ autosomes). Females produce gametes with Z or W. This is female heterogamety .
• Human Sex Determination (XY):
• 22 pairs of autosomes, 1 pair of sex chromosomes.
• Females: 22 autosome pairs + XX.
• Males: 22 autosome pairs + XY.
• Males produce two types of sperm: 50% carry X, 50% carry Y (+ autosomes).
• Females produce one type of ovum: carries X (+ autosomes).
• Fertilisation: Ovum + X sperm = female (XX); Ovum + Y sperm = male (XY).
• The genetic makeup of the sperm determines the sex of the child.
• There is always a 50% probability of a male or female child in each pregnancy. (Note: The source mentions the societal issue of blaming women for female children due to a false notion).
• Sex Determination in Honey Bee (Haplodiploid):
• Based on the number of chromosome sets.
• Fertilised egg (sperm + egg) develops into a female (queen or worker). Females are diploid with 32 chromosomes.
• Unfertilised egg develops by parthenogenesis into a male (drone). Males are haploid with 16 chromosomes.
• Males produce sperm by mitosis. Males do not have fathers and cannot have sons, but they have a grandfather and can have grandsons.

Frequently Asked Questions (FAQs)

1. What is the difference between Mendelian disorders and Chromosomal disorders?

Mendelian disorders are caused by mutations in a single gene and are inherited according to Mendelian principles. Chromosomal disorders are caused by the absence, excess, or abnormal arrangement of entire chromosomes or significant parts of chromosomes. Examples include point mutations in a gene causing sickle-cell anaemia (Mendelian) versus trisomy 21 causing Down's syndrome (Chromosomal).

1. Explain incomplete dominance and co-dominance with examples.

In incomplete dominance, the heterozygote shows an intermediate phenotype between the two homozygous parents, like pink flowers in snapdragons resulting from a cross between red and white flowered plants. In co-dominance, the heterozygote expresses the traits of both parents simultaneously, such as individuals with AB blood type expressing both A and B antigens.

1. How is sex determined in humans and honey bees?

In humans (XY system), sex is determined by the sperm; females are XX and males are XY, with males producing sperm carrying either an X or a Y chromosome that fertilises the ovum (carrying an X). In honey bees (haplodiploid system), females develop from fertilised eggs (diploid), while males develop from unfertilised eggs through parthenogenesis (haploid).