Principles of Inheritance - Class 12 Karnataka Board Botany

Principles of Inheritance and Variation

Inheritance and variation are central concepts in genetics, explaining how traits are passed from parents to offspring and why individuals differ. Inheritance refers to the transmission of genetic characteristics, while variation describes the differences among individuals. These principles are foundational for understanding genetic patterns, disorders, and the outcomes of breeding experiments.

Key Genetic Terms and Concepts

Understanding genetics requires clarity on several core terms. A character is a heritable feature (e.g., plant height), and a trait is a variant of that character (e.g., tall or dwarf). Genes control characters, and their alternative forms…

Principles of Inheritance and Variation

Inheritance and variation are central concepts in genetics, explaining how traits are passed from parents to offspring and why individuals differ. Inheritance refers to the transmission of genetic characteristics, while variation describes the differences among individuals. These principles are foundational for understanding genetic patterns, disorders, and the outcomes of breeding experiments.

Key Genetic Terms and Concepts

Understanding genetics requires clarity on several core terms. A character is a heritable feature (e.g., plant height), and a trait is a variant of that character (e.g., tall or dwarf). Genes control characters, and their alternative forms are called alleles. The genotype is the genetic constitution, while the phenotype is the observable trait.

• Homozygous: Both alleles are the same (e.g., TT or tt).
• Heterozygous: Two different alleles (e.g., Tt).
• Dominant allele: Expressed in both homozygous and heterozygous conditions.
• Recessive allele: Expressed only in homozygous condition.
• Hemizygous: Only one allele present (e.g., genes on X chromosome in males).
• Genotype vs. Phenotype: Genotype determines the genetic makeup; phenotype is the physical expression.

Mendel’s Experiments and Laws

Gregor Mendel’s experiments with pea plants established the foundation of classical genetics. He selected pure lines and contrasting traits, such as tall vs. dwarf, and performed controlled crosses.

• Monohybrid Cross: Involves one character (e.g., plant height). F1 generation shows only the dominant trait; F2 shows a 3:1 phenotypic ratio and 1:2:1 genotypic ratio.
• Dihybrid Cross: Involves two characters (e.g., seed shape and color). F2 generation shows a 9:3:3:1 phenotypic ratio.
• Mendel’s Laws:
  • Law of Dominance: In a heterozygote, only the dominant allele is expressed.
  • Law of Segregation: Allele pairs separate during gamete formation; each gamete receives one allele.
  • Law of Independent Assortment: Genes for different traits assort independently during gamete formation.

Deviations from Mendelian Ratios

Not all inheritance patterns follow Mendel’s laws strictly. Some traits show deviations due to different genetic interactions.

• Incomplete Dominance: Heterozygote shows an intermediate phenotype (e.g., red × white flowers yield pink in F1; F2 ratio 1:2:1 for both genotype and phenotype).
• Codominance: Both alleles express equally in the heterozygote (e.g., AB blood group, roan cattle).
• Multiple Alleles: More than two alleles exist in the population for a gene (e.g., ABO blood group: IA, IB, IO).
• Polygenic Inheritance: Multiple genes control a single trait, leading to continuous variation (e.g., human skin color, height).

Genetic Crosses and Calculations

Genetic crosses predict the outcome of breeding experiments. The number of different gametes produced depends on the number of heterozygous loci.

• Number of Gametes: $2^n$, where $n$ is the number of heterozygous gene pairs.
• Test Cross: Crossing F1 hybrid with a recessive parent to determine genotype.
• Back Cross: Crossing F1 with either parent (test cross is a type of back cross).
• Punnett Square: Used to visualize and calculate genotypic and phenotypic ratios.

Linkage and Recombination

Genes located close together on the same chromosome tend to be inherited together, a phenomenon called linkage. Recombination occurs when crossing over separates linked genes.

• Linkage: Closer genes = stronger linkage, less recombination.
• Recombination Frequency: Indicates distance between genes; higher frequency means genes are farther apart.
• Formula: Recombination frequency (%) = (Number of recombinants / Total progeny) × 100
• Centimorgan (cM): 1% recombination = 1 cM genetic distance.

Sex Determination Mechanisms

Sex determination varies among organisms and is based on chromosomal differences.

• Humans: XX (female), XY (male); sex determined by sperm (X or Y).
• Grasshopper: XO system; males are XO, females are XX.
• Birds: ZW system; females are ZW (heterogametic), males are ZZ (homogametic).
• Honeybee: Haplodiploidy; females are diploid (2n), males are haploid (n), males develop from unfertilized eggs (parthenogenesis).

Pedigree Analysis and Genetic Disorders

Pedigree analysis traces inheritance patterns in families to identify genetic disorders.

• Symbols: Square = male, circle = female, shaded = affected, unshaded = normal, half-shaded = carrier.
• Autosomal Dominant: Disease appears in every generation; both sexes equally affected.
• Autosomal Recessive: Disease may skip generations; both sexes equally affected.
• X-linked Recessive: More common in males; affected males inherit from carrier mothers.
• X-linked Dominant: Affected fathers pass to all daughters, not sons.

Examples of Genetic Disorders:

• Mendelian Disorders:
  • Autosomal Recessive: Sickle cell anemia, thalassemia, phenylketonuria, cystic fibrosis.
  • X-linked Recessive: Hemophilia, color blindness.
• Chromosomal Disorders:
  • Down Syndrome: Trisomy 21 (47 chromosomes), mental retardation, short stature, heart defects.
  • Klinefelter Syndrome: XXY (47 chromosomes), male with some female characteristics, sterile.
  • Turner Syndrome: XO (45 chromosomes), female, short stature, webbed neck, sterile.

Mutation and Its Types

Mutation is a sudden heritable change in the genetic material. It can be caused by physical, chemical, or biological agents (mutagens).

• Point Mutation: Change in a single base pair (e.g., sickle cell anemia: GAG → GTG).
• Frameshift Mutation: Addition or deletion of bases alters the reading frame.
• Mutagen: Agent causing mutation (e.g., UV rays, X-rays, chemicals).
• Mutant: Organism showing the effect of mutation.

Polygenic Inheritance and Pleiotropy

Polygenic inheritance involves multiple genes affecting a single trait, resulting in continuous variation. Pleiotropy occurs when a single gene influences multiple phenotypic traits.

• Polygenic Traits: Skin color, height in humans; more dominant alleles = stronger expression.
• Pleiotropy Example: Phenylketonuria—mutation in one gene affects mental development, skin pigmentation, and hair.

Application of Genetic Principles

Genetic principles are applied in breeding, disease diagnosis, and understanding evolutionary relationships.

• Test Crosses: Used to determine unknown genotypes.
• Genetic Mapping: Recombination frequency helps map gene locations.
• Pedigree Analysis: Identifies carriers and predicts risk of genetic disorders.
• Mutation Breeding: Inducing mutations to develop new plant varieties.

Exam-Focused Takeaways

• Monohybrid F2 ratio: Phenotype 3:1, Genotype 1:2:1.
• Dihybrid F2 ratio: Phenotype 9:3:3:1.
• Incomplete/Codominance F2 ratio: 1:2:1 for both genotype and phenotype.
• Gamete formula: $2^n$ (n = number of heterozygous loci).
• Recombination frequency formula: (Number of recombinants / Total progeny) × 100.
• Pedigree patterns: Recognize autosomal vs. sex-linked, dominant vs. recessive.
• Mutation types: Point (single base change), frameshift (addition/deletion).
• Polygenic inheritance: Multiple genes, quantitative traits, bell-shaped distribution.
• Pleiotropy: One gene, multiple effects (e.g., phenylketonuria).
• Sex determination systems: XX-XY (humans), XO (grasshopper), ZW (birds), haplodiploidy (honeybee).
• Chromosomal disorders: Down (trisomy 21), Klinefelter (XXY), Turner (XO).

These principles and patterns are essential for solving genetics problems and interpreting inheritance in both plants and animals.