Principles of Inheritance and Variation – CBSE NCERT Study Resources

Mendel's Law of Segregation states that alleles separate during gamete formation, ensuring offspring inherit one allele from each parent. This forms the foundation of dominant-recessive inheritance.

• Example: In pea plants, a heterozygous (Tt) tall plant produces gametes with either 'T' or 't'.
• Our textbook shows a 3:1 phenotypic ratio in F2 generation, proving allele separation.

This law explains why recessive traits reappear after skipping generations. However, it doesn’t account for linked genes or incomplete dominance.

Understanding segregation aids in predicting genetic disorders like cystic fibrosis (autosomal recessive).

Polygenic inheritance involves multiple genes controlling a single trait, resulting in continuous variation (e.g., skin color). Unlike Mendelian traits, it doesn’t follow discrete ratios.

• Example: Human skin color is influenced by 3-6 genes (e.g., MC1R), producing a spectrum of phenotypes.
• Our textbook shows a bell-shaped curve in F2 generation, contrasting Mendel’s 3:1 ratio.

This explains why siblings vary in skin tone. Environmental factors (e.g., sun exposure) further modify phenotypes.

Studying polygenic traits helps understand complex diseases like diabetes.

The chromosomal theory states genes reside on chromosomes, which segregate during meiosis. Morgan’s work on Drosophila validated this.

• Example: White-eyed male flies (X-linked recessive) crossed with red-eyed females showed sex-linked inheritance.
• Our textbook highlights 1:1:1:1 ratio deviations, proving gene-chromosome linkage.

This theory unified Mendel’s laws with cytology but couldn’t explain crossing over initially.

It laid groundwork for mapping human genes (e.g., hemophilia).

Autosomal disorders affect non-sex chromosomes (e.g., 1-22), while sex-linked disorders involve X/Y chromosomes.

• Example 1: Cystic fibrosis (autosomal recessive) affects both sexes equally.
• Example 2: Color blindness (X-linked recessive) is more common in males.

Sex-linked traits show criss-cross inheritance (mother to son), unlike autosomal traits.

Genetic counseling uses these patterns to predict risks (e.g., hemophilia in royal families).

The Hardy-Weinberg principle states allele frequencies remain constant in a non-evolving population if five conditions are met.

• Assumptions: No mutations, random mating, no gene flow, infinite population, no selection.
• Example: Textbook calculations show p² + 2pq + q² = 1 for allele frequencies.

Natural populations violate assumptions (e.g., genetic drift in small groups).

It serves as a null model to detect evolutionary forces like natural selection.

Mendel's Law of Segregation states that alleles separate during gamete formation, ensuring each gamete carries only one allele. This principle explains how traits are inherited independently.

• Example: In pea plants, the allele for tall (T) and short (t) segregate during meiosis.
• Our textbook shows a monohybrid cross (Tt x Tt) producing a 3:1 phenotypic ratio.

This law is foundational because it disproved blending inheritance. Modern genetics confirms segregation via chromosome behavior.

Understanding segregation aids in predicting genetic disorders and crop improvement.

Polygenic inheritance involves multiple genes controlling a single trait, resulting in continuous variation. Unlike Mendelian traits, these show a spectrum of phenotypes.

• Example 1: Human skin color is influenced by >3 genes.
• Example 2: Wheat kernel color exhibits a gradation from red to white.

This explains why most human traits (height, intelligence) don’t follow simple ratios. Statistical tools like histograms analyze such data.

Polygenic traits are crucial for studying complex diseases like diabetes.

Linkage refers to genes located close on a chromosome being inherited together. Morgan’s work proved genes are linearly arranged on chromosomes.

• Example: Drosophila eye color (white) and wing size (miniature) were often inherited together.
• Recombination frequency mapped gene positions, confirming linkage.

This validated Sutton-Boveri’s theory, bridging cytology and genetics. Exceptions like crossing-over were later explained.

Linkage maps now aid in genome sequencing and disease gene identification.

Human sex is determined by XX (female) and XY (male) chromosomes. The SRY gene on Y triggers male development.

• Example 1: In turtles, incubation temperature decides sex (20°C = males; 30°C = females).
• Example 2: Bonellia marine worms develop males only when larvae settle on females.

This shows evolution favors diverse mechanisms. Climate change may skew sex ratios in reptiles.

Studying these systems helps conserve endangered species.

Mutations are sudden DNA changes. They can be gene-level (point mutations) or chromosomal (structural changes).

• Example 1: Sickle-cell anemia (point mutation in HBB gene).
• Example 2: Down syndrome (trisomy 21).

While often harmful, mutations drive evolution. Our textbook shows antibiotic resistance arising from bacterial mutations.

CRISPR technology now allows targeted mutations for genetic engineering.

Dominant disorders manifest with one mutant allele (e.g., Huntington’s), while recessive ones require two copies (e.g., cystic fibrosis).

• Dominant: Huntington’s disease, Marfan syndrome.
• Recessive: Albinism, Phenylketonuria (PKU).

Dominant disorders persist despite selection pressure due to late onset. Recessive traits remain hidden in carriers.

Genetic counseling uses this knowledge to predict inheritance risks.

Pedigree analysis maps trait inheritance across generations using standardized symbols. Autosomal dominant traits appear in every generation.

• Example: A family with brachydactyly (short fingers) shows affected parents passing the trait to ~50% offspring.
• Our textbook’s pedigree charts confirm vertical transmission patterns.

This non-invasive method is vital for genetic counseling. However, incomplete penetrance can complicate analysis.

Digital pedigree tools now integrate genomic data for precision medicine.

Genetic drift is random allele frequency changes, especially impactful in small populations due to sampling error.

• Example 1: The Amish community has high Ellis-van Creveld syndrome due to founder effect.
• Example 2: Northern elephant seals lost genetic diversity after near-extinction.

Unlike natural selection, drift is non-adaptive. It can fix harmful alleles by chance.

Conservation biology uses this to manage endangered species’ genetic health.

The Hardy-Weinberg principle states allele frequencies remain constant without evolutionary influences. The equation is p² + 2pq + q² = 1.

• Example: In a population with 16% recessive trait (q²=0.16), q=0.4 and p=0.6.
• Assumptions include no mutation, migration, or selection.

Real populations rarely meet all assumptions, but deviations help identify evolutionary forces.

This model underpins population genetics and disease epidemiology studies.

The HGP (1990-2003) sequenced all 3 billion human DNA base pairs, identifying ~20,500 genes.

• Example 1: BRCA1/2 genes linked to breast cancer were mapped.
• Example 2: SNP databases now predict disease risks like Alzheimer’s.

While revolutionary, HGP showed <1.5% DNA codes for proteins, highlighting non-coding RNA’s importance.

Precision medicine and gene therapy now leverage HGP data.

Mendel's law of dominance states that in a heterozygous condition, one allele (dominant) masks the expression of the other (recessive). For example, in pea plants, the tall (T) trait dominates over the dwarf (t) trait. Thus, a plant with Tt genotype appears tall.

The law of segregation explains that alleles separate during gamete formation, ensuring each gamete carries only one allele. For instance, a Tt plant produces gametes with either T or t, not both.

These laws form the foundation of inheritance because:

• They explain how traits are passed from parents to offspring.
• They predict genotypic and phenotypic ratios in offspring.
• They highlight the role of alleles and their segregation in maintaining genetic diversity.

Incomplete dominance occurs when neither allele is completely dominant, resulting in an intermediate phenotype. For example, in snapdragons, red (RR) and white (rr) flowers produce pink (Rr) offspring.

Codominance occurs when both alleles express themselves fully in a heterozygote. For instance, human blood type AB results from codominant I^A and I^B alleles.

These phenomena deviate from Mendel's laws because:

• They show blending (incomplete dominance) or joint expression (codominance), unlike strict dominance.
• Phenotypic ratios differ (1:2:1 in incomplete dominance vs. 3:1 in dominance).

Application: Blood transfusions rely on understanding codominance to avoid agglutination.

Incomplete dominance occurs when neither allele is completely dominant, resulting in an intermediate phenotype. For example, in snapdragons, red (RR) and white (rr) flowers produce pink (Rr) offspring, showing a blend of traits.

Codominance involves both alleles expressing themselves fully without blending. For instance, in human blood groups, I^A and I^B alleles are codominant, producing AB blood type where both antigens (A and B) are present.

These concepts deviate from Mendel's laws because:

• Mendel's law of dominance assumes one allele is always dominant, but here, dominance is partial or absent.
• Mendel's laws predict clear dominant/recessive ratios, whereas these produce intermediate or joint phenotypes.

Gregor Mendel proposed two fundamental laws of inheritance based on his experiments with pea plants:

1. Law of Dominance: This law states that in a heterozygous condition, one allele (the dominant allele) expresses itself while the other (the recessive allele) remains masked. For example, in pea plants, the allele for tallness (T) is dominant over the allele for dwarfness (t). Thus, a plant with Tt genotype will appear tall.

2. Law of Segregation: This law states that during gamete formation, the two alleles of a gene separate (segregate) so that each gamete carries only one allele. For instance, a heterozygous tall plant (Tt) produces two types of gametes—one with T and another with t—in equal proportion.

These laws form the basis of inheritance because they explain how traits are passed from parents to offspring. The law of dominance clarifies why some traits appear while others remain hidden, whereas the law of segregation ensures genetic variation by allowing recessive traits to reappear in later generations.

The chromosomal theory of inheritance, proposed by Sutton and Boveri, states that genes are located on chromosomes and these chromosomes segregate and assort independently during meiosis, explaining Mendel's laws at a cellular level.

Deviations from Mendel's Laws: This theory explains exceptions like linkage and recombination. For example, Mendel's law of independent assortment assumes genes on different chromosomes assort independently. However, if two genes are located close to each other on the same chromosome, they tend to be inherited together (linkage), leading to deviations from the expected 9:3:3:1 ratio.

Example: In Drosophila, the genes for body color (black or gray) and wing size (vestigial or normal) are linked. Instead of independent assortment, these traits are often inherited together unless crossing over occurs during meiosis, producing recombinant offspring.

Thus, the chromosomal theory provides a mechanistic basis for inheritance while accounting for exceptions observed in Mendelian ratios.

Gregor Mendel proposed two fundamental laws of inheritance based on his experiments with pea plants: the law of dominance and the law of segregation.

Law of Dominance: This law states that in a heterozygous condition (e.g., Tt), one allele (the dominant allele, T) expresses itself, while the other (the recessive allele, t) remains masked. For example, in pea plants, the tall (T) trait dominates over the dwarf (t) trait. Thus, a plant with Tt genotype appears tall.

Law of Segregation: This law explains that during gamete formation, the two alleles of a gene separate (segregate) so that each gamete carries only one allele. For example, a heterozygous tall plant (Tt) produces two types of gametes: 50% with T and 50% with t. This segregation ensures genetic variation in offspring.

These laws help us understand inheritance patterns by:

• Predicting phenotypic ratios in offspring (e.g., 3:1 ratio in F2 generation).
• Explaining the reappearance of recessive traits in later generations.
• Providing a foundation for modern genetics, including Punnett squares and pedigree analysis.

Mendel's work laid the groundwork for the study of heredity, enabling advancements in fields like medicine, agriculture, and biotechnology.

Gregor Mendel proposed two fundamental laws of inheritance based on his experiments with pea plants: the law of dominance and the law of segregation.

Law of Dominance: This law states that in a heterozygous condition (with two different alleles for a trait), one allele (dominant) expresses itself while the other (recessive) remains masked. For example, in pea plants, the allele for tallness (T) is dominant over the allele for dwarfness (t). A plant with genotype Tt will appear tall because the dominant allele (T) suppresses the recessive allele (t).

Law of Segregation: This law states that during gamete formation, the two alleles for a trait separate (segregate) so that each gamete carries only one allele. For example, a heterozygous tall plant (Tt) produces two types of gametes: 50% carrying the T allele and 50% carrying the t allele. During fertilization, these gametes randomly combine to produce offspring with genotypes TT, Tt, or tt.

These laws help us understand:

• How traits are inherited from parents to offspring.
• Why recessive traits may skip generations.
• The basis of genetic variation in populations.

Mendel's law of dominance states that in a heterozygous condition, one allele (dominant) masks the expression of the other (recessive). For example, in pea plants, the allele for tallness (T) is dominant over dwarfness (t). A cross between pure tall (TT) and pure dwarf (tt) plants produces all tall offspring (Tt) in the F₁ generation, demonstrating dominance.

The law of segregation states that alleles separate during gamete formation, ensuring each gamete carries only one allele. In the F₂ generation of the same cross (Tt × Tt), the offspring show a 3:1 phenotypic ratio (3 tall:1 dwarf), proving segregation. This occurs because the alleles T and t separate during meiosis, allowing recombination.

These laws form the basis of inheritance because:

• They explain how traits are passed from parents to offspring.
• They highlight the role of alleles and their independent assortment.
• They provide a framework for predicting genetic outcomes using Punnett squares.

Understanding these laws helps in studying genetic disorders, breeding programs, and evolutionary biology.

The chromosomal theory of inheritance, proposed by Sutton and Boveri, states that genes are located on chromosomes and these chromosomes segregate and assort independently during meiosis, explaining Mendel's laws at a cellular level.

Deviations from Mendel's Laws: This theory explains exceptions like linkage and recombination, where genes located close to each other on the same chromosome do not assort independently (violating Mendel's law of independent assortment). For example, in Drosophila, the genes for body color and wing size are linked and often inherited together.

Mechanism: During meiosis, homologous chromosomes pair and may exchange segments (crossing over), leading to recombination. This creates new allele combinations, explaining why some offspring show non-parental traits. Thus, the chromosomal theory bridges Mendel's principles with modern genetics by accounting for exceptions through chromosomal behavior.