Basic Principles of Inheritance
Basic Principles of Inheritance — Study Notes
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6.1 INTRODUCTION TO INHERITANCE
Explanation6.1 INTRODUCTION TO INHERITANCE
Inheritance is the biological process through which characteristics or traits are transmitted from parents to their offspring. This phenomenon explains why members of a family share several features such as facial features, hair colour, and skin colour. These characteristics have a genetic basis, meaning they depend on the genetic information inherited from the parents. This is true for all plants and animals. The transmission of characters from one generation to the next is known as heredity. The inherited characters are present on chromosomes in the form of genes. However, although offspring inherit traits from their parents, they also show variations, which are differences between the offspring and their parents. The study of heredity and variation is called genetics. Biotechnology aims to manipulate living organisms or modify their genetic constitution to produce products that improve human life quality. For effective gene manipulation, understanding genetics and heredity is essential, including identifying the genes and their allelic forms that regulate traits. This chapter introduces the fundamental principles of inheritance.
- Inheritance is the transmission of traits from parents to offspring.
- Traits have a genetic basis and are carried on chromosomes as genes.
- Offspring show both inherited traits and variations from parents.
- Genetics is the scientific study of heredity and variation.
- Biotechnology uses genetic knowledge to manipulate organisms for human benefit.
- Understanding gene regulation is crucial for genetic manipulation.
- 📌 Inheritance: Biological transmission of traits from parents to offspring.
- 📌 Heredity: The process of passing traits from one generation to the next.
- 📌 Variation: Differences between offspring and their parents.
6.1.1 Mendel's work: The foundation
Explanation6.1.1 Mendel's work: The foundation
Gregor Johann Mendel, an Austrian monk born in 1822, is known as the father of genetics due to his pioneering work on inheritance. Mendel chose pea plants (Pisum sativum) for his experiments because they are annual plants with bisexual flowers and exhibit many contrasting traits. He selected seven pairs of contrasting characters such as tall vs dwarf plants, round vs wrinkled seeds, and yellow vs green seeds. Mendel produced pure lines for each trait by self-pollinating plants for several generations. He then performed artificial cross-pollination by transferring pollen from one flower to another using a brush. Mendel grew large numbers of plants for each cross and collected data over several generations. His experiments led to the establishment of the Law of Segregation and the Law of Independent Assortment. Though his work was published in 1866, it was largely ignored until rediscovered in 1900 by other scientists. Mendel's work forms the basis of modern genetics.
- Mendel selected pea plants for genetic experiments due to their contrasting traits and ease of breeding.
- Seven pairs of contrasting traits were studied, including plant height and seed shape.
- Pure lines were developed by self-pollination over generations.
- Artificial cross-pollination was used to study inheritance patterns.
- Mendel established fundamental genetic laws: segregation and independent assortment.
- His work was rediscovered decades after his death and is foundational to genetics.
- 📌 Pure line: A population of plants that produce offspring identical to themselves when self-pollinated.
- 📌 Cross-pollination: Transfer of pollen from one plant to the flower of another.
- 📌 Law of Segregation: Each individual has two alleles for a trait which segregate during gamete formation.
Single gene inheritance
ExplanationSingle gene inheritance
Mendel's monohybrid cross experiments focused on the inheritance of a single trait controlled by one gene with two alleles. For example, when a homozygous tall pea plant (TT) was crossed with a homozygous dwarf plant (tt), all the first filial genera
Practice Questions — Basic Principles of Inheritance
Includes NCERT exercise questions with answers
Q1.Differentiate between the following (a) Genotype and Phenotype (b) Dominant and Recessive characters (c) Hybrid and Pure individuals (d) Heterozygous and Homozygous progeny (e) Monohybrid and Dihybrid cross (f) Gene and allele (g) Incomplete dominance and codominance
Answer:
(a) Genotype vs Phenotype: - Genotype is the genetic constitution of an organism (the alleles present). - Phenotype is the observable physical or biochemical characteristics of an organism, determined by genotype and environment. (b) Dominant vs Recessive characters: - Dominant characters are expressed in the phenotype even if only one allele is present. - Recessive characters are expressed only when two copies of the recessive allele are present. (c) Hybrid vs Pure individuals: - Hybrid individuals have two different alleles for a trait (heterozygous). - Pure individuals have two identical alleles for a trait (homozygous). (d) Heterozygous vs Homozygous progeny: - Heterozygous progeny have two different alleles for a gene. - Homozygous progeny have two identical alleles for a gene. (e) Monohybrid vs Dihybrid cross: - Monohybrid cross involves one gene with two alleles. - Dihybrid cross involves two genes each with two alleles. (f) Gene vs Allele: - Gene is a segment of DNA that codes for a trait. - Allele is a variant form of a gene. (g) Incomplete dominance vs Codominance: - Incomplete dominance: heterozygote shows intermediate phenotype. - Codominance: both alleles are fully expressed in heterozygote.
Explanation:
Each pair is differentiated based on genetic principles and observable traits. Genotype is genetic makeup; phenotype is physical expression. Dominant alleles mask recessive ones. Hybrids have different alleles; pure individuals have identical alleles. Monohybrid involves one gene; dihybrid involves two. Genes are DNA segments; alleles are gene variants. Incomplete dominance shows blended traits; codominance shows both traits distinctly.
Q2.Mention the genotypic and phenotypic ratio of progeny when there is a cross between (a) $F_{1}$ progeny with pure dominant parent (b) $F_{1}$ progeny with pure recessive parent (c) $F_{1}$ progeny with $F_{1}$ progeny
Answer:
(a) Cross: F1 (heterozygous, Aa) × Pure dominant (AA) - Genotypic ratio: 1 AA : 1 Aa - Phenotypic ratio: All dominant phenotype (b) Cross: F1 (Aa) × Pure recessive (aa) - Genotypic ratio: 1 Aa : 1 aa - Phenotypic ratio: 1 dominant : 1 recessive (c) Cross: F1 (Aa) × F1 (Aa) - Genotypic ratio: 1 AA : 2 Aa : 1 aa - Phenotypic ratio: 3 dominant : 1 recessive
Explanation:
F1 progeny are heterozygous (Aa). Crossing with pure dominant (AA) results in offspring either AA or Aa, all showing dominant phenotype. Crossing with pure recessive (aa) results in half heterozygous (dominant phenotype) and half homozygous recessive (recessive phenotype). Crossing F1 with F1 gives Mendelian 1:2:1 genotypic and 3:1 phenotypic ratios.
Q3.Explain test cross through diagrammatic representation.
Answer:
Test cross is used to determine the genotype of an individual showing dominant phenotype (whether homozygous dominant or heterozygous). Procedure: - Cross the individual with a homozygous recessive individual. Diagrammatic representation: If unknown genotype is AA (homozygous dominant): Parent 1: AA Parent 2: aa Offspring: All Aa (dominant phenotype) If unknown genotype is Aa (heterozygous): Parent 1: Aa Parent 2: aa Offspring: 1 Aa (dominant phenotype) : 1 aa (recessive phenotype) Interpretation: - If any recessive phenotype appears in offspring, the unknown parent is heterozygous. - If all offspring show dominant phenotype, the unknown parent is homozygous dominant.
Explanation:
Test cross helps to reveal the genotype of an individual with dominant phenotype by crossing with homozygous recessive. Appearance of recessive phenotype in progeny indicates heterozygous genotype; absence indicates homozygous dominant.
Q4.Explain following using monohybrid and dihybrid cross. (a) Law of dominance (b) Law of segregation (c) Law of independent assortment
Answer:
(a) Law of Dominance: - In a monohybrid cross between two contrasting traits, one trait dominates over the other. - Example: Cross between pure tall (TT) and pure dwarf (tt) pea plants results in all tall (Tt) F1 progeny. (b) Law of Segregation: - During gamete formation, the two alleles for a gene segregate from each other so that each gamete carries only one allele. - Example: In monohybrid cross (Tt × Tt), alleles segregate to produce gametes T and t. (c) Law of Independent Assortment: - Alleles of different genes assort independently during gamete formation. - Example: In dihybrid cross (AaBb × AaBb), the inheritance of gene A does not affect gene B, resulting in 9:3:3:1 phenotypic ratio.
Explanation:
Monohybrid and dihybrid crosses demonstrate Mendel's laws. Law of dominance shows dominant trait expression in F1. Law of segregation explains separation of alleles during gamete formation. Law of independent assortment states that alleles of different genes segregate independently, leading to genetic variation.
Q5.What will be the genotypic and phenotypic ratio when a red and tall homozygous tomato plant is crossed to a red and tall heterozygous plant?
Answer:
Let red color (R) and tallness (T) be dominant traits. Given: - Homozygous red and tall plant: RR TT - Heterozygous red and tall plant: Rr Tt Cross: RR TT × Rr Tt Gametes: - RR TT produces only RT - Rr Tt produces RT, Rt, rT, rt Offspring genotypes: - RT × RT = RRTT (homozygous dominant) - RT × Rt = RRTt - RT × rT = RrTT - RT × rt = RrTt Genotypic ratio: - 1 RRTT : 1 RRTt : 1 RrTT : 1 RrTt Phenotypic ratio: - All red and tall (since all have at least one R and one T allele) Therefore, genotypic ratio is 1:1:1:1 and phenotypic ratio is 100% red and tall.
Explanation:
Crossing homozygous dominant with heterozygous results in progeny with genotypes combining dominant homozygous and heterozygous alleles. All progeny show dominant phenotype for both traits.
Q6.When one male and one female Drosophila, heterozygous for the two pairs of alleles AaBb, were mated, the offspring's phenotypic ratio 2:1:1:2 was obtained. (a) Explain how these ratios help in detecting linkages? (b) How degree of linkage can be determined?
Answer:
(a) Explanation of linkage detection: - In independent assortment, dihybrid cross produces 9:3:3:1 ratio. - Deviation from this ratio, such as 2:1:1:2, suggests genes are linked and do not assort independently. - Linked genes are located close on the same chromosome and tend to be inherited together. (b) Determining degree of linkage: - Degree of linkage is measured by recombination frequency. - Recombination frequency = (Number of recombinant offspring / Total offspring) × 100% - Lower recombination frequency indicates stronger linkage (genes closer together). - Higher recombination frequency indicates weaker linkage (genes farther apart). - Genetic map units (centiMorgans) are based on recombination frequency.
Explanation:
The phenotypic ratio differing from Mendelian 9:3:3:1 indicates linkage. Calculating recombination frequency from offspring data quantifies linkage degree, helping to map gene positions on chromosomes.
Q7.Make a close observation with the nature. Do you think that the phenomenon of linkage is absolute?
Answer:
No, the phenomenon of linkage is not absolute. Explanation: - Linkage refers to genes located close together on the same chromosome that tend to be inherited together. - However, crossing over during meiosis can separate linked genes, producing recombinant offspring. - Therefore, linkage is relative and can be broken by recombination. - The closer the genes, the stronger the linkage; the farther apart, the more likely recombination occurs. - Hence, linkage is not absolute but probabilistic.
Explanation:
Linkage can be disrupted by crossing over, so it is not absolute. This allows genetic variation and recombination of traits, which is important for evolution.
Q8.What is the biological process through which characteristics or traits are transmitted from parents to their offspring called?
Answer:
Heredity
Explanation:
Heredity is the biological process by which traits or characteristics are passed from parents to their offspring. This explains the similarities seen among family members and is fundamental to genetics.
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