DETAILED NOTES – 1-Chapter 4 Principles of Inheritance and Variation

Chapter 4 Principles of Inheritance and Variation

Introduction

Observations on Reproduction and Inheritance:
- Animals like elephants consistently give birth to offspring of their own species.
- Similarly, plants like mangoes produce seedlings that grow into mango plants.
- This consistency in offspring type is a fundamental aspect of genetics.
Genetics as a Scientific Discipline:
- Genetics is a branch of biology that investigates how traits are passed from parents to offspring.
- It also studies how traits vary within populations and across generations.
Inheritance and Heredity:
- Inheritance is the process through which traits are transmitted from parents to offspring.
- Heredity refers to the passing of traits from one generation to the next.
Variation and Its Importance:
- Variation refers to the differences in traits among individuals within a population.
- Understanding variation is crucial for studying evolution, adaptation, and the diversity of life.
Early Human Understanding of Variation:
- Humans have recognized the existence of variation since ancient times, dating back to 8000-1000 B.C.
- They observed that sexual reproduction played a role in generating variation.
Selective Breeding and Domestication:
- Early humans exploited natural variations in wild populations for selective breeding.
- By selectively breeding organisms with desirable traits, humans have developed numerous domesticated plants and animals.
Example: Domestication of Cows:
- An example is the domestication of cows from ancestral wild species.
- Through artificial selection, humans have developed distinct breeds like the Sahiwal cows in Punjab, India.
Limitations of Early Understanding:
- While ancient people understood the concept of inheritance and variation, they lacked a scientific understanding of these phenomena.
- They had limited knowledge of the underlying genetic mechanisms.

4.1 MENDEL’S LAWS OF INHERITANCE

Introduction of Mendel’s Work:
- Gregor Mendel made significant contributions to the understanding of inheritance in living organisms during the mid-nineteenth century.
- He conducted hybridization experiments on garden peas over a period of seven years, from 1856 to 1863.
Application of Statistical Analysis and Mathematical Logic:
- Mendel’s experiments were groundbreaking because they were among the first to apply statistical analysis and mathematical logic to biological problems.
- This approach added credibility to his findings.
Large Sample Size and Credibility of Data:
- Mendel’s experiments involved a large sampling size, which contributed to the credibility of his data.
- The extensive data collection allowed for more robust conclusions.
Confirmation of Inferences through Successive Generations:
- Mendel’s inferences were confirmed through experiments conducted on successive generations of his test plants.
- This validation supported the idea that his findings represented general rules of inheritance rather than mere speculation.
Investigation of Contrasting Traits in Garden Pea Plants:
- Mendel focused on traits in garden pea plants that exhibited two opposing characteristics, such as tall versus dwarf plants and yellow versus green seeds.
- This choice of traits allowed him to establish a foundational framework for understanding inheritance patterns.
Artificial Pollination and True-Breeding Lines:
- Mendel conducted artificial pollination and cross-pollination experiments using true-breeding pea lines.
- True-breeding lines are those that consistently exhibit the same trait across multiple generations due to continuous self-pollination.
Selection of Contrasting Traits:
- Mendel selected 14 pairs of true-breeding pea plant varieties that differed in one characteristic with contrasting traits.
- Examples of these traits include smooth versus wrinkled seeds, yellow versus green seeds, and tall versus dwarf plants.

4.2 INHERITANCE OF ONE GENE

Experimental Setup:
- Mendel conducted hybridization experiments with garden peas, focusing on the inheritance of one gene.
- He crossed tall and dwarf pea plants to study the inheritance patterns.
Observations in the First Generation (F1):
- Mendel observed that all the plants in the first hybrid generation (F1) were tall, resembling one of the parent plants.
- None of the F1 plants exhibited the dwarf trait.
Observations in the Second Generation (F2):
- Upon self-pollination of the tall F1 plants, Mendel found that in the second generation (F2), some offspring were dwarf.
- The proportion of dwarf plants in the F2 generation was 1/4, while 3/4 were tall.
- Traits did not blend in the F1 or F2 generations; rather, they followed distinct inheritance patterns.
Concept of Genes and Alleles:
- Mendel proposed the existence of discrete units of inheritance called “factors,” which we now call genes.
- Genes contain information that determines specific traits in organisms.
- Genes that code for a pair of contrasting traits are known as alleles, which are different forms of the same gene.
Dominance and Recessiveness:
- Mendel observed that in a pair of dissimilar alleles, one allele dominates over the other.
- The dominant allele is expressed in the phenotype, while the recessive allele remains hidden in the presence of the dominant allele.
- For example, in the case of height in pea plants, the tall trait (T) is dominant over the dwarf trait (t).
Monohybrid Cross and Segregation:
- Mendel’s experiments involving one gene are termed monohybrid crosses.
- During gamete formation (meiosis), alleles segregate randomly into gametes.
- The combination of alleles during fertilization determines the traits of the offspring.
Punnett Square and Probability:
- Reginald C. Punnett developed the Punnett square, a graphical tool used to predict the genotypic and phenotypic ratios of offspring in genetic crosses.
- The Punnett square demonstrates the random assortment of alleles during gamete formation and fertilization.
Law of Dominance:
- Mendel formulated the Law of Dominance, stating that characters are controlled by discrete units (genes), which occur in pairs.
- In a dissimilar pair of alleles, one allele dominates over the other.
- This law explains the expression of traits in the F1 and F2 generations and the observed ratios.

4.2.2 Law of Segregation

No Blending of Alleles:
- The Law of Segregation is based on the observation that alleles do not blend together.
- Each allele maintains its identity and is transmitted intact to the offspring.
Expression of Both Alleles in F2 Generation:
- In the F2 generation, both alleles are recovered in their original form, even if one allele is not visibly expressed in the F1 generation.
- This observation contrasts with the dominant expression of one allele in the F1 generation.
Segregation of Alleles during Gamete Formation:
- During gamete formation (meiosis), alleles segregate from each other.
- Each gamete receives only one of the two alleles present in the parent.
Homozygous and Heterozygous Parents:
- A homozygous parent, having two identical alleles for a trait, produces gametes that all carry the same allele.
- In contrast, a heterozygous parent, having two different alleles for a trait, produces two types of gametes, each carrying one allele. The proportion of each type of gamete is equal.

4.2.2.1 Incomplete Dominance

Observations in Other Plant Traits:
- In experiments with traits in plants other than peas, it was observed that sometimes the phenotype of the F1 generation did not resemble either parent and was intermediate between the two.
Example of Incomplete Dominance in Flower Color:
- The inheritance of flower color in the dog flower (snapdragon or Antirrhinum sp.) illustrates incomplete dominance.
- In a cross between true-breeding red-flowered (RR) and true-breeding white-flowered plants (rr), the F1 generation (Rr) exhibited pink flowers.
- Upon self-pollination of the F1 generation, the F2 generation resulted in a ratio of 1 (RR) Red: 2 (Rr) Pink: 1 (rr) White.
Change in Phenotypic Ratios:
- The genotype ratios in the F2 generation followed Mendelian monohybrid cross ratios, but the phenotype ratios changed from the typical 3:1 dominant : recessive ratio.
- The incomplete dominance allowed for the distinction between heterozygotes (Rr) as pink from homozygous dominant (RR) red and homozygous recessive (rr) white.
Explanation of Dominance:
- Dominance refers to the expression of one allele over another when both are present in the genotype.
- In a diploid organism, each gene contains two copies (alleles), which may be identical (homozygous) or different (heterozygous).
- The dominant allele typically produces a functional enzyme or protein, resulting in the expression of a specific phenotype.
- The recessive allele may produce a non-functional enzyme or no enzyme at all, leading to a different phenotype.
Phenotypic Effect of Alleles:
- If both alleles produce the same functional enzyme, the phenotype will reflect the combined activity of both alleles, leading to complete dominance.
- However, if the recessive allele produces a non-functional enzyme or no enzyme, the phenotype will be influenced only by the functioning of the dominant allele.
- Therefore, the dominant allele determines the observable trait, while the recessive allele remains masked in the presence of the dominant allele.

4.2.2.2 Co-dominance

Introduction to Co-dominance:
- In co-dominance, the F1 generation resembles both parents rather than showing dominance or incomplete dominance.
- A classic example of co-dominance is the ABO blood grouping system in humans.
Example of ABO Blood Grouping:
- ABO blood groups are determined by the gene I, which has three alleles: I A , I B , and i.
- Alleles I A and I B produce slightly different forms of sugars on the surface of red blood cells, while allele i does not produce any sugar.
- When both I A and I B alleles are present together, they express their own types of sugars simultaneously, leading to both A and B types of sugars on the red blood cells.
Multiple Alleles and Genotypes:
- Since humans are diploid organisms, each individual possesses any two of the three I gene alleles.
- There are six different combinations of these three alleles, resulting in a total of six different genotypes of the ABO blood types.
- The number of possible phenotypes depends on the expression of the sugars produced by the alleles.
Multiple Alleles and Population Studies:
- The example of ABO blood grouping illustrates multiple alleles governing the same character, which can only be observed in population studies.
Example of Starch Synthesis in Pea Seeds:
- Another example provided is the starch synthesis in pea seeds, controlled by a single gene with two alleles (B and b).
- Homozygotes for allele B (BB) synthesize starch effectively, resulting in large starch grains, while homozygotes for allele b (bb) have lesser efficiency and produce smaller starch grains.
- Heterozygotes (Bb) produce intermediate-sized starch grains, demonstrating incomplete dominance when considering starch grain size as the phenotype.
Dependence of Dominance on Phenotype:
- Dominance is not solely determined by the gene or its product but also depends on the phenotype examined.
- Different phenotypes influenced by the same gene may exhibit different patterns of dominance, such as complete dominance, incomplete dominance, or co-dominance.