8 Most SAQ’s of Principles of Inheritance and Variation Chapter in Inter 2nd Year Botany (TS/AP)

4 Marks

SAQ-1 : Mention the advantages of selecting pea plant for experiment by Mendel.

Introduction

Gregor Mendel, renowned as the father of genetics, notably chose the pea plant (garden pea) for his revolutionary inheritance pattern studies. The pea plant offered several advantages that made it an exemplary model for genetic experimentation.

Why Pea Plants?

Pea plants provided multiple benefits, making them ideal for Mendel’s genetic research.

1. Variety of Traits: Pea plants exhibit a range of contrasting characteristics such as seed color, seed shape, flower color, and stem length, crucial for studying trait inheritance.
2. Easy to Grow and Cross: They are relatively simple to cultivate and cross-pollinate, offering convenience for controlled experimentation.
3. Bisexual Flowers: The presence of bisexual flowers, containing both male and female reproductive organs, was key for investigating inheritance patterns.
4. Facilitates Self-Pollination: Pea plants’ structure allows for easy self-pollination, enabling Mendel to precisely control plant mating.
5. Short Life Cycle and Large Offspring Number: Their short life cycle and ability to produce many offspring allowed observation of multiple generations in a brief period.
6. Few Chromosomes: A smaller number of chromosomes in pea plants made it easier to track trait inheritance.
7. Simple Laboratory Conditions: The experiments could be conducted under straightforward laboratory conditions, simplifying Mendel’s research.

Summary

Gregor Mendel’s strategic selection of the pea plant significantly contributed to the success of his genetic studies. The plant’s diverse traits, ease of cultivation and breeding, bisexual flowers, self-pollination capability, short life cycle, fewer chromosomes, and adaptability to simple lab conditions were pivotal in laying the foundation for modern genetics.

---

SAQ-2 : Differentiate between the following: a)Dominant and Recessive b)Homozygous and Heterozygous

Introduction

In genetics, understanding the terms dominant, recessive, homozygous, and heterozygous is crucial for comprehending how traits are inherited. These fundamental concepts elucidate the transmission of characteristics from parents to offspring.

Dominant versus Recessive

1. Dominant Alleles: Dominant alleles are gene versions that are expressed in the phenotype when at least one copy is present. They mask the effect of a different allele.
2. Recessive Alleles: Recessive alleles are gene versions that are only expressed in the phenotype when two copies are present. They are masked by dominant alleles.

Homozygous versus Heterozygous

1. Homozygous Individuals: An individual is homozygous for a trait if they have two identical alleles for that trait, one from each parent.
2. Heterozygous Individuals: An individual is heterozygous for a trait if they have two different alleles for that trait, one from each parent.

Summary

Comprehending the concepts of dominance and recessiveness, and homozygosity and heterozygosity, is essential in genetics. Dominant alleles are expressed with just one copy, while recessive alleles require two copies for expression. Homozygous individuals carry identical alleles for a trait, whereas heterozygous individuals possess different alleles. These concepts form the foundation of genetic inheritance studies.

---

SAQ-3 : Explain the law of Dominance using a monohybrid cross.

Introduction

The Law of Dominance, established by Gregor Mendel, is a key principle in genetics. Mendel demonstrated this law through monohybrid crosses in pea plants, which involved breeding plants differing in a single trait.

Monohybrid Cross

1. Definition: A monohybrid cross is a breeding experiment between individuals that differ in one characteristic or trait. Mendel’s example involved crossing tall and dwarf pea plants.
2. Purpose: The aim is to study the inheritance pattern of a single gene.

Law of Dominance

1. Factors Control Characters: Traits are determined by units known as “factors,” now identified as genes.
2. Factors in Pairs: These factors or genes exist in pairs within organisms.
3. Dominance in Dissimilar Pairs: In heterozygous pairs, one gene can mask the effect of the other. The masking gene is termed the dominant gene, while the masked one is the recessive gene.

Application of Law of Dominance

1. F1 Generation: In a monohybrid cross, the dominant trait is expressed in the first filial (F1) generation, while the recessive trait remains unexpressed.
2. F2 Generation: In the second filial (F2) generation, both dominant and recessive traits reappear when F1 individuals are crossed or self-fertilized.
3. 3:1 Ratio: The F2 generation exhibits a dominant-to-recessive trait ratio of 3:1.

Summary

Mendel’s Law of Dominance, as demonstrated through a monohybrid cross, is fundamental to our understanding of genetic inheritance. This principle elucidates why certain traits are more commonly expressed than others in a population and is pivotal in predicting offspring traits in various fields like agriculture, medicine, and conservation.

---

SAQ-4 : Define and design a test-cross.

Introduction

In genetics, a test cross is an essential technique used to determine the genotype of an individual with a dominant phenotype. This method involves a cross between an individual of unknown genotype and one that is homozygous recessive.

Defining Test Cross

1. Test Cross: It’s a genetic cross between an individual with a dominant phenotype and one that is homozygous recessive for the trait in question.
2. Purpose: The main aim is to ascertain if the individual with the dominant phenotype is homozygous dominant or heterozygous.
3. Phenotype Ratio in a Monohybrid Test Cross: Involving one trait, the offspring typically exhibit a 1:1 ratio of dominant to recessive phenotypes.
4. Phenotype Ratio in a Dihybrid Test Cross: Involving two traits, the offspring display a 1:1:1:1 phenotype ratio, indicating equal probability for each trait combination.

Designing a Test Cross

1. Select an individual showing the dominant phenotype for the trait.
2. Cross this individual with one that is homozygous recessive for the same trait.
3. Observe the phenotype of the offspring.
   • If only the dominant trait is displayed, the tested parent is likely homozygous dominant.
   • If both dominant and recessive traits appear in equal proportions, the tested parent is likely heterozygous.

Summary

A test cross is a vital genetic tool for determining the genotype of an organism by analyzing the phenotypes of its offspring. This method is especially valuable for breeders in predicting the traits of future generations in plants and animals. Proper understanding and application of test crosses are key in the field of genetics and heredity.

---

SAQ-5 : Explain the Co-dominance with example.

Introduction

Co-dominance in genetics is a fascinating phenomenon where both alleles in a gene pair are equally expressed in the offspring, allowing traits from both parents to be distinctly visible, unlike the dominant-recessive inheritance pattern.

Defining Co-dominance

Co-dominance: This inheritance form occurs when both alleles for a specific trait are equally dominant. The offspring exhibit phenotypes of both parents simultaneously.

Exploring Examples of Co-dominance

1. Example 1: ABO Blood Group System in Humans
   • In the ABO blood group system, the alleles ‘A’ and ‘B’ are co-dominant. Individuals with an ‘AB’ blood type express both ‘A’ and ‘B’ antigens on their red blood cells.
2. Example 2: Seed Coat Pattern in Lentil Plants
   • Co-dominance is observed in the seed coat pattern of North American lentil plants. A cross between pure-breeding spotted and pure-breeding dotted lentils results in heterozygous lentils displaying both patterns. The F_1 hybrids show phenotypic features of both parents, indicating no dominance or recessiveness of the spotted or dotted allele.

Summary

Co-dominance is a unique and important genetic mechanism that allows both parental traits to be distinctly visible in the offspring, as seen in human blood types and plant characteristics. It provides a more complex and nuanced understanding of genetic inheritance beyond the traditional dominant-recessive model.

---

SAQ-6 : Explain the incomplete dominance with example.

Introduction

Incomplete dominance is a genetic phenomenon where neither allele in a gene pair completely dominates the other, resulting in offspring with a phenotype that is an intermediate blend of parental traits, deviating from the classical Mendelian dominance.

Incomplete Dominance

1. Defining Incomplete Dominance: This genetic occurrence happens when neither parent gene fully dominates, leading to an intermediate offspring trait.
2. Example – Flower Color in Snapdragon Plants: An exemplary case of incomplete dominance is observed in Snapdragon plants.
   • Cross-Breeding Experiment: A cross between a red flowered plant (RR) and a white flowered plant (rr) produces an F_1 generation (Rr) with pink flowers, blending red and white traits.
   • F2 Generation: Self-pollination of the F_1 generation yields an F_2 generation with 1 red (RR), 2 pink (Rr), and 1 white (rr), resulting in a 1:2:1 phenotypic and genotypic ratio.
   • Here, the ‘R’ gene does not completely dominate ‘r’, thus the Rr genotype results in pink flowers, distinct from red (RR) and white (rr).

Summary

Incomplete dominance provides a nuanced perspective on genetic inheritance, showcasing an intermediate trait that enhances genetic diversity. This concept, illustrated through the color variation in Snapdragon flowers, deepens our understanding of genetic dynamics beyond traditional Mendelian laws.

---

SAQ-7 : Write a brief note on chromosomal mutations and gene mutations.

Introduction

Mutations are critical drivers of evolution and species variation. They occur at two distinct levels: within genes or at a broader chromosomal level, each affecting organisms and species differently.

Chromosomal Mutations

1. Definition: Chromosomal mutations are changes in the number or structure of chromosomes, the cellular structures containing genetic material.
2. Types: These mutations can involve the loss or gain of a DNA segment, altering chromosome structure.
3. Effects: Such alterations lead to chromosomal abnormalities or aberrations, potentially causing various genetic disorders and syndromes.

Gene Mutations

1. Definition: Gene mutations involve alterations in the DNA sequence of bases. These changes usually affect a single gene and can modify the function of the protein it codes.
2. Types: Also known as point mutations, a well-known example is the mutation causing sickle cell anemia, where a single base change affects red blood cell function.
3. Effects: Gene mutations can cause a range of genetic diseases, influence traits, or be asymptomatic, depending on their location and impact on protein function.

Summary

Both chromosomal and gene mutations are pivotal in understanding genetic diseases, inheritance mechanisms, and evolution. While they can lead to diseases, mutations are also essential for genetic diversity, enabling species adaptation and evolution. Comprehending these mutations is key to grasping the complexities of genetics and its influence on life.

---

SAQ-8 : Define law of segregation and law of independent assortment.

Introduction

Gregor Mendel’s groundbreaking work with pea plants led to the formulation of two fundamental laws in genetics: the Law of Segregation and the Law of Independent Assortment. These laws form the basis of our understanding of genetic inheritance and trait distribution among offspring.

Law of Segregation

1. Definition: The Law of Segregation, or the ‘law of purity of gametes’, explains the separation of alleles (different forms of a gene) during gamete formation.
2. Mechanism: In a heterozygous state, two alleles for a specific trait do not blend but remain separate during meiosis, the cell division process producing gametes.
3. Result: Each gamete carries only one allele for each gene, ensuring the preservation of genetic integrity in the offspring.

Law of Independent Assortment

1. Definition: The Law of Independent Assortment states how multiple genes segregate independently from one another during the development of reproductive cells.
2. Mechanism: In organisms with multiple trait pairs, the inheritance of one pair of genes is independent of another pair’s inheritance.
3. Result: Traits segregate independently during gamete formation, leading to increased genetic variation and complexity.

Summary

Mendel’s Laws of Segregation and Independent Assortment are cornerstones in the field of genetics. They elucidate how traits are transmitted from parents to offspring and how different traits are distributed independently, contributing immensely to our knowledge of inheritance and evolutionary mechanisms.