Heredity
Understanding how traits are inherited from parents to offspring and the rules of inheritance
Introduction to Heredity
Reproductive processes give rise to new individuals that are similar but subtly different. We have seen how variations are produced during asexual and sexual reproduction. In this chapter, we study the mechanism by which variations are created and inherited.
Key Concepts
- How traits are passed from parents to offspring
- Why offspring resemble but are not identical to their parents
- How variations accumulate over generations
- The rules that govern inheritance of traits
Heredity is the passing of traits from parents to offspring. The study of heredity helps us understand why we have certain characteristics and how variations occur in populations.
Learning Outcomes
After studying this chapter, students will be able to:
- Explain how variations accumulate during reproduction
- Describe Mendel's experiments and his contributions to genetics
- Differentiate between dominant and recessive traits
- Explain the rules of inheritance
- Understand how traits get expressed at the molecular level
- Explain sex determination in human beings
- Solve basic genetic problems using Punnett squares
Period-Wise Teaching Plan
This chapter is designed to be covered over 6 periods, each lasting 45 minutes. Below is the detailed period-wise plan:
Topics: Accumulation of variation during reproduction, importance of variation for species survival.
Activities: Discussion on family resemblances, variation in classroom traits.
Topics: Mendel's work with pea plants, monohybrid cross, dominant and recessive traits.
Activities: Activity 8.1 - Earlobe study, Punnett square practice.
Topics: Dihybrid cross, law of independent assortment.
Activities: Activity 8.2 - Verifying F2 ratios, dihybrid cross problems.
Topics: Genes, DNA, proteins, relationship between genes and traits.
Activities: Discussion on how genes control characteristics, case studies.
Topics: Chromosomes, sex chromosomes, sex determination in humans.
Activities: Karyotype analysis, family pedigree studies.
Topics: Revision of all concepts, genetic problem solving, doubt clearing.
Activities: Chapter quiz, genetic problem solving, Q&A session.
Teaching Methodology
The teaching approach for this chapter should be a blend of:
- Interactive lectures with visual aids
- Hands-on activities and experiments
- Problem-solving sessions for genetic crosses
- Group discussions on inheritance patterns
- Case studies and real-life examples
- Regular assessment through quizzes and worksheets
Accumulation of Variation During Reproduction
Variations are differences in characteristics among individuals of the same species. These variations arise during reproduction and accumulate over generations.
How Variations Accumulate
Each generation inherits traits from the previous generation with some variations. When this generation reproduce, it passes on both the inherited variations and new variations to the next generation.
Variation in Asexual vs Sexual Reproduction
| Aspect | Asexual Reproduction | Sexual Reproduction |
|---|---|---|
| Amount of variation | Very little (only due to DNA copying errors) | Considerable (due to combination of genes from two parents) |
| Example | Sugarcane field - very little variation | Human population - significant variation |
| Adaptation potential | Low | High |
Significance of Variation
Variations are essential for the survival of species. They allow populations to adapt to changing environments. For example:
- Heat-resistant bacteria survive better during heat waves
- Animals with better camouflage avoid predators
- Plants with deeper roots survive droughts better
Environmental factors select variants that are better adapted, forming the basis for evolution.
Observe variations in simple traits in the classroom: height, eye color, hair texture, etc. Discuss which traits might be dominant or recessive.
If a trait exists in 10% of an asexually reproducing population and another trait exists in 60% of the same population, which trait likely arose earlier? Why?
Mendel's Contributions to Genetics
Gregor Johann Mendel (1822-1884) is known as the father of genetics. His experiments with pea plants established the fundamental rules of inheritance.
Why Mendel Chose Pea Plants
- Easy to grow and maintain
- Short life cycle
- Clearly distinguishable traits
- Ability to control pollination
Traits Studied by Mendel
Mendel studied seven contrasting traits in pea plants:
| Trait | Dominant Form | Recessive Form |
|---|---|---|
| Seed shape | Round | Wrinkled |
| Seed color | Yellow | Green |
| Flower color | Violet | White |
| Pod shape | Inflated | Constricted |
| Pod color | Green | Yellow |
| Flower position | Axial | Terminal |
| Plant height | Tall | Dwarf |
Monohybrid Cross Experiment
Mendel crossed purebred tall plants with purebred short plants:
- Parental generation (P): Tall (TT) × Short (tt)
- First generation (F1): All plants were tall (Tt)
- Second generation (F2): Self-pollination of F1 plants produced 3/4 tall and 1/4 short plants
Dominant trait: A trait that expresses itself in heterozygous condition (e.g., tallness in peas).
Recessive trait: A trait that remains hidden in heterozygous condition but expresses in homozygous condition (e.g., shortness in peas).
Observe earlobes of all students (free vs attached). Correlate with parents' earlobes and suggest inheritance patterns.
Design an experiment to confirm the 3:1 ratio in F2 generation using colored beads or other models.
Rules of Inheritance
Mendel's experiments led to the formulation of fundamental rules of inheritance that apply to all sexually reproducing organisms.
How Traits Get Expressed
Genes control the production of proteins, which in turn control traits. For example:
- Gene codes for an enzyme involved in plant hormone production
- Efficient enzyme → more hormone → tall plant
- Less efficient enzyme → less hormone → short plant
Law of Independent Assortment
Mendel's dihybrid cross experiments showed that different traits are inherited independently of each other.
Dihybrid Cross Example
Cross between round-yellow (RRYY) and wrinkled-green (rryy) seeds:
- F1 generation: All round-yellow (RrYy)
- F2 generation:
- 9/16 round-yellow
- 3/16 round-green
- 3/16 wrinkled-yellow
- 1/16 wrinkled-green
Chromosomes and Inheritance
Genes are located on chromosomes. Each cell has two copies of each chromosome (except sex chromosomes), one from each parent.
Why Germ Cells Have Half the Chromosomes
Germ cells (sperm and egg) have only one set of chromosomes. This ensures that:
- When they fuse during fertilization, the normal chromosome number is restored
- Traits from both parents are combined in the offspring
- Independent assortment of genes can occur
A man with blood group A marries a woman with blood group O. Their daughter has blood group O. Is this enough to determine which trait is dominant? Why or why not?
Design a project to find the dominant coat color in dogs by studying pedigrees of different dog breeds.
Sex Determination
Sex determination refers to how the sex of an individual is determined. Different species use different mechanisms for sex determination.
Chromosomal Basis of Sex Determination
In human beings, sex is determined by the combination of sex chromosomes:
Human Sex Chromosomes
- Females: XX (two X chromosomes)
- Males: XY (one X and one Y chromosome)
- All children inherit an X chromosome from their mother
- Father can contribute either an X or Y chromosome
- XX combination → female child
- XY combination → male child
Other Mechanisms of Sex Determination
Not all organisms use the XY system for sex determination:
| Organism | Mechanism | Details |
|---|---|---|
| Some reptiles | Environmental | Temperature during egg incubation determines sex |
| Snails | Changeable | Individuals can change sex during their lifetime |
| Birds | ZW system | Females are ZW, males are ZZ (opposite of XY system) |
| Bees | Haplodiploidy | Females from fertilized eggs, males from unfertilized eggs |
Analyze a family pedigree to determine patterns of inheritance for specific traits and predict probabilities for future offspring.
Discuss the ethical implications of sex determination technologies and the problem of sex-selective abortions.
Teaching Resources
Key Terms
- Heredity: Passing of traits from parents to offspring
- Variation: Differences between individuals of the same species
- Genetics: Study of heredity and variation
- Gene: Unit of heredity; section of DNA coding for a protein
- Allele: Different forms of a gene
- Dominant trait: Trait that expresses in heterozygous condition
- Recessive trait: Trait that expresses only in homozygous condition
- Homozygous: Having identical alleles for a trait
- Heterozygous: Having different alleles for a trait
- Genotype: Genetic makeup of an individual
- Phenotype: Observable characteristics of an individual
- Chromosome: Thread-like structure carrying genes
- Sex chromosomes: Chromosomes that determine sex (X and Y in humans)
- Autosomes: Chromosomes other than sex chromosomes
- Allosomes: Sex chromosomes
Assessment Questions
Chapter Review Questions
- How do Mendel's experiments show that traits may be dominant or recessive?
- How do Mendel's experiments show that traits are inherited independently?
- A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits - blood group A or O - is dominant? Why or why not?
- How is the sex of the child determined in human beings?
- If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
- How does the creation of variations in a species promote survival?
- Outline a project which aims to find the dominant coat colour in dogs.
- How is the equal genetic contribution of male and female parents ensured in the progeny?
Additional Resources
- Interactive Punnett square simulators
- 3D models of DNA and chromosomes
- Videos on Mendel's experiments
- Genetic problem-solving worksheets
- Family pedigree analysis exercises
- Case studies on genetic disorders
- Printable diagrams for labeling practice
- Online quizzes and interactive learning tools
