HEREDITY — QUALITY STUDY NOTES
1. Variation and Heredity
Variation
- Reproduction produces offspring that resemble their parents but are not completely identical.
- Differences among individuals of the same species are called variations.
- Variations can arise even during asexual reproduction, mainly because DNA copying is not perfectly accurate.
- Sexual reproduction generally produces greater variation because genetic material comes from two parents.
- Some variations can improve an organism’s ability to survive under particular environmental conditions.
- Environmental selection of useful variations contributes to evolution.
Heredity
Heredity is the transmission of traits from parents to offspring.
It explains why offspring:
- have the basic body design of their species, and
- inherit certain characteristics from their parents.
2. Inherited Traits
A trait is a characteristic of an organism.
Examples:
- Seed shape
- Plant height
- Flower colour
- Earlobe type
In sexual reproduction, both parents contribute genetic material to the offspring. Therefore, a child receives genetic information from both the mother and father.
3. Mendel and His Experiments
Gregor Johann Mendel (1822–1884) studied inheritance using garden pea plants.
He selected contrasting characteristics such as:
- Tall / short plants
- Round / wrinkled seeds
- Violet / white flowers
A major strength of Mendel’s work was that he counted individuals showing particular traits in each generation, allowing him to identify patterns of inheritance.
4. Mendel’s Monohybrid Cross
Consider:
Tall plant (TT) × Short plant (tt)
First generation — F₁
All offspring are tall.
There is no intermediate “medium-height” form.
This shows that one trait can be expressed while the other remains unexpressed.
Second generation — F₂
When F₁ plants reproduce by self-pollination:
- Tall : Short = 3 : 1
- Genetic combinations occur in the ratio:
TT : Tt : tt = 1 : 2 : 1
Thus:
- TT → Tall
- Tt → Tall
- tt → Short
5. Dominant and Recessive Traits
Dominant trait
A trait that is expressed even when only one copy of its relevant allele is present.
Example:
- Tt → Tall
Recessive trait
A trait that is expressed only when both copies are recessive.
Example:
- tt → Short
Therefore:
- T = dominant allele
- t = recessive allele
- TT / Tt = dominant phenotype
- tt = recessive phenotype
Exam point: A trait being common in a population does not automatically mean that it is dominant.
6. Independent Inheritance of Traits
Mendel also studied two traits together, such as:
- Seed shape: round / wrinkled
- Seed colour: yellow / green
When plants with contrasting combinations were crossed, the F₁ generation showed the dominant characteristics.
In the F₂ generation, new combinations appeared as well:
- Tall + round
- Tall + wrinkled
- Short + round
- Short + wrinkled
The observed phenotypic ratio is:
9 : 3 : 3 : 1
This demonstrates that different traits can be inherited independently, allowing new combinations to appear in offspring.
Key idea
Independent inheritance → recombination of traits → new combinations in offspring.
7. How Genes Control Traits
DNA → Gene → Protein → Trait
- DNA contains information required for making proteins.
- A gene is a section of DNA that provides information for making a particular protein.
- Proteins can influence the functioning of cells and ultimately determine characteristics.
Example: Plant height
- A gene contains information related to an enzyme.
- The enzyme helps in the production of a plant growth hormone.
- An efficient enzyme can lead to more hormone production.
- Greater hormone production can result in a taller plant.
- A change in the gene may produce a less efficient enzyme.
- Less hormone may then be produced, resulting in a shorter plant.
Therefore, genes control traits by influencing protein production and function.
8. Chromosomes and Equal Genetic Contribution
In sexually reproducing organisms:
- Body cells have two copies of each gene, with genetic material contributed by both parents.
- The two copies may be identical or different.
- Germ cells must receive only one copy from each gene pair.
Genes are located on chromosomes.
Each body cell has:
- Two copies of each chromosome
- One inherited from the mother
- One inherited from the father
During formation of germ cells, one chromosome from each pair enters the germ cell.
During fertilisation:
Male germ cell + Female germ cell → offspring with the normal chromosome number
This maintains the chromosome number of the species and explains how equal genetic contribution from the parents is achieved.
9. Sex Determination
Different organisms use different mechanisms to determine sex.
Examples mentioned in the chapter:
- In some reptiles, temperature during development can determine sex.
- Some organisms, such as certain snails, can change sex.
- In humans, sex is largely determined genetically.
Human sex chromosomes
Humans have:
- 22 pairs of autosomes
- 1 pair of sex chromosomes
Sex chromosomes:
- Female → XX
- Male → XY
The mother’s egg always contributes an X chromosome.
The father’s sperm can contribute:
- X → XX offspring
- Y → XY offspring
Therefore, according to the chromosome mechanism described in the chapter:
XX → girl
XY → boy
The paternal chromosome determines whether the offspring receives XX or XY. The expected probability is approximately 50% XX and 50% XY.
10. Most Important Ratios
| Cross / Observation | Important ratio |
|---|---|
| F₂ phenotype in a monohybrid cross | 3 : 1 |
| F₂ genotype in a monohybrid cross | 1 : 2 : 1 |
| F₂ phenotype in a dihybrid cross | 9 : 3 : 3 : 1 |
| Expected XX : XY outcome | 1 : 1 |
11. Key Terms
Heredity: Transmission of traits from parents to offspring.
Variation: Differences between individuals of the same species.
Gene: A section of DNA carrying information for a protein.
Chromosome: A DNA-containing structure carrying genes.
Trait: A characteristic of an organism.
Dominant trait: A trait expressed when its allele is present in a single copy.
Recessive trait: A trait expressed when two recessive copies are present.
F₁ generation: First generation produced from the parental cross.
F₂ generation: Generation obtained by reproduction of F₁ individuals.
Sex chromosomes: Chromosomes involved in determining sex; in humans, X and Y.
12. Exam-Ready Points
- Reproduction produces variations, and sexual reproduction generally generates greater variation.
- Variations can sometimes increase an organism’s survival advantage.
- Heredity explains the transmission of characteristics from parents to offspring.
- Mendel used pea plants to establish important inheritance patterns.
- A dominant trait can be expressed with one copy of its allele.
- A recessive trait generally requires two recessive copies for expression.
- Different traits can be inherited independently, producing new combinations.
- Genes are sections of DNA that provide information for proteins.
- Chromosomes occur in pairs in body cells, with one chromosome of each pair inherited from each parent.
- Germ cells receive one chromosome from each pair, and fertilisation restores the normal chromosome number.
- Human females are XX and males are XY.
- The paternal X or Y chromosome determines whether the child has an XX or XY combination.