Welcome to Inheritance: The Secret Code of Life!
Hello future scientists! This chapter, Inheritance, is all about how you got your traits—from the colour of your eyes to whether you can curl your tongue. It’s like studying the instruction manual for the human body.
Don't worry if the vocabulary seems heavy at first; we will break down every key term. By the end, you'll be able to predict the characteristics of offspring using simple genetic rules!
Why is Inheritance Important?
- It explains why children look like their parents (but not exactly the same).
- It is the basis for understanding genetic disorders and modern medicine.
- It forms the foundation of all evolution and biological change.
Section 1: The Genetic Building Blocks
To understand inheritance, we first need to look inside the cell at the structures that carry the instructions.
1. DNA, Genes, and Chromosomes
Imagine your body is built following a massive instruction manual. This manual is your genetic material.
a) DNA (Deoxyribonucleic Acid)
DNA is the long, spiralling chemical (the famous double helix) that contains all the instructions for making and operating an organism. It’s the material that stores the information.
Analogy: DNA is the entire instruction manual.
b) Genes
A gene is a small section of DNA that codes for a specific protein, which in turn determines a specific characteristic (or trait), such as eye colour or height.
Analogy: A gene is a single recipe within the instruction manual (e.g., the recipe for "Blue Eyes").
c) Chromosomes
To keep the DNA organized, especially during cell division, it is tightly wound up into structures called chromosomes.
In humans, these usually come in pairs:
- We have 46 chromosomes in total, arranged in 23 pairs.
- You inherit one chromosome from each pair from your mother, and the other from your father.
- Pairs 1 to 22 are called autosomes (non-sex chromosomes).
- The 23rd pair are the sex chromosomes (X and Y).
DNA is coiled to form Chromosomes. A small section of DNA is a Gene, which determines a Trait.
Section 2: The Language of Genetics (Key Terms)
These terms are the most important vocabulary in this chapter. Master them, and genetics becomes much easier!
1. Alleles: Versions of a Gene
A gene determines a trait (like hair colour). An allele determines the specific version of that trait (like brown hair or blonde hair).
Since you have two chromosomes (one from each parent), you carry two alleles for every gene.
- Example: The gene for height might have an allele for 'Tall' and an allele for 'Short'.
2. Genotype vs. Phenotype
a) Genotype
The genotype is the specific combination of alleles an organism has for a trait. It is the genetic makeup, represented by letters.
Example: BB, Bb, or bb.
b) Phenotype
The phenotype is the physical appearance or observable characteristic that results from the genotype.
Example: Brown eyes, Tall, or Curly hair.
3. Homozygous and Heterozygous
We use these terms to describe the relationship between the two alleles in a genotype:
a) Homozygous (Purebred)
If both alleles are the same, the organism is homozygous for that trait.
Examples: Two capital letters (BB) or two lowercase letters (bb).
Trick: Homo means "same" (think of a homogenised smoothie—it’s all the same consistency!).
b) Heterozygous (Hybrid)
If the two alleles are different, the organism is heterozygous for that trait.
Example: One capital and one lowercase letter (Bb).
Trick: Hetero means "different."
Section 3: Dominant and Recessive Alleles
How do we know which allele (version) actually shows up in the phenotype?
1. Dominant Alleles
A dominant allele is always expressed in the phenotype, even if only one copy is present (i.e., in heterozygous individuals).
We always represent dominant alleles with a capital letter (e.g., B for Brown eyes).
- If the genotype is BB (homozygous dominant), the phenotype is Brown eyes.
- If the genotype is Bb (heterozygous), the phenotype is still Brown eyes.
Analogy: The dominant allele is a loud voice; it always gets heard.
2. Recessive Alleles
A recessive allele is only expressed in the phenotype if two copies are present (i.e., the individual is homozygous recessive).
We represent recessive alleles with a lowercase letter (e.g., b for blue eyes).
- If the genotype is bb (homozygous recessive), the phenotype is Blue eyes.
Analogy: The recessive allele is a whisper; it is only heard if there is no loud (dominant) voice around.
A heterozygous organism (Bb) will always display the dominant phenotype. The recessive allele (b) is still present, but it is hidden. The organism is called a carrier of the recessive allele.
Section 4: Monohybrid Inheritance and Punnett Squares
A monohybrid cross involves looking at the inheritance of a single characteristic. We use a diagram called a Punnett square to predict the probability of different genotypes and phenotypes in the offspring.
Step-by-Step Guide to Punnett Squares
Let’s use an example: In mice, black fur (B) is dominant to white fur (b). We cross a homozygous black mouse (BB) with a white mouse (bb).
Step 1: Determine the Parent Genotypes and Gametes
Remember that gametes (sperm and egg) carry only one allele from each pair.
- Parent 1 (Black): Genotype BB. Gametes can only be B.
- Parent 2 (White): Genotype bb. Gametes can only be b.
Step 2: Set up the Punnett Square
Draw a 2x2 grid. Place the gametes from one parent along the top and the gametes from the other parent down the side.
| B (from Parent 1) | B (from Parent 1) | |
| b (from Parent 2) | ? | ? |
| b (from Parent 2) | ? | ? |
Step 3: Fill in the Square
Combine the alleles from the row and column headers in each box.
| B | B | |
| b | Bb | Bb |
| b | Bb | Bb |
Step 4: Determine the Ratios
Count the resulting genotypes and phenotypes:
- Genotypes: All offspring are Bb. Genotypic Ratio: 4:0 (or 100% Bb).
- Phenotypes: Since 'B' is dominant, all offspring will have Black fur. Phenotypic Ratio: 4:0 (or 100% Black).
Common GCSE Crosses (Heterozygous Cross Example)
What happens if we cross two of the offspring (Bb x Bb)?
Gametes: Both parents produce B and b gametes.
| B | b | |
| B | BB | Bb |
| b | Bb | bb |
- Genotypes: BB : Bb : bb = 1 : 2 : 1 (25% : 50% : 25%)
- Phenotypes: Black Fur (BB + Bb) : White Fur (bb) = 3 : 1 (75% Black : 25% White)
Important Takeaway: The 3:1 phenotypic ratio is the classic result whenever two heterozygous individuals (hybrids) breed.
Section 5: Sex Determination
The 23rd pair of chromosomes determines the biological sex of an individual. These are the sex chromosomes.
1. The Sex Chromosomes
- Females have two X chromosomes (XX).
- Males have one X and one Y chromosome (XY).
2. Who Determines the Sex?
The female parent can only pass on an X chromosome (gametes are X).
The male parent can pass on either an X chromosome or a Y chromosome (gametes are X or Y).
Therefore, it is the sperm (from the father) that determines the sex of the offspring.
Punnett Square for Sex Determination (XX x XY)
| X (Sperm) | Y (Sperm) | |
| X (Egg) | XX (Female) | XY (Male) |
| X (Egg) | XX (Female) | XY (Male) |
Result: The probability of having a male (XY) or a female (XX) is 50% (1 in 2).
The Y chromosome is much smaller than the X chromosome and carries far fewer genes.
Chapter Summary & Final Encouragement
You have covered the core concepts of Inheritance! Remember: Genetics is a powerful tool because it allows us to predict the likely outcomes of breeding, whether we are talking about humans, plants, or animals.
- Gene = instruction for a trait.
- Allele = version of the instruction.
- Genotype (letters) determines Phenotype (look).
- Dominant traits mask Recessive traits.
- Sex is determined by the father’s Y chromosome.
Keep practicing those Punnett squares—they are the key to unlocking this chapter!