Genetics and DNA
Understand the molecular basis of heredity — from the structure of DNA to how traits are inherited through generations using Punnett squares.
DNA Structure
DNA (deoxyribonucleic acid) is the molecule that carries the genetic instructions for all living organisms. It was first described by Watson and Crick in 1953 as a double helix — two strands twisted around each other like a spiral ladder.
Key Features of DNA:
- Double helix shape — two strands wound around each other
- Sugar-phosphate backbone forms the outer “rails” of the ladder
- Nitrogenous bases form the “rungs”, connected by hydrogen bonds
- Four bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)
Complementary Base Pairing Rules:
Genes, Chromosomes and the Genome
A segment of DNA that codes for a specific protein (or trait). Humans have approximately 20,000–25,000 genes.
A tightly coiled structure of DNA wrapped around proteins. Humans have 46 chromosomes (23 pairs) in each body cell.
The complete set of genetic material in an organism. The Human Genome Project mapped all human DNA by 2003.
Organisation: DNA → Genes → Chromosomes → Genome. Think of it like: letters (bases) make words (genes), words form chapters (chromosomes), and chapters form the complete book (genome).
Alleles and Inheritance
An allele is a variant form of a gene. Since we inherit one copy from each parent, we have two alleles for each gene. These alleles can be:
Dominant Allele (B)
Written as an uppercase letter. The trait is expressed even if only one copy is present. It “masks” the recessive allele.
Recessive Allele (b)
Written as a lowercase letter. The trait is only expressed when two copies are present (homozygous recessive).
Genotype
The genetic makeup of an organism — the alleles it carries.
- BB = homozygous dominant
- Bb = heterozygous
- bb = homozygous recessive
Phenotype
The observable trait expressed by the genotype.
- BB → brown eyes (dominant phenotype)
- Bb → brown eyes (dominant phenotype)
- bb → blue eyes (recessive phenotype)
Punnett Squares
A Punnett square is a diagram used to predict the possible genotypes and phenotypes of offspring from a genetic cross. It shows all possible combinations of alleles from two parents.
Cross: Bb × Bb (both parents heterozygous)
| Father's Alleles | |||
| B | b | ||
| Mother's Alleles | B | BB | Bb |
| b | Bb | bb | |
Genotype ratio: 1 BB : 2 Bb : 1 bb
Phenotype ratio: 3 dominant (brown eyes) : 1 recessive (blue eyes)
There is a 75% chance of brown eyes and a 25% chance of blue eyes.
Key Vocabulary
| Term | Definition |
|---|---|
| DNA | Deoxyribonucleic acid; the molecule that carries genetic instructions. |
| Allele | A variant form of a gene (e.g., B for brown, b for blue). |
| Homozygous | Having two identical alleles for a trait (BB or bb). |
| Heterozygous | Having two different alleles for a trait (Bb). |
| Genotype | The genetic makeup (allele combination) of an organism. |
| Phenotype | The observable physical characteristic resulting from the genotype. |
Worked Examples
Cross between a homozygous dominant (BB) and homozygous recessive (bb) parent.
Parents: BB (brown eyes) × bb (blue eyes)
Step 1: Parent 1 can only contribute B. Parent 2 can only contribute b.
Step 2: All offspring receive one B and one b → all genotypes are Bb.
Step 3: Since B is dominant, all offspring have brown eyes.
Result: 100% Bb (heterozygous), 100% brown-eyed phenotype. All offspring are carriers of the recessive allele.
Predicting offspring from two heterozygous parents (Tt × Tt) for tongue rolling.
Given: T = tongue roller (dominant), t = non-roller (recessive). Both parents are Tt.
Punnett Square:
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
Genotype ratio: 1 TT : 2 Tt : 1 tt
Phenotype ratio: 3 tongue rollers : 1 non-roller = 75% rollers, 25% non-rollers.
Determining an unknown genotype from offspring results.
Question: A black mouse (B?) is crossed with a white mouse (bb). Some offspring are white. What is the black mouse's genotype?
Step 1: White mice must be bb (homozygous recessive).
Step 2: The white parent can only give a b allele.
Step 3: If some offspring are white (bb), they must have received a b from the black parent too.
Step 4: Therefore, the black mouse must be Bb (heterozygous), not BB.
Answer: The black mouse is Bb. The cross Bb × bb gives 50% Bb (black) and 50% bb (white).
Knowledge Check
Select the correct answer for each question. Click “Check Answer” to see feedback.
Question 1
In DNA, adenine (A) always pairs with:
Question 2
A person with genotype Bb is described as:
Question 3
In a cross Bb × bb, what percentage of offspring will show the recessive phenotype?
Question 4
How many chromosomes are in a normal human body cell?
Question 5
If a DNA strand reads A-T-C-G, what is the complementary strand?
Key Concepts Summary
- •DNA is a double helix with complementary base pairs: A-T and C-G.
- •Genes are sections of DNA; chromosomes are coiled DNA structures; the genome is the complete set.
- •Dominant alleles (uppercase) mask recessive alleles (lowercase).
- •Punnett squares predict offspring genotype and phenotype ratios (e.g., 3:1 from Bb × Bb).
- •Genotype = allele combination; phenotype = observable trait.