LEARNING OBJECTIVES
What you will be able to do
- connect DNA, genes and proteins
- compare mitosis and meiosis
- use inheritance terms
- complete genetic diagrams
- explain codominance, sex linkage and ABO groups
AT A GLANCE
INTRODUCTION · THE BIG IDEA
How DNA information is expressed, copied and passed between generations.
Inheritance connects DNA base sequence to proteins, characteristics and the alleles passed to offspring. Cell division determines whether chromosome number is maintained or halved.
Genetic diagrams describe probabilities, not guaranteed family outcomes, and must use consistent symbols for alleles, gametes and genotypes.
SECTION 01
Genes and protein synthesis
Chromosomes are DNA containing genes. A gene is a DNA length coding for a protein; alleles are alternative forms.
DETAILED EXPLANATION
- mRNA copies a gene in nucleus, moves to cytoplasm and passes through ribosomes, which assemble amino acids.
- Proteins include enzymes, carriers and neurotransmitter receptors.
- Most cells share genes but express only those needed. Human diploid cells have 23 pairs; haploid gametes one set.
SECTION 02
Mitosis and meiosis
Chromosomes replicate before mitosis, then copies separate so identical daughter nuclei retain chromosome number.
DETAILED EXPLANATION
- Stem cells are unspecialised and can divide then specialise.
- Meiosis produces gametes, halves diploid to haploid number and creates genetically different cells.
SECTION 03
Monohybrid inheritance
Genotype is allele combination; phenotype is observable expression. Homozygous has matching alleles and heterozygous different alleles. Dominant alleles show when present; recessive alleles show only without a dominant allele.
DETAILED EXPLANATION
- Tt × Tt gives genotype 1:2:1 and phenotype 3:1.
- A test cross with a homozygous recessive identifies an unknown dominant genotype.
- Pedigrees trace characteristics across generations.
SECTION 04
Codominance and sex linkage
In codominance, both alleles contribute. ABO uses Iᴬ and Iᴮ codominantly while Iᵒ is recessive.
DETAILED EXPLANATION
- Eggs carry X; sperm carry X or Y. XX is female, XY male.
- Red-green colour blindness is X-linked and more common in males because they have one X.
STEP-BY-STEP EXAM EXAMPLE
Solving a monohybrid cross
- Let B be a dominant allele and b the recessive allele.
- Cross two heterozygous parents: Bb × Bb.
- Each parent produces B and b gametes.
- The offspring genotypes are BB, Bb, Bb and bb.
Answer: The genotype ratio is 1 BB : 2 Bb : 1 bb; the recessive phenotype probability is 1/4.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Genes code for proteins.
- Mitosis and meiosis have different outcomes.
- Genetic diagrams predict probability.
- ABO is codominant.
- X-linkage changes inheritance patterns.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- connect DNA, genes and proteins
- compare mitosis and meiosis
- use inheritance terms
- complete genetic diagrams
- explain codominance, sex linkage and ABO groups