LEARNING OBJECTIVES
By the end of this chapter, you should be able to:
- connect DNA, genes and proteins
- compare mitosis and meiosis
- use inheritance terms
- complete genetic diagrams
- explain codominance, sex linkage and ABO groups
THE BIG IDEA
How DNA information is expressed, copied and passed between generations.
This chapter builds each process from its structures, inputs and outputs. Follow the connections rather than memorising isolated facts, then use precise biological terms when explaining cause and effect.
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
- Base order determines amino-acid order, protein shape and function.
- 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
- Mitosis supports growth, repair, replacement and asexual reproduction.
- 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
- Show parental genotypes, gametes, Punnett square and offspring ratios.
- 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
- IᴬIᴬ or IᴬIᵒ gives A; IᴮIᴮ or IᴮIᵒ gives B; IᴬIᴮ gives AB; IᵒIᵒ gives O.
- 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.
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.