IB Biology (SL)
INHERITANCE
Contents
1. Gametes, fertilisation & genetic crosses
2. Key terminology: genotype, phenotype, dominant/recessive, co-dominance
3. Inheriting alleles (PKU, multiple alleles, ABO blood groups)
4. Sex determination & sex-linked disorders (haemophilia)
5. Pedigree charts
6. Continuous variation & statistical skills (box plots)
, 1. Gametes, Fertilisation & Genetic Crosses
Gametes and fertilisation
• Gametes are the sex cells of an organism — in humans, the sperm and egg (ovum).
• The egg is larger than the sperm because it stores food (yolk/cytoplasm) to nourish the
embryo; the sperm has many mitochondria to power its movement.
• Gametes fuse during fertilisation to form a zygote.
• Gametes are made by meiosis and are haploid — they contain only ONE chromosome from
each homologous pair (23 in humans), so only ONE allele of each gene.
• Fertilisation restores the diploid number: the zygote has two chromosomes of each type, and
therefore TWO alleles of each gene.
Key definitions
Homozygous — the two alleles at a locus are the SAME.
Heterozygous — the two alleles at a locus are DIFFERENT.
Fertilisation = fusion of gamete nuclei; because gametes come from two different parents, this
creates variation in offspring.
Mendel's pea plant experiments
• Gregor Mendel (Austrian monk, 19th century) is called the 'Father of Genetics'.
• He carefully transferred pollen between pea plants (artificial pollination) to control exactly
which plant fertilised which — removing uncertainty from his data.
• He investigated height, flower colour and seed coat texture across generations.
•
◦ Cross 1 (P generation, tall × dwarf): ALL first-generation (F₁) offspring were tall.
◦ Cross 2 (F₁ × F₁): the second generation (F₂) showed BOTH characteristics in a 3 : 1
ratio.
• Mendel had (without knowing it) discovered genes — he called them 'units of inheritance'.
Constructing a Punnett grid
• 1. Write down the parental phenotypes and genotypes.
• 2. Write down all possible gamete genotypes each parent can produce (draw them in a
circle).
INHERITANCE
Contents
1. Gametes, fertilisation & genetic crosses
2. Key terminology: genotype, phenotype, dominant/recessive, co-dominance
3. Inheriting alleles (PKU, multiple alleles, ABO blood groups)
4. Sex determination & sex-linked disorders (haemophilia)
5. Pedigree charts
6. Continuous variation & statistical skills (box plots)
, 1. Gametes, Fertilisation & Genetic Crosses
Gametes and fertilisation
• Gametes are the sex cells of an organism — in humans, the sperm and egg (ovum).
• The egg is larger than the sperm because it stores food (yolk/cytoplasm) to nourish the
embryo; the sperm has many mitochondria to power its movement.
• Gametes fuse during fertilisation to form a zygote.
• Gametes are made by meiosis and are haploid — they contain only ONE chromosome from
each homologous pair (23 in humans), so only ONE allele of each gene.
• Fertilisation restores the diploid number: the zygote has two chromosomes of each type, and
therefore TWO alleles of each gene.
Key definitions
Homozygous — the two alleles at a locus are the SAME.
Heterozygous — the two alleles at a locus are DIFFERENT.
Fertilisation = fusion of gamete nuclei; because gametes come from two different parents, this
creates variation in offspring.
Mendel's pea plant experiments
• Gregor Mendel (Austrian monk, 19th century) is called the 'Father of Genetics'.
• He carefully transferred pollen between pea plants (artificial pollination) to control exactly
which plant fertilised which — removing uncertainty from his data.
• He investigated height, flower colour and seed coat texture across generations.
•
◦ Cross 1 (P generation, tall × dwarf): ALL first-generation (F₁) offspring were tall.
◦ Cross 2 (F₁ × F₁): the second generation (F₂) showed BOTH characteristics in a 3 : 1
ratio.
• Mendel had (without knowing it) discovered genes — he called them 'units of inheritance'.
Constructing a Punnett grid
• 1. Write down the parental phenotypes and genotypes.
• 2. Write down all possible gamete genotypes each parent can produce (draw them in a
circle).
