3.23 understand that division of a diploid cell by mitosis produces two cells which contain identical sets of chromosomes

Mitosis – cell division resulting in two, genetically identical diploid daughter cells.

  • All cells of the body (except from sex cells) are formed by mitosis from the zygote
  • In mitosis, each copy of each chromosome is made before the cell divides.
  • During the division, each daughter cell receives a copy of each chromosome


  • Mitosis is important for the replacement of cells.
  • Eg. skin cells, gut cells and blood cells.
  • In cancer, the normal control of mitosis is lost, so the cells divide very rapidly and repeatedly.



3.22 describe the determination of the sex of offspring at fertilisation, using a genetic diagram

  • Sex is inherited by sex chromosomes.
  • 2 xx chromosomes make a female, one xy chromosome makes a person male
  • Half of the sperm contain x chromosomes and the other half contains a y.
  • The type of chromosome that fuses with the egg determines the gender of the zygote (to be baby) – therefore there is an equal chance for both genders


  • When drawing a punnett square, females are homozygous, as they have XX chromosomes – they  only have one column.  

3.18 describe patterns of monohybrid inheritance using a genetic diagram

Monohybrid inheritance – the inheritance of a single character such as seed shape or eye colour.


  • The first successful scientific study on inheritance was carried out in an Austrian monastery garden in the mid 19th Century. It was discovered by Gregor Mendel and he used pea plants to study the inheritance of characteristics such as flower colour, seed shape and plant height.
  • Pea plants carry the same phenotypic features and are controlled by a single gene with two alternative alleles.
  • Mendel could control his experiments by ‘pure-breeding’ plants by transferring pollen by hand.

3.17 understand the meaning of the terms: dominant, recessive, homozygous, heterozygous, phenotype, genotype and codominance

  • Dominant – A form of a gene that is expressed and masks the recessive gene. It gives the same phenotype in both homozygous and heterozygous conditions as it expresses itself.
  • Recessive – A form of a gene that expresses itself only in the homozygous condition. E.g. Bb, it won’t be expressed because the dominant ‘B’ allele masks it and is expressed instead. But in ‘bb’, this is homozygous recessive, so it will be expressed.
  • Homozygous – Having identical alleles for a particular trait. E.g. BB-homozygous dominant, or bb-homozygous recessive. (two copies of the same allele)
  • Heterozygous A condition where you have different alleles for a particular trait. E.g. if B codes for brown eyes (dominant allele is always upper case) and the recessive allele is b (always the lowercase of the dominant alleles’ letter), then a person with a Bb genotype for eye colour is heterozygous dominant, so will have brown eyes.
  • Phenotype – Describes the physical characteristics of an organism with respect to a particular pair of alleles (the genotype), also the physical appearance resulting from the inherited gene
  • Genotype – Describes the alleles that a cell or organism has for a particular feature.
  • Codominance – occurs when neither the allele is dominant and both contribute to the appearance of the offspring.