Essential Idea:
Alleles segregate during meiosis allowing new
combinations to be formed by the fusion of gametes
Understandings:
- One diploid nucleus divides by meiosis to produce four haploid nuclei
- Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number
- DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids
- The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation
- Orientation of pairs of homologous chromosomes prior to separation is random
- The halving of the chromosome number allows a sexual life cycle with fusion of gametes
- Crossing over and random orientation promotes genetic variation
- Fusion of gametes from different parents promotes genetic variation
Applications:
- Non-disjunction can cause Down syndrome and other chromosomal abnormalities
- Studies showing age of parents influences chances of non-disjunction
- Description of methods used to obtain cells for karyotype analysis – e.g. chorionic villus sampling and amniocentesis and the associated risks
Skills:
- Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells