Meiosis and Genetic Diversity

• Produces four genetically unique haploid cells from one diploid cell
• Two divisions: Meiosis I (reductional) and Meiosis II (equational)
• Occurs in gonads (ovaries and testes)

Key outcome: Chromosome number halved ($2n \rightarrow n$)

Crossing Over (Prophase I)

• Non-sister chromatids of homologous pairs exchange segments at chiasmata
• Creates recombinant chromosomes with new allele combinations
• Each chromatid can have a unique combination of alleles

Independent Assortment (Metaphase I)

• Each bivalent orients randomly at the metaphase plate
• For $n$ pairs: $2^n$ possible combinations of maternal/paternal chromosomes
• Humans ($n = 23$): $2^{23} = 8,388,608$ possible gamete combinations

Random Fertilisation

• Any sperm can fertilise any egg
• Humans: $2^{23} \times 2^{23} = 2^{46} \approx 7 \times 10^{13}$ possible zygote combinations (without crossing over)

Mutation

• Random changes in DNA provide raw material for new alleles

• Failure of chromosomes to separate properly during meiosis
• Can occur in meiosis I (homologous pairs fail to separate) or meiosis II (sister chromatids fail to separate)
• Results in gametes with abnormal chromosome numbers (aneuploidy)
• Examples:
  • Trisomy 21 (Down syndrome): extra chromosome 21
  • Turner syndrome (XO): missing X chromosome
  • Klinefelter syndrome (XXY): extra X chromosome

Question: Explain how crossing over contributes to genetic variation. (3 points)

Solution:

During prophase I of meiosis, homologous chromosomes pair up (synapsis). Non-sister chromatids may break at corresponding points and exchange segments of DNA at structures called chiasmata. After crossing over, each chromatid carries a new combination of alleles — some from the maternal chromosome and some from the paternal. When these recombinant chromosomes are separated during meiosis, the resulting gametes have unique allele combinations that differ from either parent chromosome. This increases genetic variation in the offspring.

• Meiosis: two divisions producing 4 genetically unique haploid cells.
• Meiosis I: homologous chromosomes pair, cross over, assort independently, and separate (reduction).
• Meiosis II: sister chromatids separate (similar to mitosis).
• Genetic variation from: crossing over, independent assortment, random fertilisation, mutation.
• Nondisjunction causes aneuploidy (e.g., Down syndrome, Turner syndrome).