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Ecosystem > Inheritance
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Stages of Meiosis


DNA replicates → Identical sister chromatids form


Meiosis I

Meiosis II (same as mitosis)


//Spindle forms
Nuclear envelope disappears
- Chromosomes shorten/thicken/condense
- Form bivalents/tetrads
Crossing-over of homologous pairs

//Spindle forms
//Nuclear envelope disappears


//Spindle complete
- Bivalents at equator
- Join to spindle (fibres) via centromere

//Spindle complete
- Chromosomes at equator


//Cytokinesis begin
Random segregation of homologues
- Intact centromeres
- Two chromatids on one chromosome

//Cytokinesis begins
Random segregation of chromatids
- Chromatids are pulled to opposite poles
- Centromeres divide


//Spindle disappears
//Nuclear envelope reforms
- 2 haploid cells
- Chromosomes still duplicated

//Spindle disappears
//Nuclear envelope reforms
4 haploid daughter cells

Principles of Mendelian Inheritance

  • Mendel's 1st Law of Segregation (Anaphase I and II)
    • During gamete formation, allele pairs (Gg) of one gene separate (G)(g)
    • Thus, only one of the alleles of one gene is present in a single gamete
    • Monohybrid inheritance (single gene - 3:1 ratio)
    • Recessive alleles can cause genetic disorders (e.g. cystic fibrosis)
  • Mendel's 2nd Law of Independent Assortment (Anaphase I and II)
    • Alleles for one gene segregate independently with the alleles of another gene (GgBb)
    • Two genes for each characteristic segregate during gamete production (GB)(gb)(Gb)(gB)
    • Independent assortment means either G / g can go with either of B / b
    • Meiosis separates alleles / homologous chromosomes
    • Dihybrid inheritance (two genes - 9:3:3:1 ratio) → occurs at different loci

Multiple Alleles

  • Human ABO group is controlled by the immunoglobulin gene I
    • The immunoglobulin gene has 3 alleles IA, IB, I0
    • These alleles code for antigen A, B, neither A/B, respectively
  • Only 2 alleles can be present in a diploid cell → IAIB is codominant, I0 recessive

Codominance (1:2:1)

  • Heterozygous allele is neither dominant nor recessive → both alleles are expressed

Sex Linkage

  • e.g. Haemophilia → clotting time of blood is longer than usual
  • Inheritance of sex in humans
    • Females are homogametic sex (X: or XX)
    • Males are heterogametic sex (XY) / Y chromosome is shorter
    • Involves whole chromosomes instead of individual genes
  • Phenotypic characteristic is inherited on X, not on Y chromosome
    • Thus, more common in males / females can be heterozygous (XAXB)
    • Thus, sex linked characteristic is never passed from father to son
  • Evidence from a tree diagram which suggests that a disease is
    • Sex linked: only seen in males / not in females
    • Recessive: unaffected parents

Application of Chi-Squared Test (x²) to Data Obtained

  • Observed Expected value
    • IMG 5-14-1
    • Degree of freedom = n - 1
  • Shows if differences between sets of data are significant or not
  • Null hypothesis states that there are no significant differences between sets of data
  • Small value / probability higher than the level of significance 0.05/5%
    • Little difference between observed and expected value
    • Likely to be extremes of the same population
    • Null hypothesis accepted
  • Large value / probability is less than the level of significance 0.05/5%
    • Significant difference between observed and expected data
    • Likely to be two distinct populations
    • Null hypothesis rejected