Inheritance

Stages of Meiosis

Interphase	DNA replicates → Identical sister chromatids form
	Meiosis I	Meiosis II (same as mitosis)
Prophase	//Spindle forms Nuclear envelope disappears - Chromosomes shorten/thicken/condense - Form bivalents/tetrads - Crossing-over of homologous pairs	//Spindle forms //Nuclear envelope disappears
Metaphase	//Spindle complete - Bivalents at equator - Join to spindle (fibres) via centromere	//Spindle complete - Chromosomes at equator
Anaphase	//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
Telophase	//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