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BYA5 SECTION 14.1

Continuity is maintained by the transmission of genetic
information from generation to generation

Table 5-14-1: 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)
  • \ 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
    • \ more common in males / females can be heterozygous (XAXB)
    • \ 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

            

    • 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

References and Further Reading
AQA (2006) GCE Biology/Biology (Human) 2006 specification, [PDF]

BYA5 SECTION: 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9 14.10
 

 

 

 

 

 

 
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