BYA5 SECTION 14.1Continuity is maintained by the transmission of genetic
information from generation to generation |
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Table 5-14-1: Stages of meiosis
| Interphase |
DNA replicates → Identical sister chromatids form |
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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 |
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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)
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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
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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
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Codominance (1:2:1) |
- Heterozygous allele is neither dominant nor recessive
→ both alleles are expressed
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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
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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
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References and Further Reading
AQA (2006) GCE Biology/Biology (Human) 2006 specification, [PDF]
