Inheritance - BiologyGuide

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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

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

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

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

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

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