Starter Activity http www youtube comwatch v8 b
Starter Activity • http: //www. youtube. com/watch? v=8 b. B 9 PPjp 04 o • In this video clip, the father is described as being ‘black’. (Q 1) Are both of his parents black? Explain your answer.
What is polygenic inheritance? • Where a single characteristic is controlled by multiple genes with a cumulative effect • Polygenic inheritance gives rise to continuous variation in the phenotype e. g. human skin colour
(Q 2)What is melanin and what purpose does it serve? • Melanin is one of the pigments that gives human skin, hair, and eyes their colour. Dark-skinned people have more melanin in their skin than light -skinned people have. Melanin gives protection against the harmful effects of the Sun.
Polygenic inheritance of skin colour • Skin colour is thought to be controlled by up to four separate genes, each with their own alleles. This is very complicated, so we’ll look at two genes (A & B) with two different alleles each (A or a & B or b). • The genes are not linked. They are on separate chromosomes. A a In polygenics, alleles can be: • Contributing (they add to the phenotype) • Non-contributing (do NOT add to the phenotype) B b Key to alleles: A = add melanin a = don’t add melanin B = add melanin b = don’t add melanin
Task: (Q 3) How many gametes are possible from the following genotype? A a B b
(A) The gametes A A B a b a B b
(A) Why so many combinations? • Alleles segregate during meiosis • Alleles of unlinked chromosomes orient randomly (independent assortment) • Crossing over during prophase 1. • There is also random fertilisation of gametes http: //www. sumanasinc. com/webcontent/animations/content/i ndependentassortment. html
Now lets consider the phenotypes… • • • In polygenics, alleles can be: – Contributing (they add to the phenotype) – Non-contributing (they don’t add to the phenotype) Q 4. Which genotype will give A: add melanin rise to people with the a = don’t add melanin darkest skin? B: add melanin Q 5. Which genotype will give b = don’t add melanin rise to people with the palest skin?
Probability Task: Polygenic inheritance gives rise to continuous variation in the phenotype aabb 0 units a. Ab. B Aab. B aa. BB aa. Bb a. Abb Aa. Bb Aabb AAbb 1 unit 2 units Palest skin a. ABB Aa. BB AAb. B AABb AABB 3 units 4 units Darkest skin
(Q 6) How would you describe this population? It follows a normal distribution
(Q 7) Express the phenotypic ratio. Phenotype: 0 units: 1 unit: 2 units: 3 units: 4 units Ratio: 1 : 4 : 6 : 4 : 1
Do you recall the starter activity? (Q 8) Explain why it is possible for non-identical twins to be different colours. Clue: Think about the gametes.
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