Variation Polygenic inheritance mutations Ch 7 PP 4

Variation, Polygenic inheritance, mutations, Ch 7 – PP 4

Key terms • Monohybrid cross • Autosomal dominant/recessive • True breeding • Test cross • Parent generation • F 1 generation • Filial generations • • • Partial dominance Co-dominance Multiple alleles Lethal alleles X Inactivation

Genetic variation • Genetic variation is necessary for evolution to take place (later in Unit 4) • Variation occurs in a number of ways • Asexual reproduction – cloning of parent DNA – no variation • Sexual reproduction – mixes alleles from both parents • During meiosis, crossing over adds to variation

Polygenic inheritance • Many characteristics are controlled by more than one gene interacting with each other • This is called a polygenic characteristic • Eg: Skin colour in humans, or height • The range of skin tones we see can be explained with multiple genes

Polygenic inheritance Aa. Bb. Cc x Aa. Bb. Cc Gametes ABC Abc Ab. C Abc a. BC a. Bc ab. C abc ABC 6 5 5 4 4 3 ABc 5 4 4 3 3 2 Ab. C 5 4 4 3 3 2 Abc 4 3 3 2 2 1 a. BC 5 4 4 3 3 2 a. Bc 4 3 3 2 2 1 ab. C 4 3 3 2 2 1 abc 3 2 2 1 1 0

Variation • Continuous variation – multiple genes involved – range of phenotypes • Discontinuous variation – one gene involved – small number of phenotypes

Mutations • Gene mutation is the process that produces new alleles of genes in various species and so produces a new genetic variation • Sometimes whole chromosomes are affected • The DNA then carries a new set of instructions • Mutations may be spontaneous or induced by mutagenic agents Eg X rays or benzene, • May be neutral, beneficial or harmful

Mutations • Mutations can occur in somatic (body) cells, in which case the trait is not heritable • Germ line mutations – that is mutations that occur in the gametes are passed onto the next generation • Covered in Ch 8

Environmental effects on heredity • Even though identical twins have the same genotype, they often show variations in their phenotype. Eg slight difference in weight, height • Environmental conditions affect gene expression

Activities Worksheet • Polygenes Plus 6 past exam questions

2006 Exam 2 2008 Exam 2 2007 Exam 2

2007 Exam 2

2011 Exam 2