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Genes and Inheritance II

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  1. Genes and Inheritance II • Revision: most genes come in more than one form (alleles) • New alleles are created by mutation and recombination • Dominant and recessive properties of an allele determine its effect on the phenotype • It’s not always that simple though

  2. Inheritance in families • Many traits can be followed in families (pedigree analysis) • There are many examples, including some human diseases and other conditions • Careful study of the pedigree shows the mode of inheritance (dominant, recessive) • See examples in textbook: figures 10.10 and 10.11

  3. Multiple alleles • A gene can have more than 2 alleles • Levels of dominance are possible • Figure 10.12 – coat colour in rabbits • There is a “hierarchy of dominance” leading to several different coat colours

  4. Thanks to Lauren Spence for the photos Sable Chinchilla Iron grey Himalayan White Dominance of alleles: C > cch > ch > c

  5. Incomplete dominance • Many alleles are not completely dominant or recessive - their effects blend together or mix • Example - the colours of snap-dragon flowers (red/pink/white) • The inheritance still follows Mendel’s laws • Figure 10.13 in textbook

  6. Co-dominance • Co-dominant alleles are ones whose effects can both be seen together in the phenotype • A good example is the human ABO blood group system • This has 3 alleles, IA, IB, IO • Their presence in a person’s blood can be detected using specific antibodies • Figure 10.14 in textbook

  7. Interactions between genes • “Epistasis” is where genes alter the effects of other genes • This is also very common • An example is mouse coat colours (figure 10.15) • Wild-type is agouti (dominant allele B) with bands on the hairs, mouse is grey • bb genotype has no bands on hairs and is black • A second gene (A,a) affects pigment production • Homozygous aa mice are albino (no pigment is produced) so effect of B,b gene cannot be seen • Aa and AA produce pigment so effect of B,b gene is seen

  8. Albino (aa) Agouti (wild-type) Black (bb, not aa) Epistasis - mice coat colours

  9. Polygenic inheritance • Many traits are influenced by several genes together (“polygenes”) • Includes human traits like height, skin colour - these are “continuous” traits (there is a spectrum of values between the 2 extremes) • Probability applies here as well - this is why most individuals are about average with few at the extremes - figure 10.17 (different in Purves 7th edition)

  10. Many genes influence the phenotype in a way modified by the environment Siamese cats again! Pigment produced because enzyme active in cool parts of body If you remove some dark fur then put the cat in a warm environment, fur grows back light-coloured The proportion of individuals carrying the gene that actually show the phenotype is called the “penetrance” Gene-environment interaction