Genes and Inheritance II

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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
• Its not always that simple though

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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

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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

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Thanks to Lauren Spence for the photos
Sable Chinchilla Iron grey
Himalayan White
Dominance of alleles C gt cch gt ch gt c
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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 Mendels laws
• Figure 10.13 in textbook

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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 persons blood can be
  detected using specific antibodies
• Figure 10.14 in textbook

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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

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Epistasis - mice coat colours
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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)

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Gene-environment interaction

• 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