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Genes and Inheritance II PowerPoint PPT Presentation


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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 It’s not always that simple though. Inheritance in families.

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Genes and Inheritance II

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Genes and inheritance ii l.jpg

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


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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 > cch > ch > 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 Mendel’s 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 person’s 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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Albino (aa)

Agouti (wild-type)

Black (bb, not aa)

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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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


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