genes and inheritance ii revision: most genes come in more than one form (alleles) new alleles are...
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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
• 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
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
Dominance of alleles: C > cch > ch > c
Sable Chinchilla Iron grey Himalayan White
Thanks to Lauren Spence for the photos
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
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
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
Epistasis - mice coat colours
Agouti (wild-type)
Albino (aa)
Black (bb, not aa)
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)
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”