Download c ch

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
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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
Thanks to Lauren Spence for the photos
Sable
Chinchilla
Iron grey
Himalayan
Dominance of alleles: C > cch > ch > c
White
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
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“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
Albino (aa)
Black (bb, not aa)
Agouti (wild-type)
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
• The proportion of
body
individuals carrying the
• If you remove some dark fur
gene that actually show
then put the cat in a warm
the phenotype is called
environment, fur grows back
the “penetrance”
light-coloured