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”