Lecture 3 - 4
Extensions and Modifications of Basic Principles
Penetrance and Expressivity
●
●
The same genotype does not always produce the same phenotype
Cause of incomplete penetrance and expressivity - due to the effects of other genes and to
environmental factors that can alter or completely suppress the effect of a particular gene
●
Penetrance - the proportion of individual organisms having a particular genotype that express the
expected phenotype
○ Variation in the population
○ Ex. incomplete penetrance - polydactyly
●
Expressivity - the degree to which a phenotype is expressed (mild to severe)
○ Variation in the individual
○
Examples
1
Environmental factors
●
●
Age, sex, temperature, chemicals
Range of phenotypes expressed by a single genotype under different environmental condition is
referred to as the norm of reaction
●
Temperature - himalayan allele - low temp produces dark fur at the extremities of the body
○ Enzyme necessary for pigment production is inactivated at higher temperatures
○ Ex. himalayan allele in rabbits produces dark fur at extremities of the body - dark pigment only
develops at low temperatures
○ Ex. siamese allele, C^s, is allele of the tyrosinase gene - protein is inactive at higher
temperature leaving a light brown background - tips of extremities which are much cooler
enzyme is active and produces normal amounts of pigment
●
Occasionally environmental factors alone produce a phenotype that is the same as the phenotype
produced by a genotype
○ Phenocopy - a change in phenotype arising from environmental factors that mimic the effects of
a mutation in a gene
○ Ex. the chemical thalidomide can produce a phenocopy of a raw dominant trait called
phocomelia n which limb development is disrupted
Many people inherit genes that lead to a propensity for heart disease, but regulating diet and exercise
can affect the occurrence (penetrace) and seriousness (expressivity) of the disease
●
Mendel’s Law of Independent Assortment
●
●
The inheritance pattern of one trait will not affect the inheritance pattern of another trait
Examined by dihybrid crosses (ex. Aa Bb X Aa Bb)
Genetic Interaction
●
●
Different combination of alleles from 2 or more genes can result in different phenotypes, because of
interactions between their products at the cellular or biochemical level
What happens when multiple genes often contribute to a single characteristic?
○ Complete dominance
○ F2 phenotypic ratio = 9:3:3:1
2
Complementation
●
●
●
●
●
●
Occurs when 2 strains of an organism with different homozygous recessive mutations that produce the
same phenotype, produce offspring of the wild-type phenotype when mated or crossed
Only occurs is mutations are in different genes
Other genone supply the wild type allele to complement the mutated allele
Ratio 9:7
Ex: Albino cats
In human pedigrees
○ About 50 genes have recessive mutant alleles that can cause deafness in humans
○ A heterogeneous trait: a mutation in any 1 of a number of genes can give rise to the same
phenotype
3
Genetic Epistasis
●
Def. the masking of the expression of 1 gene by another. No new phenotypes are produced
○ “Epistatic gene” does the masking
○ “Hypostatic gene” is masked
●
Recessive
●
Molecular Mechanism:
○ c/c: no pigment is synthesized (mice are white regardless of genotypes at the A locus)
○ Genotype at the A locus determines how to pigment is deposited (either black aa or agouti A-)
●
Dominant
4
Summary of F2 ratios
Pleiotropy
●
●
●
A single gene can be responsible for a number of distinct and seemingly unrelated phenotypic effects
Sickle cell disease
○ Respiratory problems, sickled cells, chronic infections, joint pain, enlarged spleen, stroke…
Cystic fibrosis
○ Mucus close the lungs and leads to infections
○ Mucus obstructs the pancreatic ducts creates digestion problems
Key points
Inbreeding
●
●
●
Inbred lines of experimental species are often
less vigorous than hybrid lines
Inbred lines of self-fertilized plants are
homozygous for allele that were present in the
founding line
When 2 different inbred lines are crosses, the
hybrids are heterozygous for many genes
○ These display heterosis or hybrid vigor
●
Overall increases the frequency of homozygotes and decreases the frequency of heterozygotes
5
Hardy Weinberg Principle
●
Assumption
○ Population must be large
○ Randomly mating
○ Unaffected by mutation, migration or natural selection
●
If assumptions are met
○ Prediction #1 - the allelic frequencies of a population do not change
○ Prediction #2 - phenotypic frequencies stable (will not change) after one generation in the
proportions p^2 (frequency of AA), 2pq (frequency of Aa) and q^2 (frequency of aa), where p
equals the frequency of allele A and q equals the frequency of allele a
●
Can use allele frequencies in genetic counseling - example: Tay Sachs disease
6
Sex determination and Dosage compensation
●
Mechanisms of sex detemrination
○ Genetic
○ Environmental
○ Genetic + environmental
●
In humans: the X and Y chromosomes pair
during meiosis, even though they are not
homologous (the genes located on each are
different)
● Several different mechanisms of sex
●
●
detemrination
XX-XO system
○ XX female
○ XO male
○ Grasshoppers
XX-XY system
○ XX or XO female
○ XX or XXY male
○ Mammals
●
Dosage compensation - way of equalizing gene expression in the face of different gene dosage
●
X inactivation
○ Early in development, one X chromosome is inactivated in each cell - the process is random, it
is equally likely that the maternal or paternal X chromosome will be inactivated
○ Females that are heterozygous for x-linked traits are genetic mosaics
○
○
○
Lyon hypothesis - the random inactivation of 1 female X chromosome
■ If a cell contains more than 2 X chromosomes, all but 1 of them are inactive
■ XO females have on Barr Bodies, XXY males have 1 barr body, XXX females have 2
barr bodies
Therefore, all females are functionally hemizygous for X-linked genes at the cellular level
■ Approx 50% cells express 1 allele, and 50% cells will express the other allele
What happens if there are genes on the X chromosome?
■ Ex - cali cats
● Orange gene is on x chromosome (O = orange / o = black)
7
●
Example of mosaics in humans for X-linked genes red green colourblindness
○ Females can be coloublind in only 1 eye
8