Unit 6 - Patterns of Inheritance Flashcards
Genotype
Allele combinations possessed by an organism leading to specific phenotypes
Discontinuous variation
Qualitative differences Clearly distinguishable categories (categorical) Monogenic inheritance One/two genes An allele has a large effect
Continuous variation
Quantitative differences Phenotypic diff have a wide range of variation in a pop. (sig affected by environment) Each allele has a small effect Polygenic inheritance Large number of diff genes involved
Monogenic inheritance
One gene w/ 2 or more alleles
Monohybrid cross
1 gene, 2 alleles (r and d)
Drawing genetic crosses
Parental genotype
Parental phenotype
Parental gametes
F1 ratio for genotype then phenotypes
Codominant inheritance
Involves more than one dominant allele
Multiple alleles genetic crosses
1 trait
1 gene
>2 alleles
Example of multiple allele genetic cross
Blood group
I A
I B
I O
3 ways genetic variation arises from sexual reproduction
IA of homologous chromosomes (M1)
Crossing over
IA of sister chromatids (M2)
23rd pair of chromosomes
Only pair that varies in shape and size
X - v. large and doesn’t carry genes involved in sexual development
Y - V. small, no genetic info, but carries gene that causes formation of male embryos
Sex linked genes
Characteristics determined by genes carried on X and Y
Why do sex-linked genes affect males
Y is much smaller so only has one copy of the gene, if recessive allele is found on X but no D allele on Y, male will express the recessive trait (usually condition)
Most females will have a D allele present on the 2nd X chromosome so are either normal or a carrier
Examples of sex-linked conditions
Haemophilia - blood clots v. slowly due to a lack of protein blood clotting factor
Red-green colour blindness
Dihybrid cross
Used to show inheritance of 2 diff characteristics, 2 genes at diff loci, >2 alleles on each
Expected results of a heterozygous dihybrid cross
9:3:3:1
Why may the actual ratio vary from expected
Fertilisation is random
If there is no crossing over, alleles for 2 characteristics will be inherited together if on same chromosome
Autosome
Any chromosome that is not a sex chromosome
Autosomal linkage
2 separate genes are found on the same autosome
Represented by diff letters
Linked genes are inherited together so offspring usually show same combination as parents (certain gametes are more common)
W/ no crossing over in autosomal linkage
Gametes stay in parental comb. and offspring show 3:1 phenotypic ratio
What may prevent linked genes from being inherited together
If they’re separated by chiasmata
W/ crossing over in autosomal linkage
Genotypic and phenotypic ratios are variable
Parental types > cross-over type
Proportion depends on how often cross overs ocurred between two loci
Recombinant offspring
Offspring w/ a diff combination of alleles to either parent
Closer genes are located on a chromosome …
Less likely to be separated during crossing over –> fewer recombinant offspring
Recombination frequency
Measure of amont of crossing over occured in meoisis - indicating level of linkage
Also used to map genes loci ; 1% = distance of 1 map unit on chromosome
Calculating recombinant frequency
No of recombinant offspring/ total no. of offspring
50% recombination frequency
No linkage, separate chromosomes
<50% recombination frequency
Gene linkage and IA has been hindered
Signifies autosomal linkage
Linked genes are inherited together
Crossing over produces few recombinant offspring
Homozygous
Has identical alleles on both chromosome
H0 in chi squared
There is no sig. difference between expected and observed values
Degrees of freedom in chi squared
No. of categories - 1
Epistasis
Interaction of genes at diff loci
Genes masking the expression of other genes (not alleles)
Gene regulaion is a example w. reg . genes controlling structural genes
When can epistasis be seen
Multistep reactions
Hypostatic
Gene affected by another gene
Cause the phenotype
Epistatic gene
Gene that affects the expression of another gene; can happen as a result of dominant or recessive alleles
Epistatic alleles
Another pair of alleles found at diff loci
Antagonistic epistasis
Dominant and recessive epistasis
Dominant epistasis
If there are ANY dominant alleles present in the epistatic alleles, masks expression of hypostatic alleles
Phenotypic ratio in a heterozygous dihybid cross w/ dominant epistasis
12:3:1
Recessive epistasis
Occurs when a pair of homozygous recessive alleles at one gene locus masks the expression of the hypostatic allele at a 2nd locus
Phenotypic ratio in a heterozygous dihybrid cross w/ recessive epistasis
9:3:4
Bivalent
Homologous pair of chromosomes
Chiasmata
Point representing where homologous touch and exchange genetic info
Gene pool
Total no.of genes and their alleles in a particular population
Assumptions of the Hardy-Weinberg Principle
Pop is v. large (reduced effect of genetic drift)
Mating within pop. is random - no selective breeding
No selective advantage for any genotype coded for by that allele
No mutation
No migration
Gene pool is stable
Hardy Weinberg principle
A is dominant, p = freq. of A
a is recessive, q = freq. of a
p + q = 1
p^2 + 2pq + q^2 = 1
When to use p + q = 1
When given allele frequency
When to use p^2 + 2pq +q^2
When given phenotypes
Evolution
Changes in allele frequencies over time leading to changes in species
What can affect allele frequencies
Mutations - new advantageous alleles will remain in pop
Natural selection
Effects of small population
Genetic drift
Artificial selection and selective breeding
Selection
Increase in allele frequency
Stabilising selection
Selection pressure toward the centre increases no. of individuals at the modal values
Extreme values are selected against and lost
Types of selection
Stabilising
Directional
Disruptive
Directional selection
Selection pressure towards one extreme moves the mode in this direction
Extreme value is advantageous; more likely to survive and reproduce
Disruptive selection
Selection pressure toward the extremes creates two modal values
Intermediae values selcted against - lose those alleles
Creates two distnct populations
e.g. Darwin’s finches
Genetic drift
Random events causing changes in allele frequencies
Effects are greatly increased in small pop or small gene pools
Alleles in new generation will therefore be the genes of the ‘lucky’ individuals and not necessarily healthier individuals
Polymorphic
Genes w/ > 1 allele
Effects of small populations
Founder effect and genetic bottleneck reduce genetic diversity by creating small populations
Founder effect
Occurs when a small group of migrants that aren’t genetically representative of the pop. from which they came from, establish in a new area
New population is v. small w/ an increase in inbreeding and relatively low genetic variation
Why does inbreeding cause genetic diseases
Increases impact of recessive alleles and most genetic diseases are caused by recessive alleles
Genetic Bottleneck
Big events that cause drastic reduction in a parent pop leaving a surviving pop w/ v. low genetic diversity (unless they mutate)
Events that may cause genetic bottleneck
Overhunting to the point of extinction
Habitat destruction
Natural disasters
Process leading to Genetic Bottleneck
Orig population Large no. die Reduced population (some alleles lost) Reproduction New population w/ low genetic diversity
Order of conservation
Habitat
Population
Genes
Artifical selection and selective breeding
Humans use animal and plant breding to selectively develop particular phenotypic ratios by choosing spp individuals
Occurs over several generations
Agent of selection in natural selection
Environment
Agent of selection in artifiicial selection
Human
Effect of allele frequencies in selection
Changes for both natural and artificial
Effect of evolution due to natural selection
Drives it
Effect of evolution due to artificial selection
Drives it then slows it down
Speed of natural selection
Slow
Speed of artifical selection
Fast
Ethical considerations w artificial selection and selective breeding
Health problems; certain traits may be exaggerated
Reduction of genetic diversity - more susceptible to genetic diseases caused by r alleles, potentially useful alleles for the future lost
Speciation
Formation of new and distinct species through the course of evolution
Factors that may cause directional selection
Predation
Habitat changes
Competition
Environments that cause directional selection
Slowly changing environmental conditions in one direction
‘Ingredients’ for speciation
Existing genetically varying poulation Isolation: geographical or reproductive Time Different selective pressures Large change in allele frequencies
Why do you need diff selective pressures for speciation
Changes allele frequencies in diff directions
Allopatric speciation
Geographically isolated
Gene pool is physically separated so the sep pop can then evolve independently of each other
What causes changes in allele frequency in allopatric speciation
Accumulation of diff mutations forms separate gene pools
Different biotic/ abiotic factors
Differential reproductive successes
Sympatric speciation
Reproductively isolated
Organisms inhabiting same area separated into 2 or more groups due to changes in alelles and phenotypes preventing them from successfully breeeding together
Examples of things causing reproductive isolation
Seasonal changes (Different flowering seasons) Mechanical changes (Changes in genitalia) Behavioural changes (Diff courtship rituals)
How does the presence of epistatic alleles inhibit the expression of the hypostatic allele
Epistatic allele codes for repressor protein/ TF
Product of epistatic allele binds to promoter of hypostatic allele
Product stops transcription or inhibits enzyme action of enzyme encoded by A
Causes of variation in continuous variables e.g. height
Environment
Age
Polygenic
Result of speciation
Gene flow restricted
Leads to diff specialisation
