OCR Biology A - 6.1.2 - Patterns of Inheritance
A genotype is the genetic makeup of an organism, representing the allele combinations it carries for specific traits.
Genotype
Allele combinations possessed by an organism leading to specific phenotypes
Key Terms
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 ...
Continuous variation
Quantitative differences
Phenotypic diff have a wide range of variation in a pop. (sig affected by environment)
Each allele has a small...
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
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| Term | Definition |
|---|---|
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 >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 |