Patterns of Inheritance Flashcards
What is the difference between homo and heterozygous?
Homo - Two identical alleles for a characteristic
Hetero - Two different alleles for a characteristic
What is the difference between continuous and discontinuous variation?
Continuous - Characteristic that can take any value within a range
Discontinuous - Characteristic that can only appear in specific/discrete values
What is the cause of variation, genetic control and examples in continuous variation?
Cause - genetic and environmental
Genetic control - Polygenes - controlled by a number of genes
Examples - leaf surface area, mass, skin colour
What is the cause of variation, genetic control and examples in discontinuous variation?
Cause - mostly genetic
Genetic control - one or two genes
Examples - blood group, albinism, round or wrinkled pea shape
What is monogenic inheritance?
An inheritace pattern where a single gene codes for a certain attribute
In what scenario would an offspring have a 100% change of being heterozygous?
When the parental genotypes are homozygous recessive and homozygous dominant
Explain codominance and give an example
- Occurs when two different alleles occur for a gene, and are both dominant
- Both alleles of the gene are expressed in the phenotype if present
- eg pink snapdragon flowers when the red and white allele is crossed
How is codominance written/represented?
With a big letter C that is the same for both characteristics, and the different characteristic represented as a smaller letter, like a to the power in maths
Explain what is meant by multiple alleles and give an example
- One gene, but more than two alleles
- Codominance can come within this
- eg blood type, represented by a big I with A,B, O (antigens) to the powers
Explain sex linkage and why males are more likely to be affected by alleles on the X chromosome
- Genes found on sex chromosomes (X an Y)
- Males have XY, whereas females have XX
- The Y chromosome is much smaller, so there is less likely to be the same gene with an allele to counteract it
What is dihybrid inheritance?
- Shows the inheritance pattern of 2 genes
- Each gamete carries one allele of both genes
- 4 possible phenotypes
What is the typical ratio when two heterozygous genotypes are crossed in dihybrid inheritance?
9:3:3:1
Why may there be unexpected ratios when gametes are crossed?
- Random fertilisation (egg and which sperm cell fused)
- Autosomal linkage (genes found on the same chromosome that are inherited together, no crossing over occurs to separate these genes)
What does the chi squared (X2) test test for?
- To find significant difference between expected and observed ratios of offspring
- To test if there is autosomal linkage or epistasis
When is there a significant difference between observed and expected ratio?
- When the chi squared value is bigger than the critical value
What is meant by recombinant offspring?
Different combinations of alleles from either parent due to crossing over
What is epistasis?
- Interaction of genes at different loci (different locations on different chromosomes)
- For example, gene A codes for protein A which could be an activator protein which would bind to gene B and cause it to be expressed
What is meant by dominant epistasis?
- A dominant allele results in a gene having an effect on another gene
- If an epistatic gene codes for an enzyme that modified one of the precursor molecules in the pathway
- The next enzyme in the pathway would therefore lack a suitable substrate molecule so pigment would not be produced
- All the genes in this sequence would be masked
What would be recessive epistasis?
If the presence of two recessive alleles at a gene locus led to the lack of an enzyme
What does the Hardy-Weinberg principle state, and what assumptions does it follow?
- In a stable, non-evolving population, allele frequencies stay constant
- Assumes that there is a large population size, random mating, no mutations and no selection pressure, so no evolution
Interpret the 2 Hardy Weinberg formulas
p + q = 1
where p = dominant allele frequency, and q = recessive allele
can be used to find p^2+2pq+q^2=1
p^2/q^2 = frequency of homozygous dominant/recessive genotype in population
2pq = frequency of heterozygous
What are the 4 factors affecting evolution?
Mutations
Changes in population size
Genetic drift
Selection of favourable alleles (sexual and natural selection)
In what 2 ways do changes in population size affect evolution?
density dependent factors affect population due to its size eg predation. Density independent factors affect population regardless of size, eg natural disaster
Explain genetic drift
- Occurs in small populations
- Change in allele frequency due to random mutation
- New allele will have large impact due to small population
- Can lead to founder effect, when a mutation from an existing individual gives rise to a very favourable gene/allele (rare), which outcompetes rest of population, forming a new colony over time
Explain the 3 types of natural selection
Stabilising selection - average phenotype selected/rises, shift in distribution where there are less individuals with extreme phenotypes, spike in middle of normal distribution curve
Directional selection - one extreme phenotype selected
Disruptive selection - two extreme phenotypes selected, very rare
Define a species
Organisms that can interbreed to make fertile offspring
Describe allopatric speciation
- Physical barrier a population into two different areas such as a mountain
- Therefore two different selection pressures such wet on one side, dry on the other
- Other genotypes, for phenotypes more favoured so favourable allele and more likely to survive and reproduce
- Change in allele frequency in the gene pool
- Characteristics accumulated until a new species is formed
Explain sympatric speciation
- Occurs within the same habitat
- Disruptive selection or mating preferences that have caused a new species to arrive in the same habitat
- Much more likely in animals than plants, but still rare in animals
