🔴️Stabilising selection
- occurs when the environment favours those with the most common (mean) characteristic
- those on the extreme dies out
- the frequency of the common characteristic increases
- the range of characteristic (standard deviation) will reduce
🔴️Despite being 25% likely to inherit a ff allele, why doesn’t this happen?
1 Random fusion of gametes
2 Small sample
3 Gametes not produced in equal numbers
4 Offspring ratios are probabilities not fixed
Recessive allele
Only expressed in phenotype if homozygous;
Co-dominant allele
If both present, then both are expressed in phenotype
Why are genetic disease more prevalent in small, isolated populations?
1 Reproductively isolated so there is a small gene pool.
2 Inbreeding may occur so there is a high probability of mating with person having recessive allele
Why is the actual popluation without the disease higher than predicted by the H-W calculation
1 People with the disease may die young so they may not reach reproductive age (adult)
2 There is a selective disadvantage
3 Genetic screening helps prevent genetic diesease being perpetuated.
Describe and explain speciation
1 geographical isolation of organism;
2 Separate gene pool as no interbreeding between organisms
3 Different environmental conditions in each place
4 This means there are different selection pressures in separate populations;
4 Variation in population due to chance mutation occurs, causing organisms to acquire advanatageous allele
5 This advantageous phenotype makes indivudials more suited to their environment, and so more likely to survive and repoduce
6 The beneficial allele more likely to be passed on, increasing the frequency of the allele;
7 Over a long time, the populations have changed so much that they’re unable to produce fertile offspring
Explain why Males are more likely to show a phenotype produced by a recessive allele carried on the X chromosome.
1 Males have one allele** for each gene so recessive alleles **always expressed in Male;
2 Females need two recessive allele (homozygous recessive)
3 females recessive alleles can be masked by dominant allele
Explain Phenotype
- Expression of a characteristic
- due to genotype and environment;
Dominant allele
characteristic Is always expressed in the phenotype when present
What is a sex-linked gene?
a gene carried on one of the sex chromosomes, normally the X chromosome
Directional selection
- When the environment favours those with a feature/allele as it confers advantage
- so more likely to survive and reproduce;
- The allelic frequencies increases so more individuals with feature
Species
- group of organisms with similar physiological features;
- which can interbreed to produce fertile offspring;
An allele may be present in the genotype but its effects are not seen in the phenotype. In terms of protein production, explain why
- Effect of recessive allele not seen when dominant allele present
This is true if the:
- Recessive allele codes for non-functional protein;
- Dominant allele codes for functional protein;
- Therefore the effect nullifies the recessive
Homologous chromosomes
- Capable of pairing during meiosis
- Same gene loci;
What’s the H-W principle
- The proportion of alleles** of a particular **gene
- Will stay constant from one generation to the next
- Providing no mutation, mating at random and no migration;
How do you know a disease is caused by a recessive allele?
- 2 unaffected parents (X,Y) give birth to (Z) affected
- Parents must both be heterozygous
How do you know if a disease is caused by a gene on the X chromosome
- Uaffected fathers (XY) give birth to affected sons
- Mother gives the X chromosome to son and father gives the Y
Explain how mutation may produce multiple alleles of a gene.
- Changes base sequence
- In different places within gene;
