6.1.2: Patterns of inheritance Flashcards
Types of variation
- Discontinuous
* Continuous
Discontinuous variation
- Qualitative differences between phenotypes
- Clear phenotypic categories (blood group, sex)
- Generally monogenic; if polygenic, genes interact in an epistatic way
- Different alleles at single locus have large effects on phenotype
- Unaffected by environment
Continuous variation
- Quantitative differences between phenotypes
- No distinct categories (height, weight)
- Usually polygenic; each gene provides an additive effect to the phenotype
- Strongly influenced by the environment
Polygenic (of a characteristic)
A characteristic is controlled by two or more genes
Both _______ and ________ contribute to phenotype
Both genotype and environment contribute to phenotype
Example of how genotype and environment contribute to phenotype
Height
⟶ Have genetic potential for certain height
⟶ Malnutrition can mean this potential is not reached
Variation
differences between members of the same species, arising as a result of mutations and essential in natural selection and therefore evolution.
Allele
version of a gene
Dominant allele
version of a gene that will always be expressed if present
Recessive allele
version of a gene that will only be expressed if two copies of the allele are present
Genotype
The genetic information of an organism
Phenotype
The observable characteristics of an organism
Homozygous
Two identical alleles for a characteristic
Heterozygous
Two different alleles for a characteristic
How variation plays a role in selection
- When environment changes, organisms well adapted will survive and reproduce
- They pass on ALLELES to offspring
- This forms the basis of evolution by natural selection
How is genetic variation produced?
- Prophase 1: crossing over of non-sister chromatids –> formation of recombinants
- Metaphase 1: independent assortment –> it is random which chromosomes end up at each pole of the cell
- Metaphase 2: Random orientation –> centromere splits, different alleles to each pole of cells –> large number of possible allele combinations for gametes
- Mutations (random) in DNA replication in Interphase
- Fusion of gametes in fertilisation is random
What is an example of codominant alleles in context of disease
- Sickle cell anaemia
- Missense mutation
- When haemoglobin is deoxygenated, becomes crystalline and deforms RBC, eventually impeding blood flow and leading to tissue damage
- Codominant heterozygotes: typically phenotypically symptomless ∵ presence of normal Hb in RBC prevents sickling
What is sex linkage?
When genes are carried on (either sex chromosome)
⟶ Tend to be carried on the X chromosome because Y chromosome v. short
In a classic Mendelian dihybrid cross
- The 2 genes don’t affect each other
- The 2 genes are not linked on the same chromosome
- Independently assorted at M1 of meiosis
Why do actual ratios differ from expected ratios?
• Random fertilisation
• Genes may be linked
⟶ Larger sample = closer
Recessive epistasis
A homozygous recessive genotype at 1 locus overrides the genotype at the other locus, even if there is a dominant allele present there.
Difference between epistasis and dominance?
Epistasis occurs between 2 genes coding for different characteristics
Dominance occurs between 2 alleles of the same gene
Epistasis
Interaction between gene loci; occurs between two genes coding for different characteristics.
Dominant epistasis
A single dominant allele at 1 locus overrides all alleles at the other locus.
The alleles that are masking the effect are called the…
epistatic alleles
The alleles whose effect is being masked is called the…
hypostatic alleles
Ratio that suggests dihybrid inheritance of 2 unlinked genes
9:3:3:1
Ratio that suggests recessive epistasis
9:3:4
Ratio that suggests dominant epistasis
12:3:1 or 13:3
Ratio that suggests epistasis by complementary action
9:7
9:7 suggests
Epistasis by complementary action
12:3:1 or 13:3 suggests
Dominant epistasis
9:3:4 suggests
Recessive episasis
9:3:3:1 suggests
Dihybrid inheritance of 2 unlinked genes
Hardy Weinberg equation describes populations that
are not evolving
Genotype frequencies stay the same if what conditions are met:
1) Very large population: no genetic drift
2) No gene flow: no emigration or immigrant
3) No mutations: no new alleles added to the gene pool
4) Random mating: no sexual selection
5) No natural selection: all traits aid equally in survival
Hardy Weinberg equilibrium: equations
p + q = 1.00
p² + 2pq + q² = 1.00
In the Hardy-Weinberg equilibrium equations, what does p represent?
Frequency of the dominant allele
In the Hardy-Weinberg equilibrium equations, what does q represent?
Frequency of the recessive allele
In the Hardy-Weinberg equilibrium equations, what does p² represent?
Frequency of homozygous dominant genotype
In the Hardy-Weinberg equilibrium equations, what does q² represent?
Frequency of homozygous recessive genotype
In the Hardy-Weinberg equilibrium equations, what does 2pq represent?
Frequency of the heterozygous genotype
Factors affecting evolution
- Mutation
- Sexual selection
- Gene flow
- Genetic drift
- Natural selection
How do mutations affect evolution?
Mutations lead to new alleles
How does sexual selection affect evolution?
Sexual selection leads to alleles coding for mating success.
Epistasis by complementary action
At least 1 dominant allele at each different loci needed to produce an effect
Why is genetic drift more of a concern in small populations?
- 1 individual or allele has a higher effect proportionally in a smaller population
- Less variation so more vulnerable to environmental change e.g. disease
Why is maintaining genetic diversity good?
- Useful in changing climate
- Prevention of inbreeding depression
- Promotion of hybrid vigour
- Prevent dwindling gene pool
