6.1.2 Flashcards
What is codominance?
Codominance is when two different alleles occur for a gene - both being equally dominant. This means that both alleles are expressed in the phenotype of the organism if present.
What is incomplete dominance?
Incomplete dominance is when two different alleles occur for a gene, but are only partially expressed in the phenotype. E.g. a white plant and a red plant being crossed to form a pink plant.
What are sex-linked characteristics?
Sex-linked characteristics are those that are determined by genes carried on the sex chromosomes.
In humans, what sex chromosome is larger?
X chromosome
What does the X chromosome being larger mean for sex-linked characteristics?
There are some genes in the X chromosome that males only have one copy of, meaning that if a gene is present in the X chromosome, it will be expressed whether dominant or recessive, if the Y chromosome does not have that gene.
Give an example of a sex-linked genetic disorder
Haemophilia
What is a dihybrid cross used for?
It is used to show the inheritance of two different characteristics, caused by two genes, which may be located on different pairs of homologous chromosomes.
What is an autosome?
Non-sex chromosome
What is autosomal linkage?
When two genes are linked by being on the same chromosome
If two genes are very far apart on the same chromosome, are they weakly or strongly linked?
Weakly
How does autosomal linkage affect independent assortment?
There is no independent assortment, as the linked genes are inherited as one unit
What is the only way autosomal linked genes can be separated?
Crossing over
What are recombinant offspring?
Offspring that are produced by crossing over
What is recombination frequency?
A measure of the amount of crossing over that has happened in meiosis
How is recombination frequency calculated?
Number of recombinant offspring/total offspring
What does a recombination frequency of 50% indicate?
There is no linkage and the genes are on separate chromosomes
What does a recombination frequency of <50% indicate?
There is gene linkage and the independent assortment has been hindered
What statistical test can be carried out to determine whether the observed frequencies are significantly different to the expected values?
Chi-squared
What is epistasis?
Epistasis is the interaction of genes at different loci
What is dominant epistasis?
Dominant epistasis is when a dominant allele results in a gene having an effect on another gene, masking said gene
What is a gene pool?
The sum total of all the genes in a population at any given time
What is the allele frequency?
The relative frequency of a particular allele in a population
What is the Hardy-Weinberg principle?
It states that in a stable population with no disturbing factors, the allele frequencies will remain constant from one generation to the next and there will be no evolution
What conditions are required for the Hardy-Weinberg principle to hold true?
- No mutations
- No immigration/emigration
- No selection
- Mating is random
- The population is large
How can the Hardy-Weinberg principle be expressed?
p^2 + 2pq + q^2 = 1
What factors affect evolution?
Mutation
Sexual selection
Gene flow
Genetic drift
Natural selection
What is gene flow?
The movement of alleles between populations. This is caused by immigration and emigration
What is genetic drift?
A change in allele frequency due to the random nature of mutation
What is the founder effect?
When a few individuals of a species colonise a new area, their offspring initially experience a loss in genetic variation, and rare alleles can become much more common in the population
What is the bottleneck effect?
When a major disaster of event kills or a large number of the population, so only few individuals survive.
What is stabilising selection?
When the norm of average is selected for and the extremes are selected against
What is directional selection?
When there is a change in the environment and the normal phenotype is no longer the most advantageous
What is disruptive selection?
When the extremes are selected for and the norm is selected against
What is speciation?
Formation of new species through the process of evolution
What events lead to speciation?
Members of a population become isolated and no longer interbreed with the rest of the population, resulting in no gene flow.
Alleles within the groups continue to undergo random mutations. Environments of each group may be different or change.
The accumulation of mutation and changes in allele frequencies over many generations lead to changes in phenotype
What is allopatric speciation?
Allopatric speciation is the more common form of speciation and happens when some members of a population are separated from the rest of the group by a physical barrier, so they are geographically isolated.
What is sympatric speciation?
Sympatric speciation occurs within the population that share the same habitat. It is reproductive speciation
What is artificial selection?
The selection for breeding of plants or animals with desirable characteristics by farmers or breeders.
What is inbreeding?
The breeding of closely related individuals.
