10 Genetics (HL) Flashcards
In which stage of interphase is DNA replicated?
S phase
Producing two identical sister chromatids
When are sister chromatids separated (meiosis)?
Meiosis II
When are homologous chromosomes separated (meiosis)?
Meiosis I
What are the stages of Meiosis I?
Prophase I - chromosomes condense, nuclear membrane dissolves and crossing over occurs
Metaphase I - spindle fibres from centrosomes connect to bivalents (at centromere) and allign them in the middle of the cell
Anaphase I - spindle fibres contract and split the bivalent, homologous chromosomes are moved to opposite poles
Telophase I - chromosomes decondense, nuclear membrane may reform, cell divides to form two haploid daughter cells
What are the stages of Meiosis II?
Prophase II - chromosomes condense, nuclear membrane dissolves centrosome move to perpendicular from before
Metaphase II - spindle fibres attach to centromere and line up chromosome on the equator
Anaphase II - spindle fibres contract, separating sister chromatids which move to the poles
Telophase II - chromosomes decondense, nuclear membrane reforms, cells divide into 4 haploid daughter cells
Define chiasmata
X shaped points of attachment between two non-sister chromatids - crossing over
When does crossing over occur (meiosis)?
Prophase I
What name is given to a daughter chromosome which underwent crossing over?
Recombinant
Define linked genes
Genes whose loci are on the same chromosome and hence don’t independently assort
How may the linked genes (AaBb) be represented?
AB
ab
Define recombinants of linked genes
Combination of linked genes not found in parents
What is the expected ratio for a dihybrid cross of unlinked traits?
9:3:3:1
Define monogenic traits
Characteristics controlled by a single gene loci (tend to be discrete)
Define polygenetic traits
Characteristics controlled by more than two gene loci (tend to be continuous)
Which 5 processes can lead to a change in allele frequency in a population?
Mutation
Gene flow (immigration/emigration)
Sexual reproduction
Genetic drift (chance/random event)
Natural selection (environmental pressure)
Define genetic drift
When a change in the composition of a gene pool occurs due to a random event or by chance
What are the two mechanisms by which population change may occur?
Population bottlenecks and the founder effect
What is a population bottleneck?
When an event reduces the population by an order of magnitude (~>50%)
The smaller population has less genetic variability and will be subject to a higher level of genetic drift
What is the founder effect?
When a small group breaks away from a larger group to colonise a new territory
There will be less genetic variation in this founder group and thus higher genetic drift
What are the three types of selection?
Stabilising
Directional
Disruptive
Describe stabilising selection
Where an intermediate phenotype is favoured at expense of both phenotypic extremes (removal of extremes)
Operates when env conditions are stable and comp is low
Describe a stabilising selection distribution curve
Gets thinned and taller from the middle out
Describe directional selection
Where one phenotypic extreme is selected at the cost of the other in response to gradual or sustained environmental change (usually followed by stabilising selection)
Describe a directional selection distribution curve
Shifts either far right or left
Describe disruptive selection
Both phenotypic extremes are selected for at the expense of the intermediate when fluctuating conditions favour the presence of two phenotypes
Can eventually lead to speciation
Describe a disruptive selection distribution curve
Central distribution decreases for two peaks on either side of it
What are the two types of reproductive barriers?
Prezygotic isolation
Postzygotic isolation
Give examples of prezygotic isolation
Temporal
Behavioural
Geographical/ecological
Mechanical
Give examples of postzygotic isolation
Inviability
Infertility
Breakdown is hybrid organisms
What are the two types of speciation?
Allopatric (geographical isolation)
Sympatric (reproductive isolation)
What is the most common cause of sympatric speciation?
Meiotic failure that makes reproduction no longer viable (plants may still clone despite this and therefore this is more common amongst plants)
What are the two methods for speciation?
Phyletic gradualism
Punctuated equilibrium
Describe phyletic gradualism
Speciation occurs uniformly, steadily and gradually (many intermediate forms)
Describe punctuated equilibrium
Species remain stable for long periods before undergoing rapid change (speciation)
Sudden big changes
