Chapter 2 - Meiosis and Variation Flashcards
Describe the events in meiosis 1
PROPHASE 1:
-chromatin condenses & undergoes supercoiling
-homologous pairs of chromosomes come together to form a bivalent
-non sister chromatids attach at points called chiasmata
- nucleolus disappears and the nuclear envelope disintegrates.
- a spindle forms made out of protein microtubules.
METAPHASE 1:
-bivalents line up across the equator of the spindle, attached to the centromere by spindle fibres.
-each member of the homologous pair faces opposite poles.
ANAPHASE 1:
-homologous chromosomes are pulled by the spindle fibres to opposite poles.
-centromeres do not divide
-chiasmata separates
TELEPHASE 1:
-two new nuclear envelopes form, one around each set of chromosomes. the cell divides by cytokinesis.
How does telephase 1 differ in plants compared to animals?
in most plant cells, the cell goes straight from anaphase 1 to meiosis 2.
Describe the events in meiosis 2
PROPHASE 2:
- nuclear envelopes break down again
- nucleolus disappears, chromosomes condense and spindles form.
METAPHASE 2:
-chromosomes arrange at equator of spindle. they are attached to the spindle fibre at the centromeres.
ANAPHASE 2:
-centromeres divide and chromatids are pulled to opposite poles by spindle fibres
TELOPHASE 2:
-nuclear envelope reforms
-in animals, the two cells –> four haploid cells
-in plants, a “tetrad” of 4 haploid cells formed
What is meiosis?
the type of nuclear division where the chromosome number is halved
Define “allele”
alternative version of a gene
Define “locus”
Specific position on a chromosome, occupied by a specific gene
Define “phenotype”
observable characteristics of an organism
Define “genotype”
combination of alleles possessed by an organism
Define “dominant”
the allele responsible is expressed in the phenotype, even in those with heterozygous genotypes
Define “codominant”
a characteristic where both alleles contribute to the phenotype
Define “recessive”
in which the allele responsible is only expressed in the phenotype if there is no dominant allele present. It is NOT expressed if heterozygous and masked by a dominant allele.
Explain “crossing over”
where non sister chromatids exchange alleles during prophase 1.
What is epistasis?
interactions between gene loci - one gene locus makes or supresses the expression of another gene locus
Explain recessive epistasis. How is this different to dominant epistasis?
Recessive epistasis is when the homozygous presence of a recessive allele prevents the expression of another allele at a second locus.
What do the figures mean in the chi squared test?
x^2 = sum of (O-E)^2/E
O = observed E = expected
What criteria must be met for the chi squared test? What does the value mean?
Criteria:
- sample size must be relatively large
- only raw counts (not % or ratios) can be used
The smaller the value, the more certain it is that the difference between observed and expected is due to chance & therefore not a significant difference
How do calculate the significance of the chi squared value?
Using n=1 (where n= number of classes) and a 5% critical value, we can see if the value is due to chance.
If the value is SMALLER than the value on the table, we can ACCEPT the null hypothesis - the difference is due to chance and not significant.
If the value is LARGER than the value on the table, the null hypothesis is REJECTED - the difference is significant.
What is the difference between continuous and discontinuous variation?
CONTINUOUS VARIATION eg height
- quantitative differences between phenotypes
- wide range of variation within the population
- no distinct categories
DISCONTINUOUS VARIATION eg blood type
- qualitative differences between phenotypes
- clearly distinct categories with no intermediate
Explain the genetic basis of continuous and discontinuous variation?
CONTINUOUS VARIATION
- different alleles at a single gene locus have small effects on the phenotype
- different gene loci have the same effect on the trait
DISCONTINUOUS VARIATION
- different alleles at a single gene locus have large effects on the phenotype
- different gene loci have different effects on the trait
Why is variation essential in selection?
So that when the environment changes, there will be individuals that are better adapted to the new environment. They will survive and pass on their advantageous alleles to their offspring, allowing the species to continue.
Define population
a group of individuals of the same species that can interbreed
How can you work out the frequency of a heterozygous genotype in a population?
For a heterozygous genotype, this is easy. The frequency of the genotype = the frequency of the phenotype.
What is the Hardy-Weinberg principle?
-used to calculate allele frequencies in populations
What assumptions does the hardy-weinberg principle make?
- the population is v large (eliminates sampling error)
- random mating (so no zoos)
- no selective advantage for any genotype
- no mutation, migration or genetic drift.
