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
