Principle of independent assortment
segregation of each pair of alleles is independent
Principle of independent assortment only applies when genes
are on different chromosomes or are far apart on the same chromosome
9:7 epistasis
observed when homozygous recessive mutation in either or both of 2 different genes gives same mutant phenotype
9:4:3 epistasis
when homozygosity for recessive allele at one gene masks expression of the genotype of a different gene
Noncomplementation
combination of two recessive mutations results in mutant phenotype; mutations are alleles of same gene
Complementation
results in wildtype phenotype; mutations are alleles of different genes
Mutations block enzyme production if
if both copies of gene are defective
Histones
pack DNA in chromatin
Dense chromatin
heterochromatin (very few genes); regions at centromere & telomeres of most chromosomes
Less dense chromatin
euchromatin (gene rich)
Non-dividing nuclei
DNA wraps around histone proteins, DNA-protein chain coils into fibers, & fibers loosely organized into loops
Dividing nuclei
during nuclear division chromatin highly compact to facilitate chromosome movement
Cell cycle
G1, S, G2, M
G1
pre-DNA replication
S
DNA replication
G2
post-DNA replication
Mitosis Prophase
Chromosomes condensate, individual chromosomes become visible, & nuclear envelope disintegrates
Mitosis Metaphase
Mitotic spindle forms, spindle fibers organize & attach to chromosomes, & chromosomes line up in center
Mitosis Anaphase
Centromeres divide, sister chromatids separate, & one member of each pair pull to either pole of the cell
Mitosis Telophase
two daughter nuclei organize, chromosomes reverse condensation, & after cytokinesis, daughter cells are diploid
Meiosis I: Prophase I
Genetic recombination, homologous chromosomes pair (synapsis), physical exchange of genetic material between homologous chromosomes, Chiasmata link chromosomes, & chromosome condensation begins
Meiosis I: Metaphase I
Independent assortment of genes on different chromosomes & nonhomologous chromosomes align at random on metaphase plate
Meiosis I: Anaphase I
Independent segregation (physical separation of homologous chromosomes)
Meiosis II: Prophase II
may or may not involve recondensation of chromosomes; no chromosome replication between Meiosis I & II
Chi squared equation
(o-e)^2/e
p > 0.05
fail to reject the null hypothesis
Law of segregation occurs in
anaphase I & II
Independent assortment occurs in
metaphase I