Download Homologs: behave independently in mitosis Tfm: secondary and

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Homologs: behave independently in mitosis
Tfm: secondary and tertiary sex determination fail as a male
Anticipation: Increase in severity from one generation to the next
1:2:1: ratio of 2 strand: 3 strand: 4-strand double crossovers
Sister chromatid adhesion: lost at start of Anaphase I
Sister chromatids: adhere to one another in prophase I
10 map units: interference = 1
PD=NPD: genes on non-homologous chromosomes
Essential genes: about 1/3 of all genes
5-bromouracil: a base analog
trait rare: assume unrelated individuals homozygous or hemizygous for
normal allele
auxotroph: requires nutritional supplementation beyond that required by
wild type
SNP: due to a base pair substitution
Addition rule: can be used when events are mutually exclusive
Zygotene: synaptinemal complex forms
Chromatid interference: does not exist
Indel: e.g. deletion of a few base pairs
Bivalent: physical association of two homologs
3 strand doubles: produce T asci
Z: one copy present in female
Interphase: DNA replication occurs in some, but not all, cases
Pleiotropic: one allele causes more than one phenotype
Law of Segregation: each gamete receives only one copy of each gene
Reductional division: each chromosome has 2 chromatids after division
completed, each gamete receives only one copy of each gene (Meiosis I)
Germ cell: cell that gives rise to gametes
Repulsion: alleles on different homologs
Chaisma frequency = 20%: recombination frequency = 10%
Chi-squared = sum (observed-expected)2/(expected)
Central element: appears progressively during synapsis
P value = 6%: data not significantly different from expectation
Adjacent I: homologous centromeres segregate to opposite poles
Chromatid interference: level determines fraction of double crossovers
that are 2-strand, 3-strand, and 4-strand doubles
Expressivity: degree to which phenotype is altered in affected
individuals
 Mendel’s 1st law: the law of segregation: each trait is controlled by
particulate factors that occur in pairs; during gamete formation, the
members of a pair separate from each other so that each gamete receives
only one; the double number is restored upon fertilization
 Mendel’s 2nd law: the law of independent assortment: the segregation of
each gene pair during gamete formation is independent of all others
 Metacentric: centromere near center of chromosome
 Sub-metacentric chromosome: centromere more near end
 Acrocentric chromosome: centromere almost at very end
 Telocentric chromosome: centromere pretty much at end
 Karyotype: chromosome sets as seen at end of prophase
 Matroclinous: daughter is like mother in phenotype even in criss-cross
inheritance (result from nondisjunction in anaphase)
 Patroclinous: son is like father in phenotype because of nondisjunction
 Nondisjunction: homologs fail to let go (dragged to one pole) or
homologs never pair together (no basis for segregation)
 In Drosophila, sex determined by number of X chromosomes; Y
chromosome confers male fertility (XXX and YO die, XX female, XO
and XY male)
 Hemizygous: present in only one copy (in male, such as XY)
 Moths, birds, and butterflies: ZW female, ZZ male
 Mammals: XO fertile female (Turner’s syndrome), XXY sterile male
(Klinefelter’s Syndrome), XXX female  Y chromosome determines
maleness in mammals
 Testicular Feminization Gene (Tfm): primary: gonad testes,
secondary: fails female genitalis, tertiary: fails
 Testes determining factor (TDF) and Sex determining region on Y (Sry)
 Propositus: individual that brought attention of pedigree to geneticists
(indicated by arrow)
 When alleles are together on same homolog, they are said to be in cis
with one another or in coupling.
 When alleles are on different homologs, they are said to be in trans or in
repulsion.
 Linkage: association of genes on same chromosome, genes said to be
linked
 Recombination: production of new gamete types
 Recombination frequency: nonparental gametes/total gametes * 100%
 Prophase I: leptotene (thin thread), zygotene (paired thread, beads on
string), packytene (thick thread), diplotene (2 thread)
 Synapsis: intimate and precise pairing of homologs  occurs in
zygotene and the complex is seen in packytene
 Locust: position of gene on chromosome (plural, luci)
 1% rec. = 1 map unit (centimorgan)
 undetected double crossovers lead to an underestimate of distance
between genes
 double crossovers are rarer than expected based on assumption of
independence of crossing over in adjacent intervals
 Interference: occurrence of one crossover inhibits others from occurring
nearby
 Coefficient of coincidence: (observed # doubles)/(expected # doubles)
(between 0 and 1)
 Interference: 1- coefficient of coincidence
 Interference = 1 within 10 map units, there are no doubles within 10 map
units
 3 types of tetrads: parental ditype (PD), tetratype (T), nonparental ditype
(NPD)
 NPD<<PD  genes are linked
 If not linked, PD=NPD because of Mendel’s second law
 Chromatid interference: influence of chromatids selected for 1st
crossover on chromatids selected for second  DOES NOT EXIST
 Chiasma interference: real, inhibition by one crossover of others
occurring nearby
 Even if a double crossover occurred 100% of the time between a and c,
rec. freq. would equal 50% because of no chromatid interference
 Indel: insertions of deletions of a few base pairs
 Polymorphism: presence of more than one common form in a population
 Founder effect: alteration in allele frequency due to the founding of the
population by a small # of individuals