2 Mendel & ChromosomesMendel was ahead of his time. 19th C cytology suggested a mechanism for his earlier findings. What did they find?Chromosomes and genes are both present in pairs in diploid cells.Homologous chromosomes separate and alleles segregate during meiosis.Fertilization restores the paired condition for both chromosomes and genes.
3 Chromosome Theory of Inheritance Mendelian genes have specific loci on chromosomesChromosomes are what physically undergo segregation and independent assortment.
4 Morgan first associated a specific gene with a specific chromosome. Morgan’s Fruit FliesMorgan first associated a specific gene with a specific chromosome.Why fruit flies?Breed quickly (two week generations)4 pairs of chromosomes (3 pair of autosomes, 1 pair of sex chromosomes)Females = XXMales = XY
5 Wild Type flies are the most common natural phenotype. (Red Eyes) Morgan’s Fruit FliesWild Type flies are the most common natural phenotype. (Red Eyes)After a series of crosses, Morgan produced mutants with white eyes.After a few generations, Morgan noted that only males displayed the white eyes.He concluded that certain genes are located on the sex chromosome and thus linked to sex.Sex-linked genes (ie: hemophilia)
7 Males (XY) only need one copy of recessive allele to show trait. Sex-linked TraitsMorgan concluded the gene with the white-eyed mutation is on the X chromosome. Y chromosome = no infoMales (XY) only need one copy of recessive allele to show trait.
8 Linked GenesAll genes located on the same chromosome tend to be inherited together.Chromosome passed on as a unit.Testcross results varied from those predicted by the law of independent assortment.This showed that certain genes will assort together. (on same chromosome)
10 Linked GenesBody color and wing shape are usually inherited together (same chromosome)
11 RecombinantsWhere did the other phenotypes come from? (grey-vestigial and black normal)Genetic recombination= offspring with new combinations of traits inherited from two parentsHow??independent assortment of genes (non-homologous)crossing over of genes (homologous)
12 Recombinants Mendel’s dihybrid crosses produced recombinant genotypes. 50% parental : 50% recombinant genotypes typical for nonhomologuesMetaphase IYR, Yr, yR, and yrSeed shape and color tetrads are independent from one another
13 RecombinantsLinked genes tend to move together during meiosis/fertilizationIf Independent assortment of genesExpect a 1:1:1:1 phenotype ratioIf Complete linkage of genes1:1:0:0 ratio (all parental)Observed 17% recombinant fliesSuggested Incomplete linkage of genes
14 Crossing Over Prophase I: homologous chromosomes can “swap” alleles More variable gametes than simple mendelian rules would predict
16 Linkage Maps Ordered list of genetic loci along chromosome Based on recombination frequencies between two genesHigher % of recombination = farther apartMore places in between genes for crossing over to occur and separate the genes
17 Linkage Maps The recombination frequency between cn and b is 9%. The recombination frequency between cn and vg is 9.5%.The recombination frequency between b and vg is 17%.
18 Linkage MapsMap units are the distances between genes on a chromosome.1 map unit = 1% recombination50% recombination = so far apart that crossing over is all but certainRemember, 50% recomb. = ind. assortment (non-homologous)Linkage maps show relative order/distanceMore recent studies show exact distances and order
20 X-Y Sex DeterminationX and Y behave as homologuesEach egg receives an X from XX motherOne sperm receives X and one YResults in 50/50 chance of male or femaleSRY GenePresent (on Y) : gonads develop into testes (male)Not present (no Y): gonads become ovaries (female)SRY also regulates other genes
21 Sex-Linked GenesSex chromosomes also contain other genes. (ie: drosophila eye color)Females must be homozygous recessive to display trait (XX – second X can mask recessive)Females can be carriersMales only need to inherit a single copy to show traitCan a male be a carrier?
22 Sex-Linked Disorders Duchenne Muscular Dystrophy 1/3500 males Progressive muscular weakeningDie by mid-20’sMissing X-linked geneNo production of dystrophin (muscle protein)
23 Sex-Linked Disorders Hemophilia Absence of one or more clotting factorsaffected individuals cannot stop bleeding normallytreated with protein injections
24 Barr Bodies Only one of the females X chromosomes is active The other becomes a Barr bodyWhen assorted into an ovum, the Barr body becomes activated againWhich X becomes Barr body is random in each cellApprox. 50% express each allele (if hetero)
26 Nondisjunction Errors with meiotic spindle Meiosis I: Homologous tetrad doesn’t separate ORMeiosis II: Sister chromatids don’t separateSome gametes receive two of the same type of chromosome and another gamete receives no copy
27 Aneuploidy Results from fertilization involving nondisjoined gamete(s) Trisomy three copies of a particular chromosome (2n + 1)Monosomy only one copy of a particular chromosome (2n – 1)
28 Down Syndrome Three copies of chromosome 21 1/700 children born each yearDefinite link with maternal age
29 Aneuploidy in Sex Chromosomes XXY Male (Klinefelter’s Syndrome)Male sex organs, sterile w/ femininityXYY MalesTend to be taller than normal
30 Aneuploidy in Sex Chromosomes XXX FemalesWill develop as normal femalesXO Females (monosomy – Turner syndrome)Immature females1/2500 live female births