Meiosis Know what happens in meiosis how chromosomes are separated in the two divisions and when crossing over occurs Meiosis Formation of gametes sex cells One diploid cell divides twice and forms 4 haploid cells Each daughter cell gets one chromatid from each pair of homologous chromosomes First Division Meiosis I homologous chromosomes are separated into different daughter cells tetrads form in late prophase I chromosome recombination crossing over occurs chiasma is the point where two homologous non sister chromatids exchange genetic material during chromosomal crossover during meiosis Synaptonemal complex is responsible for crossing over Pairing of homologous chromosomes is essential to meiosis Pairing depends on DNA sequence homology Second Division meiosis II Sister chromatids are separated into different daughter cells Each gamete has one chromatid from each pair of homologous chromosomes Meiosis I Prophase I Pair of homologs and synapsis pair up tetrads Crossing over DNA exchange between tetrads Metaphase I Tetrads align at the center Anaphase I separate homologs and cytokinesis starts Telophase I 2 HAPLOID cells form tetrads not together 1 Meiosis II Prophase II homologs Metaphase II homologs align Anaphase II separate sister chromatids and cytokinesis starts Telophase II 2 MORE HAPLOID cells homologs not together TOTAL 4 Haploids cells all different gametes Mistakes in Meiosis Nondisjunction failure to divide DNA Trisomy and Monosomy Downs Syndrome trisomy 21 of chromosomes 2n 10 10 Prophase Mitosis 10 Telophase Mitosis Prophase M1 tetrad 10 5 Prophase M2 5 Telophase M1 pairs Telophase M2 5 of pairs 5 5 5 tetrads 0 0 0 of chromatids 20 10 20 10 10 5 Know what homologous chromosomes are what haploid and diploid chromosome and chromatid mean Diploid 2n Somatic cells have identical number of chromosomes If 2n 10 n of chromosomes then n 5 of pairs 1 pair of homologous chromosomes 2n 2 If 2n 10 25 of different possible gametes 1 pair of chromosomes 2 possible outcomes Haploid n Sex cells Equal to the diploid number 1 of the 2 possible outcomes Homologous chromosomes Similar chromosomes same genes but different alleles versions of same gene Paired chromosomes with different alleles for the same genes how chromosomes appear in diploid cells In G1 Pairs of homologous chromosomes In G2 after replication in S phase pairs of homologous chromosome pairs with sister chromatids A pair are exact duplicates of each other The chromosomes condense after replication and each has two sister chromatids Sister chromatids 2 Crosses and inheritance Be able to do a dihybrid cross and crosses involving partial dominance codominance and X linkage Monohybrid Cross mating true breeding individuals from two parent strains Dihybrid Cross crossing two contrasting traits two factor cross Incomplete or Partial Dominance Neither allele is dominant phenotype of heterozygote is intermediate displays blended phenotype such as pink a blend of red and white In partial dominance the amount of product depends on the amount of the enzyme made Ex Human Familial Hypercholesterolemia Genetic defect in LDL receptor Defective receptor results in high blood cholesterol levels Recessive alleles usually no functional protein is coded for but if the normal functional allele by itself cannot make enough protein product to do the job then we observe incomplete dominance Ex Incomplete Dominance Cross Problem Horse coat color Chesnut brown is dominant to Cremello white cream C and Ccr Chesnut Horse CC mates with a Cremello Horse Ccr Ccr All offspring are palomino which is a light brown mix of chesnut and cremello Palomino C Ccr due to incomplete dominance Palomino Horse C Ccr mates with another Palomino Horse C Ccr Phenotypic and Genotypic Ratio ARE THE SAME 1 2 1 C Ccr CC Ccr Ccr Codominance Co for Cow Both alleles are expressed phenotype of heterozygote is a combination of the two homozygotes expressed independently at the same time and not blended but equal such as a cow s coat colors There is no recessive ness rather dominance shared between alleles 3 Ex Human ABO Blood Types Codes for a protein on the surface of RBCs three versions IA type A IB type B and I type O A and B are codominant A and B are dominant over O Rh factor is classical dominance RR and Rr Rh and rr Rh Universal donor is O and universal recipient is AB all proteins are recognized in AB and are not seen as foreign Ex Codominance Cross Problem LALA blood type A x LBLB blood type B Offspring LALB and would express both blood types A and B would have both A and B antigens LALB blood type AB x LALB blood type AB Phenotypic and Genotypic Ratio is 1 2 1 LALB LALA LBLB LINKAGE When two genes are located on the same chromosome their inheritance is linked linked genes are found close together on same chromosome and are linked because they are less likely to cross over if close together X Y Sex Chromosomes vs autosomes and XY male XX female X chromosome carries thousands of genes Y chromosome carries very few genes Females 2 copies of each gene on the X chromosome Males Only 1 copy of each gene on X All genes on the X chromosome in males are expressed whether they are dominant or recessive X Linked Traits in Humans Red green colorblindness Hemophilia Duchenne Muscular Dystrophy Cross Problems for Linked genes Look at F2 generation SCID and Fragile X syndrome Expected unlinked ratio F1 x F1 9 3 3 1 Actual ratio if linked 11 1 1 3 Expected unlinked ratio F1 x bbss 1 1 1 1 Actual ratio if linked 30 10 10 30 Be able to analyze human pedigrees to determine whether a trait is inherited as an autosomal recessive autosomal dominant X linked recessive or X linked dominant 4 Pedigrees Pedigree Clues If allele dominant or recessive o Squares are males circles are females o Darkened symbols have the trait undarkened ones do not o Recessive it skips generations o Sex linked only males affected o X linked if not on the Y affecting both female and male o Recessive skips generation o Dominant shows up a lot o More men than women affected o o All blood related men are affected if it is on the Y If equal amount autosomal If sex linked is it linked to X or Y If allele sex linked or autosomal Know that the frequency of crossing over between genes recombination can be used to map the positions of genes on chromosomes and be able to make a genetic map from a three point cross Genetic map is a graphic representation of a chromosome including the position of its genes Allows us to
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