Exam 5 Review Sex Chromosomes 04 30 2014 Autosomes Cells of diploid organisms with two copies of each o Homologous chromosomes o Each carries the same genes same location not same version necessarily Sex Chromosomes Carry genes regulating controlling sex o Different for males vs females o NOT homologous o In humans Females XX Males XY X Y differ in shape size and genes Y is smaller has less DNA less genes Share enough homology XY pairing in meiosis Sex Determination in Humans o Y carries SRY TDF SRY Sex determining region Y protein TDF Testis determining factor Both initiate chain of events leading to male sex organs Not all necessary genes on Y autosomes too o XX XY Humans and some insects o XX XO Insects o ZW ZZ Birds reptiles insects ZW female o Haploid Diploid Ants and bees o Non Chromosomal Sex Determination Temperature dependent Reptiles Cold males Sex changes Depends on life cycle stage Contact dependent Larvae become males if contact female no contact female Hermaphrodites X Chromosome Gene Expression o Holds many genes unrelated to sex determination o Since females have two they can be homo heterozygous o Males only have one hemizygous Due to only having one X chromosomes males will express whatever is on X for X chromosome traits o Recessive X linked traits therefore more common in males X linked Traits o Alleles written as superscript Capitol dominant Lowercase recessive o Females can only produce X gametes o Males produce both X and Y gametes sex determining Only pass X to daughters Only receive X from mothers Sex Linked Traits CANNOT BE CARRIERS OF X LINKED TRAITS Chromosome Gene Dosage Extra copy of Chromosome 21 Down Syndrome trisomy 21 X Chromosome Inactivation o Females have twice as much dosage of each gene o Gene expression restored due to X inactivation o Cells with more than one X one used for gene protein production and any others inactivated o Chromosome Condensation Active are unwound for easy access Inactivated chromosome is super coiled and condensed Stays this way throughout cell cycle Barr Bodies Lie along inner side of nuclear XIST Gene on X chromosome that keeps it envelope inactivated Mosaic Phenotype Linked Genes Law of Independent Assortment applies to chromosomes on DIFFERENT chromosomes o Allows formation of 2 n unique gametes n of heterozygous genes Each gene is equally probable Dihybrid cross 1 1 1 1 ratio Genes on same chromosome are physically joined o Do no assort independently o CAN be separated by crossing over When cross over has occurred the alleles from the two genes have recombined Recombinant chromosomes Distance between genes determines frequency likelihood of crossing over Farther apart more likely Linked genes show altered patterns of inheritance o Most gametes keep two alleles together less recombinant Not 1 1 1 1 suggest linkage Nomenclature in Drosophila fruit fly Genetics Wild Type WT Normal phenotype Dominant MUTANT alleles Capital letter Recessive MUTANT alleles Lowercase letter In fruit flies an autosomal mutation in the gene for wing veining results in wings lacking cross veins This mutation is recessive At another gene an autosomal recessive mutation results in white eyes Which of the following indicates the genotype of an individual heterozygous at both genes cv cv w w In fruit flies a dominant autosomal mutation results in curly wings At another gene a recessive autosomal mutation results in ebony body color Which of the following indicates the genotype of an individual that is heterozygous for curly wings and homozygous for normal body color CY CY eb eb Frequency of Recombination Indicates gene distance Calculating recombination frequency o Total Recombinant Total Offspring Calculating map distance o map units Chromosome Linkage Maps Show order and relative distance between genes on a chromosome The maximum distance you can calculate between two linked genes is 50 map units o Recombination frequency of 50 means gametes are produced in equal amounts o Genes far enough apart cross over every time Behave as though on different chromosomes Considered completely unlinked if 50 apart Completely Linked NEVER separated by crossing over o So close together on chromosome that aren t separated by breaks in chromosome Cross Over is also reduced for genes near the centrosome Assume that a cross is made between AaBb and aabb plants and all of the offspring are either AaBb or aabb These results are consistent with which of the following Complete Linkage If you examine the gametes of individuals having this arrangement of alleles on one pair of their chromosomes would you expect to see a higher proportion of gametes with AB alleles or CD alleles AB DNA Replication DNA molecules Copying the sequence of one DNA molecule to make two identical Replication occurs before every cell division S of Interphase DNA Structure Nucleic acid polymer of nucleotides o Nucleotides Phosphate group Sugar Nitrogenous Base o Ladder like structure with sugar phosphate backbone o Base Pairing Adenine Thymine AT Guanine Cytosine CG where A G are purines and C T are pyrimidines o Sequence of DNA bases contains encodes genetic info o Bases that pair together complementary o Strands are anti parallel Process is semi conservative Each new molecule is one old strand and one new strand old template to create new Built 3 to 5 Strands built in opposite directions DNA Replication Fork Where two original strands are separating Steps o 1 Topoisomerases unwind helix o 2 Helicase separates two strands o 3 Single strand binding proteins bind to each strand to keep them separated o 4 DNA Polymerase DNA Pol adds nucleotides to the free 3 hydroxyl of the new strand o 5 Added nucleotides are complementary to those in template DNA Polymerase can ONLY add nucleotides to an existing strand DNA Primase Synthesizes short sequence of RNA nucleotides DNA Polymerase addes nucleotides to the free 3 end of the RNA called an RNA primer primer Leading and lagging strands o Leading Growing in the direction that DNA is unraveling opening Direction that helicase is moving Template runs 3 to 5 o Lagging Growing AWAY from replication fork Template runs 5 to 3 Okazaki Fragments joined by DNA Ligase DNA Replication Bubbles Replication begins are Origin of Replications there are multiple on each chromosome Errors in DNA Replication Proof reading mechanisms and error repair systems exist Errors can be from o DNA Polymerase o Free Radicals o UV Light o Viral Infections Repairing Errors o Mismatch