Objectives for Cumulative Part of the Final Exam Meiosis Know what happens in meiosis how chromosomes are separated in the two divisions when crossing over occurs Meiosis Formation of gametes sex cells o One diploid cell divides twice and forms 4 haploid cells o Each daughter cell gets one chromatid from each pair of homologous chromosomes Meiosis I o Homologous chromosomes are separated into different daughter cells cuts chromosome number in half o Chromosome Recombination Occurs Crossing Over Non sister chromatids exchange pieces different combinations of alleles Occurs at the chiasma during Prophase I Meiosis II o Sister chromatids are separated into different daughter cells o No DNA replication just division o Each cell gets 1 chromatid from each pair of chromosomes homKnow what homologous chromosomes are what haploid and diploid chromosome and chromatid mean Homologous Chromosomes a pair of chromosomes having the same gene sequence o Each derived from one parent Haploid n a cell or an organism having one member of each pair of homologous chromosomes Diploid 2n a condition in which each chromosome exists in pairs Chromatid each of the two threadlike strands into which a chromosome divides longitudinally during cell division o Each contains a double helix of DNA Crosses and inheritance Be able to do a dihybrid cross and crosses involving partial dominance codominance and X linkage Dihybrid Cross two factor cross o 9 3 3 1 Phenotypic Ratio Partial Dominance o A cross between parents with contrasting traits produces offspring with an intermediate phenotype o Neither allele is dominant o Phenotype of heterozygote in intermediate o Amount of product depends on the amount of the enzyme made o 1 2 1 Phenotypic and Genotypic Ratio o Ex Tay Sachs Disease Snap Dragon Flowers Human Familial Hypercholesterolemia Co Dominance o Two alleles of a single gene are responsible for producing two distinct detectable gene products the joint expression of both alleles in a heterozygote o Both alleles are expressed o Phenotype of Heterozygote is a combination of the two homozygotes o 1 2 1 Genotypic Ratio o Ex MN blood group X Linkage o The pattern of inheritance resulting from genes located on the X chromosome o Ex Hemophilia Duchene s Muscular Dystrophy Color Blindness SCID Fragile X Syndrome etc 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 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 Sex chromosomes Know the XY sex determination mechanism the differences in the genetic makeup of the X and Y chromosomes know the name of the gene that determines sex and where it is located on the Y XY Sex Determination Mechanism Mammals and Some Insects XX female XY male Presence of Y chromosome development of testis and inhibition of the formation of the female reproductive organs Female pathway is the default pathway X Chromosome Carries thousands of genes 900 1400 genes All genes on the X chromosome in males are expressed whether they are dominant or recessive Y Chromosome Carries very few genes o All of the genes are necessary for full male fertility Determines the sex in humans Parts of the Y Chromosome o MSY Male Specific Region of the Y o SRY Sex Determining region of the Y o PAR Psuedoautosomal Region homologous with the X o Heterochromatin inert DNA that is turned off Regions on both ends of the Y chromosome that share homology with regions on the X chromosomes and synapse and recombine with it during meiosis Psudoautosomal Region PAR Homologous Regions SRY Gene Determines sex Located on the Y chromosome near the boundary in the PAR Sex Determining Region Y SRY Encodes TDF a protein that causes the undifferentiated gonadal tissue of the embryo to form testes SRY Expressed Gonads develop into testis o Testis Secrete 1 anti Mullerian hormone blocks mullerian ducts from developing into uterus and ovaries 2 Testosterone stimulates the formation of internal and external male structures SRY Not Expressed gonads develop into ovaries Testis Determining Factor TDF Caused the undifferentiated gonadal tissue to develop into testes A transcription factor a DNA binding protein that interacts directly with the regulatory sequences of other genes to stimulate their expression A master switch that controls other genes downstream in the process of sexual differentiation Be able to provide an explanation for the existence of XX males and XY females XX Males X chromosome has the region of the Y that contains SRY gene attached Have a translocation from the Y to the X XY Females Y chromosome is missing the SRY gene Have a deletion of part of the Y Know what X inactivation is what the Barr body is and what dosage compensation is X inactivation The Lyon Hypothesis One X chromosome is inactivated in each cell in females The inactivation of X chromosomes occurs randomly in somatic cells at a point early during early development in embryonic development Once inactivation has occurred all descendant cells have the same X chromosome inactivated as their initial progenitor cell Occurs during the late blastula stage Females are Genetic Mosaics patchwork where 1 of 2 X s is active in each cell X Inactivation Center Xic o Its genetic expression occurs only on the X chromosome that is inactivated o Contains the XIST Gene X inactive Specific Transcript XIST o The gene responsible for X inactivation o It is transcribed but not translated does not encode a protein o DNA of inactivated X becomes methylated Barr Body the inactivated X Highly condensed chromosome structure that is an inactivated X chromosome Regardless of how many X chromosomes a somatic cell possesses all but one of them appear to be inactivated and can be seen as Barr bodies The Number of Barr Bodies N 1 o N total number of X chromosomes present Dosage Compensation Because females have two X chromosomes and males have only one expression of genes on the X is unbalanced between the two sexes Prevents excessive expression of X linked genes Balances the dose of X chromosome gene expression in females and males Chromosome mutations Know the genetic makeup of individuals with Down syndrome Turner syndrome and Klinefelter syndrome and know the general phenotype of these syndromes Down Syndrome Trisomy 21 Three copies of the
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