Objectives for Cumulative Part of the Final Exam Genetics Spring 2012 Meiosis Know what happens in meiosis how chromosomes are separated in the two divisions when crossing over occurs Meiosis produces the reproductive cells sperm and egg formation of gametes one diploid cell divides twice and forms 4 haploid cells each daughter cell gets one chromatid from each pair of homologous chromosomes If 2n 4 then during meiosis n 2 Meiosis I Meiosis II First Division homologous chromosomes separate into different daughter cells crossing over chromosome recombination occurs Second division sister chromatids are separated into different daughter cells in both processes DNA is rst replicated and then different methods are used for separation of the replicated genetic information chromosomes before replication consist of a single DNA double helix bound by histone proteins chromatin 2n 2 copies of each chromosome 2n 2 pairs of homologous chromosomes diploid of 4 2n 4 chromosomes Know what homologous chromosomes are what haploid and diploid chromosome and chromatid mean The haploid number n is the number of chromosomes in a gamete A somatic cell has twice that many chromosomes 2n Humans are diploid A human somatic cell contains 46 chromosomes 2 complete haploid sets which make up 23 homologous chromosomes pairs Homologous Chromosomes carry the same genes but may have different alleles have same centromere position same length non homologous chromosomes would carry completely different genes exception sex chromosomes are homologous chromosomes but don t carry the same genes sister chromatids are exact duplicates a condensed chromosome ready for division mitosis has 2 arms the sister chromatids Crosses and inheritance Be able to do a dihybrid cross and crosses involving partial dominance codominance and X linkage Phenotype the physical expression of a trait Genotype the genetic makeup of an individual Homozygous both alleles for a trait are the same Heterozygous two different alleles for a trait are present True breeding homozygous for a trait Dihybrid Cross cross between F1 offspring of 2 individuals that differ in 2 traits of interest ex Bb x Bb Monohybrid Cross method of determining the inheritance pattern of a trait A cross between two parents who are true breeding for a trait ex AA x aa P1 parental generation F1 rst lial generation F2 second lial generation produced by a F1 Cross or Monohybrid Cross just cross two of the same F1 s from the P1 cross Partial dominance neither allele is dominant the phenotype of a heterozygote is intermediate ex Red ower RR x White ower rr F1 all pink owers Rr amount of product pigment depends on the amount of enzyme made which is determined by the genotype Codominance both alleles are expressed phenotype of heterozygote is a combination of the two homozygotes ex A and B blood types these are codominant but A and B are dominant over O this is also an example of multiple alleles 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 Homework 1 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 for an example problem see sample test 1 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 autosomes chromosomes that are not sex chromosomes 22 sets of autosomes sex chromosomes 23rd set of chromosomes are the sex chromosomes heterogametic sex the sex that produces gametes containing unlike sex chromosomes In mammals the male is the heterogametic sex homogametic sex the sex that produces gametes that do not differ with respect to sex chromosome content in mammals the female is homogametic XO system one X is male two X s XX is female XY system mammalian system XY is male XX is female ZW system reptiles birds some amphibians and insects ZW is female ZZ is male sort of opposite of mammalian system Differences and how these lead to male female development X chromosome is really a garden variety chromosome that codes for many different things Y chromosome has very few genes and basically just codes for male development only contains about 2 dozen genes the SRY gene on the Y chromosome codes for TDF Testis determining factor PAR Pseudoautosomal region shares homology with regions on the X chromosome and synapses and recombines with it during meiosis In humans the absence of a Y chromosome leads to female development At 6 8 weeks of development SRY gene becomes active in XY embryos SRY encodes a protein that causes the undifferentiated gonadal tissue of the embryo to form testes This protein is TDF If SRY is expressed the gonads develop into testis if its not expressed the gonads develop into ovaries The cells in the testis secrete anti Mullerian hormone this blocks the Mullerian ducts from developing into the uterus and ovaries Testosterone stimulates the formation of internal and external male structures Be able to provide an explanation for the existence of XX males and XY females XY females have a deletion of part of the Y the part containing the SRY gene XX males have a translocation from the Y to the X part containing the SRY gene could have occurred during crossing over PAR region The XY females and XX males provided evidence for the SRY gene being responsible for male sex determination Know what X inactivation is what the Barr body is and what dosage compensation is Dosage Compensation females have 2 X chromosomes and males only have 1 so expression of the X is unbalanced How is gene dosage regulated X inactivation in human females Lyon Hypothesis one X chromosome is inactivated in each cell in females during early development Barr Body inactivated X Fruit ies instead double the activity of genes on the X in males compared with females to compensate Barr Body inactivated X To nd of Barr Bodies X s 1 N 1 because you only want one active X chromosome don t know all the details on how this is achieved occurs in late blastula stage 1 of 2 X s is inactivated once that X is shut off every daughter cell made has that X turned off Chromosome mutations Know the genetic makeup of individuals with Down syndrome Turner syndrome and Klinefelter syndrome and know the general phenotype of these
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