Objectives for Cumulative Part of the Final ExamGenetics, Summer 2012Meiosis:• 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 homologouschromosomes • If 2n=4, then during meiosis n=2 • Meiosis I: • First Division • homologous chromosomes separate into different daughter cells • crossing over (chromosome recombination) occurs • Meiosis II: • Second division • sister chromatids are separated into different daughter cells • in both processes, DNA is first replicated and then different methods are usedfor separation of the replicated genetic information • chromosomes before replication consist of a single DNA double helix bound byhistone 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. Asomatic cell has twice that many chromosomes (2n).• Humans are diploid. A human somatic cell contains 46 chromosomes: 2 completehaploid sets, which make up 23 homologous chromosomes pairs.• Homologous Chromosomes- carry the same genes but may havedifferent alleles; have same centromere position, same length • non-homologous chromosomes would carry completely different genes • exception- sex chromosomes are homologous chromosomes but donʼtcarry the same genes • sister chromatids are exact duplicates; a condensed chromosome ready fordivision (mitosis) has 2 arms (the sister chromatids) • Crosses and inheritance: Be able to do a dihybrid cross and crosses involvingpartial 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 traitsof 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 = first filial generation • F2 = second filial generation • produced by a ʻF1 Crossʼ, or ʻMonohybrid Crossʼ • just cross two of the same F1ʼs from the P1 cross • Partial dominance- neither allele is dominant, the phenotype of a heterozygoteis intermediate • ex: Red flower (RR) x White flower (rr) • F1 = all pink flowers (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 acombination of the two homozygotes • ex: A and B blood types--these are codominant, but A and B are dominantover 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 geneticmap 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 sexchromosomes. In mammals, the male is the heterogametic sex • homogametic sex- the sex that produces gametes that do not differ withrespect to sex chromosome content; in mammals, the female is homogametic. • XO system- one X is male; two Xʼs (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 ismale (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 formany different things • Y chromosome has very few genes and basically just codes formale 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 theX 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 tissueof 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 externalmale 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 genebeing 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 theX is unbalanced • How is gene dosage regulated? • X-inactivation in human females--
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