Slide 1

Slide 2 Chapter 15 RQ 1.What are genes located on a sex chromosome called? 2.In fruit flies, what do we call the normal phenotype for a character? 3.Genes that tend to be inherited together are called what? 4.What is nondisjunction? 5.How do people have Down Syndrome? Slide 3 1. Describe the contributions that Thomas Hunt Morgan, Walter Sutton, and A.H. Sturtevant made to current understanding of chromosomal inheritance. Morgan selected Drosophila as organism - proposed linkage (characteristics are inherited together) Sturtevant Morgans student - probability of crossing over between genes is directly proportional to the distance between them Sutton noticed parallels of Mendels thoughts and the actual behavior of chromosomes Slide 4 2. Explain why Drosophilia melanogaster is a good experimental organism. It is easily cultured in the lab They are prolific breeders They have a short generation time Their 4 pairs of chromosomes are easily differentiated and observed Slide 5 3. Define linkage and explain why linkage interferes with independent assortment. Linked genes do not independently assort Linked genes genes that are located on the same chromosome and that tend to be inherited together - move together through meiosis and fertilization - F 2 generation doesnt show 9:3:3:1 ratio in the dihybrid Slide 6 Slide 7 4. Distinguish between parental and recombinant phenotypes. Parental Progeny that have the same phenotype as one or the other of the parents Recombinant Progeny whose phenotypes differ from either parent Slide 8 Slide 9 5. Explain how crossing over can unlink genes. During meiosis, exchange of parts between homologous chromosomes breaks linkages in parental chromosomes and forms recombinants with new allelic combinations Slide 10 6. Map the linear sequence of genes on a chromosome using the given recombinant frequencies from experimental crosses. LociRecombinantApproximate Map FrequencyUnits b vg17.0%18.5* cn b9.0%9.0 cn vg9.5%9.5 *higher because b & vg are relatively far apart and double crossovers occur between these loci and cancel each other out leading to underestimation of map distance next slide Slide 11 #6 continued 1.Establish distance between the genes farthest apart 2.Determine the frequency between the 3 rd (cn) and the 1 st (b) 3.Consider possible placements b-----------------------vg 17 cn---------b 9 cn----------b-----------vgORb----------cn-----------vg Determine recombinant frequency between 3 rd (cn) and 2 nd (vg) to eliminate b-------------cn---------------vg Slide 12 Slide 13 7. Explain what additional information cytological maps provide over crossover maps. Cytological maps locate genes with respect to chromosomal features, such as stained bands that can be viewed with a microscope Slide 14 Slide 15 8. Distinguish between a heterogametic sex and a homogametic sex. Heterogametic sex The sex that produces two kinds of gametes and determines the sex of the offspring Ex: human male (sperm X or Y) Homogametic sex The sex that produces one kind of gamete Ex: human female (egg X only) Slide 16 Slide 17 9. Describe sex determination in humans. 2 chromosomes determine sex X and Y Males carry X and Y options Females carry X only It is the gene SRY (sex determining region of Y) that triggers for complex events leading to the development of testes, etc. Default is the development of ovaries Slide 18 10. Describe the inheritance of a sex-linked gene such as color blindness. Refers to X-linked traits X is larger than Y, so there are more traits Fathers pass on X to daughter - no sex-linked traits to son Mothers give to both type of offspring - sex linked trait is a recessive allele - females show trait only is homozygous Slide 19 Slide 20 11. Explain why a recessive sex-linked gene is always expressed in human males. It is said to be hemizygous - an organism having only one copy of a gene in a diploid organism Ex: son (XY) - on X, the recessive trait for colorblindness - on Y, no dominant trait to shadow recessive Slide 21 12. Distinguish among nondisjunction, aneuploidy, and polyploidy; explain how these major chromosomal changes occur and describe the consequences. Nondisjunction meiotic or mitotic error during which certain homologous chromosomes or sister chromatids fail to separate Slide 22 Aneuploidy Aneuploidy condition of having an abnormal number of certain chromosomes (result of nondisjunction) ex: Downs syndrome (trisomy of #21) Slide 23 Polyploidy Polyploidy a chromosome number that is more than 2 complete chromosome sets (3 haploid sets 3n) Slide 24 13. Distinguish between trisomy and triploidy. Trisomy an aneuploid cell that has a chromosome in triplicate Triploidy polyploid chromosome number with 3n Slide 25 14. Distinguish among deletions, duplications, translocations, and inversions. Deletion chromosomes which lose a fragment and lack a centromere Duplications fragments without centromeres that join a homologous chromosome Translocation when the fragments join to a non-homologous chromosome Inversions when the fragments reattach to the original chromosome but in a reverse order Slide 26 Slide 27 15. Describe the affects of alterations in chromosome structure, and explain the role of position effects in altering the phenotype. Effects of alterations - homozygous deletions, including a single X in males (usually lethal) - duplications and translocations tend to have deleterious effects - reciprocal translocations and inversions between non-homologous chromosomes can alter phenotype Position effect influence on a genes expression because of its location among neighboring genes Slide 28 16. Describe the type of chromosomal alterations implicated in the following human disorders: Down syndrome, Klinefelter syndrome, extra Y, triple-X syndrome, Turner syndrome, cri du chat syndrome, and chronic myelogenous leukemia. Downs syndrome trisomy of 21; 1 in 700 children Klinefelter XXY, feminine maleness Extra Y XYY, normal male, usually taller Triple X XXX, usually fertile, normal female Turners XO, only known viable human monosomy - short and sterile, secondary sex characteristics dont develop (unless estrogen therapy is applied) Cri du chat deletion on chromosome 5, mental retardation - smaller head, unusual features, cry sounds like a mewing cat Chronic myelogenous leukemia (CML) part of chromosome 22 switches places with a small fragment from chromosome 9 (translocations) Slide 29 17. Define genomic imprinting and provide evidence to support this model. It is the process that induces intrinsic changes in chromosomes inherited from males and females; causes certain genes to be differently expressed in the offspring depending upon whether the alleles were inherited from the ovum or the sperm cell Same alleles have different effects if maternal and paternal DNA methylation is one mechanism Slide 30 Example Deletion of a particular segment of chromosome 15 If it came from dad: Prader-Willi syndrome (mental retardation, obesity, short, small hands and feet) If it came from mom: Angelman syndrome (uncontrollable laughter, motor and mental symptoms) Slide 31 Slide 32 18. Give some exceptions to the chromosome theory of inheritance, and explain why cytoplasmic genes are not inherited in a Mendelian fashion. Exceptions Extranuclear genes are found in cytoplasmic organelles like plastids and mitochondria These are not inherited in Mendelian fashion because they are not distributed by segregating chromosomes in meiosis The End !