Chapter 15 RQ

2. Chapter 15 RQ • What are genes located on a sex chromosome called? • In fruit flies, what do we call the normal phenotype for a character? • Genes that tend to be inherited together are called what? • What is nondisjunction? • How do people have Down Syndrome?

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  Morgan’s student - probability of crossing over between genes is directly proportional to the distance between them • Sutton  noticed parallels of Mendel’s thoughts and the actual behavior of chromosomes 

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 

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 - F2 generation doesn’t show 9:3:3:1 ratio in the dihybrid 

7. Parental Progeny that have the same phenotype as one or the other of the parents Recombinant Progeny whose phenotypes differ from either parent  4. Distinguish between parental and recombinant phenotypes.

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 

10. 6. Map the linear sequence of genes on a chromosome using the given recombinant frequencies from experimental crosses. Loci Recombinant Approximate Map Frequency Units b vg 17.0% 18.5* cn b 9.0% 9.0 cn vg 9.5% 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

11. #6 continued… • Establish distance between the genes farthest apart • Determine the frequency between the 3rd (cn) and the 1st (b) • Consider possible placements • b-----------------------vg  17 • cn---------b  9 • cn----------b-----------vg OR b----------cn-----------vg • Determine recombinant frequency between 3rd (cn) and 2nd (vg) to eliminate  b-------------cn---------------vg

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 

15. 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)  8. Distinguish between a heterogametic sex and a homogametic sex.

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 

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 

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 

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 

22. Aneuploidy… • Aneuploidy  condition of having an abnormal number of certain chromosomes (result of nondisjunction) ex: Down’s syndrome (trisomy of #21) 

23. Polyploidy… • Polyploidy  a chromosome number that is more than 2 complete chromosome sets (3 haploid sets  3n) 

24. Trisomy  an aneuploid cell that has a chromosome in triplicate Triploidy  polyploid chromosome number with 3n  13. Distinguish between trisomy and triploidy.

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 

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 gene’s expression because of its location among neighboring genes 

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. • Down’s 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 • Turner’s  XO, only known viable human monosomy - short and sterile, secondary sex characteristics don’t 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) 

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 

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)

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 !