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Chapter 15 RQ What are genes located on a sex chromosome called?

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


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 Loci Recombinant Approximate Map
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% * cn b 9.0% cn vg 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 8. Distinguish between a heterogametic sex and a homogametic 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)


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 - on X, the recessive trait for colorblindness
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 13. Distinguish between trisomy and triploidy.
Trisomy  an aneuploid cell that has a chromosome in triplicate Triploidy  polyploid chromosome number with 3n 

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 !

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