BIOL 3451 1st EditionExam # 2 Study Guide Lectures: 10 - 18Lecture 10 (September 30) 7.5: Dosage Compensation Prevents Excessive Expression of X-Linked Genes in Humans and Other Mammalso What is dosage compensation?- Genetic mechanism used to balance the levels of expression of the X chromosome; done by random inactivation of one X chromosome (forms a Barr body) in mammals (this is sometimes called lyonizaton)o Where are Barr bodies found and how many are in the different sex determined disorders?- Found against nuclear envelope of interphase cells- Turner (45,X) females: 0- 47,XYY: 0- Klinefelter (47,XXY) males: 1- 47,XXX females: 2- 48, XXXX females: 3- *Barr bodies follow N-1 rule*o What is the Lyon Hypothesis? Give examples.- The inactivation of X chromosome occurs in early embryonic development (blastocyst stage), and all descendent cells have same X chromosome inactivation- Thus, when looking at calico and tortoiseshell cats, you can see the patches of orange and black provide evidence for the Lyon Hypothesis where, in some areas, the X chromosome had been inactivated and in other areas, it hadn’t.o How does inactivation work?- X inactivation center (Xic) produces X-inactive specific transcript (XIST) gene to regulate inactivation 7.6 The Ratio of X Chromosomes to Sets of Autosomes Determines Sex in Drosophilao How is sex determined in Drosophila?- Determined by the ratio of X chromosomes; based on the genic balance theory that there is a threshold for maleness reached when ratio of X chromosome to haploid set of autosomal chromosomes is at least 1.2, but an additional X can cause differentiation in females (metafemales)o How does dosage compensation work in Drosophila?- X-chromosome inactivation does not occur, instead, male X-linked genes are transcribed twice the level (upregulated) of female X-linked genes 7.7 Temperature Variation Controls Sex Determination in Reptileso How is sex determined in reptiles and what are the three different types of effects?- Change in temperature (and other environmental aspects) influence sex determination; called temperature-dependent sex determination (TSD): change in temperature affects the production of steroids (mainly estrogens) In comparison to genotypic sex determination (GSD): determined genetically- Low temps: produce all females- High temps: produce all males- Low and high temps: all females with intermediate temps resulting in various proportions of females (refer to fig. 7.15) 8.1 Specific Terminology Describes Variations in Chromosome Numbero Know the following terms as they relate to Chromosome Mutations:- Aneuploidy: gains or loses one or more chromosomes, but not complete set; most changes in autosomes result in death Monosomy: loss of single chromosome (2n-1) Disomy: normal phenotype (2n) Trisomy: adding a single chromosome (2n+1); often lethal- Euploidy: complete haploid sets of chromosomes are present (normal) Diploidy: 2 sets present (normal) Polyploidy: where more than 2 sets are present Triploidy: 3 sets are present Autopolyploidy: multiples of same genome Allopolyploidy: multiples of closely related genomes; doesn’t work well for mammals, but allotetraploidy works well in some plantsa. (amphidiploidy):- Nondisjunction: paired homologous chromosomes fail to disjoin during segregation; causes all the previous chromosome mutationso How can chromosome mutations/chromosome aberrations occur?- Change in total number- Deletion/duplication of genes, missing/changing parts of chromosome- Rearranging material on chromosome- Called Chromosome mutations/aberrations (different than gene mutations) 8.2 Monosomy and Trisomy Result in a Variety of Phenotypic Effectso What is the phenotype of monosomy in autosomes and sex chromosomes?- In humans (and other mammals), monosomy of autosomes result in death; can be lethal in other species if the other allele is lethal since there is no otherdominant allele to mask the lethal allele- In plants, monosomy is better tolerated Haploinsufficiency: single copy of recessive gene may not be sufficient for adequate function for sustaining organism, so results in death.- For X and Y chromosomes, a different phenotype results that may result insterility, but not death (unless the male doesn’t get the X chromosome)o What is trisomy and phenotype(s) associated with it?- Trisomy is the addition of a chromosome, and is usually lethal when occurs with large chromosomes (because the ratios get more messed up and makes meiosis irregular), but not when with smaller chromosomes (like 21)- In plants, trisomy is tolerated more, but different phenotypes do result (jimsonweed, rice plant)- Down syndrome (Trisomy 21): only autosomal trisomy where individuals survive past first year (found by Langdon Down in 1866) 12-14 characteristics, but most only have about half of these (outward appearance is similar though) Often exhibit higher risks for respiratory disease, heart malformations, leukemia, Alzheimer’s disease Down syndrome critical region (DSCR): critical region where dosage sensitive regions of genes that are responsible for phenotypes associated with this syndrome- Patau syndrome: trisomy of 13 and Edwards syndrome: trisomy of 18 Both are very bad, led to testing of spontaneously aborted fetuses and found aneuploidy more common than originally thoughta. 6% of all conceptions have abnormal chromosome complement (our body is good at terminating what it knows will not survive well)o What are some possible ways that down syndrome occurs?- Increased incidence with increasing maternal age Possible because eggs are arrested in Meiosis 1 for longer period of time- Sometimes runs in families (called familial down syndrome) Involves a translocation of chromosome of 21 Not as severe because only involves a part of 21 More commono How does genetic counseling play a role?- Recommended for women with pregnancies later in life- Now, we can screen for all different types of genetic disorders- Amniocentesis and chorionic villus sampling (CVS): Diagnostic testing to see if has Down syndrome; can abort child but ethical issues arise 8.3 Polyploidy, in Which More Than Two Haploid Sets of Chromosomes Are Present, Is Prevalent in Plantso How does polyploidy occur in plants?- Autopolyploidy: adding one or more sets of chromosomes to the same haploid (same species)