Genetics 3000: Week 4 of Notes
Genetics 3000: Week 4 of Notes 85033 - GEN 3000 - 002
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85033 - GEN 3000 - 002
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This 5 page Class Notes was uploaded by Lisa Blackburn on Sunday February 7, 2016. The Class Notes belongs to 85033 - GEN 3000 - 002 at Clemson University taught by Kate Leanne Willingha Tsai in Fall 2015. Since its upload, it has received 26 views. For similar materials see Fundamental Genetics in Biomedical Sciences at Clemson University.
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Date Created: 02/07/16
Terms Important People Key Ideas Examples Chapter 5: Sex Chromosomes Terms: o Dosage Compensation: how the body deals with extra X chromosomes Females have two copies of the X chromosome, therefore, they would produce twice as much gene product than males (males only have one X chromosome). o Barr Body: or “X body.” Dark stained part of nucleus that is an inactive X chromosome. o X-inactivation: chosen at random, a copy of the X chromosome is “turned off” in females to help with dosage compensation. Important People: o Barr: discovered a dark stained body in the nucleus of cells in cats. Named this body the Barr Body. He noticed this body during interphase of female cat cells only. o Lyon: came up with the Lyon Hypothesis to explain what the Barr Body is. Determined that the body is inactive X chromosomes. Key Topics: o Chromosomal Determination of Sex in Drosophila and Humans: Drosophila: uses XX and XY chromosomes to determine sex of an individual. The presence of a Y chromosome does not mean the individual will be a male. A ratio is used to determine if the individual will be male or female. If there are two X chromosomes, the individual will be female. If there is one X chromosome the individual will be male. Example: XX is a female. XXY is a female. XY is a male. XO is a male. For any individual with one X, it is a male. For any individual with two X, it is a female. In humans: uses XX and XY chromosomes to determine the sex of an individual. The lack of a Y chromosome will result in the individual being a female. Example: XX is female. XY is male. XXY is male. XO is female. o Dosage Compensation: The body’s way of compensating for overproduction of the X chromosome gene product. Only need one X chromosome in an individual. Therefore, males do not have Barr Bodies, since they only have one X chromosome to begin with. Since females have two, they will “toss out” an X chromosome. In Turner’s Syndrome: there are no Barr Bodies. Females only have one X to begin with (XO) which means they cannot afford to give up the X chromosome and will result in having no Barr Body. In Klinefelter Syndrome: The only time a male will have a Barr Body. There will be enough Barr Bodies to make it so the individual only has one X chromosome. o Example: If the individual is XXY he will have one Barr Body. If the individual is XXXY he will have two Barr Bodies and so forth. o X-inactivation: heterozygous females of a loci on the X chromosome will show one allele or the other allele in a particular cell. This gives the females a mosaic like depiction of heterozygous genes on the X chromosome Example There are two alleles for the color of orange fur of a cat: X+ results in a black color while X results in an orange color. + o For males: X Y will be a black cat. X Y will be an orange cat. Males will not result in a mixture of the two colors. Will be either black or orange. For females: X X will be a black cat. X X will be an orange cat. X X will be a mixture of the two colors. This results in a “tortoiseshell” or “mosaic” like pattern. (calico cats) o Each cell determines, at random, which X chromosome will become the Barr Body during development. During mitotic division, all future daughter cells from this cell, has the same Barr Body. This allows for the colors to be expressed in different proportions on different individuals. o If the cat has a black spot: the Barr Body is the X o chromosome (orange). o If the cat has an orange spot: the Barr Body is the X + chromosome (black) X Inactive-Specific Transcript (XIST): a gene located on the X chromosome and is required. It has to be active to turn off the extra X chromosome (creating the Barr Body) Chapter 6: Chromosome Mutations Terms: o Karyotype: display of a completed set of chromosomes, usually chromosomes in st the metaphase stage due to being the most condensed at this phase. 1 chromosome is the largest and 21/22 chromosomes are the smallest. o Aneuploids: the number of chromosomes that are being altered by an addition or deletion. o Polypoids: one or more complete sets of chromosomes that are added. Autopolyploidy: all chromosome sets are from a single species Allopolypoidy: chromosome sets are from different species o Duplication: region of DNA that is repeated Tandem: is adjacent to the copied region Example: Chromosome gene sequence is AB*CDE, the tandem duplication would be AB*CCDE where the C is adjacent to the region it copied (C). Displaced: the duplicated region is located a distance away the copied region or even on a different strand of DNA. Example: Chromosome gene sequence is AB*CDE, the displaced duplication would be AB*ECDE where the E is the displaced. o Deletion: A region of DNA is taken out of the sequence o Inversions: the gene order is changed (flipped). Paracentric Inversion: when an inversion takes place within a gene sequence that does not include the centromere. Example: Chromosome gene sequence is AB*CDEFG and the paracentric inversion would be AB*EDCFG where the CDE is inverted to EDC and does not include the centromere. Pericentric Inversions: the centromere is located within where the inversion takes place. Example: Chromosomes gene sequence is AB*CDEFG and the pericentric inversion would be ADC*BEFG where the B*CD is inverted to DC*B and the centromere is located within the inversion. o Translocations: Movement of gene sequence/region from one chromosome to another nonhomologous chromosome. This is not crossing over. Nonreciprocal Translocation: there is movement of one gene from chromosome 1 to chromosome 2 (nonhomologous chromosomes), but nothing is moved from chromosome 2 to chromosome 1. It is a one way flow of genes. Example: Chromosome 1 ABC*DEFG and Chromosome 2 HIJ*KLMN → (nonreciprocal translocation takes place) Chromosome 1 C*DEFG and Chromosome 2 ABHIJ*KLMN. Chromosome 2 has a translocation from Chromosome 1 (AB) but Chromosome 1 gains nothing from Chromosome 2. Reciprocal Translocation: There is movement of one gene from Chromosome 1 to Chromosome 2 and a gene movement from Chromosome 2 to Chromosome 1 (nonhomologous chromosomes). It is a two way flow. Example: Chromosome 1 ABC*DEFG and Chromosome 2 HIJ*KLMN → (reciprocal translocation takes place) Chromosome 1 HIJ*DEFG and Chromosome 2 ABC*KLMN. Chromosome 1 has a translocation from Chromosome 2 (HIJ) and Chromosome 2 has a translocation from Chromosome 1 (ABC). Robertsonian Translocation: a translocation takes place and a deletion. Causes some forms of Down Syndrome Example: Chromosome 1 A*BCDEFG and Chromosome 2 H*IJKLMN → (Robertsonian Translocation takes place) Chromosome 1 A*H and Chromosome 2 BCDEFG*IJKLMN. Chromosome 1 is so small it often gets lost (deletion). o Aneuploidy: is a change in the number of individual chromosomes, can lose or gain an entire chromosome. Nullisomy: loss of both members of a homologous pair (from mom and dad). In humans it is lethal. It is expressed as (2n-2) Monosomy: loss of a single chromosome. In humans it results in Turner’s Syndrome. It is expressed as (2n-1). It is lethal in humans unless it causes Turner’s Syndrome, meaning if it is any other chromosomes than the X chromosome, it will kill the individual. Trisomy: gain of a single chromosome. It is expressed as (2n+1). If an individual has an extra copy of two nonhomologous chromosomes, this is a double trisomic individual. Tetrasomy: gain of two homologous chromosomes. It is expressed as 2n+2. Needs to be homologous chromosomes for it to be tetrasomy. Uniparental Disomy: both chromosomes are inherited from a single parent. Key Topics: o Chromosomal Mutations: Can be chromosomal rearrangements (change in structure of chromosome), aneuploids (addition or deletion), and polypoids (complete sets of chromosomes are added). o Duplication and Loop formed during Meiosis: During meiosis, when homologous chromosomes align, the chromosomes will align so that the same genes will line up together, the duplicated part is pushed back into a loop shape and can be seen during meiosis of cells. Why does duplication alter phenotype? There is a certain level that the chromosome will want to maintain, if it has duplication and both genes are activated, then it results in 2x more product being produced by the duplicated gene. o Deletions and why they are bad: deletions are a loss of a part of chromosome structure. Results in: Loss of essential genes, which can be lethal if the gene lost is homozygous. Deletion of a portion where the centromere is located will lead to the loss of the chromosome Heterozygous deletions: o The products of the gene will become imbalanced o Some recessive alleles that are not deleted will be shown in the phenotype (pseudodominance) If the deletion is of the dominant (wild type) allele, then the recessive will be expressed (can be lethal or alter phenotype) o Some genes require two copies to produce enough product (haploinsufficicent) Can have the wild type allele but not express it because of not having enough product created. Will wind up being in-between the dominant and the recessive phenotypes. During meiosis, a loop will be formed to line up homologous chromosomes o Issues caused by Inversions: inversions are when a segment is turned around 180 degrees. can change the gene order of a chromosome, can break a gene (if breaks in the middle the gene will no longer be functional) Positions effect: a gene may need to be in front of a certain gene to make sure that a following gene is active or inactive. If the order is switched, it can cause a gene that needs to be active to not be or a gene that should be inactive be active. Results in a loss of control over a gene. Crossing over taking place: if crossing place takes place when a gene forms a loop (so the homologous chromosomes can line up properly) then it can cause issues for when the chromosomes split during meiosis. Will create nonviable gametes. o Jimson Weed: Has several different seed casings and are all different trisomics. Caused by trisomy in different chromosomes o Human Aneuploidies: Sex Chromosome aneuploides: chromosomes have mechanisms to cancel extra chromosomes or to compensate for loss (dosage composition). XYY: has very little impact on the individual due to the lack of genetic info found on the Y chromosome. This is the most known aneuploidy overall. Turner/Klinefelter: is another one o Autosomal Aneuploids: the smaller the chromosome the more tolerated it is. Down Syndrome (21): most common autosomal aneuploids (Trisomy). Chromosome 21 is a small chromosome with not a lot of genes on it compared to the others, it is easier for the other chromosomes to balance out the extra genes. Trisomy 18: Edward Syndrome, severe, die by age 1 Trisomy 13: Patau syndrome, 50% die within 1 month, 95% die by age 3 Trisomy 8: mosaic individuals and can live normal life expectancy o Down Syndrome: 75% results from a nondisjunction in the mother. 2 of the 3 chromosome 21 comes from the mom while the third one comes from dad. Primary Down Syndrome increases with Maternal Age: the older the mother is before fertilization, the more likely the spindle fibers will snap creating nondisjunction. Familial Down Syndrome: occurs when translocation of part of Chromosome 21 occurs. This runs in families (parents have undergone Robertsonian translocation) one copy of 21 is moved to chromosome 15. The individual becomes a carrier of downs syndrome. Not dependent on gender or age
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