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Biology 212 General Genetics Spring 2007 Lectures 20 & 21: Human Genetics I & II: Chromosome Defects Reading: Text Chap. 5 to p. 168, pp. 174-189 Lecture outline: 1. Human karyotypes 2. Sex chromosomes 3. Variations in chromosome # 4. Variations in chromosome structure 5. Polyploidy Lecture: 1. Human karyotypes Karyotype: An array of all the chromosomes in a metaphase cell of an organism. Human karyotype 46, XX female 46, XY male consists of 22 pairs of autosomes, 1 pair of sex chromosomes Preparation of a karyotype: (see Fig. 5.2) 1) Treat blood cells with colchicine to arrest dividing cells in metaphase 2) Spread on a glass slide 3) Stain with Giemsa 4) Photograph 5) Use computer to cut and paste chromosome images to create karyotypes 6) Chromosomes arranged in pairs from largest (#1) to smallest (#22), followed by sex chromosomes XX or XY Chromosome painting Uses fluorescently tagged DNA markers to identify each chromosome (see Fig. 5.1) by hybridization Anatomy of a chromosome telomere shorter arm = p (petit) centromere longer arm = q (queue: a long line) telomere 1 Role of centromere Stabilize chromosome movement during cell division Site where chromosome attaches to spindle Acentric chromosome: Chromosome that lacks a centromere; segregates abnormally in mitosis or meiosis. Dicentric chromosome: Chromosome with two centromeres; unstable, usually breaks during cell division. Centromere position in normal chromosomes can vary: Metacentric: centromere in middle of chromosome Submetacentric: centromere closer to one end Acrocentric: centromere near the end of the chromosome 2. Sex chromosomes female 46, XX male 46, XY X chromosome female has twice the genetic material than males for genes on the X chromosome dosage compensation: adjust for levels of gene expression in humans and many organisms dosage compensation occurs by X-inactivation the X-chromosome inactivated in each cell is randomly chosen can observe inactive X chromosome as a Barr body in the cell nucleus Y chromosome Few genes are represented Encodes SRY, master sex-determining gene SRY encodes a transcription factor that is the testis determining factor Transcription factors help determine the level of transcription of a gene The pseudoautosomal region on the Y chromosome pairs with a region on the X The Y chromosome is progressively shortening in mammalian evolution 3. Variations in chromosome number Chromosome abnormalities occur in about 7.5% of all miscarriages. Types of common abnormalities (Table 5.1) Trisomy: one extra chromosome Monosomy: missing one chromosome 45, X (Turner syndrome) only viable human monosomy 2 Translocation: a chromosome rearrangement Polyploid: extra set of chromosomes Aneuploidy Monosomy and trisomy are examples Contains extra or missing chromosomes Have an unbalanced chromosome set Chromosomes behave unstably in cell division A common trisomy: Down syndrome = Trisomy 21 Affected individuals have 3 copies of chromosome 21 Affects 1/750 live births Symptoms: Mental retardation Short stature Similar face shape Shortened life span (<50 years) Often have heart defects Happy, responsive Major cause of Down syndrome: Non-disjunction = improper chromosome segregation in meiosis I or meiosis II Occurs more commonly in oogenesis Likelihood of conceiving affected child mainly depends on age of mother (Fig. 5.12) Minor cause: hereditary form Chromosome translocation Sex chromosome abnormalities Due to non-disjunction in meiosis 47, XXX female, mostly normal 47, XYY male, tall, phenotypically mostly normal 47, XXY male, Klinefelter syndrome, tall sterile, often mentally impaired 45, XO female, Turner syndrome, short, sterile—doesn’t reach sexual maturity 3 4. Variations in chromosome structure A. Deletion or deficiency A B C D E --> ABE Missing a segment of the chromosome Large mutations are usually lethal B. Duplication A B C D E --> A B B C D E Region of the chromosome is present twice Produced by unequal crossing over C. Inversion A B C D E --> A D C B E A segment of the chromosome with the genes in reverse order Inverted segment may include the centromere = pericentric inversion OR inverted segment may be outside the region with the centromere = paracentric inversion D. Translocation A B C D E --> A B Y Z XYZ XCDE Exchange of parts between non-homologous chromosomes Reciprocal translocation (balanced translocation) is swapping the ends of the chromosomes Translocation may not be reciprocal Individuals with chromosome defects have high risk of errors in meiosis Refer to practice problems that look at pairing of chromosomes in heterozygotes 5. Polyploidy = extra full set of chromosomes lethal in humans common in plants to have extra chromosome sets haploid = chromosome set in a gamete diploid = chromosome set in most eukaryotic cells = 2 monoploid sets 4 triploid = 3 monoploid sets tetraploid = 4 monoploid sets In evolution of plants it is common for hybridization of gametes to occur from different but related species to produce a new polyploid species In agriculture, polyploids often produce larger flowers and or larger or more desirable fruit. A few other organisms are also polyploid: trout are tetraploids 5