Chr. Aberrations _Ch. 8_

Chr. Aberrations _Ch. 8_ - Chapter 8 : Chromosome Behavior...

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Chapter 8 : Chromosome Behavior The Human Karyotype. The chromosome complement of individuals of a species is usually the same (with the exception of XX/XY chromosome difference in species where there is a chromosomal mechanism of sex determination). Not only are the numbers the same in different individuals, but also so is the chromosomal morphology. The appearance of chromosomes in a metaphase spread ( Figs. 8.1A, 8.2A) and its analysis by size and banding (Fig. 8.2B)- the karyotyp e, reveals a constancy of chromosome organization within a species. Since several chromosomes are of the same approximate size, use is made of special staining techniques (fluorescent in situ hybridization technique as shown in Fig. 8.1 or banding techniques, such as Giemsa or G-banding shown in Fig. 8.2 ) to identify the 2 homologous chromosomes. The standard nomenclature for chromosomes is “p” for the short arm and “q” for the long arm . Each band in the human karyotype is given a number ( Fig. 8.3 ). Also similar size chromosomes are grouped together (for example, chromosomes 1,2 and 3 are part of group A and chromosomes 19 and 20 are part of group F; Figs. 8.2B, 8.3). The conventional abbreviations used to describe human karyotype are shown in Table 8.1 (no need to memorize this table). Depending upon the location of the centromere in a chromosome, it is called metacentric (centromere in the middle, V-shaped at anaphase; Fig. 8.5 ), submetacentric (centromere is off-center, J-shaped) and acrocentric or telocentric (centromere near the end, l- shaped). A chromosome fragment without a centromere is called acentric and an abnormal chromosome with 2 centromeres is dicentric . Acentric and dicentric chromosomes are unstable. Pages 256-259-304 describe X-chromosome inactivation. In humans, there is dosage compensation of X-chromosomes and silencing of all but one X-chromosome in human females. This occurs on the 16 th day after fertilization. In each cell of the embryo, one of the 2 X-chromosomes is randomly silenced by compaction of the DNA (heterochromitinization). The inactive X-chromosome is visible in the female cell’s nuclei and is called Barr body . As a result, human females are a mosaic. Also, this explains why females do not show the deleterious effects of X-linked recessive mutations (such as color-blindness or hemophilia) even though only one active X-chromosome is present. Due to the random silencing of the X- chromosomes, half the cells of a heterozygous female are still normal. The condensation of the inactive X-chromosome begins at a site called X-inactivation center ( XIC ). Present day X and Y-chromosomes evolved from a common ancestral chromosome by loss of most of the Y-chromosome genes. Clearly, there still is some homology between the X- and the Y-chromosome since they can pair in meiosis and undergo crossing over. Actually there are some genes in the X-chromosome that have homologs in the Y-chromosome (these do not follow sex-linked inheritance since both males and females have 2 copies each; they behave
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This note was uploaded on 09/29/2011 for the course GENETICS 380 taught by Professor Glodowski during the Spring '08 term at Rutgers.

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Chr. Aberrations _Ch. 8_ - Chapter 8 : Chromosome Behavior...

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