PCB3063_Chapter08_Spring2018 (1).doc - PCB 3063 Chapter 8...

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PCB 3063 Chapter 8 CHAPTER 8 VARIATION IN CHROMOSOME STRUCTURE AND NUMBER INTRODUCTION CHROMOSOME STUDIES CYTOGENETICS is the field that involves the microscopic examination of chromosomes - It typically examines the chromosomal composition of a specific cell or organism - This allows the detection of individuals with abnormal chromosome number or structure - This also provides a way to distinguish between species • Refer to Figure 8.1 Types of chromosomes • Chromosomes can be divided into four main types based on their centromere location • Three main features are used to identify and classify chromosomes 1. Size 2. Location of the centromere 3. Banding patterns - These features are all seen in a KARYOTYPE CHROMOSOMAL VARIATION • Chromosomal variation can occur in two basic ways: Structure and Number • The study of chromosomal variation is important for several reasons 1. They can have major effects on the PHENOTYPE of an organism 2. They can have major effects on the phenotype of the organism’s OFFSPRING 3. They have been an important force in the EVOLUTION of species • A given abnormality may be present in all body cells, or in some of them - MOSAICISM = The presence of two or more cell lines in an individual, one or more of which are abnormal VARIATION IN CHROMOSOME NUMBER
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PCB 3063 Chapter 8 • Chromosome numbers can vary in two ways (Figure 8.15) - EUPLOIDY - Changes in the entire set of chromosomes - Occurs occasionally in animals and frequently in plants - ANUEPLOIDY - Changes in individual chromosomes rather than in the complete set - Regarded as abnorma l conditions • In general changes in chromosome number have a MORE SEVERE effect on survival than changes in chromosome structure ANEUPLOIDY • Aneuploidy includes - Monosomy = One homologue is missing - Trisomy = One extra homologue is present - Tetrasomy = Two extra homologues are present - Double monosomy = Two different homologues are missing - Double trisomy = Two different extra homologues are present - Nullisomy = A whole pair of chromosomes is missing • Note: These definitions apply to a DIPLOID cell • Aneuploidy commonly causes an abnormal phenotype - It leads to an IMBALANCE in the amount of gene products - Refer to Figure 8.16 Monosomy - Tolerated in some plants - In Drosophila - Haplo-IV = Survive but have problems! Missing one small copy of chromosome 4 have limited flight - Lethal in humans except for the X chromosome we can only tolerate 1 copy of chromosome X Turner Syndrome
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PCB 3063 Chapter 8 - Syndrome first described by Henry Turner in 1938 - Chromosomal abnormality first described by Ford et al. in 1959 - Karyotype = 45, X 45, XO - Frequency = 1 in 10,000 LIVE FEMALE BIRTHS - Features = - Short stature - Webbed neck - Underdeveloped ovaries (streak gonads) STERILE - Wide chest with widely-spaced nipples - About normal intelligence - Failure to develop secondary sexual characteristics - 40-50% of patients are mosaics - ~99% of Turner syndrome conceptions are lost prenatally - Therapy may help - Anabolic steroids - Estrogen
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