Sex Chromosomes, Barr Body and Lyon Hypothesis PDF

Sex Chromosomes, 2 Barr Body and Lyon Hypothesis by Dr.Ezat Mersal At the end of this lecture, you will be able to:- 1. To understand X inactivation 2. To determine the number of barr bodies in a human cell. 3. To explain Lyon hypothesis 4....

Sex Chromosomes, 2 Barr Body and Lyon Hypothesis by Dr.Ezat Mersal At the end of this lecture, you will be able to:- 1. To understand X inactivation 2. To determine the number of barr bodies in a human cell. 3. To explain Lyon hypothesis 4. To recognize the advantages of barr bodies determination.  Chromosomal sex is determined at fertilization  Sexual differences begin in the 7th week  Sex is influenced by genetic and environmental factors  Females(generally XX) do not have a Y chromosome  Males (generally XY) have a Y chromosome © 2015 John Wiley & Sons, Inc. All rights reserved. © 2015 John Wiley & Sons, Inc. All rights reserved.  Aneuploidy Having too many or too few chromosomes compared to a normal genotype  Monosomy Possessing only 1 copy of any particular chromosomes Monosomy, with the exception of the X chromosome is incompatible with life Xisde 90" © 2015 John Wiley & Sons, Inc. All rights reserved.  Sex Chromosomes: females XX, males XY Genes on X: females 2 , males 1  How to create equal amount of X chromosome gene products in males and females?  decrease X gene products by half in females called in human X-inactivation © 2015 John Wiley & Sons, Inc. All rights reserved.  This was shown by culturing cells with different karyotypes XY XO both with 1 X chromosome XX XXY both with 2 X chromosomes XXX XXXX both with 3 or more X chromosomes So, now:  We observe the levels of enzymes encoded by the X chromosome © 2015 John Wiley & Sons, Inc. All rights reserved.  -  G6PD, glucose 6 phosphate dehydrogenase, gene is carried on the X chromosome  This gene codes for an enzyme that breaks down sugar  Femalesproduce the same amount of G6PD enzyme as males  XXY and XXX individuals produce the same about of G6PD as anyone else © 2015 John Wiley & Sons, Inc. All rights reserved.  Only 1 X chromosome is active in any given cell. The other is inactive  In some cells the paternal allele is expressed  In other cells the maternal allele is expressed  InXXX and XXXX females and XXY males only 1 X is activated in any given cell the rest are inactivated © 2015 John Wiley & Sons, Inc. All rights reserved.  XXX embryo survives because it inactivates 2 X chromosomes and has only 1 functioning X chromosome in any given cell  Trisomy 21 can not inactivate the extra copy of chromosome 21. So you have Down syndrome The only chromosome we can inactivate is the X - chromosome © 2015 John Wiley & Sons, Inc. All rights reserved.  Ag Euchromatin = true chromatin > - Light Chromosomal regions that possess active genes dark/denser Heterochromatin > -0 - These regions stain darker than euchromatin Highly repetitive DNA with very few active genes Usually found around the centromere and near the tips of chromosomes (telomeres) Facultative Heterochromatin >uchromatineatin - Active like euchromatin in some cells and inactive like heterochromatin in other cells © 2015 John Wiley & Sons, Inc. All rights reserved. © 2015 John Wiley & Sons, Inc. All rights reserved.  sS =  Serves as a mechanism for a cell to shut off a portion or an entire chromosome to prevent gene expression  X chromosome is made up of facultative heterochromatin  Active X chromosome behaves like euchromatin, with active genes that are transcribed  Inactive X chromosome behaves like heterochromatin © 2015 John Wiley & Sons, Inc. All rights reserved.  Thecell inactivates one X chromosome by converting the entire chromosome to heterochromatin or inactive DNA Hel inactive / Chromosome  Thisinactive DNA makes up the dark staining Barr body  Thisprocess occurs in females or any individual with more than one X chromosome Y t 1X1X activ inactiv © 2015 John Wiley & Sons, Inc. All rights reserved.  In 1961 English geneticist Mary Lyon proposed this hypothesis to describe X inactivation  Consists of 5 tenants  1.Condensed X chromosome is genetically inactive  2.X inactivation in humans occurs early in development when embryo consists of about 32 cells. 1 or 2 days following fertilization & - © 2015 John Wiley & Sons, Inc. All rights reserved.  3.At this stage in each of the 32 cells one of the X chromosomes is W randomly inactivated  4. Inactivation is- mitotically stable  5.Net effect of this is to equalize phenotypes in males and females for genes that are carried on the X chromosome same level of y chromosome © 2015 John Wiley & Sons, Inc. All rights reserved.  OneX chromosome in females is inactivated early in embryonic development. © 2015 John Wiley & Sons, Inc. All rights reserved. © 2015 John Wiley & Sons, Inc. All rights reserved.  Present in somatic XX nuclei  Not present in XY nuclei Females Barr Bodies Active X XX 1 1 Males Barr Bodies Active X /1 XO 0 1 XY O 0 1 XXY 1 1 "His XXX XXXX 2 3 1 1 - XXXY 2 1 © 2015 John Wiley & Sons, Inc. All rights reserved.  Inepithelial cells: (buccal smear)  Barr body appears as darkly mass in the Wnuclear membrane (30-50% of the cell of normal female)  Inblood leucocytes: sex chromatin appears as drum stick appearance attached to the nucleus in 3% of female’s neutrophils. © 2015 John Wiley & Sons, Inc. All rights reserved.  Female XX male XXY T barody barrdy G © 2015 John Wiley & Sons, Inc. All rights reserved. By staining the sex chromatin, we can:  Diagnose the genetic sex in doubtful cases as hermaphrodism  Study abnormalities of sex chromosomes  Diagnose abnormal sexual development and maturation.  Diagnose of infertility, primary amenorrhea, spontaneous abortion.  Identification of fœtal sex by staining cells from amniotic fluid. © 2015 John Wiley & Sons, Inc. All rights reserved.  Basic genetics : a human approach / BSCS. Dubuque, IA, Kendall/Hunt Pub. Co., c1999. 147 p. QH431.B305 1999  Genes, ethnicity, and ageing. Edited by Lincoln H. Schmitt, Leonard Freedman, Rayma Pervan. Nedlands, Australia, Centre for Human Biology, University of Western Australia ; Singapore, River Edge, NJ, World Scientific, c1995. 100 p.QH455.G45 1995  Genetic polymorphisms and susceptibility to disease. Edited by M. S. Miller and M. T. Cronin. New York, Taylor & Francis, 2000. 266 p.  GENETIC ANALYSIS AN INTEGRATED APPROACH Mark F. Sanders , John L. Bowman Second edition 2015 ISBN 978 0-321-94890-8 (student edition) www.pearsonhighered.com  Boundless Biology: Modern Understandings of Inheritance https://courses.lumenlearning.com/boundless- biology/  Anthony JF Griffiths, William M Gelbart, Jeffrey H Miller, and Richard C Lewontin (1999): modern genetic analysis