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MATTERS OF SEX  Anueploidy having too many or too few chromosomes compared to a normal genotype having too many or too few chromosomes compared to a normal.

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Presentation on theme: "MATTERS OF SEX  Anueploidy having too many or too few chromosomes compared to a normal genotype having too many or too few chromosomes compared to a normal."— Presentation transcript:

1 MATTERS OF SEX  Anueploidy having too many or too few chromosomes compared to a normal genotype having too many or too few chromosomes compared to a normal genotype  Monosomy Possessing only 1 copy of any particular chromosomes Possessing only 1 copy of any particular chromosomes  Monosomy, with the exception of the X chromosome is incompatable with life

2 X chromosome dosage  How to create equal amount of X chromosome gene products in males and females? Sex Chromosomes: females XX, males XY Genes on X: females 2, males 1copy decrease X gene products by half in females (e.g. humans called X-inactivation)

3 X CHROMOSOMES  This was shown by culturing cells with different karyotypes XYXOboth with 1 X chromosome XYXOboth with 1 X chromosome XXXXYboth with 2 X chromosomes XXXXYboth with 2 X chromosomes XXXXXXXboth with 3 or more X chromosomes XXXXXXXboth with 3 or more X chromosomes  Looked at levels of enzymes encoded by the X chromosome

4 X LINKED GENE  G6PD, glucose 6 phosphate dehydrogenase, gene is carried on the X chromosome  This gene codes for an enzyme that breaks down sugar  Females produce the same amount of G6PD enzyme as males  XXY and XXX individuals produce the same about of G6PD as anyone else

5 G6PD GENE  There are variant alleles of the G6PD gene G6PD A G6PD B G6PD A G6PD B  Produce different variants of the enzyme, but both break down sugar  Female could be heterozygous for G6PD A and G6PD B  Each cell only produces 1 of the 2 forms of the enzyme

6 X CHROMOSOME  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  In XXX and XXXX females and XXY males only 1 X is activated in any given cell the rest are inactivated

7 X CHROMOSOME  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

8 Table 12.3

9 3 TYPES OF CHROMATIN  Euchromatintrue chromatin Chromosomal regions that possess active genes Chromosomal regions that possess active genes  Heterochromatin These regions stain darker than euchromatin These regions stain darker than euchromatin Highly repetitive DNA with very few active genes Highly repetitive DNA with very few active genes Usually found around the centromere and near the tips of chromosomes (telomeres ) Usually found around the centromere and near the tips of chromosomes (telomeres )  Facultative Heterochromatin Active like euchromatin in some cells and inactive like heterochromatin in other cells Active like euchromatin in some cells and inactive like heterochromatin in other cells

10 Facultative Heterochromatin  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

11 Facultative Heterochromatin  The cell inactivates one X chromosome by converting the entire chromosome to heterochromatin or inactive DNA  This inactive DNA makes up the dark staining Barr body  This process occurs in females or any individual with more than one X chromosome

12 LYON HYPOTHESIS  1961 English geneticist Mary Lyon proposed this hypothesis to describe X inactivation  Consists of 5 tenants 1. Condensed X chromosome is genetically inactive 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 2. X inactivation in humans occurs early in development when embryo consists of about 32 cells. 1 or 2 days following fertilization

13 5 TENANTS OF LYON HYPOTHESIS  3.At this stage in each of the 32 cells one of the X chromosomes is 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

14 Human Chromosomes  One X chromosome in females is inactivated early in embryonic development.

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17 PAR REGION NOT INACTIVATED  RSP4 gene in this region present on both X and Y chromosomes  Encodes a protein that makes up part of the ribosome  If this gene were inactivated it would reduce by half the number of ribosomes made and reduce the protein synthesized capacity of that cell by half

18 X REACTIVATION IN FEMALES  In the female fetus future germ cells undergo Lyonization along with somatic cells at the 32 cell stage  Following differentiation of female fetus, the inactivated X chromosomes are reactivated during female gametogenesis  When germ cells develop into oocytes and enter meiosis their inactivated X chromosomes become reactivated so that every egg produced has an activated X chromosome prior to fertilization

19 X REACTIVATION IN MALES  XXY Klinefelter males also reactivate the second X chromosome during gametogenisis  The presence of an extra X chromosome during early puberty causes death of male germ cells and testicular atrophy  This leads to low levels of testosterone

20 NONDISJUNCTION OF SEX CHROMOSOMES Extra copies of the X and Y chromosomes do not cause the severe problems that extra autosomes do Nondisjunction in mother would produce eggs that are XX or O with no X chromosome If XX egg fertilized with X sperm get an individual who is XXX. Individual will be sterile and have 2 Barr bodies

21 NONDISJUNCTION OF SEX CHROMOSOMES If XX egg fertilized with Y sperm get an individual who is XXY. Individual will be sterile male with many female body characteristics. Known as Klinefelter syndrome Occurs 1/500 male births If O egg fertilized by Y sperm the zygote is non- viable If O egg fertilized by X sperm get and individual who is XO. Turner syndrome

22 NONDISJUNCTION OF SEX CHROMOSOMES Turner syndrome Occurs 1/2000 live births Sterile Can also have non-disjunction of the Y chromosome in males Produce sperm with 2 Y chromosomes Fertilize an X egg develop into XYY male Fertile males Occurs 1/1000 males

23 MOSAICISM  Is due to a mitotic loss of 1 X chromosome in a cell early in zygotic development  This produces a combination of both XX and XO cells  The more XO cells an individual has the more severely she will be affected  Some estimates put mosaic Turner females as high as 60% to 80%

24 TURNER SYNDROME  Newborns may not be affected  Lag behind classmates in sexual development  Reach puberty they fail to menstruate  Small uterus  Rudimentary ovaries Remains a primitive streak gonad Remains a primitive streak gonad  Sterile W/O an ovary they can not produce eggs or estrogen W/O an ovary they can not produce eggs or estrogen

25 TURNER SYNDROME  Can lead fairly normal lives if they receive hormone supplements  Hormones promote breast development and other secondary sex characteristics if administered during puberty  Growth hormone adds up to 3 inches of height. Can mean the difference between an adult height of 4’11” to 5’2”


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