Sex chromosomes1


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Sex chromosomes1

  1. 1. In mammals the gender of an individual is determined by the combination of the sexchromosomes. Females posses two X-chromosomes and are said to be homogametic whereas males areheterogametic with one each of an X- and Y-chromosome. Females can only donate an X-chromosome. If themale also donates an X-chromosome the offspring will be female. If the male donates a Y-chromosome theoffspring will be heterogametic and consequently male. In mammals the sex chromosomes are very differentsizes.
  2. 2. We have said that humans cells have 46 chromosome which consistof two sets of 23. Of those 23 chromosomes in a haploid set, 22have nothing to do with the determination of sex during fetaldevelopment. They are called autosomes. However, the remainingchromosome is called a sex chromosome which can be found in twodifferent forms: X and Y. Normal females have XX combinations.Normal males have XY combinations.
  3. 3. Cells of females have two X chromosomes. Cells of males have one X chromosome and one Y chromosome.Meiosis produces female gametes which all contain one X chromosome. Half of the male gametes contain an X andhalf contain a Y chromosome.
  4. 4. The sperm cell are the male reproductive cell, this cells contain one X and one Y chromosome.
  5. 5. The egg cell are the female reproductive cell, these cells contain two X chromosome.
  6. 6. In Klinefelter Syndrome, the last pair of chromosome have an extra Y chromosome
  7. 7. Turner Syndrome affects 60,000 girls and women in the United States. This disorder occurs in 1 in 2000to 1 in 2500 live births, with about 800 new cases diagnosed each year. Symptoms include short stature and lack ofovarian development. Other features, such as webbed neck, arms that turn out slightly at the elbow, and a low hairlinein the back of the heads are sometimes seen. Women and girls with Turner Syndrome have only one X chromosome. This is an example of monosomy.
  8. 8. Before meiosis the precursor cells of sperm and ovum undergo DNA replication. Only only chromosomeis shown e.g. the sex chromosome of a male (XY). Blue: replicated Y chromsome, red; replicated X chromosome.Meiosis I results in two daughter cells, one cell contains the replicated Y chromosome, the other the replicated Xchromosome. In meiosis 2 the replicated Y and X chromosomes are separated to form the haploid sperm. Theseparation of the Y chromosome was normal, however in this example the x chromosome did not sepatate. Among thetwo resulting sperms one has no X chromosome, the other one has two X chromosomes. If an egg is fertilized with thesperm without the X chromosome a OX condition is exists (Turner syndrome) . If an egg is fertilized with the sperm thathas the two X chromosomes a condition XXX exists .
  9. 9. Trisonomy 21 also known as down syndrome have 3 chromosomes in the 21 pair of chromosome.