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Sex chromosomes1
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2. In mammals the gender of an individual is determined by the combination of the sex
chromosomes. Females posses two X-chromosomes and are said to be homogametic whereas males are
heterogametic with one each of an X- and Y-chromosome. Females can only donate an X-chromosome. If the
male also donates an X-chromosome the offspring will be female. If the male donates a Y-chromosome the
offspring will be heterogametic and consequently male. In mammals the sex chromosomes are very different
sizes.
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8. We have said that humans cells have 46 chromosome which consist
of two sets of 23. Of those 23 chromosomes in a haploid set, 22
have nothing to do with the determination of sex during fetal
development. They are called autosomes. However, the remaining
chromosome is called a sex chromosome which can be found in two
different forms: X and Y. Normal females have XX combinations.
Normal males have XY combinations.
9. 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 and
half contain a Y chromosome.
10. The sperm cell are the male reproductive cell, this cells contain one X and one Y chromosome.
11. The egg cell are the female reproductive cell, these cells contain two X chromosome.
15. Turner Syndrome affects 60,000 girls and women in the United States. This disorder occurs in 1 in 2000
to 1 in 2500 live births, with about 800 new cases diagnosed each year. Symptoms include short stature and lack of
ovarian development. Other features, such as webbed neck, arms that turn out slightly at the elbow, and a low hairline
in 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.
16. Before meiosis the precursor cells of sperm and ovum undergo DNA replication. Only only chromosome
is 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 X
chromosome. In meiosis 2 the replicated Y and X chromosomes are separated to form the haploid sperm. The
separation of the Y chromosome was normal, however in this example the x chromosome did not sepatate. Among the
two resulting sperms one has no X chromosome, the other one has two X chromosomes. If an egg is fertilized with the
sperm without the X chromosome a OX condition is exists (Turner syndrome) . If an egg is fertilized with the sperm that
has the two X chromosomes a condition XXX exists .
17. Trisonomy 21 also known as down syndrome have 3 chromosomes in the 21 pair of chromosome.
