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These include disorders like Down’s syndrome, Klinefelter syndrome, Turner syndrome, and XYY syndrome.
Down’s syndrome is also known as trisomy 21. From the name, onecan tell what the actual disorder is. Instead of the required two, thereare three (tri) chromosome 21’s. This is the smallest humanchromosome .19. Trisomy 21 is caused by a meiotic nondisjunction event. Innondisjunction, a gamete is produced with an extra copy of chromosome 21 causing the gamete to contain 47 chromosomes.Most cases of trisomy 21 result from nondisjunction in the materalgamete.20.In rare cases, some cells in the body have 46 chromosomes whileothers have trisomy 21. this is called mosaic Down syndrome. Itoccurs due to a nondisjunction event during an early cell division inthe embryo instead of the nondisjunction in a sex cell.
Down syndrome is estimated to occur at somewhere between 1 in800 to 1 in 1000 births. Maternal age greatly influences the changesof conceiving a baby with Down syndrome. For women 20 to 24 theprobability is 1 in 1562. This rockets to 1 in 19 for women over 45.22. Parthenogenesis, is an asexual form of reproduction found infemales where growth and development of embryos or seeds occurswithout fertilization by a male. The offspring produced byparthenogenesis are always female. While this process occursnaturally in some species, it has also been induced artificially inother species.
In this process, females produce eggs that develop withoutfertilization and develop without a Y chromosome thus producingonly females. Parthenogenesis occurs naturally in aphids, daphnia,rotifers, some other invertebrates, and in many plants. Somevertebrates like komodo dragons and hammerhead-sharks alsocomplete this process naturally.24.Only in the XY sex-determination system will all offspring be female.In the less common ZW sex-determination system, bothchromosomes passed on will be those associated with males. Theprocess involves the inheritance and subsequent duplication of onlya single sex chromosome.25.When parthenogenesis occurs naturally, the offspring is capable of sexual reproduction. This process of asexual reproduction hasexisted since the beginning of life on Earth which implies that issuccessful.26. As with all types of asexual reproduction, there are costs andbenefits. The cons comprise of low genetic diversity andsusceptibility to adverse mutations that might occur. The main pro isthat of reproduction without the need for a male. A vital characteristicin perhaps a dying species.