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Sex is a type of reproduction common among living things. In humans, the sex of a person depends on what sex chromosomes that person got from his or her parents.
It works by combining genes from more than one source. When a sperm and ova combine to form a fertilised egg, the baby may get either of these chromosomes from its father.
If the baby gets two X chromosomes, it will develop into a girl.
If the baby gets one X and one Y, it will develop into a boy.
There are occasional exceptions to this rule: the process of meiosis which makes the sex cells can go wrong.
This results in an individual having 3 X chromosomes, or 2 Y chromosomes or XXY instead of XY.
Some genetic disorders may produce physical or mental traits which need special treatment. Other conditions may have partly genetic causes: mental retardation, attention-deficit hyperactivity disorder (ADHD), obsessive compulsive disorder (OCD) and schizophrenia.
Before babies are born, they grow either male sex parts or female sex parts.
Sexual reproduction first appeared about a billion years ago. Sexual reproduction is a process that can only be found in eukaryotes. There are other kinds of organisms (other than animals, plants and fungi), the other eukaryotes – such as the malaria parasite – that also engage in sexual reproduction.
Some bacteria use conjugation to transfer genetic material between bacteria.
This is not the same as sexual reproduction, but it also results in the mixture of genetic traits.
In sexual reproduction the cells used for reproduction, called gametes, are either eggs or sperms. The mechanism of cell division only works when one sperm alone enters the egg.