1) they have similar chemical properties.
for example fluorine from the 17th column is a non metal so does all the elements of that group.
2)they have same valence electrons in their outermost shell
example
electronic configuration of
hydrogen 1, lithium 2,1, sodium 2,8,1 and so on.
3) have same valency.
If one person in a family has a cancerous cell, it can easily be passed through the family as cells multiply very rapidly. for exaple stem cells can change into any cell including a cancerous cell.
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It was developed by a Swedish<em> </em>Botanist<u> </u><em><u>Linnaeus</u></em> in 17th century.