Parallel and Cross Cousins
There are different relationships in this world, in which siblings relationship are studied in two different ways too. Firstly there was an only concept leading to real siblings irrespective of their gender which has evolved from your parents. But then two types of siblings came into the picture that is a parallel cousin and cross-cousin.
Parallel cousins are those cousins who have evolved either through your mother’s sister or your father’s brother. In simple words, they are the kind of siblings belonging to the same domain. Suppose your father has only one brother, so your uncle having kids either a boy or a girl will be your parallel cousin. But the concept of getting married in Parallel cousin is boycotted as it is firmly believed by many religions that due to this arrangement the whole family into the dark.
Then there is a different kind of cousin too, viz. Cross-Cousin.
Cross cousins are those cousins who have evolved just from the opposite sex of your mother or father. For example, your father’s sister kids will be your cross cousins and so is the case with your mother’s brother kids. For example, if your maternal uncle has two kids then these kids will be your cross-cousins. The concept of the cross cousin is directly related to marriage process in some of the communities. For example, in a Southernmost part of India, there has been a practice of bifurcate merging in which there is marriages in their own lineage. Muslim communities to follow this kind of marriages in their own families. Although not every community practices it, in some communities this is taboo. For example talking about the Northern most part of India, people do not encourage the marriages in outside of families which belong to the same lineage or popularly called as gotra.
Richard D. Alexander was the one who discouraged the idea of having marriages in the same lineage as it can be seen as marrying to their half-siblings. There are many cases in which there is seen that by marrying own cousins; there is high chances of some kind of mental or physical deformity. For example there are many birth defects in the children who are evolved from parallel cousins.