The Hijra – tall, athletic, muscular – is conspicuous at the street crossing near my home in Delhi. I get anxious when she approaches my car: the red coloured cheeks, matching lipstick and a glittering orange saree fail to camouflage her masculinity. Her hairy hands pound at the car window and she demands money. I feel extorted. If I were a legislator, my prejudice would pronounce her / him illegal, because my mind floods with repulsion against this shape of sexuality. But being a doctor, I try to understand: what make us so.
Most of us are heterosexual. But our sexuality can manifest in three other ways, often with overlap. First, we could be homo sexual, lesbian or bisexual; second, we could be intersex, when we carry internal organs of one gender, but external organs of the opposite; and third, we could be transgender if we have emotional urge to identify with the opposite gender. 
Let us start at the beginning. We carry 23 pairs of chromosomes in all our cells, the twenty-third pair determines our gender: females carry XX and males have XY chromosomes. At the first dance of fertilization, chromosomes split from their original mates and pair up with new partners. The sex chromosome, XX from the egg splits into X and X and the XY from the sperm, separates into X and Y. The new paring produces either a new male XY or a new female XX. 
When the embryo is six weeks old, the Y chromosome of XY pair signals to the latent embryonic tissue to start differentiating into a male. Testicular tissue sprouts and secrets testosterone, which initiates the formation of what would become future male genitalia: penis, testes and scrotum. The XX pair does not produce testosterone and the embryonic tissue continues to develop into female organs: ovaries, uterus and Fallopian tubes. 
The testosterone and other unknown chemicals also influence the growth of embryonic cells of brain; some cells in male hypothalamus become different from that of a female.
But fetal development is prone to missteps. Chromosomes may combine as XXY or XYY affecting the sexuality of the adult. Or the embryo may produce excessive or insufficient testosterone causing over or under-virilisation.
Disorder of embryonic development result in Intersex. We could carry an XX chromosome with an ovary but have male external genitals or have XY chromosomes with testes but female external genitalia. Or we could carry one ovary and one testes and undeterminable external sex organs.
Genes may also predispose a person to become homosexual, lesbian or transgender. Studies show similar sexual behavior among identical twins; chance of homosexuality in an identical twin of a homosexual male varies from 20% to 60 % in different studies.  Meta-analysis of all available data shows a presence of a marker (called Xq28) on the X chromosome of a significant number of homosexual brothers. And there is higher frequency of homosexuality among uncles and cousins from maternal side. Since these variations are not true in a hundred percent cases, scientists believe, other developmental factors may be at play.
We become sexually different because of our biology, on which we have no control. Should we make such genetic variations illegal? Prejudiced irrational legislation can push innocent individuals into trouble; section 377 of the Indian Penal Code is a prime example.
(Dr. Shiban Ganju is the Founder of Save a Mother Foundation. Opinion expressed in the article are his personal. )