Why do I look like this?

The spermatozoid of my father had 23 chromosomes and my mother's ovule too; those come together (fertilization) to form the zygote, next, by the mitosis the cells divide and then form the embryo.

My parents gave me the life and DNA (Deoxyribonucleic acid) this is inherited and contains all the information of my body: Features, personality, temperament etc. The genes that control that are called alleles.
When the sperm and the ovule join some genes are more dominant (the winners) so the baby has more features from one than the other. The gene that is not the dominant and “lose” is called recessive gene. And the one that is like tied with the other is called co-dominant gene.

Is 50% of probability if is a boy or a girl; I am a boy because my mother ovule was x, and my father sperm was y, and my sister is girl because both were x.

Down syndrome: Is caused because an ovule or a sperm gave an extra chromosome, the baby instead of have 46, have 47, and instead of have 2 chromosomes 21, have 3.

On the DNA is inherited some diseases like: cancer, diabetes, heart problems, asthma.