Basic Genetics, Twins, Chromosomal abnormalities

Basic Genetics

Genetics is the study of heredity and inheritance.
Chromosomes are long, stringy aggregates  of genes that carry heredity information.  They are composed of DNA and of protein and are located within the nucleus of our cells. Chromosomes determine everything from our hair color and eye color to our gender.
Human cells contain 23 pairs of chromosomes for a total of 46.

Twins

Twins are off springs from the same pregnancy, which can the same or opposite sex.  Twins are usually born in close succession. Due to th size of the mother's, multiple pregnancies are less likely to  be carried a full term . Twins are usually carried for 37 weeks which is three weeks less than normal pregnancies.


Chromosomal abnormalities

Chromosomal abnormalities reflect an abnormality of chromosome structure or number. There are many types of abnormalities, but they can be separated into two groups.  Group one is Numerical and group two is Structural. When an individual is missing a chromosome from a pair or has an extra chromosome different diseases can be caused. For example an individual cal have Down Syndrome  where that person has three sets of 21 chromosomes instead of two.

For more information: http://www.genome.gov/11508982#5
http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Twins_identical_and_fraternal?OpenDocument