Chromosomes & Karyotypes Genetics

What is a Chromosome? A LONG strand of DNA. Organisms that reproduce sexually (two parents) receive half of their chromosomes from their mother and half from their father. Humans have 46 chromosomes (23 from each parent). When you have a child, you will contribute 23 chromosomes!

Karyotypes We can make a picture of what your chromosomes look like! This is called a karyotype. Each pair of chromosomes is called a homologous pair. Doctors can use the karyotype to determine is certain birth defects are present. Also, you can use it to tell if you are a boy or girl.

The 23rd PAIR Your 1st through 22nd pair of chromosomes are called autosomes. They control traits that are NOT sex related. Your 23rd pair of chromosomes are called sex chromosomes because they tell whether you are male or female. If you have 2 X chromosomes, you are female. If you have 1 X and 1 Y, you are male. However, ALL humans start out female. It’s not until the 2nd or 3rd month of pregnancy that the male hormones kick in and turn the baby male. This is why boys still have nipples, even though they don’t breast feed babies! Sometimes the male sex hormones don’t kick in completely and the baby is called intersex. This means that even though it is an XY or an XX it could have both male and female characteristics. This is about as common as having red hair!

Sex Chromosome Abnormalities Sometimes, people are born with sex chromosome abnormalities. For example, a female can have Turner’s Syndrome if they are born with only one 23rd chromosome (an X). This can possibly result in short height, heart defects, and infertility. A male can have Klinefelter’s Syndrome if they are born with an extra X chromosome (XXY). This can result in low testosterone levels which can affect growth, puberty, and fertility.

The 21st Pair Sometimes babies are born with an extra 21st chromosome. This results in Down’s Syndrome, sometimes called Trisomy 21. The probability of having a baby affected by Down’s Syndrome increases with the age of the mother. A mother is termed a high risk pregnancy if she is 35 or older.