Basic Terms and Definitions

Before you can understand genetics, it's important to understand some basic terms. 

What is a Cell?

A cell is the smallest unit of life. Some living things like bacteria and yeast are made of only one cell, while plants, animals, and humans have a lot of cells. 

Each cell has its own specific job. For example, a heart cell helps the heart beat and pump blood throughout the body. A brain cell helps us think and store memories.

One human being can be made up of anywhere between 30-100 trillion cells! So many cells, in fact, that we don't know the exact amount.

What is DNA? 

Each cell contains instructions for carrying out its job in the body. These instructions are in the form of DNA. 

DNA is a molecule that looks like a spiral-shaped ladder. Scientists call this a double helix. The “sides” of this ladder are made of sugar and phosphate molecules and form the “backbone” of DNA. The “steps” of the ladder consist of pairs of nitrogenous bases. These base pairs provide the ''recipe'' for making every living thing on Earth. You can think of your DNA as a program, and each base as a bit of code in that program. 

Base pairs in DNA are two bases bonded together. There are 4 different bases:

  • Guanine (‘G’)
  • Cytosine (‘C’)
  • Adenine (‘A’)
  • Thymine (‘T’)

‘G’ can only pair with ‘C’, and ‘A’ can only pair with ‘T’.

Fun fact: Charles Darwin figured out that all living things evolved from a common species. It turns out that we share 96% of our DNA with chimpanzees, 90% with cats, 85% with mice, 61% with fruit flies, and 60% with bananas!

What is a Nucleus? 

DNA is stored in a protected part of the cell called the nucleus.

If you took all the DNA found in one human cell, you could stretch it out to about 10 feet (3 meters)! If you did the same with all the DNA from every cell in your entire body, it would be twice as long as the solar system is wide. To fit inside the nucleus, DNA gets packed into structures called chromosomes.

What is a Chromosome?

Each human cell has 23 pairs of chromosomes. You inherit one set from your mom and one set from your dad.

All chromosome pairs are numbered from 1 to 22, except for the last pair. These are the sex chromosomes, which we call the ‘X’ and ‘Y’ chromosomes. Males carry one of each type (‘XY’), while females have two ‘X’ chromosomes (‘XX’)

What is Genetic Recombination?

You inherit half of your DNA from your mom, and the other half from your dad. However, you’re not a simple blend of the two. This is because your parents’ DNA was randomly shuffled before it got passed on to you. This randomness makes you unique from your siblings even if you have the same parents. 

The one exception is identical twins, who are born with the exact same DNA.

Think of your DNA as a deck of cards. Your mom provides half the cards, and your dad provides the other half. Genetic recombination shuffles this deck of cards, giving you and your siblings a unique hand.

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