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This week's experiment is a result of me looking over the various state science standards to see where I have gaps in coverage. One big gap turned up in genetics, so I began working on a couple of genetics videos. I kept finding more and more interesting stuff, and could not wait to share some and let you in on the fun. This time, we will take a quick look at dominant and recessive traits, to see what that really means. To explore that, you will need:
- your ears
- the ears of other family members
Friends, Romans, and countrymen. Lend me your ears! I only need them for a moment, just long enough to look at your ear lobes. No, it does not matter if they are pierced or not. We just want to see how they connect to your head.
For some people, the ear lobes are connected directly to the side of their head, all the way to the bottom of the lobe. We will refer to those as attached ear lobes. I have attached ear lobes.
For other people, the ear lobe hangs down beyond the point of attachment. We will refer to those as free ear lobes.
OK, so why do some people have attached lobes and others have free lobes? That is where the science of genetics comes in, because ear lobe attachment is an inherited trait. That's right. You got your ear lobes from your parents.
Inside most of the cells in your body (not red blood cells) there is a complete set of instructions for your body and how it develops. These instructions are chemicals that fit together into very long molecules called DNA. In a human cell, the DNA forms 46 strands, called chromosomes, which fit together to form 23 pairs. Each pair of chromosomes fits together, sort of like the teeth in a zipper. Arranged along the chromosomes are sequences of chemicals that form genes. Each gene is a part of the chromosome that some of the instructions for making you you. Genes control the color of your hair, how tall you will be, the color of your eyes, the color of your skin, and many, many other characteristics, called traits.
Now, it gets even better. Some genes control more than one trait, and some traits are controlled by several genes working together. Some genes turn other genes on or off. Some genes don't do anything that we can detect so far. That does not mean that they are useless, just that we don't yet know what they do.
The connection of ear lobes seems to be controlled by a single gene. I say "seems to be" because there is still a tremendous amount that we don't know about genes. For example, many books list tongue rolling (the ability to roll up the sides of your tongue to form a tube) as a single gene trait, but recent studies suggest that it is not.
So that must mean that you can look at your ears and tell whether you got the gene for your ear lobes from your father or your mother, right? No. Actually, you got an ear lobe gene from each of them. Remember that I said your chromosomes form pairs that are zipped together. One strand of each chromosome pair is from your father, and the other strand is from your mother. So, what happens if your father has attached ear lobes and your mother has free ear lobes? Do you wind up with ears that don't match, one attached and one free?
No. Some genes are dominant, and others are recessive. Dominant genes take priority. In the case of ear lobes, free lobes are dominant and attached lobes are recessive. Since you got one gene from each parent, and there are two options for each gene; there are four possible combinations.
1. If you got a dominant, free lobe gene (we will label that E) from your father and a dominant, free lobe gene from your mother (E), then you have (EE), two dominant, free lobe genes, so your ears will have free lobes.
2. If you got a recessive, attached lobe gene from your father (we will label that with a lower case e) and a dominant, free lobe gene from your mother (E), then you have (Ee), one dominant, free lobe gene and one recessive, attached lobe gene. The dominant gene takes priority, so your ears will have free lobes.
3. If you got a dominant, free lobe gene from your father (E) and a recessive, attached lobe gene from your mother (e), then you have (eE), one dominant, free lobe gene and one recessive, attached lobe gene. The dominant gene takes priority, so, again, your ears will have free lobes.
4. If you got a recessive, attached lobe gene from your father (e) and a recessive, attached lobe gene from your mother (e), then you have (ee), two recessive, attached lobe genes, so your ears will have attached lobes.
Now, if you are reading this and realizing that you have attached lobes and both of your parents have free lobes, don't start worrying that you are adopted. Remember that each of your parents also has a pair of genes that control ear lobe attachment, and that you only get one from each parent. If your father has (Ee), then you have a 50/50 chance of getting either an E (dominant, free lobe) or an e (recessive, attached lobe) gene. The same is true for your mother, so if both are (Ee), they would both have free lobes, but there is a one in four chance that you could wind up with (ee), getting the recessive attached lobe gene from both parents.
When you add in more variables, it gets more difficult to predict the outcome. For example, hair color depends on the interaction of several genes that control different amounts of different pigments, and it is also genetically connected to skin tone and eye color. That is why it is much easier to deal with traits that are controlled by a single gene, such as ear lobe attachment.
Next time, we will look at some other traits and learn a bit more about the wonders of genetics.
Have a wonder-filled week.