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00:00Dear Tim and Moby,
00:16My parents both have brown eyes, but mine are blue.
00:19What's going on?
00:21From Tyra
00:22Hmm, I bet there are blue eyed people somewhere in your family tree.
00:27Eye color is just one of many physical traits we inherit from our parents.
00:31That passing of traits from parents to offspring is known as heredity.
00:36But remember that parents inherit traits from their parents, and so on, as far back as your
00:41family tree can go.
00:43Let's look at some pea plants to see how it all works.
00:46Yes, pea plants.
00:49Every living thing inherits traits.
00:51Back in the 1800s, a monk named Gregor Mendel used pea plants to study how traits, like
00:57height, are inherited.
00:59When Mendel bred short plants with tall plants, he noticed that all the offspring were tall.
01:05Whatever produced those short plants seemed to disappear.
01:08So Mendel called the tall height trait dominant, since it dominated, or covered up, the short
01:15height form.
01:16And he called the short height trait recessive.
01:19When he bred the next generation of plants together, the recessive short trait cropped
01:23up again.
01:25The belief at the time was that traits simply blended together, but Mendel's results showed
01:30that was wrong.
01:32After tons of research, he noticed a pattern.
01:34On average, one out of every four pea plants was short.
01:39He could explain the results by imagining that the traits from each parent stay in separate
01:44packages.
01:45For each trait, offspring get a set of two packages, one from each parent.
01:50Those sets are what we now call genes, and the packages within them aren't necessarily
01:55identical.
01:56They come in different versions, called alleles.
01:59So a pea plant can get a tall version of the height gene from one parent, and a short version
02:04from the other.
02:05The tall allele, represented here by a capital T, is dominant, meaning it takes only one
02:11of them to make a plant tall.
02:13It takes two short alleles, represented by a lowercase t, to make a plant short.
02:19In the 19th century, a British geneticist named Reginald Punnett also experimented with
02:24peas.
02:25He developed something called the Punnett square.
02:28Punnett squares show you all the possible combinations of alleles when two individuals
02:33reproduce together, which helps you predict what you're likely to get if you breed a
02:37tall plant with a short one.
02:40Peas can get one allele from each parent, so they end up with two per gene.
02:45Every plant produced by these parents will be tall, because each has one of those dominant
02:50tall alleles.
02:51But what happens if two of these new plants produce offspring?
02:55Most likely, we'd get three tall plants and one short one.
03:00Mendel only figured this stuff out after carefully studying 30,000 individual pea plants over
03:05nearly a decade.
03:08Well, I guess it's pretty reasonable to assume that he had peas for dinner every night.
03:15I think you're missing the point.
03:18Moby?
03:20Moby!