Genetics vocabulary | Inheritance and variation | Middle school biology | Khan Academy

We know that any sexually reproducing organism is getting DNA from both its male parent and its female parent, and that's true also for human beings. You might know we have 23 pairs of chromosomes, but let's zoom in on one of those pairs. So, let's say this is one of the chromosomes in a pair, and let's say that this is the other chromosome in the pair. So, one came from one parent, the other came from the other parent.

Chromosomes are nothing more than bunched up strands of DNA plus other things that help package it. So, let's unravel a little bit to understand how these relate to genes, and then we'll dive into a little bit more terminology around genetics. You have this big long strand of DNA, and segments of this code for different genes. Maybe this part right over here is related to how tall you become. So, let's say that part right over there might be related to hair color.

In other videos, we talk about how these genes can essentially be translated eventually into proteins which are expressed in forms like height, or hair color, or eye color, or many, many, many, many other ways. But each of these chromosomes actually code, generally speaking, for the same genes. So, if this one has a region that codes for that gene, this one will as well, and this one will also. The other chromosome in the pair will also have a region that codes for this other gene.

Now, as you can imagine, the genes that get translated into proteins that eventually show up in your hair color, they can come in different versions. So what we call the different versions of a gene is an allele. So, for example, from this parent, you might have gotten the allele here that would describe, let's say, blonde hair. I'll denote that with lowercase b, while from this parent, you might have gotten the allele that's associated with, say, brown hair.

So, they're both associated or they're both coding for the same type of protein that might play a role in hair color, but they're different versions of it. You could view an allele as versions or a version. I'll say alleles are versions of a gene. Now, if you know which versions you have, then you're able to say, “I know my,” or “I know the organism's genotype.”

So, in this situation, we know we have the capital B allele, which is for brown hair, and we have the lowercase b allele, which is for blonde hair. And so, the genotype in this situation, where you actually know what alleles you have, you would say that is you have a capital B from one parent and you have the lowercase b from another parent.

Now, the thing you might be asking is, “But what does this organism look like? How does this genotype actually get expressed?” And if you're asking that question, you're asking a question of phenotype. Phenotype is what is expressed, and this is usually observable in some way.

So, the phenotype in this situation might be—and it's seldom this simple in genetics, but we're going to make it simple—in this scenario that if you have a brown allele and a blond allele, then maybe the brown allele is dominant. Which is that if you have even one brown allele, at the end of the day, you’re going to see brown. So, I'll just write that down for this scenario, although that's not actually going to be the case. Genetics gets a lot more complicated than that, but certain genes, certain alleles, they do—they are that simply dominant or recessive.

Now, you can imagine there's many different genotypes based on what I just described that could result in this phenotype. You could have this genotype that we just showed you; you could get a brown allele from both parents. Either of these genotypes would express themselves as the phenotype brown, if we assume—and that is a big if—if we assume that this is the dominant version of the gene or the dominant allele. If that is, then in either case, it will express itself as the phenotype of brown.

The only way you would get blonde hair in this scenario is if you had two of the recessive gene or the recessive version of the gene or the recessive allele. This would lead to a blonde phenotype right over here. We've talked about dominant alleles, we've talked about recessive alleles, and another piece of terminology that you will hear when you talk about genetics are notions of homozygous homozygous and or versus heterozygous genotypes.

And when people are talking about homozygous or heterozygous, they're talking about the genotype. Homozygous, you might know, the prefix homo refers to same. So, this is a situation where both the alleles are the same, same alleles. Heterozygous is a situation where hetero, that's referring to different, different alleles. So what would we call this scenario right over here? We would call this a heterozygous genotype for this gene.

This would be homozygous dominant; we have two of the same dominant allele, and this would be homozygous recessive. I'll leave you there. You're now armed to talk a lot about genetics.