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Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

An organism's genotype is its pairing of alleles for a specific gene. Genotypes, unlike phenotypes, can be homozygous or heterozygous. If an organism is heterozygous for a gene, or possesses one of each allele, then the dominant trait is expressed. A recessive allele is only expressed if an organism is homozygous for that trait, or posses two recessive alleles.

One of the standard terms using the study of genetics is the word genotype. It is the combination of alleles that an organism has. I always think of it as simply the type o genes you posess and it's your genotype that determines your phenotype which is showing on the outside of your body.

So there's several different combinations of genes that you can use. So you could describe somebody as being homozygous dominant. The root where homo means the same, zyg refers to the zygote where the two different versions of an allele that you have came together. And so homozygous dominant means that the two alleles that came together, you are the same. Dominant means that they're the dominant one. So if we use big r, say for example the tongue rolling allele we would write big r big r. Remember with genotypes when you're writing them out you use the first letter of the dominant trait like roll versus a non-roller. You use the letter r and you use the capital letter for the dominant version. And the lower case version for the recessive. Somebody's homozygous recessive, again homo, means the same, so two of the same recessives. A non-roller would be little r little r. Their genotype would be homozygous recessive.

Somebody who's heterozygous, hetero means other different. So you have to have two of the different. That's why you don't say heterozygous dominant or heterozygous recessive because you simply have big r little r. This person got a big r maybe from mommy and a little r from daddy. Typically though heterozygous and homozygous dominant individuals show the same outward expression because they can roll their tongue and they can roll their tongue.

It gets a little bit more complicated when you're looking at things like sex link traits. With the sex link trait, you have to include the sex chromosome itself. So a woman can be woman can be big h big h, x big h x big h. This would be some a woman who is homozygous dominant for hemophilia, say for example. Or actually she would have normal blood. She could be homozygous recessive and actually show our hemophilia trait.

Males on the other hand because they only have one x chromosome and a y, they can only have one copy. So their genotype instead f being called homozygous or heterozygous or whatever, it's called hemizygous. Hemi meaning half. So only half of the chromosomes came together in that case.

What if you're talking about the genotype of something that is looking if someone that is got many traits. So you're looking at say not just tongue rolling, but whether or not you have free earlobes or attached earlobes. In that case you just simply write out all the letters. So I'll write big r little r. Let's say for example we're talking about me. I'm heterozygous, I can roll my tongue but my mum can't so she gave me the little r. I have free earlobes which means that they are not stuck into my jawline like this, they're free. So I've got a big e. But let's suppose that my dad gave me his attached earlobe gene. But I am not a hemophiliac. So this is how you write out somebody's genotype. Would you then say this person is homozygous, these other things or heterozygous usually not. If you're going into more than one gene people'll just say and I'll use the letters because that's the easy way and I like it that way.