Short-ish List of Important Concepts in Biology

OK, this isn't everything you need to know about biology, but it's the list of key terms and concepts you need for this class, specifically for the section of genetics. For many of you, that list will be enough to remind you of your basic biology from high school. If you would like a more in-depth explanation of the terms, or are confused by the definitions, please come talk to me in office hours or watch some of the videos in the Crash Course Biology series (particularly videos 9-20).

DNA - deoxyribonucleic acid, the genetic material you inherit from your parents. Some people refer to it as the "building blocks" of life. I prefer to think of it as an owners manual, as we'll discuss in class. You get half of your DNA from your mom, and half from your dad (roughly).

Meiosis - this is a type of cell division that creates sex cells (sperm and eggs). The critically important aspect of meiosis for our purposes is that the resulting cells only have half of a person's genetic material. When you create sperm or an egg, you're putting a (mostly) random half of your DNA into that cell, in the hopes that it will merge with a random half of your partner's DNA in his/her sperm/egg and create a new person who carries genes from both of you.

Gamete - this is, essentially, the end product of meiosis. It's a sperm or egg cell, ready to meet its perfect match and make a new life.

Gene - a segment of DNA that codes for an amino acid. A gene can be expressed -- meaning it's actually "turned on", making the amino acid(s) that it is supposed to make -- or not. Whether or not it is expressed usually depends on environmental cues.

Allele - a variant of a gene. For example, you have genes that control your blood type. You may have a gene for type A, type O, or type B blood. Each of those gene types is a separate allele.

Genotype - literally, just the two genes (the specific alleles that you received from your mom and dad) that exist in a particular location on your DNA. If you received a gene for type A blood from both your mom and your dad, then your genotype is AA.

Phenotype - this describes the physical expression of your genotype, what someone could see or measure about you as a result of the genes you have. For example, if you have two genes for type A blood (for a genotype of AA), then your phenotype is type A blood. A description of your hair color, height, skin pigmentation, and ear wax stickiness would also be a description of your phenotype.

Heterozygote - if an individual has two different alleles at a particular location on their DNA, they are a heterozygote (hetero=different, zygote=a fertilized egg). If you received a gene for type A blood from your mom, but a gene for type O blood from your dad, you're heterozygotic for that trait.

Homozygote - if an individual has the same alleles at a particular location on their DNA, they are a homozygote (homo=same, zygote=a fertilized egg). If you received a gene for type O blood from both your mom and your dad, then you're homozygotic for that trait.

Mendelian trait - a trait controlled by one pair of alleles. These are simple traits, such as blood type, where the presence or absence of the trait depends almost entirely on the presence or absence of particular alleles. The influence of the environment is often minimal. For example, if you have an allele for type A blood, then your body produces the structures on your blood cells that are characteristic of type A blood. Other genes, and the environment, can modify that to some extent, but in most cases these traits are simply reflections of the genotype.

Dominant - a dominant allele is an allele that is expressed when it is present. For example, if you have an allele for type A blood, then your body will create the structures on your blood cells characteristic of type A blood. That allele is dominant and it is expressed through the creation of those structures.

Recessive - a recessive allele is an allele that is suppressed or not expressed when paired with a dominant allele, but it will be expressed when a person is homozygous recessive, that is, when they have two paired recessive alleles. For example, the allele for type O blood is recessive. If you have two alleles for type O blood, one from each parent, then you will have type O blood. However, if you received an allele for type O blood from your mom, but your dad gave you an allele for type A blood, then you'd have type A blood because the A allele is dominant.

Incomplete dominance - if an allele shows incomplete dominance, it means that the trait for which it codes is not fully expressed when it is paired with another dominant allele. For example, in some plants, mixing the alleles for petal pigments of red and white would lead to pink. This creates a third possible phenotype in the offspring, not just the dominant phenotype or the recessive phenotype, but a mixture.

Codominance - if two alleles are codominant, that means they are both expressed. For example, the alleles for type A blood and type B blood are both dominant. If an individual has one of each, the alleles are codominant, and the person has type AB blood. Again, this allows a third possible phenotype in the offspring, not just the dominant phenotype or the recessive phenotype, but an expression of two dominant phenotypes.

Monohybrid Cross - in the context of this class, this phrase literally just means "we're going to make a Punnett square looking at one trait". For example, a Punnett square that shows the possible offspring of a couple with blood type genotypes of AB and Ao (ABxAo) would be a monohybrid cross.

Dihybrid Cross - again, in the context of this class, this phrase means "we're going to make a Punnett square looking at two traits". For example, a Punnett square that shows the possible offspring of a couple with blood genotypes of AB and Ao and earwax genotypes of WW and ww. Usually this would be written as ABWWx Aoww.

Pleiotropic - we won't actually spend that much time on Mendelian genetics and Punnett squares in this class, partly because the really interesting material -- the stuff we really care about when it comes to human adaptations, diversity, behavior, and evolution -- is much more complex. The behavior of most genes can't be described as simply as the terms above suggest. Most genes are pleiotropic, meaning they affect a variety of (often seemingly unrelated) traits. For example, the disease PKU comes from a mutation on a single gene, but it affects brain growth, skin pigmentation, hair growth, digestion, and a number of other characteristics.

Polygenic - just as most genes aren't as simple as Mendelian genetics and Punnett squares suggest, most traits aren't that simple, either. Most traits are polygenic, meaning they're influenced by a number of different genes. Traits like height or skin pigment are affected by dozens of genes, and these genes are expressed differently depending on environmental influences. Many traits are not "controlled" by genes at all, but rather are influenced in ways big and small by the behaviors and physical structures made possible by our genes.