A Dance of Genes

Hope everyone had a lovely Mother’s Day and showed their mothers and motherly guardians how much they appreciate their hard work. We had scones, worked on a puzzle and watched Guillermo del Toro’s best Mother’s day film, Mama.

Here comes Mama.

Speaking of Mama/mamas, I still owe a post or two on additional exceptions to Mendelian inheritance. These last exceptions include epistasis, mitochondrial inheritance, genomic imprinting and triplet repeat extension.

When one gene is dependent on another for expression, we call that epistasis and the best example is color expression in hair. Because I can, we’ll use Danaerys Targaryen’s magnificent silvery mane as an example.

Maybe she’s born with it, maybe it’s epistasis…Actually, yeah, she’s born with it.

While Dany’s phenotype may be white hair, her genotype for hair color could be a completely different color. She could even be homozygous dominant for black hair, BB. Again, I’m using a simplified version of hair genetics, but Dany’s fireproof and owns dragons, I think I can pretend her hair genetics work like those of mice for the time being. OK, so now she’s genetically black haired. However! She has a separate set of genes that determine whether or not that hair color is expressed; C for color, c for no color. Since Dany does not have black hair, we can safely infer her genotype is BBcc. Without the allele for color expression, Dany’s hypothetical raven waves are in lockdown.

Mitochondrial inheritance is also called maternal inheritance and refers to a set of genes that are always and entirely inherited from the mother. Why? Because I said so. Actually, because the egg is the only gamete (sex cell) to contribute organelles. Sperm have no organelles, just some egg excavation tools and DNA. Eggs, on the other hand, have organelles besides the nucleus and one organelle in particular possesses its own set of DNA: the mitochondria. These are not junk genes, either, mitochondrial DNA includes genes involving cellular respiration and metabolism. Problems with these are rare, but very serious.

The last one did not have any Game of Thrones references but bear with me! It’s Monday and I’m sleepy and y’all don’t pay me, dammit. Geez, I work and I slave…

Source

-. 2010. GRE Subject Test: Biology 5th Ed. Kaplan, New York.

A Song of Genes and More Genes

Contemplated continuing the saga of molecular genetics yesterday, but then I discovered Hyperbole and a Half and sort of broke. Mostly fixed now (but still occasionally and quietly giggling to myself) so I’ll continue in the vein of Reasons Why Mendel is Wrong Even Though We Love Him. Starting with codominance.

Codominance occurs when an individual is heterozygous for a certain trait and the two different alleles are fully expressed. The easiest example is the A, B and O alleles for blood type. They code for different identity proteins in the blood, so you can have blood type AA, AB, BB, BO, AO, BO or OO. Alleles A and B are codominant, so your body equally produces both proteins if your phenotype is AB. Since O is recessive (and not codominant), genotypes AO and BO display A and B blood types, respectively. Genotype OO results in blood type O. Confusing enough? Better not be, I haven’t even gotten to incomplete dominance!

Incomplete dominance is when a heterozygous genotype results in incomplete expression of the dominant allele in the individual’s genotype. Basically, the dominant allele works and gets expressed, but it would need a second dominant allele to actually get the job done. Instead, the dominant allele produces whatever proteins it can while the recessive allele dinks around and makes nothing. Or at least nothing that works. Teamwork, people. A good example is Daenerys Targaryen.

KHALEESI ❤

Let’s look at her phenotype for flammability; Dany is completely fireproof (I call her Dany, it’s OK, we’re homies). Her genotype is homozygous dominant, FF. Although it is uncommon for a dominant allele to be rare, the fireproof allele (F) is very rare. So Dany’s baby gets a dominant allele from her and a recessive allele from Daddy Drogo. This recessive allele codes for a defective protein so that Daddy is highly flammable (considering his people cremate their deceased, this makes funeral preparations easier). But Baby Rhaego is genotypically heterozygous, so he has (or SPOILERS! would have had) Drogo’s defective proteins bumbling around with Dany’s highly effective proteins. So he is only partially fireproof because the one dominant allele cannot produce enough protein by itself to make him fully fireproof.

There are more exceptions to Mendelian genetics, but I’ll let all y’all absorb this stuff for now.

Mendel, there are too many exceptions. Daenerys is tired of your shit.

Source

-. 2010. GRE Subject Test: Biology 5th Ed. Kaplan, New York.

Clash of Genes

Since I named him the other day, I may as well explain Gregor Mendel’s laws of genetics. Gregor Mendel was an Augustinian friar in the mid-nineteenth century who spent WAY too much time with his pea plants. Except maybe not too much time, because by observing their traits and “mating” certain plants, he laid the foundation for modern genetics with his study of genetic inheritance. His laws include the Law of Dominance (which I explained Monday), the Law of Segregation and the Law of Independent Assortment.

The Law of Segregation is not racist, don’t you folks worry, it just describes the separation of alleles during meiosis (remember the Hand Jive?). When developing sperm and eggs divide, the final product only has one copy of one allele for one trait. Using the creepy-ass Lannister Twins as an example, Jaime’s bb (blonde blonde) gene is segregated during meiosis, with one b allele in each new sperm cell. Jaime’s niece/nephew-children do not get two blonde alleles from him, they get one from him and one from Mother/Aunt Cersei. This is especially important to understand in cases of the segregation of a heterozygous genotype, when the alleles are different.

Now, the Law of Independent Assortment sounds suspiciously similar to the Law of Segregation. The laws themselves are very different, but many a biology student has been tripped up remembering the name of each law. The Law of Independent Assortment refers to the manner in which different genes for different traits sort independently of one another during meiosis. Meaning the genes are not linked together and inheritance of one trait does not guarantee the inheritance of the other. So Cersei’s sass-brows will not necessarily be inherited with her hair color.

 

However! Since Mendel, we have discovered there are many linked genes, some autosomally linked (linked gene on a non-sex chromosome) or sex-linked (linked gene on a sex chromosome, usually the X because it holds more). For instance, sass-brows or no, any blonde kiddos of Cersei will also have her blue eyes since the two genes are autosomally linked.

Although that is not all for genetics, that is all for me today. I am tired from avoiding homeless men, cyclists and petitioners. Time for that mini wine.

 

Source

-. 2010. GRE Subject Test: Biology 5th Ed. Kaplan, New York.

Game of Genes

Today, I’m going to pretend I know pop culture. You kids watch that there Throne Games, right? Game of Thrones? Whatever. All I know is Daenerys Targaryen is a badass and I want pet dragons (Oh Khaleesi… ❤ ). Sadly, Danny is not as useful in discussing genetics as is Eddard Stark.

 

No, EDDARD STARK.

Don’t give me sass, you look FABULOUS in a dress.

OK, Eddard Stark, in an age that did not possess Gregor Mendel or his famous pea plants, figured out something was amiss with Queen Cersei’s abundance of blonde babies despite a brunette husband. Reading the family album and all the who-begat-whos with what hair color, Ned found that, historically, brunette + blonde = more brunettes. Ned was inferring that brunette hair is a dominant trait while blonde hair must be recessive. Assuming for a moment that hair color genetics is that simple (it’s not, not at ALL), this means that genes from a parent (mom, dad, doesn’t matter) with brunette hair will trump blonde genes from the other parent. These aren’t different genes, these are alleles, different forms of a gene for a single trait, in this case, the trait is hair color and the alleles are brown and blonde hair. So if Rob and Cersei had actually had children together, they would have two different alleles (one from each parent) but Rob’s would be dominant. That is, Rob’s alleles would be expressed and Cersei’s would be repressed.Their genotype (the alleles possessed by the kiddos) would be Bb (B= brunette, b=blonde, lowercase letters designating the recessive allele) and they would be heterozygous dominant (two different alleles, one dominating) for brown hair. Brown hair being the phenotype, or the physical expression of their hair color genes. As it actually stands, Cersei and her twin, Jaime pulled an Arkansas (apologies to anyone from Arkansas) and had beautiful, inbred, homozygous recessive (two same alleles, both expressed) and pheotypically blonde babies together. We’re all…so happy for them.

Except not.

 

 

Source

-. 2010. GRE Subject Test: Biology 5th Ed. Kaplan, New York.