What do you know about the genes

introduction

Red hair may seem rare, but the fear of a mass extinction is unfounded. While it's true that the gene variant for red hair is not as common, it can still be passed down through generations. So, rest assured, the fiery-haired flame is not going out anytime soon. Let's rely on scientific studies and genetics to debunk these sensationalized stories.

Let's discuss genes, which are individualized sections of DNA found on the chromosomes of cells. Chromosomes are the lengthy strands of genetic material in the nuclei of our cells. Typically, humans have a total of 46 chromosomes, or 23 pairs of two. However, genes are the segments along those chromosomes that carry instructions. Humans have over twenty thousand genes spread across our 46 chromosomes. Every living organism possesses genes, although some have more genes than others. For instance, the nearly microscopic freshwater flea has over thirty thousand genes. Most genes contain specific instructions for producing proteins. When these instructions are read, a process known as gene expression, they influence the appearance and functioning of an organism. Each gene exists in different versions called alleles, which determine various traits such as the shape of a seed or a person's blood type. When we refer to different versions or variants of genes, we are talking about alleles. Scientists often use these terms interchangeably. The reason why we all look different from one another is that we possess different versions of the same genes, these alleles. An organism's complete collection of alleles is called its genotype, which accounts for its entire genetic makeup. On the other hand, the phenotype encompasses all the observable traits in an organism. For example, your genotype may include alleles for both a long big toe and a short big toe, but if you have a short big toe, that is the trait that will be part of your phenotype.

The offspring acquires a distinct combination of alleles from both parents. These alleles are the same genes, but they are different versions and come from different chromosomes of a matching pair. The traits that are ultimately expressed are influenced by the interactions of these alleles and, in many cases, the organism's environment. The process by which traits are passed from one generation to the next is known as patterns of inheritance. Most traits are a result of multiple genes interacting with the environment, almost like mosaics. However, some traits follow simpler patterns. For instance, the length of a cat's fur is determined by a single gene known as "fibroblast growth factor 5," or FGF5. There are several allele variants of FGF5, including four different long-hair alleles. For simplicity, let's group them together as one. Consider the big L as a short-hair allele, which is dominant. When the dominant allele meets the recessive long-hair allele, the dominant one prevails. A cat only needs one dominant allele to have short hair, while two recessive copies are required for long hair. When one allele completely dominates the other, it is referred to as autosomal dominance or Mendelian inheritance, named after Gregor Mendel, who first observed this pattern in pea plants over 150 years ago. Let's now examine how this pattern of inheritance works. Meet Mortimer, a cat with short hair, white markings resembling a tuxedo, and a fondness for long naps in the sun. On the other hand, Bagel has long orange fur, sharp claws, and a habit of spilling coffee on keyboards. Despite their differences, Mortimer has one dominant short hair allele, masking one long hair allele, making him heterozygous.

The male cat, Mortimer, has two different alleles for the hair length gene, making him heterozygous. On the other hand, the female cat, Bagel, has two recessive alleles, making her homozygous. By knowing the genotypes of our cats, we can predict the probabilities of their hypothetical kittens inheriting different combinations of alleles and therefore having either long locks or sleek fur. This can be represented using a Punnett square, which shows that there is a fifty percent chance that the kitten will inherit one copy of each allele, making it heterozygous dominant and having short hair. The other fifty percent chance is that the kitten will turn out homozygous recessive, inheriting two copies of the recessive allele from both parents, resulting in long hair. However, if Mortimer has two copies of the dominant allele, then the kitten will definitely have short hair. In some cases, alleles do not completely overshadow each other, and instead, they blend together in what is known as incomplete dominance. This can lead to an expressed trait that is a mixture of the traits associated with each allele. For example, the white spots on Mortimer's tuxedo and Bagel's socks are determined by an incompletely dominant allele for white spots. A cat with two dominant alleles will be mostly covered in white, while a heterozygous cat with just one copy of the allele will have less than half of its body covered in white patches. Since both Mortimer and Bagel are heterozygous for spots, their kitten has a twenty-five percent chance of inheriting two dominant alleles and being mostly white, as well as the same odds of inheriting two recessive alleles.

But if I were a gambler, I’d put my bet on it being heterozygous like its parents.

There’s a fifty percent chance of that, and a one hundred percent chance of it being adorable.

Sometimes, instead of mixing together, traits linked to two alleles show up separately in what’s called codominance:

Kind of like wearing a Hawaiian shirt and a sequined jumpsuit.

They’re both jostling for attention. Calico cats — with their blotches of

orange and black fur — owe their looks in part to codominance.

But they’ve got another pattern of inheritance going on at the

same time, called sex-linked inheritance. That’s where a gene is carried on only one of the chromosomes that influences an organism’s sex. For most mammals, sex chromosomes

come in two versions, X or Y. But a cool and important thing to know:

XY is /not/ the only system out there. And some organisms don’t have sexes at all.

Anyway, in the XY system, an organism with two X chromosomes usually develops as female, while an organism with an X and a Y usually develops as male.

So when a gene shows up on the X chromosome and not the Y, its traits will show up at different rates in females and males. Now back to those calico kitties.

A color gene on cats’ X chromosome comes in two alleles: an orange version (XB) and a black version (Xb). If a kitten inherits

both alleles (XBXb), each cell only expresses one at a time.

So the fur ends up looking like Joseph’s Technicolor Dreamcoat.

It’s more straightforward for the XY carrier, who gets only one X chromosome.

Inherit one allele, and it’s orange fur; inherit the other, and it’s black.

That’s why calico cats are almost always XX. And while codominance isn’t always dependent on sex, it is in the case of calico kitties.