Summer has come and passed, the innocent can never last. Wake me up when September ends. Hi everyone, I'm Mr. Yeoman and today we'll be discussing non-Mendelian patterns of heredity. Gregor Mendel laid a strong foundation for biologists when he discovered the dominant and recessive inheritance patterns, however, that has created a misconception that this is how all genetic traits work.
Studies vary on how often it occurs, but research agrees that very few genes actually follow this single gene dominant recessive behavior. Depending on the organism, this can be estimated to be as as high as 20% to below 10% of all genes. Some common examples of this misunderstanding is how eye color, height, and even psychiatric diseases are taught as Mendelian traits, even though they may have multiple genes or environmental influences.
Here are a number of inheritance patterns that do not follow Mendel's predictions, so they are referred to as non-Mendelian or complex patterns of heredity. This first example, incomplete dominance, is very similar to Mendelian genetics in that it only uses one gene with two alleles. The difference in this case is that one allele does not completely mask the other allele.
This can happen since each cell has two copies of the gene. In the image of the flowers, this gene will produce a protein that reflects red light. The red flower has two copies of the gene, copies of a version of the protein that makes the red pigment, and in this case, both copies are being expressed. This produces a higher concentration of red pigment, resulting in a red phenotype. The white flower is not producing any red pigment for whatever reason, perhaps the promoter region was damaged, or a mutation substituted a premature stop codon, or… I don't know, there are lots of ways to break a protein.
The result is that there is zero pigment being produced, hence the white phenotype. In Mendelian traits, a single functioning copy of the gene is enough to produce the protein because it is regulated differently, oftentimes by a feedback loop where more protein is made until a desired amount is produced. If there are one or two copies of the allele, the result is the same because protein synthesis will be stopped when the concentration gets high enough. In incomplete dominance, since there is only one allele producing the red pigment, there will be half as much pigment produced as the red flower, but twice as much as the white flower.
The result is an intermediate phenotype between the two extremes. We can still use a Punnett squared to determine the probabilities of offspring, but it has to be interpreted slightly differently. Pink flowers are heterozygous, possessing one red allele, the capital R, and one white allele, the lowercase r. If we were to cross two pink flowers, each plant would have a 50-50 chance of giving their offspring a red or white allele. The probability in the offspring would be 25% homozygous for the red allele, 25% would be homozygous for the white allele, and 50% would be heterozygous.
If we were interpreting this as a Mendelian trait, then we would expect a 3 to 1 ratio of red to white flowers. However, since this is not a Mendelian trait, we would expect a 1 to 2 to 1 ratio of red to pink to white, since the pink flowers are expressing that incomplete dominance. Another example could be to have red-producing alleles and blue-producing alleles, which makes the heterozygote appear a purple color, which is a mixture of the two extremes.
Our next example can also be interpreted using our standard Punnett squares, but its expression looks different from incomplete dominance. In the example of an Appaloosa horse, instead of a brown allele and a white allele producing a light brown coat, some cells will produce brown hair, while other cells produce white hair. Both variations from each parent are fully present in the offspring. Blood types can also express codominance when different proteins are produced on the surface of each red blood cell, but we'll take a look at that a little bit later.
We can still use a Punnett square to determine probability, but we have to represent our alleles a little differently. Some people will use two separate capital letters, in this case a B for brown and a W for white. This will get the point across, but it isn't much of a problem. my preference because it can lead to avoidable misconceptions. By using two different letters, then it can make multi-gene crosses more difficult.
A common letter ensures that we are referencing the same gene. In this case, I might use a capital C for coat or color. I could then follow it up with a superscript of capital B or W to show that we're talking about codominants. But there shouldn't be any confusion about whether it's the same gene. If I'm crossing two Appaloosa horses, then I can predict their offspring since all Appaloosas are heterozygous.
Just like in our incomplete dominance example, we end up with 25% brown, 25% white, and 50% Appaloosa, just like the parents. This visual is a classic test for trying to determine if a person is colorblind. Colorblindness can be a big problem.
comes in a variety of forms, but red-green color blindness is one of the most common and is an example of a sex-linked or X-linked trait. Sex-linked traits are characteristics that are more common in one biological sex over another. That's because these genes are typically located on the X chromosome.
With the exception of statistically rare situations, individuals with two X chromosomes will typically develop as a biological female, whereas possessing an X chromosome as well as the SRY gene on the Y chromosome will typically result in a biological male. Since chromosomal males possess a single X chromosome, and if that gene is damaged, there's no backup allele to make that protein. When males receive the gene that causes red ring colorblind, they will not produce one of the three cones in the eye that interprets colors of light. As long as a female has two X chromosomes, she would still be able to produce all three cones in the eye as long as one of her two alleles is functional, since the functional gene is dominant over the non-functional gene. Chromosomal females can still be red-green colorblind, but they would need to have two copies of the nonfunctional gene.
Since she received one of her two X chromosomes from her father, then that would mean he would also have to be red-green colorblind. Our Punnett square is still useful, but we'll have to not only list the genes, but also the chromosome that the gene is present on. If I had a man who was red-green colorblind and he was going to have children with a woman who was a carrier for red-green colorblindness, meaning she had one functional and one non-functional gene, the mother will give the afflicted chromosome 50% of the time.
The father will give his damaged chromosome 50% of the time, but his Y chromosome the other 50% of the time. This results in a 1 to 1 to 1 to 1 ratio, where they would have a color vision female 25% of the time, a color vision male 25% of the time, a colorblind female 25% of the time, and a colorblind male 25% of the time. But Mike, you say, sex-linked traits appear in one sex more than the other.
That's right. To show that, let's perform another cross between a female carrier and a colored vision male. The chances of having a male child with red-green colorblindness is still the same, 50-50.
However, since dad does not have an X chromosome with a damaged gene, then none, not a zero of his daughters will be red-green colorblind. Though red-green colorblindness is more common than other types of colorblindness, it's still relatively rare. The likelihood of a colorblind male and female carrier having children together is statistically low in a large population.
That's why we see many more males than females with red-green colorblindness. Our next example continues to change up our understanding of Mendelian genetics. Everything we've looked at so far has a single gene with two alleles.
Sometimes, however, there can be multiple alleles. Blood type alleles come in three varieties, type A, type B, and a recessive form that produces no surface antigens. These are represented by the letter I, which stands for isoglutinogen, which is just a fancy way to say antigen. Since types A and B are codominant, then they are represented by a capital I followed by a capital A or B superscript.
The recessive form is represented by the lowercase i. If we know the genotypes of a couple, we can still predict the probability of having children with certain phenotypes. Each parent only possesses two of the three possible alleles, but that increases our possible combinations. Thank you for watching!
Individuals could be homozygous for type A blood or heterozygous having one A allele and one recessive allele. The same is true for type B blood. A person could be homozygous or heterozygous for type B blood, but they would need at least one dominant allele. If they possess a capital IA allele and a capital IB allele, they would have type AB blood where both antigens are being produced on the cell surface. This cell blood type is also demonstrating co-dominance.
It's not incomplete dominance because the blood type isn't same. some halfway between A and B blood type. If a person has two copies of the recessive eye, they will have type O blood where there are no surface antigens.
The reason this is an issue in blood transfusions is because depending on your blood type, you will produce corresponding antibodies. If you have type A blood, you will produce anti-B antibodies. So your body will attack and clot any blood that has type B antigens present. If you have type O blood, blood, you produce both anti-A and anti-B antibodies, which is why people with type O blood can only receive type O blood. Otherwise, their body will attack it.
People with type AB blood do not produce either anti-A or anti-B antibodies, which is why they are considered the universal receiver. Here are two quick crosses to demonstrate the possible outcomes. An example of this is where we can take two individuals who are type A and type B, but just because we know their blood types doesn't necessarily mean that we know what their genotypes are. That's what makes having multiple alleles a little bit more complicated. And so in this situation, we could have...
A type A individual and being type A could look one way, so where they have two dominant alleles. for the type A blood type, or they could be type B and have two type B alleles for the B blood type. And so when we cross these, there's only one possible outcome.
In this situation, you can only have type AB offspring. But if the genotypes of these parents were different, so still a type A. and a type B cross, but this time if instead there was a dominant A allele, and then the recessive allele, that recessive I, and then over here we have a parent that has type B blood, but they also have the recessive I allele, then we can have a lot more of these different combinations. And so here we can actually produce all of the different feelings.
phenotype. So we'd have a chance of having a child who is type AB. We'd also have a 25% chance of having a child with type B blood, a 25% chance of having a child with type A blood, or a 25% chance of a child having type O blood.
The remaining examples are not able, or at least not reasonably able, to use a pun at square to determine the expression of that character. A very common example of this is polygenic inheritance. Many of our traits are controlled by multiple genes, poly, interacting rather than a single gene, showing simple dominance or recessiveness.
Height, weight, hair color, eye color, skin color are all examples of traits that have multiple genes that impact them. Even if we know all the genes that were involved, involved, they don't all have an equal influence, and the number of genes involved would make predicting the traits of offspring incredibly complicated. For example, in 2012, Richard Sturm and David Duffy identified that there are at least 34 genes that are responsible for skin color in humans. With two possible alleles that assort independently for 34 genes, we would take two to the 34th power, giving us just over 17 billion possible genotype combinations per parent. And that would require a punnett square with 295 quintillion boxes in order to cross those parents in reality some of these genes would be on the same chromosome so this estimate isn't entirely accurate but you get the idea this creates a huge amount of variation this is how humans can express such a spectrum of skin colors and why there can be such wide variation even within families you Pleiotropy is almost the exact opposite of polygenic inheritance.
Instead of multiple genes influencing a single trait, in pleiotropy, a single gene can influence multiple, sometimes seemingly unrelated traits. Niccolo Paganini was a 19th century Italian violinist who was so talented that it was rumored he had sold his soul to the devil. Modern biologists read descriptions of Paganini and his family and assume that he suffered from a genetic condition called Ehlers-Danlos Syndrome. This syndrome can have multiple causes, but many of them are single mutations that cause connective tissue to be built incorrectly. This can cause incredibly stretchy skin, joint pain, and flexibility that could allow your fingers to bend and play chords that other violinists couldn't.
It can also impact the valves in your heart since they are also made of connected tissue. Epistasis is another example where genes can interact. In this case, one gene can influence or prevent the expression of another gene. When discussing protein synthesis, our class used a mutation of the M-series.
C1R, TYR, and OCA genes to cause albinism. Individuals with albinism still have genes to produce skin, hair, and eye color, but the TYR gene prevents the production of melanin. So even if their DNA says that they should have black hair, lots of melanin, Their hair will be blonde due to the lack of melanin.
If or when a person with albinism has children, as long as the children don't inherit two copies of the albinism gene, their melanin production will be similar to their non-albino relatives. Epigenetics is a newer field of research that shows how our DNA can actually change its expression based on factors like stress. When DNA is in the euchromatin form, it is loosely packed and easily read by RNA polymerase.
When an organism is stressed, it can add small molecules to the euchromatin to turn it into heterochromatin. This process is called methylation. Heterochromatin is DNA that is tightly packed, so the RNA polymerase is unable to transcribe that DNA, affecting the DNA. effectively silencing that gene. This regulates gene activity without altering the gene sequence itself.
This is what happens to holly bushes when they are chewed on by herbivores. The stress of the leaf causes methylation of the DNA and changes the leaf expression from smooth to spiky. This is a selection adaptation that helps prevent further feeding on the plant.
Over time, the DNA can return to its demethylated form and produce smooth leaves again. However, recent studies have shown that certain types of stress can cause methylation, and then the offspring of those organisms continue to show a stress response to stresses they've never experienced. Epigenetics is just one way that the environment can change the expression of genes. In certain species of hydrangeas, the flowers will change color. based on soil pH. In acidic soil, the flowers will turn a lavender blue color.
In basic soil, the flowers will turn a pink color. The reason for this will rely on us to remember some items about protein structure. We know the shape of a protein determines how it will behave.
Some of the amino acids in a polypeptide chain are acidic or basic. So when we change the pH of the environment it's in, the protein will fold differently. In extreme situations, this can cause the protein to be acidic. to denature and no longer function.
In this case, the change in shape causes the protein to reflect different frequencies of light. Other environmental changes can impact protein synthesis less directly. In Siamese cats and Himalayan rabbits, melanin production is influenced by temperature. In areas that are colder, the ears, face, and paws, gene regulation increases the amount of melanin produced there.
A likely explanation is that dark colors absorb more light, so this adaptation might help keep these extra... extremities slightly warmer. The arctic fox will produce more melanin in the summer months than in the winter. This helps with camouflage. Many trees will reabsorb chlorophyll in order to save and reuse the magnesium in it.
These behaviors can be initiated by temperature or the length of the day depending on the species. This is just an introduction to the variations in gene expression, but I hope it helps you see how complex and amazing biology really is. As always, stay curious and seek truth.