When scientists discovered Gregor Mendel’s work on the basics of genetics, it was hailed as a major breakthrough. For the first time, scientists could reliably predict and describe what was happening when two organisms with known traits were bred to produce offspring.
Things were going great until scientists noticed something funny happening—not all the traits that they predicted in test crosses panned out as expected. What could possibly be the explanation? Was Mendel wrong? Did they need to go back to the drawing board?
We know now that Mendel was correct in his ideas—but the big picture of genetics is a lot more complicated. Mendel described the first simple part of a huge, shifting puzzle. He gave us the tools to understand the basics of how genetics work, but scientists had to figure out what else was happening.
What are non-Mendelian genetics?
Non-Mendelian genetics are basically any inheritance patterns that don’t follow one or more laws of Mendelian genetics. Let’s review those laws quickly:
- Mendel’s First Law (Law of Segregation) – A parent who has two alleles for a gene can only pass on one allele or the other to each offspring.
- Mendel’s Second Law (Law of Independent Assortment) – Two or more traits are inherited separately from each other; they don’t always occur together.
- Mendel’s Third Law (Law of Dominance) – One dominant allele will take charge over a recessive allele and “mask” it. The only way recessive alleles can be seen is if an individual possesses two copies of the recessive allele.
These are the basic rules of Mendelian genetics, but as scientists began exploring more and more test crosses, they found tons of traits that didn’t match up nicely with what these laws predicted.
Some traits exhibited a kind of blending, where the offspring of organisms with two different traits didn’t have one or the other form from the parents—they had something that was sort of in the middle. This implies that certain alleles aren’t dominant over the other ones. Rather, they share roles like harmonious friends.
Some traits seemed to be controlled by complex inheritance patterns. We know now that traits can be controlled by more than one gene, or genetic material may pass down from parent to offspring in different ways than what Mendel predicted with his Law of Segregation.
Finally, non-Mendelian inheritance patterns might just be caused by mistakes in reproduction. There are a ton of different processes all happening together, like a coordinated dance, and all it takes is one thing to trip or mess up and the whole thing goes haywire. There’s so many things that could go wrong that frankly it’s a wonder that we all end up here in one piece!
Let’s take an in-depth look at some of the different types of non-Mendelian genetic inheritance patterns.
Did you know that your chromosomes determine whether or not you’re male or female? Everyone has two sex chromosomes, and your gender is determined by which two you possess. Females have two copies of the X chromosome (XX), while males have one copy of the X chromosome and one copy of the Y chromosome (XY).
Because males only have one X chromosome, any genes that are on it will be present. This is because there isn’t a backup copy of the gene on another X chromosome to mask it if it’s recessive, like women have. Men also have their own special Y chromosome that women don’t possess.
But don’t go celebrating about your own special chromosome just yet, dudes: the Y chromosome is much, much smaller than the X chromosome, so technically women have more DNA! And, since any gene on the X or Y chromosome will be expressed regardless of if it’s dominant or recessive, men are far more likely to get sex-linked genetic diseases than women.
Some common sex-linked genetic diseases in men include things like male pattern baldness and red-green color blindness. These disorders are carried on the X chromosome and can only be passed on to males through their moms. Let’s look at a Punnett square to see why:
In this case, the mom is a carrier of the sex-linked trait. She does not have the diseases because her other, normal X chromosome masks it. Half of her kids will get the normal chromosome and be totally fine, but the other half of her kids will get the diseased chromosome.
Whether or not her kids with the diseased allele actually develop the disease is now up to the dad: if he passes on an X chromosome, the child will be a daughter and be safe, although she can still pass on the disease to any of her offspring.
If the dad passes on a Y chromosome, the child will be a son and has a 50/50 chance of having the disease because he has no backup X chromosome.
Codominance is a direct violation of the Law of Dominance—thank goodness there’s no gene police to tell it that, though!
When the alleles for a particular trait are codominant, they are both expressed equally rather than a dominant allele taking complete control over a recessive allele. This means that when an organism has two different alleles (i.e., is a heterozygote), it’ll express both at the same time.
Have you ever seen speckled flowers? This is a common case of codominance, where the plant breeder has bred two different colors of flower together, resulting in a speckled hybrid that has patches of color from both parents.
One really good example of this in humans is blood type. The most important blood type is the ABO system, because if you get a blood transfusion with the wrong type of blood, you could develop a severe allergic reaction and die!
The A and B blood types are codominant. Thus, if two people with AA and BB blood type alleles have children, it’ll look like this:
In this case, every single child (male or female) from this couple will be heterozygotes. Both alleles will be expressed equally, meaning that every blood cell in their little bodies will have both A antigens and B antigens present. One allele is not dominant over the other in this case.
With codominant alleles, both traits are expressed at the same time. With incomplete dominance, the same thing occurs—but the traits are blended together just like paint mixed together, rather than occurring in discrete patches like the speckled flowers.
Going back to our flower example, if flower color shows incomplete dominance then two different flowers crossed together will produce a hybrid that’s in between both of the parents. So, for example, if you cross a white flower with a red flower, you would get a pink flower if the two alleles showed incomplete dominance.
Up until this point we’ve been talking about traits that are controlled by alleles from one gene and fit neatly into our Punnett square. But, some traits are controlled by many genes. Scientists estimate that your height is controlled by more than 400 different genes, for example!
The reason human height is controlled by so many different genes is because height isn’t a simple on/off, yes/no-type trait. We’re actually pretty complex critters for some types of traits!
There’s a lot of things that have to happen to make people tall—blood vessels, muscles, nerves, and bones have to grow and elongate; more blood has to be produced to accommodate the extra tissue; the brain needs to send out hormones to coordinate everything, etc. It’s a big job and it’s no wonder there are a lot of genes that come into play!
Many other human traits are controlled through polygenic inheritance, such as IQ, skin color, eye color, etc. Can you think of some of the things that might need to happen to produce these traits?
If the above examples are in direct conflict with the Law of Dominance, then gene linkage is in direct conflict with the Law of Independent Assortment!
When Mendel broke ground with his pea experiments, he was looking at traits that just happened to be located on different chromosomes. Thus, when he looked at two traits, they were inherited separately because they were on different chromosomes. Green peas were equally likely to occur on short plants as they were on tall plants, and wrinkled peas were equally likely to be green or yellow, for example.
But, each chromosome can have hundreds or thousands of genes on it. You have upwards of 2,000 different genes on Chromosome 1 alone, for example. That’s a lot of genes! And because they’re all on the same chromosome, they’re inherited pretty much as a package deal.
Many experiments have been done on gene linkage in fruit flies (hopefully the scientists didn’t have too much rotten fruit in their offices!). For example, some combinations of wing shape and body color are inherited together. Fruit flies that have brown bodies are more likely to have normal wings, while fruit flies that have black bodies are more likely to have itty-bitty wings, in one example.
Did you know that some organisms don’t even need to reproduce to pass on their genetic material? It sounds strange but it’s true! Some types of bacteria can pass on their genetic material directly to their neighbors, sort of like trading baseball or Pokemon cards.
This obviously makes it very hard to predict genetics of some bacteria, because they can do whatever they want with their genetic material! It’s also one reason why bacteria can evolve very quickly—rather than waiting for a whole new generation, bacteria can pass on their chromosomes to their comrades instantaneously and evolve within a single generation.
Your nuclear DNA lives inside the nucleus in your cells, but did you know you also have other DNA outside your nucleus? It’s true—it lives in your mitochondria, and it’s called mitochondrial DNA. Plants even have their own version too, that lives in their chloroplasts.
Because mitochondria and chloroplasts have their own DNA and reproduce on their own inside each cell, they’re thought to be ancient bacteria that eventually evolved to live inside our cells and provide power. Animals and plants might actually be an amalgamation of several different species!
Mitochondrial DNA is passed down from a mother to her offspring because the mitochondria in sperm cells don’t make it into the egg. Thus, all of the mitochondrial DNA in your body—whether you’re male or female—originally came from your mom!
Mitochondrial diseases are rare, but when they do happen any children that a woman has will also have the disease too, because her mitochondria are passed on unchanged from mother to offspring.
This has given rise to a new phenomenon: three-person babies. If a mother has a mitochondrial disorder and doesn’t want to pass it on to her kids, she can conceive a baby using some pretty amazing science.
First, doctors take a donor egg from a healthy woman and remove the nucleus—leaving behind an empty shell with plenty of healthy mitochondria inside. Next, they take the nucleus out of one of the biological mom’s eggs and implant it in the empty shell egg.
That way, the new egg has healthy mitochondrial DNA from the donor mom, plus all the nuclear DNA that actually makes up a person from its biological mom. The egg can then be fertilized, implanted, and carried to term just like any other test-tube baby.
Why study non-Mendelian inheritance?
As we’ve seen here, some cases of genetic inheritance can be far more complex than simple Mendelian inheritance. Both types of inheritance are equally important to unlocking the clues hidden away in our own DNA.
Because non-Mendelian inheritance patterns are so complex, there’s plenty of room for new geneticists in the field—maybe you could be the one to discover how an important gene is inherited?