Do you know your classical Mendelian genetics inside and out? If not, then read on, because Mendelian genetics is always a crucial part of the AP® Biology Exam. All forms of life are composed of DNAs, which Mendelian genetics can explain, and this crash course can help you out with the studying.

Mendel’s famous pea plant experiments have launched the study of genetics into an intense research subject area that has saved lives. So while this man’s experiments started with a pea plant, the knowledge gained by many grew to so much more. In this AP® Biology crash course we will illustrate Mendel’s discoveries.

What is Mendelian Genetics?

Mendelian genetics and characteristic intelligence is a fancy term for the way that genes get information from your parent’s sex cells to integrate it into your characteristics. At the very heart of Mendelian genetics is heredity, which is the passing on of genetic traits from parent to offspring. The definitions for Mendelian genetics and heredity are similar, because Mendel based his research off of this idea. He researched what happens genetically when parents mate to form offspring. Mendelian genetics sounds like a complex concept, but it is the foundation of everything that modern science knows about genetics. Genetics is what gives your eyes their color and what makes you tall or short.

Gregor Mendel, in 1865, published a hypothesis about inheritance of the characteristics of the peas in his garden. Mendel did this in order to prove the popular blending hypothesis of the time invalid. Before, it was believed that the traits of both parents blended to produce a hybrid offspring that was a perfect mix of both parents.

An example of this is that a white flower mated with a purple flower would produce a light purple offspring. Mendel disproved this theory, because when he bred his flowers, the F1 generation, were all the original deep shade of purple. The colors did not mix as what was believed in the blending hypothesis. Mendel claimed that genes will retain their individuality generation after generation as a result of his experiment.

This is also why the human gene for red hair color may skip generations. The red coloring is not blended with the hair color of the other parent. Instead, the red is only masked by another version of the gene within the chromosome.

Chromosomes and Genes

But what are chromosomes? To put it simply, chromosomes are coils and coils of genetic information. Humans have forty-six chromosomes that are combined in pairs, which make it twenty-three pairs all in all. This happens, because humans are diploid organisms, meaning that we gain a chromosome from each parent’s haploid sex cells, which contain twenty-three chromosomes in the sperm and another twenty-three in the egg cell. Once they combine, then the forty-six chromosomes come together to create a full genetic code. These chromosomes pairs are homologous, as these encodes for the same traits, and they link up because of the genes housed in their DNA.

Genes housed within the DNA are made up of specific parts of a chromosome and are used to determine the characteristics of an organism. Genes code for specific functions and characteristics in the body.

Within the body, genes sometimes work together to form a trait. This is called a polygenic trait. Other genes, called pleiotropic genes, affect multiple characteristics with only the single genes. Even less genes are encoded to form a single trait. This is the type of trait that Mendel studied with his peas, and we refer to it as a Mendelian Trait.

Determining Traits

Traits are determined by which form, or allele, of the gene is expressed. These alleles can be dominant, meaning they overpower the recessive gene. The expression of the trait is called the phenotype. Therefore, if the purple and white flowers bred together and produced purple flowers, then the purple coloring would be the phenotype.

Through Mendelian genetics, we can also determine that the purple flower is the dominant trait, because it was expressed over the white. The genotype is the combination of alleles within the gene. After gaining a chromosome from Mom and one from Dad you have two alleles. The genotype shows these alleles. This is usually noted as two letters, each representing an allele. Dominant traits are capitalized and the recessive is lowercase (Bb). For example, if your mother has blue eyes, a recessive trait, and your father has blue eyes, then each parent will have a genotype of two recessive alleles for blue eyes, or “bb”. They will pass one of each recessive allele down to you, causing you to also have two recessive alleles for blue eyes. Having two recessive alleles allows the recessive trait to show, causing the genotype to be homozygous recessive.

If your parents have brown eyes, the dominant trait, then it gets a bit more complex. Your parents could either have homozygous dominant, or BB, for brown eyes or heterozygous for brown eyes, or Bb. This is because the brown trait is expressed over the blue trait. This gives the child a fifty percent chance of being heterozygous, or brown-eyed, a twenty-five percent chance of being homozygous dominant, or brown-eyed, and a twenty-five percent chance of being homozygous recessive, or blue-eyed. This can be determined by a Punnett Square, which is a visual representation of trait distributions.

Another example of a trait that can be laid out like this is the ability to curl your tongue. We will say that the ability to curl your tongue is dominant (C) and the inability to curl your tongue is recessive (c). Classical Mendelian genetics dictates that if your parents are both homozygous dominant, then all offspring will be able to curl their tongues. If the parents are homozygous recessive, then none of the offspring will be able to curl their tongues. If the parents are heterozygous, then the twenty-five percent will not be able to curl their tongues and seventy-five percent will be able to curl their tongues.

Not only did Mendel prove that the blending hypothesis was invalid, but he also provided a method to predict the outcome of the genes of the offspring by utilizing Punnett squares. After Mendel proved this he also proved that allele pairs segregate independently. Because of this, your eye color has no effect on your hair color. This is important, because without this, one faulty gene could alter your entire body composition! Mendel proved this with his peas as well when he simultaneously studied the shape of the pea and the color of the pea. He determined that the alleles for each of these traits did not affect each other. This can be represented in a dihybrid Punnett square as seen to the right. As you can see, the color expressed by the pea does not affect the shape expressed by the pea.

Why is This Important to AP® Biology?

These Punnett Squares show the variations on the genotypes and the chances of the gene being expressed as a certain phenotype. While this may seem trivial, this ability to predict how offspring will turn out is vital. Mendelian genetics began on a small level of crossing two single traits in a monohybrid Punnett Square. Mendel paved the way for scientists to uncover why certain genetic impairments act the way they do. Deafness, for example is a recessive trait; therefore, through Mendelian genetics scientists could determine why deafness could not be present in the parents but was present in the child. This was because the parents could be heterozygous and both pass on the recessive allele, causing the offspring to be deaf. This is one of the great applications of Mendelian genetics that has truly led to bettering the knowledge of genetics as a whole.

Mendelian genetics is also important to AP® Biology and this AP® Biology crash course, because knowing how genetic variation works sets the foundation for studying evolution. By understanding Mendel’s Laws that state that alleles are individually segregated and that alleles separate as gametes, or sex cells, form, then Charles Darwin’s Theory of Evolution is confirmed and even further supported. After all, how could the survival of the fittest come to be so fit without their initial genetic variation? It would not make any sense.

You were created by the ideas that the laws of Gregor Mendel convey. Your body started out as a single cell, a combination of a sperm cell and an egg cell. You not a clone of your parents, but you are your own individual with both the same and unique traits from each parent. This allows for genetic diversity in the population, which allows for each generation to become stronger than the last.

So yes, Mendelian genetics unlocked the study of genes and DNA to the rest of the scientific community. Those pea plants spurred curiosity, and in turn, changed Gregor Mendel into the properly named father of genetics. Did he figure out every aspect of genetics? No, of course not, but Gregor Mendel did build the groundwork for other scientists to build upon his work, which is science at its best.

Do you have a question about Mendelian genetics? Please ask us about it. Do you have a strange trait that has been passed down through your family? Please share!

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