The Study of Genetics: The Story of Crossbreeding Plants

FROM THE LECTURE SERIES: Understanding the Misconceptions of Science

By Don Lincoln, Ph.D., Fermi National Accelerator Laboratory (Fermilab)

The study of genetics is one of the most fascinating fields of modern science. Not only is it the study of how people inherit traits from their parents, but it also connects people together, showing how people with heritages from across the world are both the same and different. Do humans have any connection with the animal species?

An illustration showing the human skeletal frame next to those of a gibbon, chimpanzee, gorilla, and orangutan.
Humans have a connection with other animal species like chimpanzees, orangutans, and bonobos. (Image: TimVickers/Public domain)

Human Connectivity with Other Species

There is a connectivity of humans to the rest of the tree of life. Humans are most closely related to chimpanzees and bonobos. The genetic variation within humanity is under 0.1 percent, meaning that humans share 99.9 percent common genetic material. For chimpanzees and humans, the overlap is probably only 99 percent similar. The orangutan is different from humans, but the genetic overlap is about 98 percent. Cats have a 90 percent genetic overlap with humans.

Humans and Bananas Connectivity

The connectivity between humans and bananas is a technical question as the human genetic code is complicated. There are genes that are active and make humans what they are with a 50 to 60 percent overlap with bananas. There is also a ton of junk DNA in the human genetic code, which is genetic material that used to be useful but is not anymore.

Learn more about the most jarring misconceptions of science.

Updated DNA Versions

New tools have revolutionized scientists’ ability to manipulate the genetic code. Scientists can literally reach in and snip out existing bits of DNA and replace them with new versions. This provides the possibility of editing out genetic defects.

Diagram showing the difference between normal cells and sickle cell anemia.
Sickle cell anemia, which has an African connection, can affect a huge population and is caused by a single gene. (Image: joshya/Shutterstock)

Sickle cell anemia is a disease affecting one in 500 Americans, caused by a single gene, and it is a real possibility that this gene could be edited out. This means that a parent with sickle cell anemia could have a child who is genetically theirs, but without the sickle cell mutation, with none of their descendants having the mutation ever.

The Study of Genetics

The complexity of the study of genetics and the vitality of this field is vast. Learning about genetics, most people will have encountered Punnett squares, which invented in the early 1900s by Reginald Punnett, a British geneticist. They were a formalized way to understand observations made by Austrian friar and scientist, Gregor Mendel who did experiments with pea plants, testing various properties of the plants, breeding and looking at their height, pod shape and color, seed shape and color, and the position and color of flowers.

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True-Breeding Peas

Gregor Mendel studied pea color using yellow and green peas. True-breeding yellow peas always had yellow offspring. The same was true of true-breeding green peas, they always had offspring with green peas. When he crossbred yellow and green peas, they always produced offspring with yellow peas. However, looking at the next generation, he got a different result. He found that the second-generation plants, essentially the grandchildren of the pure plants, had seed colors that appeared with the ratio of one green to three yellow.

Punnett Squares

Diagram of a Punnett square explaining the process and outcomes of crossbreeding a plant.
In a Punnett square the results of crossbreeding help in knowing whether the offspring has dominant or recessive traits. (Image: Designua/Shutterstock)

The modern understanding of the process can be tied to Punnett squares. It is known that the color of this particular breed of peas is set by a single gene. Genes are just a set of instructions held in the center of cells. When plants or animals reproduce, they get a copy of each gene from their parents. In animals, one copy is from the mother and another from the father. It is similar, although not as obvious, in plants where it is considered as just two parents as plant gender is a complication.

An individual has a copy of the gene from the mother and another from the father. In the case of the true-breeding green seed pea plants, both copies were green and of true-breeding yellow plants, both copies were yellow. Breeding green with green, the offspring was two greens as well.

Learn more about the misconceptions about genetics and the Punnett squares.

Crossbreeding Peas

When crossbreeding a green and yellow pea plant, the offspring has one copy of the green gene and one of the yellow gene, and the offspring will no longer be genetically pure. But Mendel discovered these plants had only yellow seeds. He coined the term ‘dominant’ to indicate that if an organism had any of that trait, it would win. In modern terms, a plant with any yellow genetic seed material will yield yellow seeds. The green seed genetic material isn’t strong enough to win. Thus, the green seed genetic material is called recessive. Only plants with exclusively green genetic material end up with green seeds.

Knowing Phenotype and Genotype

There are two important words, one is known as ‘phenotype’, which is what an organism looks like. The other is ‘genotype’, the organism’s genetic makeup. In the case of peas, having a phenotype that is green, it is sure that the genotype is two green genes. If the phenotype is yellow, it is unclear if the genotype has two yellow genes, or one yellow and one green.

If there are two parent plants that are purely green, it means that both their genes are green. To fill in the genetic outcomes of the offspring in a two-by-two grid, one parent is put on the top and one on the left. The plants are crossbred, and the offspring gets a green gene from the top parent and a green gene from the bottom one. The same thing happens for pure yellow gene parents. In the case of crossing a purely green plant with a purely yellow, the Punnett squares represent the offspring that are all a mixture, with one green gene and one yellow gene. Mendel found that yellow genes are dominant, having at least one yellow gene, the plant looks yellow with both having the same phenotype.

This is a transcript from the video series Understanding the Misconceptions of Science. Watch it now, on The Great Courses Plus.

Offspring of Crossbreeds

Punnett squares also help understand why, when breeding plants that were crossbred themselves, their offspring were three times more likely to be yellow than green. By setting up the Punnett square, there is a plant on the top with one yellow and one green gene and the same on the left. Either a green or a yellow gene is taken from one plant and combined with the other plant. One option is an offspring with two yellow genes and another is an offspring with two green genes.

Plants with two green genes produce green seeds that are purely yellow or a yellow/green mix, looking yellow. According to the genetic explanation of Mendel’s observation, yellow-looking beans occur three times more often than green ones.

Useful Punnett Squares

Punnett squares are very useful to understand the connection between genetics and what people actually see. A plant with one yellow and one green gene resulting in a yellow pea is not so obvious but making a yellowish-green colored pea is.

Common Questions About Genetics

Q: What is genetic?

The study of genetics is one of the most fascinating fields of modern science. It studies how people inherit traits from their parents, and shows how people with heritages from across the world are both the same and different.

Q: How much DNA do humans share with bananas?

Human DNA has an overlap of 50% to 60% with banana DNA.

Q: What is a genotype versus a phenotype?

Phenotype is what an organism looks like. In the case of peas, having a phenotype that is green, it is sure that the genotype is two green genes. Genotype on the other hand is the organism’s genetic makeup. If the phenotype is yellow, it is unclear if the genotype has two yellow genes, or one yellow and one green.

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