In this explainer, we will learn how to recall Mendel’s laws of inheritance and interpret genetic diagrams of monohybrid crosses.
Have you ever noticed that most people look a bit like their biological mother and a bit like their biological father? You might be wondering why you have inherited your dark hair from your father but your green eyes from your mother. The patterns and laws of inheritance help us explain this. Inheritance is an integral part of biology. It refers to how genetic material is passed from parent to offspring.
A characteristic is an observable, heritable feature of an organism.
A trait is a variation of a characteristic.
One of the most important advancements in the study of genetics and inheritance came from the garden experiments of an Austrian monk, which began in the mid 1850s. Gregor Mendel was interested in the breeding of plants, the variation they showed, and the use of mathematics to explain natural processes.
Mendel decided to carry out his investigations using pea plants. Pea plants were particularly useful for these kinds of experiments. They have very distinct, contrasting traits; for instance, some pea plants have purple flowers, and others have white flowers. Some pea plants have green pods, whereas others have yellow pods. It is also very easy to breed pea plants by self-pollination and cross-pollination.
Self-pollination occurs when pollen grains containing the male gametes (sex cells) fertilize the female gametes (sex cells) of the same plant.
Cross-pollination occurs when pollen grains containing the male gametes (sex cells) of one plant fertilize the female gametes (sex cells) of a different plant.
Figure 1 outlines one of Mendel’s experiments. He noticed that the offspring from the parents (known as the F1 generation) all had purple flowers, even though one of the parent plants had white flowers. Mendel concluded that the purple flowers were the dominant trait and the white flowers were the recessive trait. However, when F1 plants were self-pollinated, they produced approximately 1 plant with white flowers for every 3 plants with purple flowers. In fact, when Mendel performed this experiment, 705 of the F2 plants produced had purple flowers and 224 had white flowers. Importantly, he could show that the recessive trait had not been destroyed or blended into the more dominant trait but rather hidden in the F1 generation to then reappear in the F2 generation.
It is worth remembering that at the time of Mendel’s experiments, more than 150 years ago, DNA had not yet been discovered. People did not know about genes, alleles, or the organization of genetic material within the cells. Some of the terms that Mendel used, such as “heritable factor,” have now been given proper scientific names, such as “gene.”
A gene is a section of DNA that contains the information needed to produce a specific protein.
An allele is an alternative version of a gene.
Mendel’s experiments demonstrated monohybrid inheritance, which is the inheritance of a characteristic that is controlled by a single gene. In this case, the characteristic is flower color, which has two traits: it can be either white or purple. The expression of these traits is controlled by the combination of alleles an organism inherits.
Definition: Monohybrid Inheritance
Monohybrid inheritance is the inheritance of the alleles of a single gene.
Let’s have a look at the genetics behind Mendel’s experiments using what we know now about inheritance.
In figure 2, we have taken the same cross shown in figure 1 and added the alleles of each plant as well as the color of the plants’ flowers. Alleles are different forms of the same gene, and an organism will generally carry two alleles for a particular trait. For instance, one of the genes that controls your eye color may have one allele that codes for brown eyes and one allele that codes for blue eyes! The alleles that an organism has is known as its genotype. The genotype of an organism will determine its physical appearance, and the observable characteristics we can see in an organism are known as its phenotype.
The genotype is the genetic makeup (alleles) of an organism.
The phenotype is the observable traits of an organism and is determined by its genotype.
Alleles in simple monohybrid inheritance usually come in two forms: dominant and recessive. Dominant alleles are those that, when present in the genotype, are always expressed in the phenotype. They are represented by uppercase letters (e.g., “P”). Recessive alleles are alleles that are only expressed in the phenotype when two copies are present in the genotype or, more simply, when there is not a dominant allele present. They are represented by lowercase letters (e.g., “p”).
Definition: Dominant Allele
A dominant allele is an allele that is always expressed in the phenotype if present in the genotype.
Definition: Recessive Allele
A recessive allele is an allele that is only expressed in the phenotype if two copies are present or if a dominant allele is not present.
As we can see from figure 2, the F1 generation all have the combination Pp. This means the F1 generation is known as heterozygous, as the two alleles for a particular gene are different. Because P, the allele for purple flowers, is dominant, this is the color expressed in the phenotype. However, when the F2 generation is produced, around of the plants will end up with the combination pp. Having the genotype “pp” results in these individuals being homozygous because both alleles for this gene are the same. Because there are now two copies for the recessive allele that gives white flowers, this is expressed in the phenotype. This helps us understand the genetics behind what Mendel observed more than 150 years ago!
An individual is heterozygous for a characteristic if they have two different alleles for a gene.
An individual is homozygous for a characteristic if they have a pair of identical alleles for a gene.
We can model the inheritance of traits using Punnett squares. A Punnett square is a highly useful tool to show how the alleles within gametes combine and the genotypes of subsequent offspring. If we know what traits the alleles correspond to, we can also determine the possible phenotypes of the offspring.
How To: Constructing a Punnett Square to Show Monohybrid Inheritance
A Punnett square is an incredibly useful tool to show the inheritance of alleles by an organism from their parents. When organisms reproduce, their gametes (sex cells) combine in a process called fertilization. Gametes contain half the genetic material of a normal cell and so will contain one allele for each gene. The top row and the left-hand column of a Punnett square should include the alleles of both parents, as shown in the diagram.
We then complete the Punnett square by filling in each of the cells. We take the letter that is in the column header and the letter that is in the row header and form a combination of the two letters. This is outlined in the diagram below.
It does not matter which parent you select as A or B.
Let’s say we have a set of parents that are reproducing to produce an offspring. One parent has two alleles for brown eyes (BB) and the other has two alleles for blue eyes (bb). We start by completing the top row and first column of the Punnett square to show how the alleles would be divided between the gametes of each parent.
Then, we just need to fill in the cells of the table with the correct combination of alleles. For each cell, we take the column head and the row head. Therefore, each cell should have a combination of 2 alleles. It is generally good practice to put the dominant alleles (those with capital letters) first.
Our Punnett square is complete! From here, we can use this to predict the genetic makeup and the physical appearance of the offspring born to these parents.
Figure 3 shows a Punnett square for the production of the F1 generation in our example with the pea plant flower color.
As we can see from this Punnett square, all the genotypes are Pp. This means that 4 out of 4, or of the offspring, will inherit a dominant allele that gives the trait of purple flowers. We can conclude that the phenotype of this generation will be purple flowers or, as a ratio of dominant to recessive, .
Example 1: Calculating Phenotypic Ratios from a Completed Punnett Square
State the phenotypic ratio (dominant to recessive) for the Punnett square provided
Let’s first understand the key terms in the question. The question asks for a “phenotypic ratio.” A phenotype is the observable traits of an organism. The phenotype is directly influenced by the genotype, which is the genetic makeup (the alleles) that an organism possesses. Alleles are written following a certain convention. An uppercase letter refers to a dominant allele, which is an allele that is always expressed in the phenotype when present. In this case, the dominant allele is “G.” A lowercase letter refers to a recessive allele that is only expressed in the phenotype when two copies are present. In this case, the recessive allele is “g.”
In this Punnett square, we are shown the possible genotypes of the offspring produced when parent organisms with the genotypes Gg and Gg reproduce.
Looking at this Punnett square, we can see that the offspring have a 1-in-4 chance of having the genotype “GG,” a 2-in-4 chance of having the genotype “Gg,” and a 1-in-4 chance of having the genotype “gg.”
Using what we know about phenotypes, a genotype that has even one dominant allele (G) present will show the dominant phenotype. The only way a recessive phenotype can be observed is if two recessive alleles (gg) are present. So, this means the offspring have a 3-in-4 chance of demonstrating the dominant phenotype and only a 1-in-4 chance of demonstrating the recessive phenotype.
The question asks for our answer in the form of a ratio. 3 out of the total of 4 phenotypes are dominant, and 1 out of the total of 4 is recessive.
Therefore, the ratio of dominant to recessive phenotypes is .
Let’s look at another example of a Mendelian experiment.
The pea plants that Mendel used in his experiments would produce seeds of two distinct colors: either green or yellow. Again, Mendel began with parent plants that had been self-pollinated to produce seeds of one color. He then cross-pollinated these to produce the F1 generation and self-pollinated individuals from this generation to produce the F2 generation. His experiment is outlined in figure 4.
Let’s use what we know about genotypes to derive some Punnett squares from these results. We will represent the dominant allele for yellow seeds with “Y” and the recessive allele for green seeds with “y”.
Figure 5 shows the Punnett square that demonstrates the inheritance of alleles from the parent plants by the F1 generation.
As we can see from the Punnett square, all the offspring in the F1 generation have one dominant allele and one recessive allele. Even though they only have one allele for the yellow color, when they produce seeds, the seeds will be yellow because this allele is dominant.
Example 2: Using Punnett Squares to Calculate the Probability of Inheriting a Specific Phenotype
In a species of pea plants, the allele for green pods is dominant to that for yellow pods. The diagram provided shows two parent pea plants being crossed. What is the probability, in percent, that the offspring will have a green pod?
When we are looking at inheritance of traits, the easiest thing to do is to use a Punnett square to model inheritance of alleles. In this example, we have been given the phenotype and the genotype (alleles) of each plant that produces these colored pods.
Let’s start by drawing our Punnett square.
In the topmost boxes and those down the left-hand side, we will put the alleles that will be found in the gametes produced by the parent plants. Remember, this is only one allele per gamete for a trait. It is also useful to remember it does not matter which plant you select as parent A or parent B.
Then, we just need to fill in the cells of the table with the correct combination of alleles. For each cell, we take the column head and the row head. Therefore, each cell should have a combination of 2 alleles.
This question is asking us for the probability, in percent, that an offspring plant produced by these parents would produce green pods. For this, we need to understand what combination of alleles would produce green pods. We can see from the parent plants that the plant with Gg has green pods, whereas the plant with gg produces yellow pods. We can conclude from this that the dominant allele “G” must be present for the plant to produce pods that are green.
Going back to our Punnett square, we can see that there is a 2-in-4 chance the offspring will inherit a “G” allele and therefore produce green pods. There is a 2-in-4 chance that the offspring will only inherit “g” alleles and therefore produce yellow pods. Now, we just need to convert this to a percentage.
We know that 2 is half, or , of 4. Therefore, the probability in percent that the offspring produced from this cross will produce green pods is .
Figure 6 shows the inheritance of alleles for seed color by the F2 generation in figure 4 from the F1 individual self-pollinating.
As we can see from this Punnett square, the F2 generation has a more varied combination of alleles. There is a 3-in-4 chance that the offspring will have a dominant allele and produce seeds with a yellow color. However, there is a 1-in-4 chance that they will inherit two recessive alleles and produce seeds with a green color.
Example 3: Calculating Phenotypic Ratios from Mendel’s Experimental Data
Gregor Mendel bred a population of pea plants and recorded the results of the crosses. He determined that 6 022 of the seeds produced by the offspring were yellow and 2 001 were green. What percentage of the total offspring displayed the dominant trait? Give your answer to the nearest whole number.
Let’s first understand some of the key terminology in this question. The question asks for the percentage of offspring that displayed the “dominant” trait. A trait is a variant of a characteristic. So in this example, the characteristic is seed color and the trait is yellow seeds or green seeds. We can see from the numbers that 6 022 plants produce yellow seeds, whereas only 2 001 produced green seeds. This leads us to conclude that yellow seeds must be the dominant trait. It is also useful to remember that dominant traits are caused by dominant alleles, which are alleles always expressed in the characteristics of an organism even if only one copy is present.
Now, we need to calculate the percentage of the total offspring that are displaying this dominant trait. First, we need to calculate the total offspring; we will do this by adding 6 022 to 2 001, which gives us 8 023 plants. To calculate the percent that will produce yellow seeds, we need to divide this number by the total number of offspring. So, . Let’s multiply this by to convert it into a percentage.
Therefore, the percentage of the total offspring that displayed the dominant trait of yellow seeds is .
From Mendel’s experiments and subsequent investigations into inheritance, three laws of inheritance were produced. We have seen that in Mendel’s experiments, when pea plants with purple and white flowers were crossed, purple flowers appeared much more frequently in the offspring than white flowers and the flower colors did not “mix.” This led to the proposal of the law of dominance, which states that in a pair of alleles, one allele will be dominant over the other. By using Punnett squares to analyze these experiments, it has been determined that the offspring will inherit one allele for each gene from each parent. This led to the law of segregation, which states that a gamete produced by a parent organism will only contain one allele for a characteristic. Through his experiments, Mendel also discovered that traits were inherited independently of each other. So, for instance, the inheritance of purple flowers happens independently of the inheritance of wrinkled seeds. This led to the law of independent assortment, which states that the inheritance of alleles for one trait happens independently of the inheritance of alleles for a different trait.
Law: The Law of Dominance
The law of dominance states that a dominant allele will be expressed in the phenotype over a recessive allele.
Law: The Law of Segregation
The law of segregation states that two alleles for a characteristic separate from each other during gamete formation, so a gamete will only contain one allele for each gene. An offspring will inherit one allele from their biological mother and one allele from their biological father for each characteristic.
Law: The Law of Independent Assortment
The law of independent assortment states that each pair of alleles segregate into gametes independently of any other pair of alleles.
When we look into more examples and models of inheritance, we will see that these laws are not always completely true, but they are a good basis for understanding genetics.
In the early 1900s, Mendel’s work was further confirmed by the discovery of Boveri and Sutton that chromosomes were the genetic material responsible for Mendelian inheritance. Their work, known as the chromosomal theory of inheritance, supports Mendel’s laws of inheritance, as they made the following observations:
- During the formation of sex cells (gametes) by meiosis, homologous chromosome pairs will move independently of other chromosome pairs.
- The sorting of chromosomes into gametes is a random process.
- The gametes that a parent produces contain half the genetic material of their body cells.
- When gametes (sperm and egg cells) combine in fertilization, they produce offspring that have the same number of chromosomes as their parents.
Let’s recap the key points we have learned so far.
- Mendel carried out experiments in pea plants to observe the inheritance of certain traits.
- We now understand that the expression of these traits is controlled by different versions of genes, called alleles.
- Dominant alleles will always be expressed in the phenotype when present in the genotype, but recessive alleles will only be expressed when two copies are present or a dominant allele is not present.
- By using Punnett squares, we can model inheritance and calculate phenotypic ratios and probabilities.
- Mendel’s work led to the development of the laws of inheritance, which include the law of dominance, the law of segregation, and the law of independent assortment.
- Sutton and Boveri used Mendel’s work to develop the chromosomal theory of inheritance, which states that chromosomes are the genetic material responsible for Mendelian inheritance.