Question

The ability to roll the tongue into almost a complete circle is conferred by a dominant gene, while its recessive allele fails to confer this ability. A man and his wife can both roll their tongues and are surprised to find that their son cannot. Explain this by showing the genotypes of all three persons.

Short answer

Both parents have the genotype Rr, which means they can roll their tongues but also carry the recessive gene for the inability to roll the tongue. It is possible for their son to inherit the recessive 'r' allele from each parent, resulting in the genotype rr and the inability to roll his tongue. This follows Mendel's laws of inheritance and demonstrates the existence and inheritance of hidden traits in genetics.

Step-by-step solution

Understanding Dominant and Recessive Genes

In genetic terminology, the trait that can roll the tongue is conferred by a dominant gene. We can call this trait 'R'. On the other hand, the inability to roll the tongue is conferred by a recessive gene, which we will denote as 'r'. For a person to express a recessive trait (like the inability to roll their tongue), they must have two copies of that recessive gene (rr). If they have one dominant and one recessive gene (Rr), they will be able to roll their tongue but would also be capable of passing the recessive trait to their offspring.

Determining the Parents' Genotypes

Since both man and his wife can roll their tongues, that means they each have at least one dominant gene (R). But since their son has the recessive phenotype (rr, can't roll the tongue), that means both parents must be carriers of the recessive gene. Therefore, both parents have the genotype (Rr).

Understanding Generational Crosses

In genetics, the cross between parents is represented by creating a Punnett square, which helps us visualize how the alleles can be paired in the offspring. Based on the parents' genotypes (Rr x Rr), the Punnett square would look like this: | | R | r | |---|---|---| | R | RR | Rr | | r | Rr | rr | Here, the alleles in the first row (R and r) represent the potential gametes (sperm) from the father, and the alleles in the first column (R and r) represent the potential gametes (eggs) from the mother.

Determine the Possible Genotypes for the Offspring

Based on the Punnett square above, we can see that there are four potential combinations for the offspring. This means that each child of these parents has a 25% chance of having the genotype RR (can roll tongue and will not carry the recessive trait), a 50% chance of having the genotype Rr (can roll tongue but carries the recessive trait), and a 25% chance of having the genotype rr (cannot roll tongue).

Explaining the Son's Genotype

Given that the son cannot roll his tongue, his genotype would then be rr. This means that he received the recessive allele 'r' from both his mother and father. Though it may seem surprising because both parents can roll their tongues, it is entirely possible as per Mendel's laws of inheritance. Even though the trait seems not to be present, it got passed down to him, showing the existence and inheritance of hidden traits in genetics. This concludes the reason why the son is unable to roll his tongue into a circle despite both his parents being able to.

Key concepts

Genotype

The term "genotype" refers to the genetic makeup of an organism, essentially the combination of alleles inherited from the parents. Alleles are different forms of a gene, and they determine specific traits.

For example, in our tongue-rolling scenario, the allele for the dominant trait (ability to roll the tongue) is represented by 'R', and the allele for the recessive trait (inability to roll the tongue) is represented by 'r'.

Each parent provides one allele, so the possible genotypes for the tongue-rolling trait include:

• RR - Two dominant alleles, which means the ability to roll the tongue.
• Rr - One dominant and one recessive allele, which still results in the ability to roll the tongue since the dominant trait prevails.
• rr - Two recessive alleles, leading to the inability to roll the tongue.

Understanding these genotypes is crucial for predicting the traits of the offspring.

Phenotype

While genotype refers to the genetic makeup, "phenotype" is the observable characteristics or traits of an organism resulting from the interaction of its genotype with the environment.

In simpler terms, it's what you can see, like being able to roll the tongue versus not being able to.

Despite having the same phenotype, individuals can have different genotypes. For example, both RR and Rr will result in a person who can roll their tongue. Here, the effect of the dominant gene masks the effect of the recessive gene. Only the phenotype 'cannot roll the tongue' indicates a genotype of rr, as there are no dominant alleles present to mask the recessive trait.

Remember, phenotype can be influenced by many factors beyond just genotype, such as environment or interactions with other genes.

Punnett square

A "Punnett square" is a simplified tool used by geneticists to predict the possible genotypes and phenotypes of offspring in breeding experiments.

It visualizes all possible alleles combinations that parents can pass to their offspring. For the tongue-rolling example, if both parents have the genotype Rr, we use a Punnett square to understand all potential outcomes for their children.

By aligning the possible gametes from each parent along rows and columns, the Punnett square gives us:

• 25% chance of RR - meaning the child will be able to roll their tongue and will not carry the recessive trait.
• 50% chance of Rr - the child will roll their tongue but will carry the recessive trait.
• 25% chance of rr - the child will be unable to roll their tongue.

This is a vital tool in genetics education, providing a clear and manageable depiction of inheritance.

Inheritance

"Inheritance" describes how genetic traits are passed from parents to offspring. The laws of inheritance were first described by Gregor Mendel through his work on pea plants.

Each parent passes down one allele to their offspring, which can result in dominant or recessive traits being expressed.

In our example, both parents are able to roll their tongues, suggesting they have at least one dominant R allele. However, because they pass on the allele randomly, there's a chance they both pass down the recessive r allele. This results in their son receiving rr, the genotype that expresses the phenotype of not being able to roll his tongue.

Even though the dominant trait is visible in both parents, recessive traits can still appear in the children, showing how genetic variation is maintained across generations.

Mendel's laws

"Mendel's laws" refer to principles of heredity that describe how traits are inherited through generations. His laws include the Law of Segregation and the Law of Independent Assortment.

The Law of Segregation states that allele pairs separate during the formation of gametes (sperm and egg cells), and randomly unite at fertilization. This explains why the son in our tongue-rolling scenario could end up with the rr genotype even though both parents can roll their tongue.

The Law of Independent Assortment describes how different genes independently separate from one another when reproductive cells develop. While this doesn't directly affect our tongue-rolling situation, it explains how multiple traits are inherited independently of one another, contributing to genetic variation.

Mendel's work laid the groundwork for modern genetics, helping us understand how traits are inherited and predicted.