In this presentation, we will cover inherited traits, determination of traits, the role of alleles in traits, and the relationship between alleles and genes.

We will initially look at a unique story involving the inherited traits of twin girls.

In 2005, a pair of unique fraternal twin girls was born. What was unique about them? One of the twin girls was born with dark skin and the other was born with pale skin. How did this happen? Fertility treatments? In-vitro fertilization?

No. It was just natural childbirth. The two girls are just as related to each other as any other set of fraternal siblings. So what did happen, and why was it so unusual?

We will try to answer this question in this presentation as we explore the basics of inheritance of traits.

Which of the following is not an example of a trait?

In 2005, a pair of unique fraternal twin girls was born. What was unique about them? One of the twin girls was born with dark skin and the other was born with pale skin. How did this happen? Fertility treatments? In-vitro fertilization?

No. It was just natural childbirth. The two girls are just as related to each other as any other set of fraternal siblings. So what did happen, and why was it so unusual?

We will try to answer this question in this presentation as we explore the basics of inheritance of traits.

Which of the following is not an example of a trait?

A trait can be any type personal feature. Those features can range from physical appearance to behavior. Many of our traits are heritable, meaning that they are passed on from generation to generation.

Now let’s think about how traits are determined. For this part, we should become familiar with alleles. Often, when people say that "my genes are different", they actually mean that their alleles are different.

What is the relationship between a gene and an allele?

A trait can be any type personal feature. Those features can range from physical appearance to behavior. Many of our traits are heritable, meaning that they are passed on from generation to generation.

Now let’s think about how traits are determined. For this part, we should become familiar with alleles. Often, when people say that "my genes are different", they actually mean that their alleles are different.

What is the relationship between a gene and an allele?

Alleles are variants of genes. In this sense, we all have the same genes. For example a gene for hair. However, we might possess different alleles for that gene. A person with straight hair has a straight allele. And a person with curly hair has a curly allele.

In the “What is a Trait?” module, they present hitchhiker’s thumb as an example of a human trait with two alleles. One for a straight thumb and one for a backwards bending “hitchhiker’s thumb.”

The recessive allele, h, is allele that corresponds to which thumb trait?

In the hitchhiker’s thumb trait, the dominant allele, written with a capital H, produces a straight thumb, and the recessive allele, written with a lower case h, is the allele that produces the hitchhiker’s thumb.

Since each of us has two alleles, what happens if a person has one dominant allele and one recessive allele?

In the hitchhiker’s thumb trait, the dominant allele, written with a capital H, produces a straight thumb, and the recessive allele, written with a lower case h, is the allele that produces the hitchhiker’s thumb.

Since each of us has two alleles, what happens if a person has one dominant allele and one recessive allele?

If a person possesses two different types of alleles for the same gene, we would say that the person is heterozygous for that gene.

If one of the alleles is dominant, meaning that it masks the other allele, then a heterozygous person will show the dominant trait. This is the case with hitchhiker’s thumb, as the straight thumb allele is dominant to the hitchhiker’s thumb.

How are traits inherited?

If a person possesses two different types of alleles for the same gene, we would say that the person is heterozygous for that gene.

If one of the alleles is dominant, meaning that it masks the other allele, then a heterozygous person will show the dominant trait. This is the case with hitchhiker’s thumb, as the straight thumb allele is dominant to the hitchhiker’s thumb.

How are traits inherited?

Each of us has two alleles for every gene, because we inherited one from our mother and one from our father.

What determines which alleles we inherit? It is random, so an offspring has a 50% probability of inheriting either of a parent’s alleles.

As a result of this rule, we can actually calculate the probability of an offspring having particular traits, as long as we have some information on the parents. Let’s try an example.

What is the probability that two parents who are heterozygous for the hitchhiker’s thumb trait have a child with hitchhiker’s thumb?

The probability of any pair of heterozygotes having a child that has two copies of the recessive allele, like being homozygous recessive, is 25%.

This answer can be determined in a few ways. One is just to do the math from above. If there is a 50% chance of inheriting the recessive allele from the mother and a 50% chance of inheriting the recessive allele from the father, then we can just multiply those two numbers together to get 0.5 times 0.5 equals 0.25.

Alternatively, a lot of biology students use the Punnett square to solve these questions. It is a good tool for exam questions. In a Punnett square, you draw a box in four quadrants and put the parents’ genotypes on the top and left side. Then you fill in the quadrants with one allele from each parent.

In this Punnett square, we see that there is a 1 out of 4 chance (25%) of the offspring inheriting the homozygous dominant genotype. There is a 1 out of 4 chance of the offspring inheriting the homozygous recessive genotype. There is a 2 out of 4, or 50%, chance of the offspring having the heterozygous genotype.

Let’s try another question. What are the odds of a child having the homozygous dominant genotype, if the father is homozygous dominant for the trait and the mother is heterozygous for the trait?

http://ricochetscience.com/genetics-videos/

The odds of the offspring having a homozygous dominant genotype from homozygous dominant and heterozygous parents are 50%. Looking at the Punnett, we see that the father only had one dominant allele to donate to the offspring, so the child had a 100% chance of having at least one dominant allele. The mother is heterozygous and has a 50% chance of donating a dominant allele and a 50% chance of donating a recessive allele. In the end, the couple has a 50% chance of having a homozygous dominant offspring and a 50% chance of having a heterozygous offspring.

Now let’s consider whether we can use Punnett squares to predict inheritance of complex traits, like a sense of humor. If the mother and father are both heterozygous for a sense of humor, does that mean that they have a 25% chance of having a humorless offspring? Not quite. The inheritance of complex traits, like height or personality traits, cannot be represented by a simple four box Punnett square.

Why do you think that inheritance of complex traits cannot be analyzed with a Punnett square?

The odds of the offspring having a homozygous dominant genotype from homozygous dominant and heterozygous parents are 50%. Looking at the Punnett, we see that the father only had one dominant allele to donate to the offspring, so the child had a 100% chance of having at least one dominant allele. The mother is heterozygous and has a 50% chance of donating a dominant allele and a 50% chance of donating a recessive allele. In the end, the couple has a 50% chance of having a homozygous dominant offspring and a 50% chance of having a heterozygous offspring.

Now let’s consider whether we can use Punnett squares to predict inheritance of complex traits, like a sense of humor. If the mother and father are both heterozygous for a sense of humor, does that mean that they have a 25% chance of having a humorless offspring? Not quite. The inheritance of complex traits, like height or personality traits, cannot be represented by a simple four box Punnett square.

Why do you think that inheritance of complex traits cannot be analyzed with a Punnett square?

Each of the four options given to the previous question is a reason why most traits cannot be described by a simple Punnett square.

Now, let's go over some of the terms as to why some traits cannot be explained.

Incomplete Dominance: Some alleles are not dominant or recessive. They blend. A curly hair allele and a straight hair allele may make wavy hair.

Multiple Alleles: Some genes have many alleles. For example, our blood type genes have three: A, B, and O.

Environmental Effects: The environment can also play a role. For example, our skin tones can be influenced by exposure to the sun.

Multiple Genes: Finally, most of our traits are governed by multiple genes. This brings us back to our black and white twins.

Skin color is believed to be controlled by several genes, which not only makes it very difficult to predict the skin tone of one’s child; it also provides an abundant array of skin tone options.

Take a look at the figure, which uses a simplified genetics of skin tone including only three genes, each with only two alleles, dark and light. Even in this simplified version, there are no less than 64 different genetic combinations giving rise to seven different skin tones.

This example shows how two parents, who are heterozygous for each of the three genes, could have a child with any of the seven skin tone combinations. This is exactly how the black and white fraternal twins were born, as one of the twins inherited very few dark alleles compared to the other. This outcome is not common, but it is possible as long as both parents are heterozygous for the trait.

Hopefully, we now have a basic understanding of inheritance of simple traits and an understanding of why the inheritance of complex traits cannot be easily understood.

Please use the quizzes and Connect assignments to make sure that you are comfortable determining the probabilities of single gene trait inheritance.