“The child had his mother’s eyes, his mother’s nose and his mother’s mouth, which leaves his mother with a pretty blank expression.
~ Robert Benchley
Remember, It’s All in the Genes!
In the last activity, you looked at meiosis and how that process results in genetic variation due to crossing over. You also explored the randomness of independent assortment. This means that each gamete produced is genetically unique, and so different versions of genes, known as alleles, are passed on to the zygote upon fertilization. It is this variation in the genes, received through sexual reproduction, that gives offspring their traits. It is this passing on of traits, or inheritance that you will explore in this activity. You will also look at how various genetic disorders are passed on from parent to offspring.
It is important that you understand the basics before we take a detailed look at how the transfer of genes makes us who we are. Take a look at these short videos from the Learn Genetics program at the University of Utah to learn more about inheritance and traits: You will need to go to the website above and use the right side menu to find the videos.
The Father of Genetics
Most of the content within this activity will be based upon Mendelian inheritance. Gregor Mendel was an Austrian monk and biologist whose observations and experiments with pea plants gave us the basic principles of heredity and set the foundation of modern genetics (I told you earlier that pea plants were special!). Mendel would crossbreed pea plants with specific traits to see what traits would show up in the next generation.
Based on his work, Mendel came up with the following inheritance fundamentals (most of which you have already learned about in earlier activities):
- Inheritance involves the passing of genes from parents to offspring.
- The alleles that determine traits are separated into reproductive cells through meiosis and randomly reunite during fertilization.
- Genes located on different chromosomes will be inherited independently from each other.
Through his observations, Mendel produced his Law of Segregation. This law states that:
- organisms inherit two copies of each gene, one from each parent (remember the homologous genes from meiosis?); and
- organisms donate only one copy of each gene to their gametes (also due to the assortment that occurs in meiosis).
At the end of this activity, you will see that some inheritance does not follow the basic ideas presented above. This is classified as non-Mendelian inheritance.
The following video is a nice overview of the concepts you will investigate in the Action section. Don’t worry about keeping track of new terms at this point, as all the new information presented in the video will covered in detail throughout this activity.
You are now ready to wade into the gene pool to see what heredity is all about. Come on in, the water is fine!
Well, maybe not exactly, but there are ways to determine the probability of certain traits and genetic disorders being passed onto the next generation by using the fundamentals of Mendelian inheritance. The higher the probability, the greater the chance that the trait will be seen in the offspring. Once the mechanism of inheritance was better understood, humans were able to manipulate those probabilities to produce populations of species with the most desirable traits, or prevent various genetic disorders from impacting the next generation.
By breeding individuals with preferred traits, we are able to increase the likelihood that those traits will show up in the next generation. For example, drought resistant crops and designer dogs that can be carried in a purse have been bred this way. Selective breeding has had a considerable impact on our way of life. It’s here to stay.
As you move through the unit, you will see how human interactions with the genetic process, such as selective breeding, help to shape the world we live in…for better or for worse. You will also see how understanding inheritance patterns can help prevent genetic disorders from moving to the next generation.
The Lingo of Life
There will be a number of new terms presented in this activity. Many you have heard before, but you may not know their true definitions. You will need to learn these terms in order to understand inheritance fully. The fun never stops!
Let’s use the common genetic trait of tongue rolling to illustrate these definitions.
Biology and other branches of science often rely on simplifications of concepts in order to help students understand the basic principles behind them.
The genetic traits that we look at in this course are not always as cut and dried as they may appear to be. Many characteristics do have a genetic connection passed from parent to child, but these traits can also be influenced by other factors. Normally, your traits are a combination of genes and environmental factors.
For example, tongue rolling does indeed have a genetic connection, but people who are born without the ability to naturally roll their tongues can learn to do so if they repeatedly practice. Although the examples used may be oversimplified, please know that the main genetic ideas behind them are accurate. We just aren’t incorporating external factors such as this into the mix.
So, it isn’t lying if I TELL you that I am bending the truth a bit, right?
Back to Tongue Rolling!
As you know, you receive two alleles of each gene upon fertilization: one from each parent. Remember that alleles are different versions of the same gene. In this case, you can be given the ‘tongue rolling’ allele or the ‘non-tongue rolling’ allele from your parents.
It can be a bit of a pain to keep saying tongue rolling or non-tongue rolling, so letters are used to represent different alleles to simplify the language. We can represent the tongue rolling allele with ‘R’ and non-tongue rolling with ‘r.’ It is important to use the same letter for each possible allele of the same characteristic because we’re describing the same gene. You can choose the letter, but be sure to be consistent with your labelling. You will learn more about the letters and the information they represent soon.
Using these letters, we can represent both alleles that an individual has for a specific gene. Each letter represents one allele from each parent. So, an individual can be RR, Rr, or rr for the tongue rolling characteristic, depending on the combination of alleles passed on from the parents. This two-letter code is known as the genotype for that characteristic.
If both parents pass on the R allele (genotype of RR), you would expect that the offspring would also be able to roll his/her tongue. When an individual receives the same allele from both parents, she/he is said to be homozygous for that characteristic.
The same idea holds true if both parents passed on the allele that doesn’t result in the tongue rolling ability (genotype of rr). Since the same allele is present for that characteristic from both parents, that individual would be homozygous for that characteristic.
Although you cannot see the actual genotype of a characteristic, you can see or detect the outward expression of the characteristic. In this case, you can actually see if a person can roll her/his tongue or not. This observable expression of a characteristic is called the phenotype.
Now, what if the genotype for that trait is Rr? In this case, the individual has one allele for tongue rolling and one allele for non-tongue rolling. When you have two different alleles for a characteristic, it is known as heterozygous.
When an individual has the heterozygous genotype for a characteristic, what will the expressed phenotype be? Will it be a mixture of the two or will one win out?
In cases like this, the phenotype will be based on the dominant allele. When an allele is dominant, that is the trait that will be expressed. Even if only one of the alleles for a characteristic is the dominant allele, you will see that trait come through. In the heterozygous condition, the non-dominant allele is called the recessive allele.
Dominance doesn’t mean that the allele is the most plentiful in the population. Instead, it means that the allele dominates over or hides the recessive alleles. The dominant allele will always be expressed over a recessive allele.
How will you know which allele is dominant? This can only be determined based on experimentation and the observation of each trait. Since you most likely don’t have access to a genetics lab, the allele which is dominant and that which is recessive for a particular characteristic will be clearly stated in the examples and questions. When you are given the two letter genotype, the capital letter represents the dominant allele. In the case of the tongue rolling, the dominant allele is the ability to roll the tongue, so that allele will be R. The recessive allele is the r, or the inability to roll the tongue. When you have the homozygous condition, you need to state if the genotype is dominant or recessive. So, RR would be homozygous dominant for the tongue rolling characteristic. You do not need to specify which allele is dominant when an individual has the heterozygous condition, since you have one of each allele.
Remember to stay consistent with your letters, and base them on the dominant allele if possible. For example, say that white fur is dominant and black is recessive. It is the convention to take the expression of the dominant allele, and use that as the letter. Since white is dominant, the dominant allele will be W. Now, be careful! Many people then label the recessive allele as b for black. The use of a lowercase letter is correct since that allele is recessive, but you need to use the same letter. So, the recessive allele will be w, or the lowercase version of the dominant allele. This probably sounds confusing, but just be sure that the letter that represents a specific gene is the same for both the dominant and recessive alleles.
It is important to remember that although a certain trait is expressed, an individual may still be able to pass on the recessive allele to the next generation. This occurs with the heterozygous condition. If the genotype is Rr, the dominant R allele will be expressed, but there is still the recessive allele r in that genotype. During meiosis, some gametes will receive the R allele, while others will receive the r allele. This explains why it is possible for a son or daughter to have a trait that is not expressed in either parent or for traits that appear to skip a generation. This will become clearer as we move on through the activity.
Genotype and Phenotype
Continuing with tongue rolling, here are all the genotypes and associated phenotypes possible for that characteristic:
Whew! That was a good amount of terminology so far! Using good independent work skills, check your understanding of the terms by answering the following questions about genetic characteristics. If you have any trouble, be sure to go back and review the concepts or ask your teacher for support.
Genotype and Phenotype Questions
- A man has hair on his knuckles. Is this describing a genotype or a phenotype?
- Straight hair is recessive to curly hair. Based on the labelling convention of alleles, how would you represent the dominant allele?
- An individual’s genotype for cheek type (dimpled or flat) is Dd. Which condition is recessive?
- In the question above, is the individual heterozygous or homozygous for the trait?
- An individual is homozygous recessive for tongue rolling. Rolling is dominant. What is the person’s genotype?
Finding Probabilities: Monohybrid Crosses
Now that you have a good understanding of genotypes and phenotypes, you can use this information to find the probability of a trait being expressed in the next generation. You know what would make this interesting? More terminology!
To find how specific traits were inherited, Mendel had to perform MANY crosses of true breeding pea plants, which were homozygous for the trait being studied. He would start with two plants that differed in only one characteristic such as flower colour, seed colour, seed shape, or flower position. He called these parent plants the P generation.
Mendel would cross pollinate the plants so that they would produce seeds. He would first remove the stamens from the receiving plant in order to prevent self-pollination.
The offspring of these fertilizations were hybrids of the two parent plants and were called the filial or F1 generation. The ‘F’ from the F1 generation comes from the Latin word filius, which means ‘son.’ Mendel would then plant the seeds produced, wait for the F1 generation to mature. Then he would make observations of the phenotypes of the characteristic being studied. Thank goodness that pea plants grow quickly!
From there, he would then cross plants from the F1 generation to produce the F2 generation, and on and on…
These experiments are known as monohybrid crosses, since the P generation plants differed only in one specific trait.
You can go back to the end of the Minds On section for the video about Mendel’s pea plants for a review of his work if needed.
Be There or Be Square!
As mentioned, Mendel performed an incredible number of crosses to study inheritance. From his data, he was able to determine probabilities of the different phenotypes possible from a monohybrid cross. Thankfully, you won’t need to spend years in the garden to determine these probabilities!
A Punnett square is a diagram used to predict all the possible phenotypes from a cross quickly and easily. This tool is named after Reginald C. Punnett, an English geneticist, who used the square to track genetic combinations while he worked.
How Do Punnett Squares Work?
As mentioned, Punnett squares are quite simple to use. You already know that for offspring to be produced, a gamete from each parent must join together to form a zygote. So, in order to make predictions about the potential offspring, you need to have some information about the gametes that are passed on. The nice thing is that this is easy to determine, as well.
Let’s look at an example. In pea plants, a yellow seed is dominant over a green seed.
If yellow seeds are dominant over green seeds, what are all the possible genotypes and associated phenotypes?
One of the parent plants produces green seeds, so it must be homozygous recessive, yy. During meiosis, each gamete receives only one of the homologous alleles. In this case, the recessive allele is the only allele present, so the only possible allele that each gamete can have is y. See, I told you it was easy!
The other parent plant produces yellow seeds. This means the genotype can be homozygous dominant, YY, or heterozygous, Yy. Since Y is dominant, each case results in yellow seeds.
If the plant has the YY genotype, all gametes would receive a Y, as that is all that is available. If the plant had the Yy genotype, then there are two possibilities: either a Y or a y.
Filling in the Square
In our example, let’s say that one parent is recessive and produces green seeds, and the other parent is homozygous dominant and produces yellow seeds.
The parental cross can be represented as follows, with with the ‘x’ in the centre meaning ‘cross with.’
Remember that we have already determined that the possible gametes for the first parent are y and y, and Y and Y for the second parent.
Since this is a monohybrid cross, there are only two possible gametes for each parent. This means we need a 2 x 2 Punnett square. The gametes of one parent are written across the top of the square, and the gametes from the other parent are written down the side of the square.
The next step is to perform the cross. Use the following interactive to see the possible offspring from the YY x yy cross.
Type the gametes from the YY parent along the top of the Punnett square and the gametes from the yy parent along the side to see the possible offspring genotypes.
Punnett Square – https://dna.frieger.com/punnett.php
It is important to understand that each square does not represent an individual offspring. Instead, each square represents one possible genotype that an offspring can inherit. Since there are four squares, each genotype has a probability of ¼ or 25%.
As you can see, the result of each cross is the genotype Yy, which results in the yellow seed phenotype (since Y is the dominant yellow allele). This means that the probability of producing an offspring with yellow seeds is 4/4 or 100%. There is only one possible outcome from this cross.
It is important to remember that these are only predictions that are based on probabilities. This time, you have only one possible outcome, but you will see that this is not always the case. Use the interactive above to try different gamete combinations to see the possible offspring genotypes that can result.
Before you try it yourself in the next section, make sure you are comfortable with the concepts. If not, go back and read through the last section or contact your teacher for help.
You Try It!
As long as you can determine the gametes produced by each parent, you can use the Punnett square to predict the outcome of a monohybrid cross. Now it is time for you to try an example.
For the flower colour of a pea plant, purple is dominant over white. If you cross two parent plants, each with a heterozygous genotype for this characteristic, determine the possible genotypes and associated phenotypes of the F1 generation.
Once you attempt this question, watch the following video about Punnett squares and you will see the answer for this example. Be sure to try it on your own first! You can complete the Punnett square in a number of ways. You can draw it out on paper, use the interactive above, or use an online Punnett square calculator.
Again, these values are only probabilities. If you did this monohybrid cross for real, you would have a ¼ or 25% chance of producing a white flower. Also, each offspring is independent of each other. So, if you ended up with three plants with purple flowers, there is still only a 25% chance that the fourth flower in the F1 generation will be white. Remember, these numbers represent the likelihood of that result and are not written in stone.
The Glencoe Online Learning Center has a number of Punnett square questions for you to attempt. Use good responsibility skills and be sure to check your answers at the end. Contact your teacher if you are having any issues with the concepts.
If you would like some additional practice or need another explanation of monohybrid Punnett squares, use your good independent work skills and take some time to watch this video. Not only does it go over the use of the squares, but it provides a review of the terminology that you learned so far in this activity.