Christine has taught college Biology and Anatomy, and has a Master's degree in Anatomy.
Some genetic characteristics are inherited in predictable patterns. In this lesson you will learn what monohybrids are and how to predict the kinds of characteristics that their offspring will show.
It's All In the Genes
Genetics - we all know it takes two parents to get things started, but after that things seem to get pretty complicated. Ever wonder why that cute puppy is the only one of the litter that looks like the mother? Or why no one in that big family down the street has their father's red hair? Many of the mechanisms of inheritance can be complex, but there are some characteristics that can be followed from one generation to the next in a fairly straightforward way. In 1865, long before our modern understanding of genetics and DNA, a scientist named Gregor Mendel discovered some of the secrets of inheritance and became the first to describe the monohybrid cross.
The Monohybrid Cross
Let's take a look at the parts of the term. A cross refers to the process of fertilization and production of a new generation, and the prefix mono means 'singular'. Mendel started off by focusing his studies on singular inherited characteristics, or traits, from crosses of his pea plants.
Mendel raised hundreds of pea plants for his experiments, and he learned a lot about how they passed on traits from one generation to another. He noticed that for some traits, like flower color or seed shape, different plants would show only two forms of the trait. For example, plants had either white or purple colored flowers, or rounded or wrinkly seeds. He could also produce a pure line of plants that would only produce offspring with one form of a single trait, so that purple flowered plants always had purple flowered offspring.
A hybrid, then, is a plant that is the result of a cross between two plants that are both pure for different forms of the same trait. Gardeners do this all the time, and most of us are familiar with the term 'hybrid' from the beautiful colors that are produced in hybrid roses.
Mendel's hybrids didn't act the way roses do, though. There was no blending of colors or seed type - in fact, the offspring of these pure crosses always, 100% of the time, looked like only one of the parent plants, never the other. So Mendel decided to find out what would happen in the next generation if he crossed the monohybrids with each other - creating the monohybrid cross.
Dominant and Recessive
Mendel noticed that one of the forms of the trait occurred more often in nature - this was the form that always appeared in his first generation of hybrids. He called it the dominant form of the trait; the other was called recessive because it seemed to disappear from some generations. Although the first cross of the two pure strains always resulted in all of the offspring showing the dominant form of the trait, the second generation, the monohybrid cross, always resulted in a three to one ratio of dominant to recessive offspring.
Mendel's ratios were consistent because he used such large numbers of plants (recall that if you flip a coin a few times it may not result in a 50/50 ratio of heads to tails, but the more tosses you do, the closer you will come to that number).
Now this all happened before our modern knowledge of how DNA works to pass along information to every cell in every living organism. Because all living things had two parents, it made sense to Mendel that at least two copies of the information were present for each particular trait. This turned out to be true, and we now call the DNA information for a single trait a gene, and the two copies of the genes which are inherited from each parent are called alleles. For traits that are inherited according to this model (remember, not all traits are inherited this way), organisms can have three possible sets of genetic information: either two of the same alleles for one of the forms of the trait, or one each of the two different possible alleles.
Over 79,000 lessons in all major subjects
Get access risk-free for 30 days,
just create an account.
Today we know that every allele that is inherited this way is either dominant or recessive. If you received both alleles for the dominant form of the trait from your parents, then you will express, or display, the characteristics of the dominant form (like the pure purple flowered peas). If you have both alleles for the recessive form, then the recessive form of the trait will be expressed (like the pure white flowered peas). The description or notation of which allele an organism has for any gene is called the genotype, and the word phenotype is used to describe which of the two forms of the trait for that gene is expressed or displayed in the individual: it's what you see.
If you happen to have both a dominant and a recessive allele, then you will always display the dominant form of the trait - the expression of the recessive allele is masked, and you appear exactly the same as someone who is pure for the dominant trait (or who has both dominant alleles). This is commonly referred to as being a carrier for a trait, a phrase sometimes used to describe the inheritance of an illness. The allele for the recessive trait is still there, still part of a person's DNA, and can be expressed in a later generation if a recessive allele is also passed on from another parent.
Remember, the recessive form of the trait is only expressed when there are two recessive alleles present. It is pure chance which allele is passed on from either parent, and the three to one ratio only holds true when there is a large group of offspring.
Example: The Fishbaums
Let's look at an example of a monohybrid cross, demonstrated by Fred and Rita Fishbaum (any similarity to living or dead fish is purely coincidental). They are monohybrids, so they each have one dominant allele and one recessive allele. In our example, the trait we are following is body length - Fred and Rita have long bodies, and the recessive form is a short body.
To do this we'll use a Punnett square, a tool which shows the genotype of a cross by using letters to represent the different alleles - capital letters represent dominant alleles and lowercase letters represent recessive alleles. The top row shows the two alleles of one parent, the left column shows the two alleles of the other parent, and the four boxes with two letters illustrate the only possible genotypes of the offspring. Letters B and b represent the alleles for the fish body - B is dominant, long body, b is the recessive form, or short body allele.
Monohybrid Cross of the Fishbaums
Now let's look at the possibilities. The possible genotypes and possible phenotypes are:
BB = long body, both dominant alleles
Bb = long body - the recessive allele is masked
bb = short body, both recessive alleles
So, a monohybrid cross is the creation of a new generation from two organisms that are hybrids for a single characteristic, or trait. They became hybrids because they had parents that were pure for the two different forms of the trait, and they now have the same mixed genetic information for the trait - they have two different alleles, or forms of the gene for that trait. These monohybrid parents can then pass on either allele to the next generation, and the different combinations result in three possible types of genetic information, or genotypes: either two dominant alleles, two recessive alleles, or one of each.
Did you know… We have over 200 college
courses that prepare you to earn
credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the
first two years of college and save thousands off your degree. Anyone can earn
credit-by-exam regardless of age or education level.