Gregor Mendel and Mendel’s principles are introduced in this area of the AP Biology Mendelian Genetics, which improves on our knowledge of heredity and meiosis. The Laws of Dominance, Segregation, and Independent Assortment provide guidelines for the inheritance of diverse alleles, allowing us to anticipate the result of many matings as long as a few alleles are known.
We’ll also learn how to use Punnett squares to anticipate the results of various crossings in this part. We’ll look at how Punnett squares may be used to estimate the genotypic and phenotypic ratios of offspring in monohybrid and dihybrid crossings, as well as other situations. We’ll also look at how a test cross may be used to figure out which alleles a creature with an unknown genotype has. Finally, we’ll use chi-squared analysis to examine whether our genetic outcome theories are supported by our observations of real populations!
Mendelian Genetics Overview
Believe it or not, the earliest human understanding of genetics originated from a Catholic priest who experimented with and observed the features of pea plants. Gregor Mendel, who is today known as the father of genetics, began with the common notion that crossbreeding plants and animals might result in offspring with diverse features. Mendel discovered that crossing pea plants with specified qualities resulted in predictable ratios of different traits in the progeny. By working backwards from this, Mendel constructed a set of criteria for how characteristics are passed down from parents to offspring. These are known as “Mendel’s Laws.”
Mendelian genetics is the study of Mendel’s laws and the likelihood of distinct genes being passed down in the offspring of two sexually reproducing organisms. Despite the fact that our knowledge of genetics has progressed well beyond Mendel (into non-Mendelian genetics), we can still utilise Mendel’s Laws to predict how kids will acquire a variety of features based on how genes are sorted during meiosis and recombined during fertilisation. The AP exam will cover Mendelian genetics and probability testing using chi-squared analysis. So stay with us as we go through all there is to know about Mendelian Genetics!
We need to know a little bit about genetics and evolution before we can appreciate Mendel’s rules. First, Gregor Mendel, although being a humble monk born in the 1820s, had a fundamental knowledge of heredity. Farmers have known for thousands of years that kids have the possibility of acquiring their parents’ characteristics. Early people used this method to selectively breed everything from cattle to the food we ate. Gregor had no clue how these features were handed down through the generations on a molecular level, but he did know that they were inherited from parents.
While this was all Gregor Mendel needed to get started, we now know a few more things that will be very useful as we begin to study Mendel’s Laws and Heredity. We now know that two molecules, DNA and RNA, carry genetic information.
The information is carried by DNA, which is transcribed into an RNA molecule that can exit the nucleus and then translated into a protein molecule by a ribosome. Mendel was studying protein molecules as biological machines, creating the outward features (called phenotypes) that Mendel was studying.
We also know that these ribosomes can be found in all domains of life, from the smallest bacterial cells to multicellular eukaryotic cells. Not only do all domains of life share this fundamental underlying process for protein formation, but they also share many other metabolic pathways, including the generation and use of carbohydrates, the synthesis of phospholipids for cell membranes, and many other essential tasks. All of this data points to the continuation of existence from a single common ancestor who lived billions of years ago! With this in mind, let’s look at Mendelian Genetics and the rules discovered by Mendel.
Consider this… While a great number of genes impact various features and situations, other diseases may be caused by a single genetic mutation. These hereditary illnesses may have serious consequences. However, using basic and very inexpensive genetic testing, we can quickly anticipate if two parents have the potential to transmit certain hereditary disorders on to their offspring. Keep these genetic circumstances in mind as we begin to look at the likelihood of inheritance using Mendel’s Laws!
Pea plant experiments
When he began his pea plant research, Mendel knew two essential facts about the pea plants he was dealing with. He realised that pea plants reproduced via pollen being created in the male stamen organs and transported to the female stigma organ. Because each pea plant blossom includes both male and female components, selfing allows a single flower to reproduce by itself. Mendel avoided this by cutting the stamens from one of the plants he wished to replicate in order to perform his tests. He carefully transferred pollen from one bloom to the other using a paintbrush, making sure the first blossom was pollinated by a plant of his choosing.
In one of his most renowned and significant experiments, Mendel crossed a strain of plants that only produced purple flowers with a line of plants that only produced white blooms. When he went to pick and plant the peas, he saw that they all had purple blooms. Despite the fact that the parental generation had both white and purple flowers, the F1 generation had exclusively purple flowers. However, Mendel discovered that crossing two of these F1 plants into a new generation (the F2) resulted in at least 1/4th of the plants having white flowers.
Law of Dominance
This is how Mendel came up with his first law, the Dominance Law. This rule asserts that in a heterozygous organism with 1 of each gene, certain alleles (such as the one that causes purple flowers) may be masked by other alleles (such as the one that causes white flowers). However, the Law of Dominion has been broadened to cover more than simply full dominance. Incomplete dominance occurs when the heterozygote produces an entirely different phenotype, such as a pink flower when a red homozygote and a white homozygote are combined. Both homozygous features are manifested in the heterozygote in codominance.
Law of Segregation
But what about Mendel’s laws two and three? Various alleles are split into different gametes, enabling dominant and recessive alleles to be inherited independently, according to the Law of Segregation. Mendel’s most famous flower colour experiment may also be used to derive the rule of segregation. Because the F2 generation had white flowers but the previous generation didn’t, it’s safe to believe that the white flower alleles were hidden in the F1 generation and separated from the dominant purple alleles before being inherited.
Law of Independent Assortment
The Law of Independent Assortment, on the other hand, can only be detected when two separate features are examined simultaneously. Different genes are inherited independently of one another, according to this rule. The gene that controls a plant’s bloom colour, for example, is unrelated to the gene that controls pod colour. Purple blooms and yellow pods, white blossoms and green pods, or any other combination of the two features may be inherited by a plant.
While Mendel was fortunate in that he was able to choose many qualities that were not physically related, we now know that the Law of Independent Assortment only applies to genes on distinct chromosomes. If two genes are found on the same chromosome, there’s a significant probability they’ll be passed along together. If you need a reminder, these steps were previously discussed in section 5.2.
Another scientist, Reginald Crundall Punnett, did not create a method to assess the chance that an offspring would get a specific allele in a mating between two species until almost 50 years after Mendel’s tests. The Punnett square was this instrument.
The Punnett square is a basic tool for predicting the result of a genetic cross by putting what we know about meiosis and sexual reproduction into a table. The Punnett square contains four squares when considering just one trait: a vertical column for each copy of each paternal allele and a horizontal row for each copy of each maternal allele. The alleles are then distributed to each box, representing a possible fertilisation event between gametes containing these alleles. This provides us with each prospective offspring’s genotype.
There are two alleles for this characteristic, which is bloom colour in pea plants. The “B” allele completely outnumbers the “b” allele. As a result, each square containing a “B” will be purple. This contains both homozygous dominant and heterozygous dominant squares. Any squares with two “b” alleles that are homozygous recessive will be white.
The chance of each genotype and phenotype for various sorts of crossings is what the Punnett square truly gives us. Because both parents are hybrids (or heterozygotes) and we’re only interested in one characteristic, this is a monohybrid cross. The genotypic ratio is the same in all monohybrid crossings. The ratio of 1 homozygous dominant to 2 heterozygotes to 1 homozygous recessive is always the same.
When alleles with total dominance are involved, however, the phenotypic ratio is the same. In this situation, the dominant phenotype will be present in 3 out of 4 children, whereas the recessive phenotype will be present in 1 out of 4 offspring. Regardless of how many individual kids are produced by each cross, this provides us with the chance of each phenotype and genotype.
Furthermore, a Punnett square’s main concept may be enlarged simply by adding additional squares. Consider a dihybrid cross, which is a cross of organisms that are heterozygous for two characteristics and display total dominance in both. In this extended Punnett square, each row and column represents a possible gamete, and each square represents a hypothetical fertilisation event between two gametes.
The chance of an offspring inheriting a given mix of characteristics may be calculated using the Punnett square. While anything more complicated than a trihybrid cross is impossible to calculate by hand, geneticists may utilise automated Punnett squares to predict a variety of features.
Furthermore, using the Punnett square to determine which genes an unknown creature possesses via a test cross is simple. We may cross a recessive phenotype with a plant with an unknown genotype since we always know the genotype of organisms with a recessive phenotype. We know the unknown genotype included one recessive gene if the kids had the recessive trait. We know the unknown genotype was homozygous dominant if the offspring solely had the dominant phenotype.
Consider a flower colour gene. This characteristic, we believe, demonstrates total domination. We can readily estimate a 3:1 phenotypic ratio using a Punnett square—three dominant phenotypes and one recessive trait. If we assess 100 kids, we should anticipate 75 dominant phenotypes and 25 recessive traits.
However, since the alleles each gamete receives are distributed randomly, we anticipate some departure from these “perfect” Mendelian ratios owing to random chance. Let’s imagine we uncover 70 dominant phenotypes and 30 recessive traits after watching 100 children. So, how can we know whether these variances support or refute our hypothesis?
Chi-squared testing comes in handy in this situation. Chi-squared testing is a basic statistical approach for determining if our data supports a certain hypothesis or anticipated value. We divide by the anticipated value for each “class” after subtracting the expected value from the actual value (in this case, each phenotype). The total chi-squared value is then calculated by adding all of the classes together. “Chi-squared is the sum of observed values minus anticipated values, squared, divided by expected values for each class,” says the formula.
To get the chi-squared value for our experiment, we simply sum observed minus expected, squared, divided by anticipated for each class, and then multiply by expected. Our chi-squared value is 1.33 when we do this. But what exactly does this imply?
To determine if our chi-squared result supports our hypothesis, we must compare it to a crucial values table. First, we determine how many degrees of freedom our chi-squared value has. The number of classes minus one is the “degrees of freedom.” We only had one degree of freedom since we only had two lessons.
Then we look at the table to see where our chi-squared value belongs. Between the P-values of 0.5 and 0.1, 1.33 fits into the table. This suggests that between 50% and 10% of the time, we should anticipate big variances like the ones we saw. We may take our findings as evidence for our hypothesis, since most scientists think that P-values over 5% indicate support for a hypothesis.
Remember that a chi-squared analysis may test practically any number of classes. The AP exam will almost certainly require you to compute something more sophisticated than this, so take the quiz in the Resources section to see whether you’re ready!