UNIT 5

5.1 Meiosis

Meiosis is the process that all organisms go through in order to produce gametes, or sex cells. The process differs from mitosis, the process of somatic cell division, in a number of key ways. Always remember that the purpose of meiosis is to create variation within the population. As you'll see, there are many different mechanisms to create variation in the sex cells. Instead of creating an exact replica of its mother cell, in meiosis, the daughter cells look similar to its parent cells but not quite identical. Meiosis involves one round of DNA replication and two rounds of cellular division. The resulting cells are all genetically unique from one another and from the parent cell. The resulting cells are also haploid, meaning that they have half of the genetic content of a typical somatic cell.

A normal human being has 46 chromosomes, but if a gamete was produced with two diploid cells (meaning each sex cell contained 46 chromosomes), the resulting gamete would have 92 chromosomes! If we even have one extra chromosome, our body doesn't know how to deal with it, so imagine having twice as many chromosomes in your body! 💣💣

This is why during the process of meiosis, each sex cell only contains n amounts of chromosomes, so that the resulting gamete would be 2n, or a diploid cell. In the human's example, the sex cells would have 23 chromosomes each, so that the gamete has 46 chromosomes, which is the normal amount of chromosomes a human ought to have.

Meiosis II consists of Prophase II, Metaphase II, Anaphase II, and Telophase II.

Meiosis I

Prophase I

Prophase is like regular prophase from mitosis. Like mitosis, DNA replicates and coils into nice chromosomes and the nuclear membrane disappears. But these chromosomes then go on to find their counterparts, or homologous pairs. Homologous pairs are pairs of the chromosomes that contain and code for the same information. These homologous pairs may also cross over, which is sharing information with each other. This creates more variation with the sex cells.

Metaphase I

Metaphase I is also like good old metaphase from mitosis. But instead of lining up in the center individually, the chromosomes line up in homologous pairs. For example, chromosomes that contain DNA coding for hair color would line up together as homologous pairs while chromosomes containing information on eye color might line up together. Keep in mind, one of each within the homologous pairs comes from the mother, while the other comes from the father's DNA. 🧬🧬

Also note that alignment is random, so the mother's DNA could end up on the right for one homologous pair, while the father's DNA could end up also on the right for another homologous pair. This also increases variation!

Anaphase I

Each homologous pair is separated and moved to its respective poles, just like in regular anaphase. However, these chromosomes are stay intact, so the chromosomes are not split into chromatids like in anaphase.

Telophase I

Two daughter cells are created with 46 chromosomes! It almost might feel like mitosis because nothing really changed in chromosome numbers but keep in mind! The chromosomes now contain variation, which is the sole purpose of meiosis!

Meiosis II

Prophase II- New spindles are formed and the chromosomes start coiling.

Metaphase II- The sister chromatids are lined up at the center of the cell.

Anaphase II- The chromatids are pulled apart.

Telophase II- The cell divides into two and voila! Four cells are created with different DNA from all four grandparent cells! This is what you call diversity! Now, the cells have n amount of chromosomes, or 23 chromosomes in each cell.

So basically, long story short, one cell duplicates its DNA, mixes the DNA and separates into normal-number-chromosome cells. Then, it divides once more to have half number of chromosomes in each of the four sex cells. You could then ask, why didn't the cell just divide into half? Well, remember, the whole point of meiosis is diversity, so mixing the DNA in meiosis I is crucial in serving its purpose!

A more simplified version (to understand the whole process) can be seen below!

1️⃣ Step 1

Step 1 shows the replication of DNA, as the cells now have the signature “X” formation of a duplicated chromosome.

2️⃣ Step 2

Step 2 shows homologous chromosomes pairing up. Homologous chromosomes are different versions of the same chromosome. For example, humans have two versions of 23 chromosomes, for a total of 46 chromosomes. During this step of meiosis, our two different versions of our 23 chromosomes would pair up with one another in preparation for division.

3️⃣ Step 3

Step 3 shows the action of crossing over between homologous chromosomes, a process that will be thoroughly discussed in the next section.

4️⃣ Step 4

Step 4 shows the first round of cellular division, resulting in the splitting of homologous chromosomes. The chromosomes that remain are referred to as sister chromatids.

5️⃣ Step 5

Step 5 shows the second round of cellular division, resulting in four genetically unique daughter cells. In comparison to the starting cell on the left hand side, the daughter cells have half the amount of chromosomes.

The AP exam does NOT require you to know the specific phases of cellular division (Prophase, Metaphase, Anaphase, and Telophase), so focus your energy on remembering how the chromosomes generally move and how the process contributes to genetic diversity.

5.2 Meiosis and Genetic Diversity

There are a few key concepts within meiosis that contribute to genetic diversity. Remember that diversity is the key to life. When all else fails, focus on diversity in your answers for the AP exam. Some key contributors to genetic diversity in meiosis are the concepts of crossing over, independent assortment, and random fertilization.

Crossing Over

Crossing over is an incredibly important process that takes place during the first round of cellular division in meiosis.

During this process, homologous chromosomes share genetic material. Remember that homologous chromosomes are two different versions of the same chromosome. For instance, chromosome 2 might have the gene for eye color. Each individual has two versions of chromosome 2, one from mom and one from dad. One version might have a dominant allele for eye color, and the other might have a recessive allele.

During the process of crossing over, homologous chromosomes exchange parts of their chromosome at the same location, therefore, not adding or subtracting genes, just exchanging versions of the gene.

This exchange of genetic material leads to numerous possibilities for the separation of chromosomes and the resulting daughter cells. This can also be seen at the end of the diagram shown in section 1.

Independent Assortment

Independent assortment refers to the way that chromosomes line up for both the first and second rounds of division in meiosis. In the example below, there is a 50% chance that both blue versions of the chromosomes will line up on the same side, as shown by possibility 1. There is a 50% chance that a blue and a red version of each chromosome will line up on the same side, as shown by possibility 2.

Depending on the original orientation of these chromosomes, different daughter cells will form. This can be shown by the four unique combinations of chromosomes shown in the row of gametes.

The random alignment of chromosomes during metaphase contributes to an immense amount of variation. The amount can be quantified using the following formula:

For humans, independent assortment results in 2^23, or 8,388,608, unique egg or sperm that one individual can produce. This does not include the variation that crossing over and random fertilization contribute as well.

Random Fertilization

Random fertilization simply means that there is a random chance that each egg and sperm will join one another. There are potentially thousands of sperm that can fertilize the one mature egg, and the genetics in each of them is distinct. The specific sperm that joins the specific egg for each fertilization is random, meaning that the same two parents are not going to produce the same child twice.

Nondisjunction

There is a special type of "genetic diveristy" which involves meiotic errors 😨. Though not ideal with meiosis, nondisjunction creates cells with too many or too little chromosomes. This can happen if the chromosomes failed to separate properly during anaphase I or II.

If nondisjunction happens during meiosis I, the all of the resulting haploid cells will have abnormal amounts of chromosomes. If nondisjunction happens during meiosis II, only two haploid cells are affected. The other two will have the normal amount, n, but the other two will either have one extra or one less.

When a gamete is produced with abnormal number of chromosomes, they often end in miscarriages or genetic defects. A prime example of nondisjunction is Down sydrome. Individuals with Down syndrome have an extra copy of the 21st chromosome, so they would be the example of the n+1 situation.

Ending Notes

Remember, there are 3 factors of genetic diversity that College Board focuses on: crossing over, independent assortment, and random fertilization. The sole purpose of meiosis is creating genetic variation, so these three are crucial to understand in order to understand the purpose of meiosis. Chances are, you'll see a question about genetic variation on a FRQ.

Nondisjunction is a meiotic error which causes most genetic defects. Depending on when the nondisjunction happened, you'll have 4 or 2 haploid cells that have too many or too little chromosomes.

5.3 Mendelian Genetics

The Father of Modern Genetics

Gregor Mendel, the father of modern genetics, came up with some really important laws, including the law of independent assortment, that allows for scientists to determine how genes are inherited from generation to generation.

DNA

DNA and RNA are carriers of genetic information. This RNA is used to create proteins, so ribosomes (protein factory) is found in all forms of life.

Law of Segregation

The law of segregation states that the two alleles from each parent are segregated during gamete formation. Essentially, each gamete gets only one of the two copies of the gene.

Law of Independent Assortment

The law of independent assortment states that the two alleles get split up without regard to how the other alleles get split up. This means that you can get your father's copy of genes for eye color, but that doesn't mean you'll also get your father's copy of genes of hair color; you might get your mother's.

Essential Vocabulary

Phenotype - the physical appearance of an organism, or the actual depiction of a trait (think: phenotype, PHYSICAL). Ex. red, purple, white, sparkly, spiky.

Genotype - the alleles that make up an individual trait (think: genotype, GENES). Ex. AA, Aa, aa OR homozygous dominant, heterozygous, homozygous recessive.

Allele - a version of a gene. Usually an allele can be dominant or recessive. For Mendelian genetics, all genes have two alleles. Homozygous Recessive - an organism that has two recessive alleles. The organism will have the recessive phenotype.

Dominant - a trait that produces enough protein or product in order to overtake another trait.

Recessive - a trait that does not produce enough protein or product and is overpowered by dominant traits.

Homozygous Dominant - an organism that has two dominant alleles. The organism will have the dominant phenotype.

Heterozygous - an organism that has one dominant and one recessive allele. The organism will have the dominant phenotype.

Punnett Squares

Note that only the homozygous recessive genotype leads to the recessive phenotype. All of the vocabulary above is used frequently and should be memorized and thoroughly understood.

Because of the rules that Mendel created, the frequency of inheritance can be determined when two individuals are crossed. This can be shown with a Punnett Square.

As shown in the Punnett square above, when a heterozygous (Yy) and homozygous recessive (yy) individual is crossed, there is a 50% chance that the offspring will show the dominant (yellow) phenotype and a 50% chance that the offspring will show the recessive (green) phenotype. This can be done for any trait that has a simple inheritance pattern. By knowing the genotype of the parents, the various possible offspring can be calculated with their frequencies.

The probability of having children with a certain trait can be calculated by the laws of probability. In most cases, you'll multiply the probability of having a certain trait with another trait. For example, the probability of having a child with brown eyes or blue eyes will be just multiplying the probability of a child having brown eyes times the probability of a child having blue eyes.

Unfortunately, most traits do not have a simple dominant/recessive inheritance pattern and, therefore, do not fit Mendel’s rules. These traits, referred to as Non-Mendelian traits, are explained next.

Pattern of Inheritance

Like regular Punnett square inheritance, these other patterns can be found using Punnett squares. The example shown above is an example of monohybrid inheritance.

Dihybrid

This inheritance pattern is just like monohybrid, except two genes are looked at. When you look at the Punnett square, you'll get 16 offspring.

If both genes are crossed, you'll actually get a magic ratio of 9:3:3:1 for phenotypes (beware, this is not genotype!), which can be useful when you don't really want to actually calculate out the full Punnett square.

Sex-linked

Sex-linked genes involve genes that are linked to our X and Y chromosomes instead of our other chromosomes. Traits such as color blindness and hemophilia are sex-linked traits. With these sex-linked traits, you are affected if all your X chromosomes have the sex-linked gene. Since males only have one X chromosome, they are more likely to be affected. This is why men are more likely to be color-blind as opposed to women. If a female has a X chromosome that is affected by the gene, she won't express it because it'll be recessive. In order to be color-blind, she would have to have two X chromosomes that are both affected by the gene. A female with only one color blind X chromosome is called a carrier, because though she herself is not color-blind, she can still pass it onto her children.

In the above Punnett square, you can tell a carrier female and normal male have a 50% chance of having a non-color-blind child. They have a 25% chance of having a carrier daughter and 25% chance of having a color-blind son.

5.4 Non-Mendelian Genetics

Most traits actually do not follow Mendel’s laws of dominant and recessive inheritance. The inheritance of these traits is referred to as Non-Mendelian genetics. A few important Non-Mendelian inheritance patterns are multiple alleles, sex-linked traits, incomplete dominance, and codominance.

Multiple Alleles

A lot of human traits are said to have multiple alleles. As opposed to just having a dominant and recessive version of an allele, there may be more than two versions of a gene that contribute to the overall phenotype.

In humans, blood type is a strong example of a trait that has multiple alleles. Another common example of multiple alleles is fur color inheritance in a certain species of rabbits. This is highlighted below.

As shown in this image, there is a dominant C allele and three different recessive c alleles. The combination of inheritance of these various alleles results in four different phenotypes.

Sex-Linked Traits

Sex-linked traits are traits that exist on a sex chromosome, X or Y. Most frequently, these traits lie on the X chromosome. Because of this, males are more likely to inherit these disorders because they only have one X chromosome, and, therefore, cannot be heterozygous.

Common examples of sex-linked traits are colorblindness and hemophilia. Both of these disorders are carried on the X chromosome. An example of how a Punnett square for a sex linked trait would be set up is shown below.

This Punnett square shows a mom (XBXb) who is heterozygous for color blindness. Because color blindness is recessive, she has normal color vision, but is a carrier of the recessive allele. There are also two boys. One has the dominant allele, XBY, and will have normal color vision. The other has the recessive allele, XbY, and will be colorblind.

The dad (XBY) has the dominant allele for this trait and, therefore, has normal color vision as well. Yet, when these two parents are crossed, there is a possibility that some of their offspring will be colorblind.

In this Punnett square there are two girls on the left-hand side. One is homozygous dominant (XBXB) and one is heterozygous (XBXb). Both will have normal color vision.

Incomplete Dominance

Incomplete dominance refers to traits where neither allele is dominant over the other. A good example of this is with flower colors. There are some species of flowers that have both red and white coloration, but neither is dominant. Heterozygous individuals have a combination of both colors, creating pink flowers.

Even though there are capital and lowercase letters shown above, neither of these alleles is dominant, hence why there isn’t one color over the other in the heterozygous offspring.

Co-dominance

Co-dominance refers to traits in which both alleles are equally dominant (think: co-captains). A good example of this is spots on certain breeds of cow. Some cows of this species are red, some are white, and some have red and white spots. Both alleles show up equally in the heterozygous offspring.

As you can see, all of the heterozygous offspring have a mix of both parental phenotypes, showing that neither is more dominant than the other.

Non-Nuclear Inheritance

Some traits result from non-nuclear inheritances, which are inheritances from organelles.

chloroplasts and mitochondria are randomly assorted, so the traits determined by chloroplast and mitochondrion do not follow Mendelian rules

mitochondria are inherited from the maternal side (your mitochondrion actually comes from your mother, maternal grandmother, etc.) so this doesn't follow Mendelian rules either

chloroplasts are inherited from the maternal side (ovule) in plants just like animals, so chloroplast-determined traits are maternally inherited.

5.5 Environmental Effects on Phenotype

Natural Selection

Environmental conditions may play a role in which phenotypes are more prevalent for a species in an ecological community. This connects back to the concept of natural selection, where heritable variations lead to differential reproductive success. In other words, some individuals inherited traits or adaptations that raise their fitness—allowing them to survive and reproduce, thereby passing on their genes to offspring.

Remember that organisms have a genotype, an individual's genetic makeup, which is the specific combination of alleles (alternative versions/variants of a gene) for a particular trait. Offspring inherit their genotype as a combination of the alleles received from each parent. An individual's genotype is different from their phenotype, which is the physical expression of their genes, including the traits that they actually display.

Environmental Changes

A classic example of the selective pressure of environmental changes on phenotype is the coloration of certain mice species. In an environment that has been covered in permafrost for the past few thousand years, the majority of mice are light in color. This allows these mice to easily blend into the environment that they live in and, therefore, protects them from predators. They have a higher chance to survive and reproduce, making the light fur phenotype dominant over mice with black or dark brown fur.

However, due to global warming trends in the changing climate, the layer of permafrost begins to melt, and the dark volcanic soil underneath is exposed. Based on the change in the habitat of the mice, mice with light fur would become more susceptible to predation, as they are easier to spot! Now, the darker mice have a phenotypic advantage to blend into the environment, meaning they are better adapted to survive and reproduce more frequently. Thus, the environmental change causes a shift in the allele frequency to favor the trait of darker-colored fur.

Phenotypic Plasticity

Environmental factors can also influence the physical expression of genes, which results in the phenomenon of phenotypic plasticity. Phenotypic plasticity occurs when individuals with the same genotype exhibit different phenotypes in different environments. Therefore, an organism with phenotypic plasticity can change its physical traits or characteristics in response to changes in the environment. These changes include, but are not limited to, the organism's appearance, behavior, and physiology.

Why is phenotypic plasticity notable? Phenotypic plasticity allows organisms to exploit new or different ecological niches. More than that, it helps individuals adapt to variable environmental conditions.

Phenotypic plasticity can be influenced by both genetic and environmental factors. Some organisms are more genetically predisposed to exhibit plasticity than others, while environmental conditions can also play a significant role in determining the extent of plasticity that an organism exhibits.

Environmental Changes

The arctic fox uses phenotypic plasticity to adapt to living in the harsh and cold environments of the arctic tundra. During the winter months, the arctic fox has a thick, white coat that helps it to blend in with the snow and ice. This environmental camouflage helps arctic foxes avoid predators and hunt for prey, as they are less visible to hares and other small mammals that the arctic fox consumes. However, in the summer, the arctic fox's coat turns a brown or gray color, which helps it to blend in with the rocks and vegetation of the tundra.

This change in coat color is an example of phenotypic plasticity, as it allows the arctic fox to adapt to the changing seasons and to better camouflage itself in different environments. The change in coat color is triggered by changes in daylight and regulated by the hormone melatonin.

5.6 Chromosomal Inheritance

As described before, chromosomes are inherited from both parents following the rules of genetics. There is an equal chance that either version of a gene may be inherited in offspring due to the law of Independent Assortment. Random fertilization allows for even more variation in that it is simply by chance that a certain egg and a certain sperm combine to form a zygote.

Crossing over in the first stages of meiosis leads to a number of different chromosomal combinations that increase the amount of variety in a population.

All of these components lead to the immense amount of diversity that we see on our planet. There will never be another you, because there is no chance that the genetic combination that created you will be created again. Remember, the only reason we don't do mitosis when reproducing is because variation in the population keeps the population more stronger when combatting diseases. It is an advantage to survival, so that's why we need to go through the pain of dividng twice with the process of meiosis.

Punnett Squares

Punnett squares allow us to determine the specific probability that two parents will create offspring with certain phenotypes. These are frequently used to determine probabilities, but only work for traits that have a simple inheritance pattern.

The ability of a parent to pass on genes to their offspring is fundamental to life. It creates the process of natural selection, as some individuals are able to survive and pass on their genes more frequently than others.

Our DNA is coiled into chromosomal shapes because it allows more easier distribution. It also helps us with understanding the patterns of inheritance. Because we rely on chromosomes for inheritance, genetic disorders can come from mutated alleles or nondisjunction as discussed earlier.