[LS3-2] Genetic Variation
This standard focuses on how and why genetic variations occur within a population. The big idea is that variation allows organisms to adapt to a changing environment. As such, DNA replication and processes during meiosis often result in changes in the DNA code. Mutations can be beneficial or deleterious, leading to increased fitness or decreased fitness, respectively.
Resources for this Standard:
For Teachers Only
Here’s the Actual Standard:
Make and defend a claim based on evidence that inheritable genetic variations may result from: (1) new genetic combinations through meiosis, (2) viable errors occurring during replication, and/or (3) mutations caused by environmental factors.
This standard has 3 specific parts that we will look at in detail:
Recombination during Meiosis
The process of genetic combination during meiosis happens in two different ways. First, the Law of Segregation says that diploid chromosomes are separated into individual haploid gametes. In essence, this separates homologous chromosomes contributed by your mother and father into separate reproductive cells. This allows new combinations of alleles to come together in the future generations, and the process can also hide recessive alleles through generations.
Second, the Law of Independent Assortment says that the separation of chromosomes does not happen along maternal and paternal lines. In other words, alleles for different traits are randomly assigned to each gamete, and they do not always resemble the pattern seen in parents. For example, if your mother had green eyes with black hair and your father had blue eyes with blonde hair, you could have any combination of those traits. (Genes can show linkage when they are found on the same chromosome, this reduces the chance they get inherited separately.)
Lastly, the process of crossing-over during meiosis mixes up your maternal and paternal chromosomes, ensuring that the set of alleles you inherit is very unlikely to have only your mother’s or only your father’s DNA. This also contributes to the Law of Independent Assortment.
Errors during Replication
The process of DNA replication is a very accurate process, though a small degree of errors is still made. There is complex cellular machinery composed of many different proteins and molecules that seek to find and correct mutations, though some mutations are not fixed or detected by these systems.
These errors can take many forms including deletions, insertions, and point mutations. However, these mutations must be present in gamete-producing cell lines in order to be passed to the next generation.
Some mutations are caused by chemicals in the environment called mutagens, such as chemicals found in tobacco and vaping products, that can mutate or damage DNA outside of normal replication events. Mutagens can work in many different ways to damage DNA. Although the cellular machinery is constantly trying to find and fix mutations, some mutations inevitably sneak through.
Mutations in a germ-line cell can be spread to the next generation, whereas somatic cell mutations are most commonly expressed as cancer. Most cancers start as minor mutations in cells which cause an increase in cellular division, leading to the growth of a tumor. Over time, these cancers can accumulate more mutations which can lead them to metastasize, or spread through the body.
A little clarification:
The standard contains this clarification statement:
Emphasis is on using data to support arguments for the way variation occurs.
Let’s look at this clarification a little closer to understand what is not included:
Supporting Arguments for Origins of Variation
The original arguments supporting the methods of variation came from Gregor Mendel and his famous “Pea Plant” experiments. During the experiments, Mendel showed empirically that offspring inherit different combinations of traits than their parents expressed. This lead to the Law of Segregation and the Law of Independent Assortment, which together explain why offspring exhibit variation in characteristics not seen in the parental generation.
Other simple arguments can include easily observable human traits, such as eye and hair color. In a class of 30, it should be possible to locate at least 1 (if not more) student that has a different eye color / hair color combination compared to either of their parents. This shows that traits can be distributed across maternal and paternal lines and explores the same basic mechanisms Mendel was looking at in pea plants.
What to Avoid
This NGSS standard also contains the following Assessment Boundary:
Assessment does not include the phases of meiosis or the biochemical mechanism of specific steps in the process.
Here’s a little more specificity on what that means:
Biochemical Mechanisms of Variation:
Meiosis is covered in more depth in other NGSS standards. The focus here should be on the results of meiosis, rather than the biochemical processes that make recombination and variation possible. For instance, students do not need to know about the specific proteins and processes involved in DNA repair. For the purposes of this standard, they simply need to know that the process is imperfect, which introduces variation (good and bad) into the population.