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2.7: Genetics, Cellular Biology, and Variation (Part 1)

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    By the middle of the 20th century, microscopes were good enough to actually see the "characters" that Mendel discovered, and by the end of the turn of this century, an outline of the human genome was completed, and now new alleles are discovered daily, and some even intentionally created.

    We could easily do a whole class on this section, but you should focus on trying to understand the mechanisms of human variation, which we'll be dealing with for the rest of the class. What is the cause of human variation? How does it happen at the cellular level? Darwin knew that variation was essential for the functioning of natural selection, but he had no clue how it worked. Mendel's research in heredity suggested principles of simple variation, but failed to explain 90% of what you see in life. In order to understand where most of variation comes from you need a microscope, and understand basic cellular biology and genetics.

    I find it helpful to get an overview of the scale of human variation. How big is the thing we're talking about? Go back and watch the 10 minute Powers of Ten movie, and focus on the cellular section.

    Even the pictures can get confusing until you get a sense of how everything fits together. The same chromosomes can look different at different stages. During protein synthesis it's hanging loose, and during cell division it's all wound up like a dreadlock. Here's another graphic that goes from small to big and mentions a few stages:


    Figure \(\PageIndex{2}\) - Chromosomes are made of DNA. Chromatin Structures By Richard Wheeler via Wikipedia (CC-BY-SA-3.0)

    Cell size can vary drastically, but ova are big and sperm are small

    A pair of sister chromatids and centromere and a pair of homologous chromosomes are about the same size; you get two chromatids because of DNA replication in preparation for cell division; you get two homologous chromosomes because of fertilization, one from Dad, and one from Mom.

    All cells arise from pre-existing cells.


    You are made up of about a trillion cells. Each cell has organelles inside them. A very important organelle is the nucleus. Inside the nucleus are your chromosomes. Your chromosomes direct protein synthesis, which determine how the cells interact with each other, and how you as an individual function.


    The ribosomes manufacture proteins. The mitochondria convert and store energy so the cell can use it. There are many others, but these are the most important to understanding human variation.


    The nucleus contains the DNA, and is metaphorically like the brain of the cell.


    There are 23 pairs of chromosomes inside the nucleus of most human cells for most of the time. You have a pair, because as Mendel discovered, you get one from your dad and one from your mom. There are two kinds of chromosomes, autosomes and sex chromosomes. The 22 pairs of autosomes are named for auto which means "self"; they're the chromosomes that stay with yourself. They are numbered from biggest to smallest. The last pair, the sex chromosomes, are named because they tend to determine the sex of an individual. The number of chromosomes doesn't make much difference. For an analogy, think of hard drive that can be formatted into different sectors.

    A genome is an entire set of genes.

    Different species have different number of chromosomes. Humans have 23 pairs, other apes have 24 pairs, hermit crabs have 127 pairs. Compare humans to a table of different species according to their number of chromosomes


    Mitochondria are the power plants of the cell, and they have their own separate DNA. The history of how mitochondria came to be is fascinating. We think they used to be independent living creatures swimming around, until 2 billion years ago, an ancestor of eukaryotic cells swallowed one, but instead of digesting it, that mitochondrion survived and began a symbiotic relationship with the host cell, reproducing inside the host's cytoplasm and being passed on to the next generation as the cell divided.


    Mitochondrial DNA (mtDNA) can be used for genealogy and for dating the migrations of pre-historic populations.

    Mitochondria are like cells within cells. Because, our cell's DNA is in the nucleus, and the mitochondrion in the cytoplasm, the mitochondrial DNA (mtDNA) was separated from the nuclear DNA of the host cell during reproduction. When sexual reproduction began, the eggs were bigger and they were almost always the source of mitochondria for the zygote. This means that you get your mtDNA from your mom, and it is inherited through matrilineal descent. Your mtDNA come:

    from your great-great-great-great-great-great grandmother,

    to your great-great-great-great-great grandmother,

    to your great-great-great-great grandmother,

    to your great-great-great grandmøther,

    to your great-great grandmøther,

    to your great grandmøther,

    to your grandmøther,

    to your møther,

    to yøu.

    And because of matrilineal descent, if your great-great-great grandmother had a mutation (o→ø) in the mtDNA of an egg, that mutation would be passed down to all of the descendants of that egg, and you would share the mutation with your mom, and your siblings, all those aunts and uncles, and fourth cousins on your great-great-great grandmother's side. The mutations in mtDNA accumulate and become markers to show ancestry, as well as demonstrate the evolutionary forces of migration and genetic drift. Because mitochondria are so simple, they have almost no functional variation – they either work or they don't – and without variation, natural selection doesn't happen. When you control for natural selection, the rate of neutral mutations of mtDNA becomes like a constant (one or two mutations every half-a-dozen millennia), and you can count how many different mutations two individuals have, and approximate how many generations ago they had a common ancestor. And by comparing large samples of indigenous populations, you can approximate where the mutation took place. We can correlate the genetic "when" and "where" with archaeological and historical data to test fascinating hypotheses of how humans moved across the globe.


    National Geographic's Genographic Project

    Skim Wikipedia: Mitochondria

    Mitochondrial diseases

    Cell Division

    Explore Sex cells have one set of chromosomes; body cells have two.


    Mitosis is the production of body cells for growth and healing. In mitosis, cells copy their chromosome and copy themselves, so that each daughter cell has the same number of chromosomes as the parent cell. Variations in the body cells can continue to be copied through mitosis (e.g. cancer), but the variations will not be passed down to the next generation.

    Watch an 8 second Mitosis Movie (Try manually moving the cursor to see it slowly)


    Meiosis is the production of gametes for sexual reproduction. In meiosis, cells copy themselves twice, but only copy their chromosomes once, so each of the viable daughter cells ends up with half the number of chromosomes as the parent cell. Individuals get the full number of chromosomes when two gametes combine during fertilization. Variations in gametes will be passed on the next generation. This is why when you get an x-ray, they put a lead blanket over your gametes – to block the radiation, and decrease birth defects.

    Watch a quick Meiosis Movie


    Oogenesis makes ova or eggs.


    Spermatogenesis makes sperm.


    Meiosis is important because it increases variation by recombining your parents' genetic information.

    Your genetic information comes in a small number of little packets, called chromosomes, and they were passed down from grandparent to parent to child. They come in pairs. One from one parent, one from the other. Meiosis splits the pairs, and shuffles them randomly so for example you might get one of your 3rd chromosomes from your paternal grandmother and one of your 4th chromosomes from your maternal grandfather.

    My genome approximates my ancestry. If you know what to look for you can see which chromosomes came from my Mom and which from my Dad. My maternal grandparents were mostly descended from Britain and Ireland, and show up as light and dark blue on this chart. My paternal grandparents were mostly descended from Ashkenazim and show up as dark green. So for the first chromosome pair, the top one came from Dad and the bottom one from Mom. For chromosome pair 22, the top one is from Mom and the bottom one from Dad. For the sex chromosomes I got the Y from my Dad, and the single X from my Mom.

    If I had my grandparents DNA, I could figure out whether the X chromosome that I got from my Mom, came from my maternal grandmother or my maternal grandfather. I definitely know that my Y chromosome came from my paternal grandfather. Each of my 46 chromosomes came from some great, great, great, ... grand-parent up in my family tree.

    Crossing Over

    Notice that that most of the chromosomes above aren't solid colors. The interspersed segments come from crossing over. During meiosis the homologous chromosomes are brought very close to each other. Because they are the same chromosome and have the same genes, pieces of one chromosome can "cross-over" to the one next to it.

    Recombination includes the shuffling of chromosomes that you're getting from each parent, and a specific kind of recombination, called crossing over, where the chromosomes themselves can change, and genes can cross over from one grandparent's chromosome to another's. The discrete packages of chromosomes don't stay the same every generation, they open up and traits move from one to another.


    While meiosis sorts and delivers the packets of genetic information we call chromosomes, one part of the process where variation can occur is that meiosis can deliver an extra packet, and we call this non-disjunction.

    During meiosis the homologous chromosomes are brought together, and then pulled apart, but sometimes they aren't pulled apart hard enough and they stick to each other, and both chromosomes are pulled into one gamete, and the other gamete gets none. This is called non-disjunction; the junction between homologous chromosomes that is usually broken during meiosis is not. Having the wrong number of chromosomes is usually lethal, and the fertilized egg just doesn't reproduce, and you just don't get pregnant that month. Many people survive and do fine with more or fewer than 46 chromosomes. Chromosomes are numbered by size, so non-disjunction with higher numbered chromosomes tends to be less lethal. Down syndrome is also called Trisomy 21, having three of the somatic chromosome number 21.


    • Read the Down Syndrome Association of San Diego, Down Syndrome Facts
    • New treatments for people with Down syndrome

    This page titled 2.7: Genetics, Cellular Biology, and Variation (Part 1) is shared under a CC BY-NC-ND 4.0 license and was authored, remixed, and/or curated by Arnie Daniel Schoenberg via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.