Many people who came to play volleyball brought their families. father and son It was interesting to look at all the people and try to figure out, by looks, which children were siblings or which adults were their parents. In some cases, the genetic similarities were easier to see, while in other cases the mixture of genes from the two parents made it more difficult to visually see relationships between family members.

Mendelian and Human Genetics

There used to be a TV show whose punch line was always, “Smile! You’re on Candid Camera.”

smiles

Can you tell which of these people are related? (Hint, look at noses and cheeks as well as smiles.)

Dr. Fankhauser in Mendel’s Greenhouse Gregor Mendel, an Austrian monk, is considered to be the “Father of Modern Genetics” for the discoveries he made by breeding pea plants. Mendel figured out that we get a copy of each gene from each parent. He also found that some alleles (alternate forms for a gene) are dominant while others are recessive, such that, if a person (or other organism) has a dominant allele from one parent and a recessive allele for the same gene from the other parent, the dominant trait will be expressed.

Here, Dr. Fankhauser is eating a picnic breakfast while sitting on the remains of Gregor Mendel’s greenhouse in Brno, Czech Republic. A statue of Mendel is to his left, between the bushes, back by the monastery wall.

An individual who has two identical alleles (two dominant OR two recessive alleles) is called a homozygote, while someone who has two different alleles (one of each) is called a heterozygote. Gametes (eggs and sperm) have only one copy of each gene, therefore only one allele for each gene, and when a sperm fertilizes an egg, the condition of having two is restored. An organism’s genotype is its actual genetic makeup for the gene(s) under consideration (for example, “heterozygous dominant” or “homozygous recessive,” etc.), while its phenotype is how those genes are expressed (sort-of, “What does it look like?” except that not all genes code for things that can be seen, visually).

Purple Pea Flower White Pea Flower To make it easier to keep track of genetic crosses, rather than having to explain something like, “I crossed a homozygous purple-flowered pea plant with a homozygous white-flowered pea plant, and all the offspring were heterozygous and had purple flowers,” geneticists came up with the idea of using letters to represent the various alleles. Dominant alleles are represented by capital letters and recessive alleles are represented by the same lower case letters. Thus, in our example, if purple flowers are dominant and white is the recessive allele of that same gene, we could use capital P (for purple) to represent “purple” and lower case p to represent “white.” We could then rephrase our description as, “I crossed a PP pea plant with a pp plant and got Pp offspring.”

Geneticists often use Punnett squares (named after Reginald Crandall Punnett) to keep track of genetic crosses. For example, suppose a heterozygous brown-eyed man (brown is dominant) marries a blue-eyed woman, and we want to figure out what eye color their children might have. If the man is heterozygous, we could express his genotype as Bb (using B for “brown”), and the woman’s genotype would have to be bb if she has blue eyes. When the man’s body is making sperm, half of his sperm will get his B allele and half of his sperm will get his b allele. When the woman’s body is making eggs, half of her eggs will get her b allele, and half will get the other b allele. In a Punnett square, the possible types of sperm are listed across the top and the possible types of eggs are listed down the left side. A Punnett square is as big in either direction as the number of possible gametes that parent can produce. In this case, since we’re dealing with two types of sperm from the father, our Punnett square will have two columns, and since we’re (sort-of) dealing with two types of eggs from the mother, it will have two rows (yes, technically, we could get by with one row). Thus, the initial set-up of our Punnett square would look like this:

    B   b 
 b         
 b         

The boxes in the center represent what the possible children’s genotypes would be if that sperm fertilized that egg. Thus, the genotypes of the sperm are copied down the appropriate column, like this:

    B   b 
 b B  b  
 b B  b  

Similarly, the genotypes of the eggs are copied across the appropriate row, like this:

    B   b 
 b   b  b
 b   b  b

When it’s all put together, and both the sperm and the eggs have been added, the final result looks like this:

    B   b 
 b Bbbb
 b Bbbb

We can then see that half of their children would be expected to be genotype Bb or “heterozygous dominant” or “heterozygous brown” eyes, while the other half would be bb or “homozygous recessive” or “homozygous blue.” In terms of phenotype, we would say that half of their children should have brown eyes and half should have blue. Understand, that’s not saying everyone will have exactly four children, but rather, it’s a prediction, an average. This couple might only have three children, and perhaps two of those will have brown eyes while the third has blue eyes. Maybe they’ll have two children, both of whom end up with blue eyes. Maybe they’ll have four, three of whom will have brown eyes and only one blue. (It’s like, if you would toss a coin 100 times, you might not end up with exactly 50 heads and 50 tails, but your results would probably be close to that.)

When you start to “dig deeper,” there are some interesting inheritance patterns that show up. As one example, humans and other mammals have a number of “regular” chromosomes called autosomes plus a set of sex chromosomes. Typically, females have two X chromosomes (genotype XX), while males have an X and a Y (genotype XY), and just like all the other chromosomes, the X and the Y have genes on them that code for various traits. However, the X chromosome is much larger than the Y, and contains a number of genes that have no counterpart on the Y chromosome. While the Y chromosome is much smaller than the X, it actually does have a few genes on it that are not represented on the X. Genes that are located on only the X or Y chromosome are called sex-linked genes. For example, in humans, red-green colorblindness and hemophilia are two traits that are carried on the X chromosome, while one variety of six-fingeredness and hairy earlobes are carried on the Y chromosome.

Sex is also a phenotype, and just like any other phenotype, is a matter of how the person’s alleles are expressed. It’s not merely a matter of whether the person has two X chromosomes or an X and a Y, but rather, what alleles the person has for the genes which are located on those chromosomes, and how those alleles are expressed. An example, described in more detail on the Biol. 105 Sex-Linked Genes Web page (link below), is the case of a genetic condition called androgen insensitivity syndrome (AIS). On everyone’s X chromosomes, there is a gene that codes for people’s cells to build receptor sites to receive androgen (testosterone), and there is no corresponding gene on the Y chromosome, so this is an X-linked or sex-linked gene. To briefly summarize, a person with AIS has an X and a Y chromosome, but on that one and only X chromosome, has an altered allele that does not allow the testosterone receptor sites to form/function properly. Thus, even though that person, due to the influence of other genes on the Y chromosome, has undescended testes which make lots of testosterone, the rest of the person’s body never receives the testosterone “message,” and by default, other than the undescended testes and a missing uterus, she develops totally, unambiguously female traits. Often, girls with AIS don’t even discover that they have it until, despite otherwise normal development at puberty, they fail to menstruate. The “opposite” is also possible: I’ve been told there was a case where a couple was not able to conceive a baby and thus were going through various testing to determine why. The results of the tests showed that the very masculine, bearded husband was, chromosomally, XX.

There are a number of other genetic and chromosomal mutations which can affect humans (see link, below). Among many others, these include such things as


Background Information

Links to Related Information on Our Web Server

The following Web pages contain information related to various aspects of genetics.

Bio Lecture Genetics
General information on Mendelian genetics and typical patterns of inheritance
Bio Lecture Genetics Practice Problems
A page of practice Punnett squares
Bio Lecture Linked and Sex-Linked Genes
Information on genes located on the same chromosome as each other and information on genes located on the X or Y chromosomes, including discussions of hemophilia and androgen insensitivity syndrome
Bio Lecture Human Genetics
Information on a variety of human genetic and chromosomal abnormalities
Bio Lab Genetics Activity
An activity using coin tosses to simulate genetic crosses

Your Assignment
Genetics “Practice”

There will be only one, combined assignment for this week’s topics (mitosis and meiosis, genetics, and DNA). Thus even though this will appear on each of those three pages to remind you, you only need to do it once. Genetics is one of those things that just needs lots of practice to “get it.” The grading criteria for this assignment are given below, and you should also refer to those as you work on the assignment. A total of 32 points is possible.

  1. Mitosis
    1. Read through the Biol. Lecture Web pages on mitosis and meiosis to become familiar with those processes, how they are the same, and how they differ.
    2. Find/collect a group of “similar” but distinguishable objects such as coins, pieces of string or yarn, socks, or whatever is handy. These will be used to represent the chromosomes in the nucleus of a cell. Also, obtain several, longer pieces of string or yarn to represent cell and nuclear membranes.
    3. Make a cell. Use a longish piece of string to make a circle to represent the cell membrane. Use a shorter piece to make a smaller circle inside to represent the nuclear envelope. This organism will have 6 chromosomes (3 from the father and 3 from the mother). For this you will need 3 pairs of something; for example a pair each of black, red, and blue socks or a pair each of pennies, nickles, and dimes (or whatever is handy that will suit the purpose). Put these 6 “chromosomes” into the nucleus of the cell.
    4. Just before mitosis happens, the chromosomes replicate, but the halves (called sister chromatids) stay attached. Simulate this by stacking 6 more identical objects (well, come as close as you can...) on top of the existing 6 “chromosomes”. For example, stack another penny on top of each of the two existing pennies, another nickle on top of each of the two existing nickles, etc.
    5. In prophase of mitosis, one thing that happens is that the nuclear envelope disintegrates. To demonstrate this, remove the string that’s the nuclear envelope and set it aside.
    6. In metaphase, all the chromosomes line up along the “equator” of the cell. Line up your 6 “chromosomes” (each with its partner still on top) in a row (single-file) across the middle (“equator”) of your cell.
    7. In anaphase, the halves of the chromosomes separate and travel to opposite poles of the cell. For each of your 6 “chromosomes,” now is the time to separate the partners. For each of the 6 stacks of 2, move one of the two items to the “north pole” of the cell and one to the “south pole” of the cell. When you’re done, each pole should have a collection of 6 objects/“chromosomes” identical to the 6 with which you began.
    8. In telophase, the nuclear envelopes re-form and the cell divides into two. First, find a piece of string with which to form a circle around each of the two groups of “chromosomes” to show the nuclear envelope re-forming. Then, near the “equator” of the cell, pinch/poke/move the string that represents the cell membrane in toward the center until the cell is divided into two. Optionally, you could replace that one string with two separate ones to remind yourself that you now have two separate cells.
    9. Congratulations! You have done mitosis.
  2. Meiosis
    1. OK, now try meiosis... Make another cell just like the previous one. Give it a cell membrane and nuclear envelope, again, as well as the same 6 chromosomes.
    2. As above, just before meiosis happens, the chromosomes replicate, as they do in mitosis, so add the matching halves back on top, again.
    3. In prophase I of mitosis, one thing that happens is that the nuclear envelope disintegrates. To demonstrate this, remove the string that’s the nuclear envelope and set it aside. Something else, very important, happens during prophase I: the chromosomes pair up. Move your “chromosomes” around so that the matching ones are next to each other. For example, put the two stacks of pennies (or the two stacks of black socks) next to each other, the two stacks of nickles next to each other, etc.
    4. In metaphase I, the chromosomes line up along the “equator” of the cell, again, but this time still in their pairs. Line up your 3 pairs of “chromosomes” (each with its partner still on top) in a row (double-file, side-by-side) across the middle (“equator”) of your cell.
    5. In anaphase I, the pairs of chromosomes separate and travel to opposite poles of the cell. For each of your 3 pairs of “chromosomes,” now is the time to separate the pairs. Keeping the partner halves still stacked together, move one whole stack from each of the 3 pairs to the “north pole” of the cell and one to the “south pole” of the cell. When you’re done, each pole should have 3 stacks of objects/“chromosomes,” one of each of the kinds with which you began.
    6. In telophase I, as before the nuclear envelopes re-form and the cell divides into two. Similar to what you did above, re-form the nuclear envelopes and divide the cell into two. When you have the cell membrane all the way “divided,” go ahead and substitute two pieces of string for the one, to represent the fact that you now have two, separate cells.
    7. This time, however, you’re not done yet. There is another cell division yet to go. Once again, in prophase II, the nuclear envelopes disintegrate, so remove those from both of the cells.
    8. In metaphase II, the chromosomes line up along the “equator” of the cell, in single-file, similar to what happened in mitosis. In each of your cells, once again, line the 3 “chromosomes” up, single-file, along the equator of that cell.
    9. In anaphase II, you finally get to separate your stacks of “sister chromatids.” From each of your stacks, move one partner to the “north pole” and one to the “south pole” of that cell. Between the two cells, you should now have a total of 4 groups of 3 items.
    10. In telophase II, once again, the nuclear envelopes re-form. You’ll now need 4 pieces of string so you can make a circle around each of the new nuclei. Also, in each cell, once again, pinch in the middle to form 2 cells out of each one, then (optionally) replace each of those cell membranes with 2 separate cell membranes (strings) for each of the new daughter cells. When you are done, you should end up with 4 daughter cells, each with 3 chromosomes.
    11. Congratulations! You have done meiosis and you now have 4 eggs or sperm.
  3. Fertilization
    1. Don’t get rid of your eggs/sperm just yet! To do this the “official” way, you could go through the whole process of meiosis with a different “parent” so that you end up with 4 sperm from one parent and 4 eggs from the other parent. However, to simplify things, from the 4 you have sitting there, now, pick one to be an “egg” and one to be a “sperm,” and think of them as having come from different parents.
    2. If necessary (if they're a distance apart), the sperm will have to “swim” over to where the egg is, until they are touching. That might be easier to do if you slide the nucleus and chromosomes onto a sheet of paper so you can move it as one unit. To make the next part easier, you might want to reposition the cell membranes of the egg and the sperm cells so the loose ends of the strings meet where the egg and sperm are touching.
    3. Now, the whole sperm nucleus (just the nucleus, not the whole cell) has to go inside the egg cell, leaving its cytoplasm and cell membrane behind. If you previously placed the nucleus on a sheet of paper, just slide the whole thing over, into the egg cell, and as close to the egg nucleus as you can get it. When that step is complete, both nuclei should be within the egg cell, and the egg cell membrane should be “closed” all around (the hole where the sperm nucleus entered closes up).
    4. Then, the sperm and egg nuclei unite, so put all of the chromosomes from both into one nucleus (and set aside the spare string). How do the number and types of chromosomes compare with what you started with before meiosis?
    5. Congratulations! You have just conceived a baby!
  4. Think about and summarize How are mitosis and meiosis similar, and how are they different? Where in a person’s body does mitosis happen, and where does meiosis happen? In what way(s) are meiosis and fertilization the “opposite” of each other?
  5. Genetics
    1. Genes are located on chromosomes. Thus, as the chromosomes move around in meiosis and segregate into the daughter cells, they carry with them all of the chromosomes located on them. For example, in the meiosis demonstration you just did, suppose the first set of “chromosomes” (the pennies?) contained the gene for eye color. If the individual was heterozygous for eye color, one chromosome would carry a B allele for “brown”, and the other chromosome would carry a b allele for “blue”. When the chromosomes replicate, they make an exact copy of themselves, so the coins/socks/yarn you stacked on top of each other would carry the same allele as each other. Suppose the nickles (the second pair of “chromosomes”) carry a gene for tongue-rolling, but if the individual is heterozygous there, too, one nickle would have an R allele (for “rolling”) while the other one would have an r allele (for “non-rolling”). Suppose the third pair of “chromosomes” (the dimes?) contain a gene for ability to taste a certain kind of test paper called PTC paper, and suppose, again, that the individual is heterozygous for this gene, too. Then one dime “chromosome” would carry a T allele (for “taster”) and the other would carry a t allele (for “non-taster”).
    2. If it helps you to visualize what’s going on, here, set up the meiosis demonstration, again, but this time, go ahead and label the “chromosomes” with the appropriate genes they contain. When you get to metaphase I and you’re lining up the pairs of chromosomes in the center of the cell, don’t be concerned whether all of the B, R, and T alleles are on the same “side” or not, because in “real life” it’s pretty much a 50:50 chance for each pair which one will line up on the “north” side and which will line up on the “south” side. This means that when the chromosomes do their first division in anaphase I, it’s a 50:50 chance for each pair which one will wind up at whichever pole of the cell. Thus, for example, if you end up with BrT at the “north pole,” that’s just one possible example of what might happen in real life.
    3. Understanding how genetic crosses work is best accomplished by working practice problems and Punnett squares. Spend time working with the Genetics Practice Problems Web page until you feel comfortable working these kinds of problems.
    4. When you submit your work for this assignment, the data-submission Web page will automatically generate several genetics problems which you will be asked to work out.
  6. DNA Do a Web search to find out more information on one kind of genetically-modified organism (GMO) and summarize what you found out. Terms for which to search might include “GMO”, “genetic engineering”, “genetically-modified”, “frankenfood”, “monoclonal antibody”, “rituxan”, “rituximab”, “epratuzumab”, “galiximab”, “campathin”, “alemtuzumab”, and/or “roundup-ready”, etc. Make sure you tell what organism was modified, what gene(s) were inserted, where those genes came from (what other organism), and the reason why somebody thought it would be a good idea to do that. What are some of the advantages or benefits that proponents claim will result from this, and what are some of the disadvantages or problems that opponents claim will come as a result of this. Based on what you’ve found out, would you say that, ethically/morally, this is a “good” thing or a ”bad” thing? Give scientific reasons to justify your point-of-view.
  7. At this point, if you are a registered student, you should submit your work.

Grading Criteria

1.   Mitosis and Meiosis:
2 — The student clearly demonstrated that (s)he knows the difference between mitosis and meiosis
1 — The differences between mitosis and meiosis were included and was at least partially correct
0 — Mitosis and meiosis were incorrectly distinguished from each other or the distinction between the two was not included
2 — The student, obviously, went beyond the minimum requirements of the assignment
1 — The student adequately completed the assignment
0 — The student completed considerably less of the assignment than what was required
2.   Genetics Problems (for each problem × 3 problems):
2 — The male gametes (sperm) were correctly specified (genotypes) and placed
1 — The genotypes and/or placement of the sperm were partially incorrect
0 — The genotypes and/or placement of the sperm were wrong or missing
2 — The female gametes (eggs) were correctly specified (genotypes) and placed
1 — The genotypes and/or placement of the eggs were partially incorrect
0 — The genotypes and/or placement of the eggs were wrong or missing
2 — The genotypes of the offspring were correct
1 — The genotypes of the offspring were partially incorrect
0 — The genotypes of the offspring were wrong or missing
3.   Genetically-Modified Organism:
2 — Thorough/complete information on the chosen GMO was included
1 — Adequate information on the chosen GMO was included and was at least partially correct
0 — Information on the chosen GMO was too sketchy or absent or was incorrect
2 — An ethical point-of-view was included and was backed up by thoroughly-researched facts
1 — The student’s point-of-view was included, but was backed up only by personal opinions/beliefs and/or partially incorrect or skimpy facts
0 — The student did not include his/her ethical point-of-view, or there was no justification given for how/why that opinion was reached
2 — The student, obviously, went beyond the minimum requirements of the assignment
1 — The student adequately completed the assignment
0 — The student completed considerably less of the assignment than what was required
4.   Overall:
2 — The grammar, English usage, punctuation, and spelling were very good
1 — The grammar, etc. were OK
0 — The grammar, etc. were poor
2 — It is evident that the student used much insight, thoughtfulness, and critical thinking when completing this assignment
1 — The student adequately thought about the assignment – there was, perhaps, a bit of “fuzzy thinking” in a couple places
0 — The assignment gives the appearance of being “slapped together” just to get it done, with little evidence of thoughtfulness
Total Possible:
32 — total points

Copyright © 2006 by J. Stein Carter. All rights reserved.
This page has been accessed Counter times since 15 Nov 2006.
(By the way, the man on the left is the father of the three people in the center. The woman on the right is the wife of the second man.)