In human beings, by the end of meiosis, each egg or sperm has _____ chromosomes.

Before a cell divides to make two cells, it copies all of its chromosomes. These copies, called sister chromatids, are identical. Until the cell divides, the identical copies stay connected with each other by their middles (centromeres.) When the cell divides, the copies are pulled apart, and each new cell gets one identical copy of each chromosome.

This type of cell division is called mitosis, and it produces cells with a total of 46 chromosomes. Beginning soon after fertilization (see below), all of the cells in your body were made this way. Thus, every cell in your body has an identical set of chromosomes.

It is important to understand meiosis in a way that will show us how we get the variation among individuals and yet why we look more like our relatives than strangers.

Every cell in your body has chromosomes. In humans we have 46. Every chromosome in your cells has a matching chromosome. So what you really have is 23 homologous pairs of chromosomes. They're called homologous because they are very similar to each other-they are the same type of chromosome, they have the codes for the same type of information, its just different information (8.13).

Your body is the result of the fusion of two gametes. One from each parent. Each of those gametes had 23 chromosomes, so that after the union of those gametes, when you were just a single cell, you had 46 chromosomes. Or, 23 pairs of chromosomes. All the rest of the cells in your body descended from that first cell (through mitotic divisions) so they all also have 46 chromosomes.

We call the cells will two sets of genetic material (in humans, the ones with 46 chromosomes) diploid cells (2n). The cells with a single set, (in the human example the gametes, sperms and eggs) are called haploid cells (n). (see 8.14)

All the cell division that takes place in your body for everyday growth and repair is mitotic cell division; each cell has the same genitic material as the parent cell. But when gametes are produced they are produced by meiotic cell division (8.15). The cells produced by meiotic cell division have half as many chromosomes (they are haploid cells).

Summary so far

All of our cells really have two sets of chromosomes, 23 homologous pairs. They resulted from the fusion of two haploid cells (called gametes) and a lot of subsequent mitosis. We produce gametes by meiotic cell division. We reduce the number of chromosomes to one half. And not just any half. Each of the four cells produced by meiosis has all of the 23 chromosomes, half a set of homologous pairs. You should read about the stages of meiosis on pages 140 - 141 in the text.

Next: How do we get variation among individuals in a population?

Ultimately, all the genetic variation among individuals is attributable to mutations. We have covered a lot of material about mutations. You should know about what causes them, and what types of mutations there are and a lot of stuff. Keep in mind that when we considered mutations earlier in the semester, we were thinking in terms of things happening within one individual. We talked about a single cell miscopying it�s DNA during the S phase of its life and then dividing by mitosis and passing off that bad copy to one of the two daughter cells. And we talked about the DNA of a cell being altered after being exposed to a mutagen then the altered DNA would be copied and handed down to both daughter cells after mitosis.

The same factors that can cause mutations in somatic cells can cause mutations in germ cells. Or, to put that another way, mutations can occur in the DNA of a cell that is about to divide (by meiosis) to form gametes. If my skin cell DNA mutates to form skin cancer I�m in trouble. But that�s a problem within my own body. If a mutation occurs in a cell that�s dividing into gametes, that mutation can be passed on to the next generation. A mutated DNA sequence in a gamete will become a part of every cell of the individual that arose from that gamete. Furthermore, when that individual goes on to produce gametes, the gametes can also have that altered sequence of DNA. Its not as awful as it might sound. In fact mutations are the basis of many of the differences we see among individuals. Green eyes and blue eyes result from slightly different versions of genes. Red hair is the result of a mutation in a hair color gene. There are so many examples that its pointless to list them. ALL THE GENETIC VARIATION IN A POPULATION RESULTS FROM MUTATIONS.

But what about sexual reproduction?

The advantage of sexual reproduction (besides the fun parts) is that new combinations of genes are handed down to offspring. The name of the game (the life game) is to pass your genes on to subsequent generations. As many as possible. You "win" if your genes outnumber the other guy's in the future. Then why not hand down an exact copy of your own genes? That�s what asexually reproducing organisms do. Its fast; you don�t need a mate. But the world is constantly changing. What may be a perfectly good combination of genes for you could become obsolete in the future, or perhaps just not as good as they are now. By mixing your genes up a bit, you may combine them with some others that have advantages. This is a big part of the biology of mate choice. Also by mixing them up there may be the potential for the expression of genes that you possess but that aren't expressed in you. (This is often the case with dominant and recessive traits which we will learn about soon)

I know its difficult to think of life "trying to pass on genes to the future" and I know that this idea of variability being a good thing is also hard. One thing that might help you to understand is to think of the uncertainty in life. There are some many things beyond your control or your ability to predict. If you were trying to predict what type of genes would be really useful in the future, you might think that the ones you have now have served you very well and that that�s your best bet. But as unpredictable as the future is, there is one thing that is absolutely true and absolutely predictable - the world will change. So as good as your current set of genes may be for you in this generation, there is a real advantage to building in some sort of mechanism to rearrange them every once in a while, as a hedge against that inevitably changing world. That�s where sexual reproduction comes in.

There are three sources of genetic variation that arise from sexual reproduction. Or, more accurately, three ways in which sexual reproduction rearranges the variation that's already there (from mutation). One is independent assortment of chromosomes, another is called crossing over, and the third way variation is achieved is through random fertilization.

Independent assortment

Remember that all the diploid cells have pairs of chromosomes, one from each parent. During metaphase 1 the homologous pairs line up along the metaphase plate with one chromosome of each pair on either side of the line. All the chromosomes of maternal origin do not go on one side with all the paternal ones on the other. Maternal and paternal chromosomes are randomly distributed on either side of the metaphase plate. So for each pair there's a 50/50 chance as to which side the homologs will go on (8.17). We have 23 pairs each with 50/50 probabilities. That works out to 223 possible combinations of gametes from one human individual. That's over 8,000,000 (8 million). That's a lot. But that's not all.

Crossing over

Independent assortment should yield gametes with many possible combinations of maternal and paternal chromosomes, but because of a process known as crossing over the chromosomes that actually end up in the gametes are neither exclusively maternal or paternal. When the homologous chromosome pairs come together during prophase I of meiosis the chromatids are linked in spots called chiasmata. Crossing over occurs when parts of non-sister chromatids (the ones from opposite chromosomes of the homologous pair) switch places (8.19). This further increases the genetic variation that results from sexual reproduction.

Random fertilization

We've seen that, even without considering the variation introduced by crossing over, a given sperm cell is one of about 8,000,000 possible combinations. Consider the possible combinations when that sperm cell is united with an egg cell that is also 1 of 8,000,000 combinations. The resulting diploid zygote is one of over 64,000,000,000,000 possible combinations of the genes of the two parents.

What follows are my notes on meiosis. The pictures in the book will help, too.

There are 2 parts, meiosis I and meiosis II, so we have prophase I, metaphase I, anaphase I..all the way to ......telophase II. First, meiosis I, in which the chromosome number is halved.

Prophase I - Chromatin forms into denser chromosomes. Homologous pairs get together and form tetrads. (There are four sister chromatids in a pair of homologous chromosomes, hence the name tetrad). It�s here where crossing over takes place.

Metaphase I - Tetrads align on the metaphase plate.

Anaphase I- Chromosomes migrate toward opposite poles. Note that chromosomes are moving in anaphase I of meiosis. In anaphase of mitosis, just one chromatid moved. The result here in meiosis is that the chromosome number is halved.

Telophase I - The chromosomes are at opposite poles of the dividing cell. Cytokinesis is also taking place. The chromosomes probably won�t unravel at this stage since they�re about to divide again in meiosis II.

Meiosis II is really no different than a mitotic division of the two daughter cells produced by meiosis I. There�s a prophase II, metaphase II, anaphase II and telophase II. Just remember that it�s in meiosis I where the chromosome number is reduced from 2n to n.

Gregor Mendel was an Augustinian monk who lived from 1822-1884 in Austria. He became interested in why there is variation in the appearance of living things and why related individuals look alike in some ways and different in others. One mechanism of inheritance that some people considered possible at that time was called blending inheritance. The offspring of two different parents would be a blend of both. This was unlikely because, although it explains why individual look like their parents, it does not explain differences among individuals. (After a few generations of blending every one would look the same.)

Mendel developed the idea of particulate inheritance. In this model the parents pass discrete, inheritable units to their offspring. These units are what we call genes.

Mendel worked with pea plants and as a monk he had a lot of time. He may not have needed to be a monk if he knew about fruit flies. Terminology: Character is the heritable feature of interest. Trait is one of the variants of the character. Mendel studied peas. They can have purple or white flowers. So flower color is the character, purple is a trait. White is also a trait.

Mendel's experiments

Mendel crossed pea plants with different traits and looked at the offspring. He chose to examine only characters that had "either-or" type variation (9.2d). He started with "true breeding plants", for example he used purple flowered plants that, when self pollinated, gave rise to only purple flowered plants. He learned most of the important stuff by looking not at the offspring of the first generation but at their offspring.

Crossing purple flowered plants with white flowered plants was done by taking the pollen from the purple plants and putting it on the flowers of the white plants (after first removing the pollen making structures of the white flowers (9.2c)). The seeds from this cross pollination were planted and the types of offspring were noted. They were all purple.

The white did not disappear, however. When this first generation (F1) was mated some white flowered plants appeared in their offspring (9.3a). Imagine trying to figure this out without the knowledge that we have today.

Mendel reasoned that each plant got something from each parent during fertilization. So during the first cross each offspring plant got some white and some purple but the purple was what he called the "dominant" trait. We call the different versions of a gene (in this case the flower color gene) alleles. In his first crosses he got plants with white flower alleles and purple flower alleles. We know that each of those alleles was on one half of a homologous pair of chromosomes (Mendel didn't).

When the purple flowers of the first generation were crossed with each other they yielded one white flowering plant for every three purple flowering plants (9.3a&b). From this evidence he developed the following four ideas:

1. Alternative versions of genes account for the variations in inherited characters. We call those alternative versions alleles. We now know that DNA at the same locus on each of a homologous pair of chromosomes can have different information.

2. For each character, an organism inherits two genes, one from each parent. Mendel didn't even know what you know about meiosis. You know that diploid organisms get one of each chromosome from the parents and that's how we get two alleles for each character.

3. If the two alleles differ, then one, the dominant allele, is expressed in the organisms appearance. The recessive allele does not show up.

4. The two alleles for each character segregate during gamete production. So if an individual has a dominant allele and a recessive allele, the gametes may get either one; they will separate. The gametes could have either the dominant or the recessive allele. This is called Mendel's law of segregation.

Some more terminology: By convention, we use an upper case letter to represent the dominant allele and a lower case letter to represent the recessive allele. An individual with two of the same alleles is called homozygous for that character. If an individual possesses two different alleles we say it is heterozygous for that character. Mendel had purple flowering plants that were "true breeders", that is, when self pollinated they always produced purple flowered plants. The plants were homozygous for the purple allele. The purple flowering plants in the first generation were heterozygous. They had white recessive alleles. The phenotype of both purple flowered plants was the same; they're genotypes differed. Phenotype can be determined by observation, it is the appearance of an individual. The genotype is the underlying genetic makeup of an individual. (9.3b)

More About Dominance

Not all alleles are completely dominant or recessive. An example from the text is the color of snapdragons. When homozygous red flowers (RR) are crossed with homozygous white flowers (rr) the F1 generation is all pink. The colors appear to have "blended", but the genetic material, the genes, have not blended. This is an example of incomplete dominance. Evidence that the genes haven't blended can be found in the f2 generation (9.10a).

Dominant alleles don't subdue the recessive alleles. Consider Tay-Sachs disease. People with the disease can't metabolize a lipid that accumulates in the brain. These people are homozygous recessives (tt). Heterozygotes and homozygous dominant individuals (Tt, TT) appear normal. So we say T is dominant. What's really going on is that the T allele has the genetic information to produce the enzyme necessary to metabolize the deadly lipids. TT individuals make a lot of the enzyme and Tt individuals make some too. tt individuals don't make any. TT and Tt individuals make enough, so we call them normal, although if you view this phenomenon at the biochemical level you'll see it as incomplete dominance.

(Something to note:Recessive alleles are not always the least common and they are not always the bad ones.)

Multiple alleles

There aren't always just two alleles of a gene in the population. Sometimes there's only one, other times there may be many. A diploid individual can only have two alleles at once of course, but there may be more out there in the population. The ABO blood groups are a good example. There are three alleles in this system: IA, IB & i. IA and IB are dominant so an IAi individual has type A blood and so does an IAIA individual. ii is type O and IAIB is type AB. The alleles code for coating on the blood cells. IA produces one type of coat, IB another. ii individuals get no coats. Type AB individuals get both. (See section 9.11)

The Real World

Many characters in the real world don't work as neatly as Mendel's peas. Like people's height or skin color. If you looked at a "population" made up of individuals you would find individuals that exhibited a broad range of traits. For instance, you don�t encounter only short and tall individuals in human populations. There are short people, tall people and people of all heights in between. Thus, variation in height is said to be continuous, and a character, such as height (or milk yield of dairy cows, sprint speed of scorpions, etc.), is referred to as a quantitative character. Two things help explain these types of characters which exhibit continuous variation.

1) Polygenic inheritance is the situation in which a character, like skin color, is controlled by many genes. In Mendelian inheritance we typically find ratios of phenotypes. With quantitative characters we find continuous distributions of phenotypes. There was once a dispute between the adherents of these two approaches, but as we see from figure 9.13 in the text, the continuous distribution of characters can be explained by the effects of alleles at multiple loci. This is just one simple example. Many of the phenotypic aspects that we refer to as "characters" result from the effects of many genes at many loci.

2) Environmental effects can also play a strong role in how a trait is expressed. One example is that genetically similar Hydrangeas produce a range of flower colors depending on soil pH. Or consider height. A genetically "tall" plant might grow up short when grown in poor soil. Or a person with tall parents, from a long line of tall ancestors, might grow up pretty short if deprived of milk or protein as a child. Or how about the elusive character "intelligence"? It's a difficult character to define, and it is also very difficult to determine how much of a person's "intelligence" (or lack thereof) has been inherited or results from environmental effects such as learning, nutrition, and cultural differences.

When we look at cells that are undergoing mitosis we can see the genetic material all bunched up into what we call chromosomes. We can photograph all those chromosomes and then cut them out of the photograph and rearrange them into homologous pairs. The resulting display is called a Karyotype (13.2).

Look at the picture of the human karyotype (page 231). There are 23 pairs (46 chromosomes). Notice how that last pair looks different? That�s because the karyotype shown is from a male. Males have an "X" and a "Y" chromosome. (They look like xs and ys in other photos).

When the mother�s chromosomes line up on the metaphase plate during metaphase I, there will be an X on either side. That�s because females have a pair of Xs for that homologous pair. Men have an X and a Y and thus their gametes can contain either an X or a Y. This is why it is said that the father determines the sex of the child. He doesn�t really determine the sex but rather the sex is determined by the contribution of either x or y in the sperm. These X and Y chromosomes are called "sex chromosomes".

You know that the traits expressed in individuals are a result of genes they have on their chromosomes. You also know that individuals with two of the same alleles for a character (homozygotes) sometimes appear different than individuals with 2 different alleles for a character. Suppose a character results from genes that are found on one of the sex chromosomes, say, the X chromosome for example. Females could be heterozygous or homozygous for this character. Males on the other hand will only have one chromosome that carries the gene for that character. A female could have two different alleles on her X chromosomes, one dominant and one recessive and she might still have the appearance of the dominant trait. If she passes on her dominant trait-bearing X to a son, the son will also be of the dominant trait type. If she passes the recessive trait-bearing to the son, he will exhibit the recessive trait. He cannot be heterozygous for the character because he only has one X chromosome. Many hereditary conditions follow this pattern and are called sexed linked traits.

That's it for now. There will be a little more ��

How many chromosomes are present in a human egg or sperm at the end of meiosis 1?

How many chromosomes are in meiosis?

What is the end result of meiosis?

What is the end result of meiosis in males and females?