Meiosis Tutorial 4: Sex Determination, Nondisjunction, and Human Chromosomal Variation

1.Introduction

Now that we understand how meiosis works, we can look at some meiosis-related issues:

  1. How, in mammals, chromosomes determine whether we’re male or female.
  2. How the process of meiosis can malfunction through a process called nondisjunction, which results in sperm or egg cells with extra or missing chromosomes.
  3. How these egg and sperm cells, if they go on to produce a zygote (a fertilized egg), can produce people with missing or extra chromosomes, resulting in conditions such as Down Syndrome, Turner Syndrome, and Klinefelter syndrome.

2. Sex Determination in humans and other mammals

[qwiz style=”width: 600px !important;” repeat_incorrect=”false”] [h]

Identifying female and male chromosomal patterns

[q] Let’s start by examining the image below. One set consists of the chromosomes of a human male, and the second consists of those of a human female. Can you tell the difference? Make a guess, then click “Show the answer” to see the next slide.

[c*] Show the answer

[f] Just by looking at these images, it’s pretty much impossible. But in the next image, I’ll present these chromosomes as a karyotype, an image that shows the chromosomes arranged by size. Click “Got it” when you’re ready to proceed.

[q labels = “top”]Here are the karyotypes: Now give it a try.

 

[l]male

[f*] Excellent. You must have noticed the “X” and “Y” chromosome in the bottom right of this karyotype. 

[fx] Sorry, that’s not correct. Take a look at the bottom right of this karyotype. Notice that the last pair of chromosomes is not matched, with the larger one labeled as “X” and the shorter one as “Y.” This is the pattern associated with people who are biologically male. 

[l]female

[f*] Good! You must have noticed the two “X” chromosomes in the bottom right of this karyotype. 

[fx] No, that’s not correct. Take a look at the bottom right of this karyotype. Notice that the last pair of chromosomes is matched, with two equally sized chromosomes, both of which are labeled as “X.” This is the pattern associated with people who are biologically female. 

[x]So, we’ve established that the chromosomal pattern of males and females is a bit different. Let’s dig deeper. Read below.

[/qwiz]

As we’ve seen earlier in this series of tutorials about meiosis, humans have 23 pairs of chromosomes, with one member of each pair contributed by the mother, and the other contributed by the father.

Here are two karyotypes. The one on the left is of a person who is chromosomally male. The one on the right is for a female.

Male Karyotype Female Karyotype

Chromosomes one through 22 are called autosomes. They have the same size, the same banding pattern, and the same genes (though the specific alleles and DNA base pair sequences may differ).

The 23rd pair are they sex chromosomes, and that pair is what determines sex.

  • Males have an X chromosome (the larger of the two) and a Y chromosome.
  • Females have two X chromosomes.

So how does sex get determined? Answer the question below

[qwiz style=”min-height: 200px !important;”]

[q multiple_choice=”true”] Sex is determined by

[c] The type of egg your mother passed on to you.

[f] No. Sex is determined by the sperm cell that fertilized your mother’s egg. Read more below.

[c*] The type of sperm that fertilized your mother’s egg.

[f] Yes! The sperm that fertilized your mother egg determined whether you’re male or female. Read more below.

[/qwiz]

Homologous pairs paired up (but about to split up) during metaphase 1 of meiosis 1

Here’s how it works. As you know, during meiosis homologous pairs pair up and then get split apart. The image below shows the pairing up of homologues that would happen in a cell with four chromosomes (two homologous pairs).

Adapted from an image from Penn State Biology

As far as meiosis goes, the sex chromosomes are also homologues, and like every other homologous pair, they pair up during prophase 1, line up along the cell equator during metaphase 1, and split apart during anaphase 1. Diagrams “1” and “2,” at right, show the the chromosomal situation in the gamete producing germ cells in the parents that are about to undergo meiosis, with “1” representing the father and “2” representing the mother. Note that the notation “44 + XY” means that there are 44 autosomes, plus and X chromosome and a Y chromosome (for a total of 46).

Diagrams “3,” “4,” and “5,” show the gametes that the male and female germ cells can produce.

In a germ cell within a man’s testes, the X and Y chromosomes will pair up, and then separate. The result will be that half of the haploid sperm cells will carry an X chromosome (as shown in “3”), and half will carry a Y chromosome (as shown in “4”).

In a female germ cell, where two X chromosomes are paired up, the only possible result is a haploid egg with an X chromosome, as is shown in “5.”

Diagrams “6” and “7” show the two types of zygotes that can result. If an X-chromosome carrying sperm (cell “3”) fertilizes the X-chromosome carrying egg, then a diploid zygote with two X chromosomes (XX) results. That zygote will develop into a female. If a Y-chromosome carrying sperm (cell “4”) fertilizes the egg, then the result will be a zygote with an X and a Y chromosome (XY), which will develop into a male.

3. Learning About Sex Determination in some non-mammalian animals

Note that this X-Y chromosome method of sex determination is not universal among our animal cousins. Here are a few links for you to explore to learn about some of this variation.

  • Some reptiles, like turtles and crocodiles, have a temperature dependent sex determination.
  • Birds (as well as other animals) do have a chromosomal system for determining sex, but not an X-Y system. It’s called the ZW system, and it’s a system where the egg, not the sperm, determines the sex of the offspring.
  • Bees (and some other insects) have a system called haplodiploidy, in which males develop from unfertilized eggs, while females are diploid.

For now, however, let’s consolidate our understanding of sex determination in people. Take the quiz below (which will only focus on mammals).

4. Human Sex Determination Quiz

[qwiz random = “true”] [h]

Sex Determination 

[i]

[q]  In the diagram below, the father’s germ cell is represented by
 

[textentry single_char=”true”]

[c*] 1

[f] Correct! The father’s germ cell is represented by “1.”

[c] Enter word

[f] No.

[c] *

[f] No. Here’s a hint. The father’s germ cell will have a total of 46 chromosomes, two of which will be “X” and “Y.”

[q]  In the diagram below, the mother’s germ cell is represented by
 

[textentry single_char=”true”]

[c*] 2

[f] Correct! The mother’s germ cell is represented by “2.”

[c] Enter word

[f] No.

[c] *

[f] No. Here’s a hint. The mother’s germ cell will have a total of 46 chromosomes, and two of these will be X chromosomes.

[q]  In the diagram below, the sperm cell that will produce a female is represented by
 

[textentry single_char=”true”]
[c*] 3

[f] Correct! A sperm cell that will create a female zygote is represented by “3.”

[c] Enter word

[f] No.

[c] *

[f] No. Here’s a hint. Find a cell that has a 22 chromosomes, plus a “Y” chromosome.

[q]  In the diagram below, the sperm cell that will produce a male is represented by
 

[textentry single_char=”true”]

[c*] 4

[f] Correct! A sperm cell that will create a male zygote is represented by “4.”

[c] Enter word

[f] No.

[c] *

[f] No. Here’s a hint. Find a cell that has a 22 chromosomes, plus an “X” chromosome.

[q]  In the diagram below, an unfertilized egg cell is represented by
 

[textentry single_char=”true”]
[c*] 5

[f] Correct! An unfertilized egg cell is represented by “5.”

[c] Enter word

[f] No.

[c] *

[f] No. Here’s a hint. Find a cell that has a 22 chromosomes, plus an “X” chromosome.

[q]  In the diagram below, a female zygote is represented by
 

[textentry single_char=”true”]

[c*] 6

[f] Correct! An female zygote is represented by “6.”

[c] Enter word

[f] No.

[c] *

[f] No. Here’s a hint. Find a zygote (bottom row only) that has a 44 chromosomes, plus two “X” chromosomes.

[q]  In the diagram below, a male zygote is represented by
 

[textentry single_char=”true”]

[c*] 7

[f] Correct! An male zygote is represented by “7.”

[c] Enter word

[f] No.

[c] *

[f] No. Here’s a hint. Find a zygote (bottom row only) that has a 44 chromosomes, plus an “X” chromosome and a “Y” chromosome.

[q] In the diagram below, sperm “1” must be carrying a(n) ____ chromosome.

[textentry single_char=”true”]

[c*] Y

[f] Correct. Sperm cell # 1 must be carrying a Y chromosome (because the egg it fertilized gave rise to a boy).

[c] Enter word

[c] *

[f] No. Sperm # 1 fertilized the egg at “3,” resulting in the boy at “5.” In order for that to happen, would the sperm have carried an “X” or a “Y” chromosome?

[q] In the diagram below, sperm “2” must be carrying a(n) ____ chromosome.

[textentry single_char=”true”]

[c*] X

[f] Correct. Sperm cell # 2 must be carrying an X chromosome (because the egg it fertilized gave rise to a girl).

[c] Enter word

[c] *

[f] No. Sperm # 1 fertilized the egg at “3,” resulting in the boy at “5.” In order for that to happen, would the sperm have carried an “X” or a “Y” chromosome?

[q] In the diagram below, the eggs at “3” or “4” must be carrying a(n) ____ chromosome.

[textentry single_char=”true”]

[c*] X

[f] Correct. Unfertilized eggs can only carry an X chromosome.

[c] Enter word

[c] *

[f] No. Unfertilized eggs from from female germ cells with two X chromosomes (“XX”). When those X chromosomes separate during anaphase 1, each resulting haploid egg will have an X chromosome (never a Y). Remember that the next time you see this question.

[q] In the diagram below, which number refers to a cell or organism that would have an X and a Y chromosome?

[textentry single_char=”true”]

[c*] 5

[f] Correct. The boy at “5” is a boy because he has an X and a Y chromosome in each of his cells.

[c] Enter word

[c] *

[f] No. Here’s a hint. The sperm at “1” had to carry a “Y” chromosome. and the egg at “3” had to carry an “X” chromosome. Who has to have both an X and a Y?

[x][restart]

[/qwiz]

5. Down Syndrome and Maternal Age: What’s the connection?

Down Syndrome is a condition that occurs in about 1 in every 700 babies born in the United States. The syndrome is characterized by (source for below is the National Down Syndrome Society, which has a great website)

  • varying degrees of cognitive delays
  • low muscle tone
  • small stature
  • an upward slant to the eyes
  • a single deep crease across the center of the palm.
  • an “increased risk for certain medical conditions such as congenital heart defects, respiratory and hearing problems, Alzheimer’s disease, childhood leukemia and thyroid conditions.”

Ninety five percent of Down Syndrome cases are caused by an error in meiosis called nondisjuction, which results in people with Down Syndrome having an extra copy of chromosome 21, as shown below.

[qwiz style=”width: 600px !important;”]

[q] Take a minute and study the chart below. Then, on your student learning guide (or any piece of paper), write down the correlation, which you should formulate as follows: “As ________, ______.”

[c*] Click to see the answer

[f] You probably wrote something like “As maternal age increases, the incidence of Down syndrome increases.”

Let’s see how nondisjunction works, so we can figure out why this is.

[/qwiz]

6. Nondisjunction

Meiosis is all about coming together, and then separating.

  • During prophase 1 (image “2,” above), homologous chromosomes come together.
  • During anaphase 2 (image “4,”, above), homologous pairs are pulled apart.
  • During anaphase II (image “10,” above), doubled chromosomes, consisting of two sister chromatids, are pulled apart.

Nondisjunction is the term used to describe what happens when homologous pairs or sister chromatids fail to separate in the typical way. Pulling the word “nondisjunction” apart will help you to understand and remember it.

  • Junction: when two things come together, as in “railroad junction.” That’s what happens during prophase 1. It also results from the duplication of chromosomes into sister chromatids that happens during interphase 1 (the interphase that precedes Meiosis I). In this case, the junction between the two sister chromatids is the centromere.
  • Disjunction: A disjunction is: “a sharp cleavage disunion, separation , as in the disjunction between theory and practice.” (Merriam Webster). In meiosis, a disjunction is what’s supposed to happen during anaphase 1 (where homologous pairs are pulled apart) and anaphase 2 (where sister chromatids are pulled apart).
  • Nondisjunction is when the separation of homologous pairs or sister chromatids that are supposed to happen don’t happen. As a result “one daughter cell has both and the other neither of the chromosomes.” (Merriam Webster)

Let’s walk through some ways in which nondisjunction can happen, and then look at the consequences.

Modified from an image created by the bioninja

To keep things simple, we’ll use a cell with two homologous pairs (shown at “a”). Meiosis I (“1”) should separate the homologous pairs, which happens correctly in cell “g.”

But look at what happens if we follow the branch to the left. In cell “b,” the bottom homologous pair fails to separate As a result, we have three doubled chromosomes on the left side of the cell, and one double chromosome on the right.

During meiosis II (“2”), the sister chromatids separate. Because of the nondisjunction that happened during meiosis I, the cell on the left (“c”) has three chromosomes that are being pulled apart (instead of two), and the cell on the right (“d”) has one chromosome that’s being pulled apart (instead of two).

During cytokinesis 2 (“3”) cell “c” divides to form haploid gametes. But each resulting gamete (the cells labeled “e”) will each have an extra chromosome. When cell “d” divides, the two gametes that result will be missing a chromosome.

At this point, we need to review the notation that biologists use to talk about meiosis, both when it proceeds, correctly, and when it goes awry, as in nondisjunction.

  • A diploid cell, with its two sets of chromosomes, can be referred to as “2n” (two sets).
  • A haploid cell, with one set of chromosomes, can be referred to as “n” (one set).

So, look at the two gametes labeled “e.” They should have a single set of chromosomes (n). But they have, because of nondisjunction, an extra chromosome. We notate that as “n+1.” The two gametes at “f” are missing a chromosome, and we’d annotate that as “n-1.”

That’s one way that nondisjunction can happen. But if you return to the diagram above, you can see that there’s another pathway as well. Look at the cell at “g.” It’s the successful product of meiosis 1 (“1”). However, during meiosis II (“2”), sister chromatids are supposed to separate. But in cell “h,” anaphase was unsuccessful, and you can see that the result is three sister chromatids on the left side of the cell, and only one on the right). When cytokinesis 2 occurs, the gamete at “j” will be n+1, while the gamete at “k” is n-1. Note that because anaphase happened correctly in cell “i,” the resulting gametes at “l” both have the correct number of chromosomes: they’re haploid cells, with a chromosome number of “n.” Also note a hidden step that’s not shown in the diagram: the doubled chromosome in the bottom left of cell “h” separates into two, full-fledged chromosomes.

If a gamete with a atypical number of chromosomes (n-1 or n+1) is fertilized by another gamete with a typical number of chromosomes (n) then the resulting zygote will similarly have an extra or a missing chromosome.

  • An extra chromosome is called a trisomy: 3 chromosomes instead of a homologous pair. The cell at “3” at left has a trisomy.
  • A missing chromosome is called a monosomy: 1 chromosome instead of a homologous pair. The cell at “4” at left has a monosomy.

Take a look at the karyotype below:

This is trisomy 21. It’s the chromosomal cause of most Down syndrome cases, and it’s caused by a nondisjunction that occurs during meiosis.

7. Nondisjunction: Checking Understanding

[qwiz random = “true”] [h]

Nondisjunction: Checking Understanding

[i]

[q] In the diagram below, which number shows a cell that is “2n”?

[textentry single_char=”true”]

[c*] a

[f] Excellent. The cell at “a” is diploid, or “2n.”

[c] Enter word

[f] No, that’s not correct.

[c] *

[f] No. Find a cell that has two sets of chromosomes.

[q] In the diagram below, which number shows a gamete that is “n + 1,” and which resulted from nondisjunction during meiosis I”?

[textentry single_char=”true”]

[c*] e

[f] Excellent. The cell at “e” is n + 1, and resulted from a nondisjunction that occurred during meiosis 1 (between cell “a” and cell “b.”)

[c] Enter word

[f] No, that’s not correct.

[c] *

[f] No. Start by finding a gamete that has an extra chromosome (making it n + 1). Then, make sure that this cell resulted from nondisjunction during meiosis 1 (which is when homologous pairs are supposed to separate).

[q] In the diagram below, which number shows a gamete that is “n – 1,” and which resulted from nondisjunction during meiosis 2″?

[textentry single_char=”true”]

[c*] k

[f] Excellent. The gamete at “k” is n – 1, and resulted from a nondisjunction that occurred during meiosis 2 (between cell “g” and cell “h.”)

[c] Enter word

[f] No, that’s not correct.

[c] *

[f] No. Start by finding a gamete that has a missing chromosome (making it n – 1). Then, make sure that this cell resulted from nondisjunction during meiosis 2 (which is when sister chromatids are supposed to separate).

[q] In the diagram below, which number shows gametes that, when fertilized, will produce a zygote with a trisomy? (Choose one of the two possible answers and enter it below)

[textentry single_char=”true”]

[c*] e; j

[f] Excellent. The cells at “e” and “j” are n + 1. When fertilized by another gamete that has one haploid set of chromosomes (n), the resulting gametes will have a trisomy? [c] Enter word

[f] No, that’s not correct.

[c] *

[f] No. Start by finding the gametes that have an extra chromosome (making them n + 1).

[q] In the diagram below, which number shows cells that, when fertilized, will produce a zygote with a monosomy? Choose one of the two possible answers and enter it below)?

[textentry single_char=”true”]

[c*] f; k

[f] Excellent. The cells at “f” and “k” are n – 1. When fertilized by another gamete that has one haploid set of chromosomes (n), the resulting gametes will have a monosomy (they’ll be missing a chromosome).

[c] Enter word

[f] No, that’s not correct.

[c] *

[f] No. Here’s a hint: you need to  find a gamete that has a missing chromosome (making it n – 1).

[q] In the diagram below, which number represents meiosis 1?

[textentry single_char=”true”]

[c*] 1

[f] Excellent. Number “1” represents meiosis 1, which is when homologous pairs separate.

[c] Enter word

[f] No, that’s not correct.

[c] *

[f] No. Meiosis 1 has two key features: it separates homologous pairs, and the chromosomes that are separated are composed of sister chromatids (so they’re doubled).

 

[q] The non-separation of either homologous pairs or sister chromatids that can happen during meiosis is called[hangman]

[c] nondisjunction

[f] Excellent!

[q] During meiosis II chromatids, sister ___________ are supposed be separated.  [hangman]

[c] chromatids

[f] Good!

[q] If an organism has a missing chromosome (instead of a homologous pair) it’s called a [hangman]

[c] monosomy

[f] Great!

[q] If an organism has three chromosomes (instead of a homologous pair) it’s called a [hangman]

[c] trisomy

[f] Great!

[/qwiz]

8. Nondisjunction, Down Syndrome, and other chromosomal variations.

So, we’ve established that

  • 95% of children with Down syndrome have an extra 21st chromosome.
  • Incidence of Down syndrome increases with maternal age, from 1 in 2000 births in 20 year old mothers, to 1 in 30 births in 45 year old mothers.

What’s the connection? Let me start by saying that the explanation that follows is still being developed. But the thinking is like this. Click here or here to dig a bit deeper than what you’ll read below.

  1. A woman’s germ cells (the cells that produce her eggs) are formed when she’s an embryo.
  2. During embryonic development, a woman’s germ cells begin the process of meiosis, and then pause in the middle of the process (after the DNA doubling that precedes meiosis 1, but before separation of homologous pairs during Meiosis 1 or sister chromatids during Meiosis 2).
  3. The chromosome separations and cell divisions that characterize meiosis resume during a woman’s ovulatory cycle. In other words, a 25 year old woman is creating haploid eggs from a germ cell that’s been paused for 25 years. A 45 year old woman is creating haploid eggs from a germ cell that’s been paused for 45 years.

The longer these cells have been paused, the longer the “junction” between homologous pairs or sister chromatids. That pause seems to increase the chance of nondisjunction, with resulting monosomies or trisomies.

In addition to Down syndrome, there are a few other monosomies or trisomies that you should be familiar with.

  1. Trisomy 18: this causes significant developmental problems (much more severe than those associated with Down syndrome). Read more about it here.
  2. Turner Syndrome: This is a monosomy of the X chromosome (sometimes annotated as “XO” to indicate that women with this condition have a single X chromosome, as opposed to two). Read more about it here.
  3. Klinefelter Syndrome: This is also called XXY syndrome. Individuals with Klinefelter are male, but have an extra X chromosome. Read more about Klinefelter syndrome here.
  4. XYY syndrome. Men with XYY have an extra Y chromosome in every cell of their bodies. Read more about XYY here.

9. Quiz: Sex determination, Nondisjunction and Chromosomal Variation

[qwiz random = “true”]
[h]Quiz: Nondisjunction and its consequences
[i]
[!!!!!!] question 1 +++++++++[/!!!!!!]
[q] In the diagram below, the name for what’s happening at the blue arrow is

[c] synapsis
[c] crossing over
[c] non-separation
[c*] nondisjunction
[f] No. Synapsis is the name for the pairing up of homologous chromosomes. This pairing up of chromosomes is followed by their separation. What you see at the blue arrow is that the sister chromatids failed to separate, and both went to one of the daughter cells (leaving the other daughter cell without any chromosomes). Look over the choices, and see if you can find a name for this process.
[f] No. Crossing over is when homologous chromosomes exchange pieces of DNA, creating new, recombinant chromosomes in the gametes. That’s not visible here. What you can see is that the sister chromatids failed to separate, and both went to one of the daughter cells (leaving the other daughter cell without any chromosomes). Look over the choices, and see if you can find a name for this process
[f] No. You chose a very logical name for what you’re observing (and you might, in fact, have been misled by the hints for the first two choices above. There’s a more formal name for this non-separation process. Look carefully at the choices, and see if you can figure it out next time.
[f] Yes. What you can see is that the sister chromatids failed to separate, and both went to one of the daughter cells (leaving the other daughter cell without any chromosomes). The name for this error is non-disjunction.
[!!!!!!] question 2 +++++++++[/!!!!!!]
[q] In the diagram below, which of the numbered daughter cells has a trisomy.

[c] 1
[c] 2
[c*] 3
[c] 4
[f] No. A trisomy means 3 chromosomes, instead of a homologous pair. This first cell has a homologous pair (two single chromosomes). One member of the pair came from the sperm, and one from the egg. Next time, look for a fertilized egg (these are all on the bottom row) that has three chromosomes, instead of a homologous pair.
[f] No. A trisomy means 3 chromosomes, instead of a homologous pair. This second cell has a homologous pair (two single chromosomes). One member of the pair came from the sperm, and one from the egg. Next time, look for a fertilized egg (these are all on the bottom row) that has three chromosomes, instead of a homologous pair.
[f] Excellent. A trisomy means three chromosomes, instead of a homologous pair. That’s exactly what you see in cell number 3.
[f] No. A trisomy means 3 chromosomes, instead of a homologous pair. This fourth cell has a single chromosome,instead of a pair, a condition called a ‘monosomy.’ Next time, look for a fertilized egg (these are all on the bottom row) that has three chromosomes, instead of a homologous pair.
[!!!!!!] question 3 +++++++++[/!!!!!!]
[q] In the diagram below, which of the numbered daughter cells has a monosomy.

[c] 1
[c] 2
[c] 3
[c*] 4
[f] No. A monosomy means one chromosome, instead of a homologous pair. This first cell has a homologous pair (two single chromosomes). One member of the pair came from the sperm, and one from the egg. Next time, look for a fertilized egg (these are all on the bottom row) that has just one chromosome, instead of a homologous pair. You can also look, in the row above, for an unfertilzed egg that’s missing a chromosome (which in this case, is shown by an empty nucleus).
[f] No. A monosomy means one chromosome, instead of a homologous pair. This second cell has a homologous pair (two single chromosomes). One member of the pair came from the sperm, and one from the egg. Next time, look for a fertilized egg (these are all on the bottom row) that has just one chromosome, instead of a homologous pair. You can also look, in the row above, for an unfertilzed egg that’s missing a chromosome (which in this case, is shown by an empty nucleus).
[f] No. A monosomy means one chromosome, instead of a homologous pair. This third cell has a three chromosomes, a condition called a trisomy. Next time, look for a fertilized egg (these are all on the bottom row) that has just one chromosome, instead of a homologous pair. You can also look, in the row above, for an unfertilzed egg that’s missing a chromosome (which in this case, is shown by an empty nucleus).
[f] Excellent. A monosomy means one chromosome, instead of a homologous pair, which is exactly what you see in cell number four.
[!!!!!!] question 4 +++++++++[/!!!!!!]
[q] The image below is a karyotype for

[c*] a chromosomally normal male
[c] a chromosomally normal female
[f] Yes. Look at the last pair of chromosomes, and note that one is large, the other is small. These are the sex chromosomes of a male (X and Y)
[f] No. As you can see by the last pair of chromosomes, this person has an X and a Y chromosome (one large, on small). A female would have two equally sized chromosomes.
[!!!!!!] question 5 +++++++++[/!!!!!!]
[q] The image below is a karyotype for

[c] a chromosomally normal male
[c*] a chromosomally normal female
[f] No. Look at the last pair of chromosomes, and note that one is large, the other is small. These are the sex chromosomes of a male (X and Y)
[f] Yes. As you can see by the last pair of chromosomes, this person has two ‘X’ chromosomes, making her female.
!!!!!!] question 6 +++++++++[/!!!!!!]
[q] Which of the following statements about sex chromosomes and gametes is true?
[c] Males pass on a ‘Y’ chromosome in their gametes. Females pass on an ‘X’ chromosome.
[c] both males and females can create gametes that carry a ‘Y’ chromosome or an ‘X’ chromosome.
[c*] When a man produces gametes, half of his sperm will carry an ‘X’ chromosome, and half of his sperm will carry a ‘Y’ chromosome. By contrast, all of a woman’s eggs will carry an X chromosome.
[f] No. A man’s sex chromosomes are ‘XY,’ During meiosis, the X and Y chromosomes, which are homologous, will separate. All of the resulting gametes will either have an X chromosome, or a Y chromosome. You are, however, right about the female: a female’s sex chromosomes are ‘XX.’ As a result, all of the eggs that she produces will have an X chromosome.
[f] No. You’re right about the male, but not about the female. Here’s why: A man’s sex chromosomes are ‘XY,’ During meiosis, the X and Y chromosomes, which are homologous, will separate. All of the resulting gametes will either have an X chromosome, or a Y chromosome. A female’s sex chromosomes are ‘XX.’ As a result, all of the eggs that she produces will have an X chromosome.
[f] That’s right. A man’s sex chromosomes are ‘XY,’ During meiosis, the X and Y chromosomes, which are homologous, will separate. All of the resulting gametes will either have an X chromosome, or a Y chromosome. A female’s sex chromosomes are ‘XX.’ As a result, all of the eggs that she produces will have an X chromosome.
[!!!!!!] question 7 +++++++++[/!!!!!!]
[q] Which of the following statements about sex determination in humans is true.
[c] The egg determines the sex of a female. The sperm determines the sex of a male.
[c*] The sperm determines the sex of the offspring.
[c] The egg determines the sex of the offspring.
[f] No. A man’s sex chromosomes are ‘XY,’ During meiosis, the X and Y chromosomes, which are homologous, will separate. All of the resulting gametes will either have an X chromosome, or a Y chromosome. A female’s sex chromosomes are ‘XX.’ As a result, all of the eggs that she produces will have an X chromosome. If an X carrying sperm fertilizes an egg, the offspring will be XX, or female. If a Y carrying sperm fertilizes an egg, the offspring will be XY, or male. Use this information to choose a different answer next time.
[f] Correct. A man’s sex chromosomes are ‘XY,’ During meiosis, the X and Y chromosomes, which are homologous, will separate. All of the resulting gametes will either have an X chromosome, or a Y chromosome. A female’s sex chromosomes are ‘XX.’ As a result, all of the eggs that she produces will have an X chromosome. If an X carrying sperm fertilizes an egg, the offspring will be XX, or female. If a Y carrying sperm fertilizes an egg, the offspring will be XY, or male. Thus, the sperm determines the sex of the offspring.
[f] No. A man’s sex chromosomes are ‘XY,’ During meiosis, the X and Y chromosomes, which are homologous, will separate. All of the resulting gametes will either have an X chromosome, or a Y chromosome. A female’s sex chromosomes are ‘XX.’ As a result, all of the eggs that she produces will have an X chromosome. If an X carrying sperm fertilizes an egg, the offspring will be XX, or female. If a Y carrying sperm fertilizes an egg, the offspring will be XY, or male. Use this information to choose a different answer next time.
[!!!!!!] question 8 +++++++++[/!!!!!!]
[q] The image below shows the sex chromosomes of male and female fruit flies. Based on the diagram, you can infer that in fruit flies

[c] The egg determines the sex of a female. The sperm determines the sex of a male.
[c*] The sperm determines the sex of the offspring.
[c] The egg determines the sex of the offspring.
[f] No. It’s exactly like humans. A male’s sex chromosomes are ‘XY,’ During meiosis, the X and Y chromosomes, which are homologous, will separate. All of the resulting gametes will either have an X chromosome, or a Y chromosome. A female’s sex chromosomes are ‘XX.’ As a result, all of the eggs that she produces will have an X chromosome. If an X carrying sperm fertilizes an egg, the offspring will be XX, or female. If a Y carrying sperm fertilizes an egg, the offspring will be XY, or male. Use this information to choose a different answer next time.
[f] Correct. It’s exactly like humans. A male’s sex chromosomes are ‘XY,’ During meiosis, the X and Y chromosomes, which are homologous, will separate. All of the resulting gametes will either have an X chromosome, or a Y chromosome. A female’s sex chromosomes are ‘XX.’ As a result, all of the eggs that she produces will have an X chromosome. If an X carrying sperm fertilizes an egg, the offspring will be XX, or female. If a Y carrying sperm fertilizes an egg, the offspring will be XY, or male. Thus, the sperm determines the sex of the offspring.
[f] No. It’s exactly like it is in humans. A male’s sex chromosomes are ‘XY,’ During meiosis, the X and Y chromosomes, which are homologous, will separate. All of the resulting gametes will either have an X chromosome, or a Y chromosome. A female’s sex chromosomes are ‘XX.’ As a result, all of the eggs that she produces will have an X chromosome. If an X carrying sperm fertilizes an egg, the offspring will be XX, or female. If a Y carrying sperm fertilizes an egg, the offspring will be XY, or male. Use this information to choose a different answer next time.
[!!!!!!] question 9 +++++++++[/!!!!!!]
[q] What’s the best diagnosis of the karyotype below?

[c] a chromosomally normal male
[c*] Turner syndrome
[c] Klinfelter syndrome
[c] Down syndrome.
[f] No. Look at the sex chromosomes. A chromosomally normal male would have an X and a Y chromosome. Next time, choose a different answer.
[f] Yes. As you can see, this female has a single X chromosome, which is the chromosomal cause of Turner syndrome.
[f] No. Look at the sex chromosomes. You can see that there’s a single X chromosome. Klinefelter is a syndrome that impacts males, and it’s caused by an extra X chromosome (as opposed to a missing X chromosome, which is what you see here.
[f] No. Down syndrome is caused by an extra 21st chromosome (trisomy 21). This person only has two 21st chromosomes.
[!!!!!!] question 10 +++++++++[/!!!!!!]
[q] What’s the best diagnosis of the karyotype below?

[c] a chromosomally normal male
[c] Turner syndrome
[c*] Klinfelter syndrome
[c] Down syndrome.
[f] No. Look at the sex chromosomes. There are two X chromosomes, and a Y chromosome. A chromosomally normal male would have just one X chromosome, and one Y chromosome. Next time, choose a different answer.
[f] No. Look at the sex chromosomes. There are two X chromosomes, and a Y chromosome. A person with Turner syndrome has a single X chromosome (XO). Next time, choose a different answer.
[f] Exactly. Look at the sex chromosomes. There are two X chromosomes, and a Y chromosome. This is the chromosomal cause of Klinefelter syndrome (XXY).
[f] No. Down syndrome is caused by an extra 21st chromosome (trisomy 21). This person only has two 21st chromosomes.
[!!!!!!] question 11 +++++++++[/!!!!!!]
[q] What’s the best diagnosis of the karyotype below?

[c*] Down syndrome.
[c] Turner syndrome
[c] Klinfelter syndrome
[f] Yes. Down syndrome is caused by an extra 21st chromosome (trisomy 21).
[f] No. A person with Turner syndrome has a single X chromosome (XO). In terms of the sex chromosomes, this person is a normal male, with an X and a Y chromosome. The thing to look for here is the 21st chromosome. Take a look, and choose a different answer next time.
[f] No. A man with Klinefelter syndrome has two X chromosomes, and a Y chromosome (XXY). In terms of the sex chromosomes, this person is a normal male, with an X and a Y chromosome. The thing to look for here is the 21st chromosome. Take a look, and choose a different answer next time.
[!!!!!!] question 12 +++++++++[/!!!!!!]
[q] The chromosomal pattern associated with Turner syndrome is
[c] Trisomy 21
[c] XXY
[c*] XO
[c] XYY
[f] No. Trisomy 21 is associated with Down Syndrome. Turner syndrome is found only in females, and involves inheriting a single X chromosome. Use that information to choose a different answer next time.
[f] No. XXY is associated with Klinefelter syndrome. Turner syndrome is found only in females, and involves inheriting a single X chromosome. Use that information to choose a different answer next time.
[f] Yes. Turner syndrome is caused by inheritance of a single X chromosome, represented by the notation XO.
[f] No. XYY is caused by inheritance of an extra Y chromosome. Turner syndrome is found only in females, and involves inheriting a single X chromosome. Use that information to choose a different answer next time.
[!!!!!!] question 13 +++++++++[/!!!!!!]
[q] The chromosomal pattern associated with Klinefelter syndrome is
[c] Trisomy 21
[c*] XXY
[c] XO
[c] XYY
[f] No. Trisomy 21 is associated with Down Syndrome. Klinefelter syndrome occurs in males who inherit an extra X chromosome. Use that information to choose a different answer next time.
[f] Yes. XXY is associated with Klinefelter syndrome. Klinefelter syndrome occurs in males who inherit an extra X chromosome.
[f] No. Turner syndrome is caused by inheritance of a single X chromosome, represented by the notation XO. Klinefelter syndrome occurs in males who inherit an extra X chromosome. Use that information to choose a different answer next time.
[f] No. XYY is caused by inheritance of an extra Y chromosome. Klinefelter syndrome occurs in males who inherit an extra X chromosome. Use that information to choose a different answer next time.
[!!!!!!] question 14 +++++++++[/!!!!!!]
[q] The chromosomal pattern associated with Down syndrome is
[c*] Trisomy 21
[c] XXY
[c] XO
[c] XYY
[f] Yes. Trisomy 21 is associated with Down Syndrome. Trisomy 21 is another way of saying that there are three chromosomes, instead of two (a homologous pair).
[f] No. XXY is associated with Klinefelter syndrome. Down syndrome is caused by an extra chromosome, but not in the sex chromosomes. See if you can find that extra chromosome in this image when you see this question again.
[f] No. XO is associated with Turner syndrome, and it involves a missing X chromosome. Down syndrome is caused by an extra chromosome, but not in the sex chromosomes. See if you can find that extra chromosome in this image when you see this question again.
[f] No. XYY is caused by inheritance of an extra Y chromosome. Down syndrome is caused by an extra chromosome, but not in the sex chromosomes. See if you can find that extra chromosome in this image when you see this question again.
[x]
[restart]

[/qwiz]

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