Genetic Diversity I: Why we are unique
MEIOSIS, SEXUAL REPRODUCTION AND GENDER
Why you are unique.
01/09/12
Primary Learning Objectives:
1. Understand terms and concepts crucial to comprehending the process of
meiosis
2. Obtain a clear understanding of how the process of meiosis decreases the
number of chromosomes from two sets of 23 (46) to one set of 23 and the
number of copies of each gene to 1.
3. Understand the processes that create the genetic diversity possible in an
individual’s gametes. Why are you unique?
4. Know the differences between somatic cell division (mitosis), and gamete cell
division (meiosis) and understand the functions of both.
Document Outline
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
IX.
X.
Introduction to Meiosis
Purpose of Meiosis #1: Decrease the Number of Chromosomes and Copies of Genes
Purpose of Meiosis #2: Create Gametes that Vary Genetically
Fertilization
Summary of Genetic Variability due to Meiosis and Sexual Reproduction
X Inactivation
Reexamine the genes that are inactivated on the Barr Body:
Females vs. Males: Sex-linked Possibilities
Sex-influenced and Sex-limited Traits
Gender Development
Additional or Missing Chromosomes
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Part 1
I. Introduction
A. Karyotype showing Replicated Homologous Chromosomes
1. Replicated Chromosomes are produced through DNA Replication. Each
replicated chromosome contains two Sister Chromatids.
2. Sister Chromatids vs. Homologs
Figure 1
2
B. Gametic cells vs. Somatic cells
C. Diploid vs. Haploid
1. Diploid: 2n cell, or a cell containing TWO sets of chromosome (one from
mother, one from father, the maternal and paternal homologous pairs). Human
somatic cells are diploid.
a. Homozygous vs. Heterozygous
- For any given gene, the condition of having two of the same
allele for a particular gene
- For any given gene, the condition of having two different alleles
for a particular gene
b. Human somatic cells remain diploid after cell division.
2. Haploid: A cell containing ONE set of chromosomes (maternal and paternal
homologous pairs. Gametes (sex cells: eggs & sperm) are haploid
D. Functions of Meiosis:
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Part 2
II. Purpose of Meiosis #1: Decrease the # of Chromosomes and the # of Copies of each gene in
Gametes
A.
Fig. 2
Initial cell in the ovaries (oogonia) or
testes (spermatogonia):
1. Diploid or Haploid?
2. Are the Chromosomes Replicated or Unreplicated?
What does that mean?
3. Total number of Chromosomes?
4. # of copies of each gene?
After Meiosis I:
1. Diploid or Haploid?
2. Are the Chromosomes Replicated or
Unreplicated? What does that mean?
B.
3. Total number of Chromosomes?
4. # of copies of each gene?
C.
After Meiosis II- Final Gametes:
1. Diploid or Haploid?
2. Are the Chromosomes Replicated or Unreplicated? What does that mean?
3. Total number of Chromosomes?
4. Number of copies of each gene?
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Part 3
III. Purpose of Meiosis #2: Create Gametes that Vary Genetically
Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced from cells
in the ovaries or testes. In meiosis, after only one round of DNA replication, there are two
successive cellular divisions.
Four stages can be described for each of the two divisions (Prophase I & II, Metaphase I & II,
Anaphase I & II, Telophase I & II). The following figures describe a few details of meiosis
and illustrate how it creates so many possible genetic variations in an individual’s gametes, that
no one can create genetically identical eggs or sperm.
Fig. 3
A. Meiosis I (Reduction Division)
See Fig. 2 A-B.
1. Prophase I
a. Homologs PAIR together.
b. CROSSING OVER occurs (See Fig.4).
c. Nuclear membrane disappears and chromosomes
condense.
d. Spindle fibers form from the centrioles and attach to
the centromeres of the chromosomes.
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Fig. 4
Crossing Over
Recombination
2. Metaphase I
(See Fig. 3)
a. Homologous pairs line up, together, on the metaphase plate
b. Remember, one member of a homologous pair is from the mother
(maternal) and the other member is from the father (paternal)
c. It is completely random on which side of the metaphase plate the
maternal and paternal member of each homologous pair align themselves
d. Each homologous pair is INDEPENDENT of all other homologous
pairs as far as which side of the metaphase plate the maternal and
paternal chromosomes go.
This random, independent alignment of maternal and paternal
chromosomes on the metaphase plate during metaphase I of
meiosis is called INDEPENDENT ASSORTMENT. (See Fig. 5.)
3. Anaphase I
The two members of each homologous pair are pulled to opposite poles of
the cell by the spindle fibers.
The sister chromatids STAY TOGETHER!
4. Telophase I: (See Fig. 2B and Fig. 3)
DNA uncoils, spindle disappears, nuclear envelopes reform & cytokinesis occurs.
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Fig. 5
Independent Assortment
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Part 4
B. Meiosis II (See Fig. 2B-C)
Fig. 6
Prophase II: Similar to Prophase I
but no crossing over.
Metaphase II
-Chromosomes line up on the
metaphase plate.
-Remember, there are no longer
homologous pairs in the cell; only
one of each.
Anaphase II
-Centromeres separate so that sister
chromatids are separated. Each sister
chromatid is now a sister chromosome,
unattached to each other.
- The sister chromatids (now sister
chromosomes) are pulled to opposite
poles of the cell.
Telophase II
Similar to Telophase I, but now
with a total of 4 cells.
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IV. Fertilization
"Following fusion of the fertilizing sperm with the oocyte, the sperm head
is incorporated into the egg cytoplasm. The nuclear envelope of the sperm
disperses. In vertebrates, other sperm components, including mitochondria,
are degraded rather than incorporated into the embryo.
Chromatin from both the sperm and egg are soon encapsulated in a
nuclear membrane. The image to the right shows a one-cell rabbit embryo
shortly after fertilization - this embryo was fertilized by two sperm, and would
likely die within a few days.
(http://arbl.cvmbs.colostate.edu/hbooks/pathphys/reprod/fert/fert.html)
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Part 5
V. Summary: Genetic Variability Due To Meiosis and Sexual Reproduction
A. Crossing Over: An event during prophase I of meiosis when homologs exchange parts.
Four gametes are formed. Therefore, there are four Chromosome #1 possibilities.
Chromosome #1 has approximately 263 million base pairs. Crossing over could occur
between any of those base pairs. To calculate the number of genetic possibilities through
crossing over on the Chromosome #1 pair (If there is only one cross over)…
263,000,000 x 2 + 2 = 526,000,002 (5.26 x 108)
(# bp/chromosome #1) x (# of variable chromatids; i.e., 2 variable sister chromatids/cross-over) + 2 nonrecombining sister chromatids.
If we calculate all of the chromosomes:
Chromosome # Length (bp in Millions)
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
X and Y
255
214
203
194
183
171
155
145
144
144
143
114
109
106
90
92
85
67
72
50
56
Don’t use.
# of genetic possibilities due to crossing over
(once on that pair)
255,000,000 x 2 + 2 = 510,000,002
214,000,000 x 2 + 2 = 428,000,002
203,000,000 x 2 + 2 = 406,000,002
194,000,000 x 2 + 2 = 388,000,002
183,000,000 x 2 + 2 = 366,000,002
171,000,000 x 2 + 2 = 342,000,002
155,000,000 x 2 + 2 = 310,000,002
145,000,000 x 2 + 2 = 290,000,002
144,000,000 x 2 + 2 = 288,000,002
144,000,000 x 2 + 2 = 288,000,002
143,000,000 x 2 + 2 = 286,000,002
114,000,000 x 2 + 2 = 228,000,002
109,000,000 x 2 + 2 = 218,000,002
106,000,000 x 2 + 2 = 212,000,002
98,000,000 x 2 + 2 = 196,000,002
92,000,000 x 2 + 2 = 184,000,002
85,000,000 x 2 + 2 = 170,000,002
67,000,000 x 2 + 2 = 134,000,002
72,000,000 x 2 + 2 = 144,000,002
50,000,000 x 2 + 2 = 100,000,002
56,000,000 x 2 + 2 = 112,000,002
Cross-overs neglible; only a small area involved
Multiply the results for all pairs to calculate the combined genetic possibilities if each pair crossed over
once per meiosis.
= 6.7408 x 10184
*
* Current world population http://www.census.gov/cgi-bin/ipc/popclockw
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B. Independent Assortment: A term used to describe the arrangement of maternal and
paternal homologs during metaphase I of meiosis. Each homologous pair lines up on the
metaphase plate, with the maternal chromosome on one side and the paternal
chromosome on the other side. The maternal and paternal homologs separate during
anaphase I and will eventually end up in different gametes. The side maternal and
paternal chromosomes are on is random and will not influence other homologous pairs.
Therefore, gametes are a mixture of the maternal and paternal chromosomes of the
parents.
C. Fusion of egg and sperm during fertilization: Combines a haploid genome of the egg
with a haploid genome of the sperm creating a diploid zygote from two individuals with
their own genetic ancestry, crossing over, independent assortment, generation after
generation.
D. Spontaneous mutation during DNA replication before
meiosis begins.
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VI. X inactivation
A. Inactivation occurs at the 16-cell stage of development.
B. It is random and permanent in each cell.
C. Barr body
D. Turner’s syndrome: Turner’s syndrome led researchers to additional information
about genes on the Barr body.
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Fig. 7
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So how many active copies do a
female or male have of these
genes?
VII. Reexamine the genes that are inactivated on the Barr Body (these are the genes that are not
found on the Y chromosome: Females vs. Males: Sex-linked Possibilities
Female
Genotypes
Inactivation
Example: Systemic Disease
-X-linked SCID
No inactivation
Example: Systemic Disease
-X-linked SCID
XAXA
XaXa
XAXa
Male
Genotypes
XAY
XaY
Example: Color-blindness
Most people can see the primary colors of a traffic light because of a gene that codes for color vision. But an error in
that gene's DNA code causes some viewers to see red and green as shades of yellowish brown. This is an X-linked
trait. Roughly 10 million men and 550,000 women in the United States are color blind.
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VIII. Sex-influenced and Sex- limited Traits
A. Sex-influenced Traits: Either dominant in one sex and recessive in the other AND/OR
affects one sex earlier in life or more severely.
-Often a result of hormonal influences.
Male Pattern Baldness in 4 Generations of The Adams Family
B. Sex-limited Traits
Only affects one sex because the other gender does not have those parts of functions.
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IX. Gender development
Conception
7th week
After 7th week
Early childhood (earlier? later?)
XX males and XY females. How?
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X. Additional or Missing Chromosomes
A. Aneuploidy: extra or missing chromosomes
-Trisomy vs. Monosomy
Autosomes
-Aneuploids that may survive birth
1. Down syndrome (trisomy 21)
-Most common aneuploidy.
-Symptoms vary greatly: profoundly retarded to functional; kidney, heart, and
digestive system defects common; increased risk of leukemia, increased risk of
Alzheimer disease.
2. Trisomy 13 (Patau syndrome)
-Few survive to birth; of those that do, most die in the first months of life.
-Defects in all organs, mental retardation, extra fingers and toes.
3. Trisomy 18 (Edward syndrome)
-Few survive to birth; of those that do, most die in the first months of life.
-Defects in all organs, mental retardation.
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Sex chromosomes
4. Turner syndrome (X0)
-99% die before birth.
-Those that live may have subnormal intelligence, underdeveloped secondary
sexual characteristics, usually must take hormone supplements, usually sterile.
5. Klinefelter syndrome (XXY)
-Underdeveloped secondary sexual characteristics (often helped with testosterone
injections). May develop female secondary sexual characteristics.
-Usually sterile.
-Long arms and legs, large hands and feet.
-May have subnormal intelligence.
-As the number of X's increase, the symptoms become more severe.
6. Triplo-X (XXX)
-Only symptoms: usually very tall, irregular menstration, may have subnormal
intelligence.
-May produce eggs with 2 X's.
7. Jacob's syndrome (XYY)
-Tall, acne, may have speech/reading problems.
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B. Nondisjunction: Failure of chromosomes to separate properly during meiosis.
Meiosis I Nondisjunction:
Homologous pairs do not separate properly.
Fig. 8
Fertilization…
With Egg #1
1
.
Aneuploid
Egg
2
Normal
Sperm
Zygote
X
Y
With Egg #2:
X
Y
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Meiosis II Nondisjunction: Sister chromatids do not separate into different cells properly.
Fig. 9
1
2
3
5
4
6
Fertilization…
Normal
Egg
With Gamete #1:
X
With Gamete #2:
X
With Gamete #3:
X
With Gamete #4:
X
With Gamete #5:
X
With Gamete #6
X
Sperm
Zygote
Are any of the
zygotes normal?
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Post-Test Questions
1.
Some definitions are given below. Write the appropriate word for each of the definitions
below.
a. Random alignment of maternal and paternal homologous chromosomes on the
metaphase plate during meiosis I. This creates a huge amount of genetic variation in eggs
and sperm.
b. The process of decreasing the number of chromosomes by half to produce gametes;
this process also is responsible for much of the great genetic variation between
individuals.
c. Process that occurs during Prophase I of meiosis where homologous chromosomes pair
together, come in direct contact, and exchange parts. This produces variation (different
genetic combinations) in the offspring.
d. Exact copies of one chromosome produced during DNA replication are called… While
still attached at the centromere, these copies make up one chromosome; they are
separated during anaphase II of meiosis, and only then each become their own
chromosome.
e. A cell that contains only one member of each homologous pair; therefore, these cells
only have 23 chromosomes.
f. These chromosomes come in pairs, one from the mother (maternal) and the other from
the father (paternal). They are the same size, have the same genes for the same traits at
the same loci, and have centromeres in the same locations. Each member of the pair
undergoes DNA replication to form sister chromatids.
g. A cell (all somatic cells) with both members of all homologous pairs (2 sets of
chromosomes); therefore, these cells have 46 chromosomes.
h. A condition of having two of the same alleles for a particular gene.
i. A condition of having two different alleles for a particular gene.
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2. Another name for homologous chromosomes is
.
3. State whether the following are diploid or haploid.
1) Cells immediately (in the ovaries or testes) before meiosis:
2) Cells (in the ovaries or testes) after meiosis I:
3) Cells (in the ovaries or testes) after meiosis II:
4) The first cell after fertilization:
4. State the number of copies of each gene that are contained in the following.
1) Cells before meiosis:
2) Cells after meiosis I:
3) Cells after meiosis II:
4) The first cell after fertilization:
5. State the total number of chromosomes in the cells.
1) Cells before meiosis:
2) Cells after meiosis I:
3) Cells after meiosis II (gametes):
4) The first cell after fertilization:
6. What is the difference between maternal and paternal chromosomes in one homologous pair?
(Hint: This is too easy.)
maternal:
paternal:
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7. Describe the difference between the separation of chromosomes during anaphase I and
anaphase II.
anaphase I:
anaphase II:
8. In humans, how many possible chromosome combinations, due to independent assortment
only, are possible in each gamete? What is the formula for calculating this number?
9. List one event in each of the stages of meiosis.
Meiosis I
Prophase I:
Metaphase I:
Anaphase I:
Telophase I:
Meiosis II
Prophase II:
Metaphase II:
Anaphase II:
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Telophase II:
10. Write the stage of meiosis where the following takes place.
a. Homologous chromosomes are pulled to opposite poles of the cell
b. Sister chromatids are pulled to opposite poles of the cell
c. Homologous pairs line up on the metaphase plate together, the maternal member on one side
and the paternal member on the other side
d. Crossing over
e. Cells undergo cytokinesis resulting in 4 daughter cells
f. Nuclear membrane disappears in one cell
h. Chromosomes line up on the metaphase plate in single file
i. Homologous chromosomes pair together
j. Cells containing one copy of each homologous chromosome and one copy of each gene are
produced
k. Cells containing one copy of each homologous chromosome and two copies of each gene are
produced
l. Spindle fibers attach to the centromeres of each chromosome
m. DNA replication
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11. List the four processes that produce genetic variation.
12. Clearly describe in complete sentences what happens during crossing over. At what stage
of meiosis does crossing over occur? DRAW representative chromosomes undergoing crossing
over (you may want to use colored pencils). How does crossing over contribute to genetic
variability?
13. Describe fertilization and how it restores the human genetic complement (number of
chromosomes and ploidy-the number of maternal and paternal chromosomes in each pair).
14. Clearly describe INDEPENDENT ASSORTMENT during metaphase I of meiosis. How
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does this increase genetic variability in gametes (and therefore offspring)?
15. Are the chromosomes in the karyotype below replicated or unreplicated? How do you know?
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Write the correct terms for the descriptions below.
16. Traits that only affect one sex because the other sex doesn't have those parts or functions.
17. Traits that are either dominant in one sex and recessive in the other AND/OR affect one sex
earlier in life of more severely.
18. What sex an individual "feels" like.
19. Sex-determining region of the Y chromosome; if present, the embryo will form testes and
will become a male.
20. The inactivated X chromosome in females.
21. "Feeling" like the opposite sex.
22. In the 16-cell stage XX embryos, the random ‘turning off’ of genes in one of the X
chromosomes.
23. Trait controlled by a gene on the X chromosome.
24. Area of the 7-week embryo that eventually will become the mature ovaries or testes.
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25. What syndrome led researchers to believe that at least some of the genes on the inactivated X
chromosome are necessary for proper functioning in females? Why?
26. One of the X chromosomes in each female cell is inactivated. Males only have one X
chromosome. Theoretically, males and females both only have one functioning copy of all of the
genes on the X chromosome. Why, then, are males disproportionately affected by recessive X
chromosome genetic disorders?
27. List one sex-influenced trait and one sex-limited trait.
28. List factors responsible for the development of testes or ovaries in males and females.
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29. Draw the chromosomes in the diagram below depicting nondisjunction during meiosis I.
30. Draw the chromosomes in the diagram below depicting nondisjunction during meiosis II.
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31. Observe the karyotype.
Female or male?
Does this individual have a normal chromosome complement? If not, explain. If an aneuploidy
is present, what is it called?
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32. Observe the karyotype.
Female or male?
Does this individual have a normal chromosome complement? If not, explain. If an aneuploidy
is present, what is it called?
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33. Observe the karyotype.
Female or male?
Does this individual have a normal chromosome complement? If not, explain. If an aneuploidy
is present, what is it called?
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http://www.biology.arizona.edu/human_bio/activities/karyotyping/karyotyping.html
34. Go to the above website.
Read the introduction and complete the patient karyotypes for patient A, B and C.
Use the back arrow to return to the activity if you place the wrong chromosome.
Remember to observe banding patterns along with size when placing the chromosomes.
When you have successfully completed the karyotypes, answer the following:
I. Patient A
What notation would you use to characterize Patient A's karyotype?
Diagnosis:
II. Patient B
What notation would you use to characterize Patient B's karyotype?
Diagnosis:
III. Patient C
What notation would you use to characterize Patient C's karyotype?
Diagnosis:
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