Aneuploidy is a change in the number of chromosomes resulting from nondisjunction during meiosis.

Antonarakis, S. E., et al. Chromosome 21 and Down syndrome: From genomics to pathophysiology. Nature Reviews Genetics 5, 725–738 (2004) doi:10.1038/nrg1448 (link to article)

FitzPatrick, D. R. Transcriptional consequences of autosomal trisomy: Primary gene dosage with complex downstream effects. Trends in Genetics 21, 249–253 (2005) doi:10.1016/j.tig.2005.02.012

Hassold, T., & Hunt, P. To err (meiotically) is human: The genesis of human aneuploidy. Nature Reviews Genetics 2, 280–291 (2001) doi:10.1038/35066065 (link to article)

Jacobs, P. A., & Strong, J. A. A case of human intersexuality having a possible XXY sex-determining mechanism. Nature 183, 302–303 (1959) doi:10.1038/183302a0 (link to article)

Mao, M., et al. Global up-regulation of chromosome 21 gene expression in the developing Down syndrome brain. Genomics 81, 457–467 (2003) doi:10.1016/S0888-7543(03)00035-1

Patterson, D., & Costa, A. C. S. Down syndrome and genetics—A case of linked histories. Nature Reviews Genetics 6, 137–147 (2005) doi:10.1038/nrg1525 (link to article)

Figure 2: Examples of polytene chromosomes

Pairing of homologous chromatids results in hundreds to thousands of individual chromatid copies aligned tightly in parallel to produce giant, "polytene" chromosomes.

The greatest impact of Sutton's work has far more to do with providing evidence for Mendel's principle of independent assortment than anything else. Specifically, Sutton saw that the position of each chromosome at the midline during metaphase was random, and that there was never a consistent maternal or paternal side of the cell division. Therefore, each chromosome was independent of the other. Thus, when the parent cell separated into gametes, the set of chromosomes in each daughter cell could contain a mixture of the parental traits, but not necessarily the same mixture as in other daughter cells.

To illustrate this concept, consider the variety derived from just three hypothetical chromosome pairs, as shown in the following example (Hirsch, 1963). Each pair consists of two homologues: one maternal and one paternal. Here, capital letters represent the maternal chromosome, and lowercase letters represent the paternal chromosome:

• Pair 1: A and a
• Pair 2: B and b
• Pair 3: C and c

When these chromosome pairs are reshuffled through independent assortment, they can produce eight possible combinations in the resulting gametes:

• A B C
• A B c
• A b c
• A b C
• a B C
• a B c
• a b C
• a b c

A mathematical calculation based on the number of chromosomes in an organism will also provide the number of possible combinations of chromosomes for each gamete. In particular, Sutton pointed out that the independence of each chromosome during meiosis means that there are 2n possible combinations of chromosomes in gametes, with "n" being the number of chromosomes per gamete. Thus, in the previous example of three chromosome pairs, the calculation is 23, which equals 8. Furthermore, when you consider all the possible pairings of male and female gametes, the variation in zygotes is (2n)2, which results in some fairly large numbers.

But what about chromosome reassortment in humans? Humans have 23 pairs of chromosomes. That means that one person could produce 223 different gametes. In addition, when you calculate the possible combinations that emerge from the pairing of an egg and a sperm, the result is (223)2 possible combinations. However, some of these combinations produce the same genotype (for example, several gametes can produce a heterozygous individual). As a result, the chances that two siblings will have the same combination of chromosomes (assuming no recombination) is about (3/8)23, or one in 6.27 billion. Of course, there are more than 23 segregating units (Hirsch, 2004).

While calculations of the random assortment of chromosomes and the mixture of different gametes are impressive, random assortment is not the only source of variation that comes from meiosis. In fact, these calculations are ideal numbers based on chromosomes that actually stay intact throughout the meiotic process. In reality, crossing-over between chromatids during prophase I of meiosis mixes up pieces of chromosomes between homologue pairs, a phenomenon called recombination. Because recombination occurs every time gametes are formed, we can expect that it will always add to the possible genotypes predicted from the 2n calculation. In addition, the variety of gametes becomes even more unpredictable and complex when we consider the contribution of gene linkage. Some genes will always cosegregate into gametes if they are tightly linked, and they will therefore show a very low recombination rate. While linkage is a force that tends to reduce independent assortment of certain traits, recombination increases this assortment. In fact, recombination leads to an overall increase in the number of units that assort independently, and this increases variation.

While in mitosis, genes are generally transferred faithfully from one cellular generation to the next; in meiosis and subsequent sexual reproduction, genes get mixed up. Sexual reproduction actually expands the variety created by meiosis, because it combines the different varieties of parental genotypes. Thus, because of independent assortment, recombination, and sexual reproduction, there are trillions of possible genotypes in the human species.

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1

Which of the following steps would NOT lead to variation of genetic material? A. crossing over of homologous chromosomes B. crossing over of sister chromatids C. the random alignment of the chromosomes during metaphase I

D. the combination of sperm and egg genes

2

Which of the following is NOT a difference between anaphase I and anaphase II? A. At the end of anaphase I, each chromosome is composed of two chromatids and at the end of anaphase II, sister chromatids have separated, becoming daughter chromosomes B. Anaphase I occurs in a haploid cell while anaphase II occurs in a diploid cell. C. Sister chromatids separate during anaphase II while homologous chromosomes separate during anaphase I.

D. The cell undergoing anaphase II is genetically different from what it contained while undergoing anaphase I.

3

The cell formed through fertilization of an egg by a sperm is called a/an A. gamete. B. sperm cell. C. zygote. D. egg cell.

E. ovum.

4

It could be said that males are able to provide gametes with more genetic diversity than females for reproduction. One main reason would be: A. Males provide more genes in sperm than females provide in eggs. B. Crossing over occurs more often in the formation of sperm than in eggs. C. Spermatogenesis in males results in four functional sperm while oogenesis in females results in only one egg and three structures that contain genetic information that is lost when they disintegrate.

D. Sperm that contain a recombination of genes are usually more successful in fertilizing an egg.

5

Determining the number of sperm in the individual If a sperm cell contains 8 chromosomes, it comes from an animal that has ______ chromosomes. A. 4 B. 8 C. 12 D. 16

E. 24

6

Another name for a tetrad is a/an A. homologue. B. bivalent. C. öocyte. D. gamete.

E. polar body.

7

If a cell contains 12 chromosomes at the end of meiosis I, how many chromosomes will the daughter cells contain at the end of meiosis II? A. 3 B. 6 C. 12

D. 24

8

During which stage of meiosis does crossing-over occur? A. prophase I of meiosis I B. anaphase I of meiosis II C. telophase I of meiosis I D. prophase II of meiosis II

E. anaphase II of meiosis I

9

Homologous chromosomes are similar in all of these characteristics EXCEPT: A. similar in size. B. carry the same alleles for all traits. C. carry genes for the same traits.

D similar in shape and location of the centromere.

10

What is the importance of crossing-over? A. It provides extra genetic material for the daughter cells. B. It increases the likelihood that daughter cells contain different genetic material. C. It produces the proteins that are associated with DNA in chromosomes. D. It increases chromosome condensation.

E. It separates the homologous chromosomes.

11

The overall function of meiosis includes all of the following EXCEPT A. gamete production. B. reduction of chromosome number (from 2N to N). C. providing genetic variation in sexually reproducing organisms.

D. growth of the overall individual.

12

Sources of genetic variation in a sexually reproducing population include(s) which of the following? A. crossing over in Prophase I of meiosis B. independent assortment in Metaphase I of meiosis C. fertilization D. All of the choices are sources of genetic variation.

E. None of the choices are sources of genetic variation.

13

To what does the term chiasma refer? A. the process of fertilization B. the life cycle of a fungus C. the process of crossing-over D. a structure that holds together homologues during crossing-over

E. the period between meiosis I and meiosis II

14

During which stage of meiosis are the bivalents arranged along the equator? A. prophase I B. metaphase II C. anaphase II D. prophase II

E. metaphase I

15

At which stage of meiosis is each chromosome composed of a single chromatid? A. prophase I B. metaphase II C. anaphase II D. prophase II

E. metaphase I

16

During which stage of meiosis does the homologue separation occur? A. prophase I B. anaphase I C. telophase I D. prophase II

E. anaphase II

17

Which statement is NOT true about homologues in meiosis I? A. Homologous chromosomes pair to form a tetrad. B. Homologous chromosomes separate and go to different daughter cells. C. Each homologue's centromere splits to form two chromosomes. D. Homologues exchange genetic material between non-sister chromatids.

E. Homologues interact with the spindle as if they were one chromosome.

18

The genus Lacerta is composed of a species of lizards that are female and do not mate. They undergo "endomitosis" where one extra chromosome replication results in a tetraploid cell before meiosis begins. Normal female 2n offspring result without fertilization. What change(s) from regular meiosis (in preparation for fertilization) would be required to produce this system? A. The haploid cell products of meiosis II fuse. B. Meiosis stops after meiosis I and does not proceed to meiosis II. C. Cytokinesis does not follow "endomitosis" that results in a tetraploid cell. D. During anaphase II, the sister centromeres fail to separate and daughter cells will not form.

E. During anaphase II, the daughter chromosomes are non-disjunctive and are all pulled to one daughter cell.

19

Interkinesis is different from interphase in which way? A. Interkinesis occurs after a cell finishes a nuclear division. B. Interkinesis is the stage that precedes a prophase stage. C. Interphase involves DNA replication and interkinesis does not.

D Interkinesis can be variable in length.

20

Which does NOT occur in meiosis? A. two daughter cells at completion B. four daughter cells at completion C. two nuclear divisions

D. formation of bivalents

21

Meiosis occurs during all of the following EXCEPT A. gametogenesis. B. oogenesis. C. pangenesis.

D. spermatogenesis.

22

The polar body is A. another name for an egg cell. B. a precursor cell that becomes an egg cell. C. a nonfunctional cell rudiment formed at the same time as an egg cell.

D. the cell produced when fertilization occurs.

23

Why do polar bodies form? A. They nurse the egg as it leaves the follicle. B. This is extra chromosomal material representing the X chromosome in each female cell. C. They orient the sperm toward the egg. D. They allow a reduction in chromosomes while preserving most of the cytoplasm for one egg.

E. They orient the egg for penetration by the sperm.

24

Which of the following is/are true about sexual reproduction? A. At the cellular level is the opposite of reproduction since it involves two cells fusing to become one. B. It requires the development of organs such as the uterus, which are of no immediate survival advantage to the individual but are advantageous to the species. C. It regularly produces a wider array of adaptations to the external environment in a shorter period of time. D. It regularly produces a wider array of adaptations to the internal environment (disease agents, parasites) in a shorter period of time.

E. All of the choices are true.

25

Species X reproduces asexually by fission and species Y reproduces sexually. Consider that all other relevant characteristics are similar between these species. When the environment changes, then A. species X and Y will have an equal chance of surviving. B. species Y should have a better chance of surviving than species X. C. species X should have a better chance of surviving than species Y.

D. neither species should have an advantage in surviving since organisms often become extinct when the environment changes.

26

Before the time of Gregor Mendel and genetics, sexual reproduction was thought to produce a blending or equal mixing of the parents' traits. Today we know that A. offspring will be identical to one another and demonstrate traits exactly halfway between the parents' traits. B. offspring can vary from receiving over 99% of one parent's genes to receiving over 99% of the other parent's genes. C. offspring inherit essentially 50% of their genes from each parent, but two sibling offspring may share with each other from zero to 23 chromosomes in common from each parent, and further variation may occur due to crossing-over. D. offspring inherit copies of the same 23 pair of chromosomes from each of their parents but the rate of crossing-over makes them very dissimilar.

E. there is one chance in 23 of getting identical sets of chromosomes from one parent, times two because there are two parents; therefore, two siblings out of every 46 are really identical except for crossing-over.

27

There is a species of desert lizard where only females are known to exist-there are no males known. It is nevertheless necessary for two females to court and for one to assume the posture of a male to stimulate the female to produce eggs. No fertilization can occur, and the eggs develop into female lizards. What is the probable evolutionary mechanism for this occurring? A. The species is probably going extinct. B. The desert is relatively uniform and there is little advantage to maintaining variation, but the animal has not been able to completely evolve away from its heritage of sexual reproduction. C. This switch in mating behavior is the direct physical reflection of crossing-over.

D. This is probably an asexual organism attempting to mimic sexual reproduction.

28

In human females, when is meiosis II completed? A. at ovulation B. immediately after the sperm penetration of the secondary oocyte C. immediately after the sperm penetrates the primary oocyte

D. None of the choices are correct.

29

Where in the human male does spermatogenesis occur? A. ovaries B. prostate gland C. epididymus

D. testes

30

Which is NOT true about daughter cells of mitosis or meiosis? A. In meiosis, daughter cells are haploid. B. In meiosis, there are four daughter cells. C. In mitosis, there are two daughter cells. D. In mitosis, the daughter cells are genetically identical.

E. In meiosis, the daughter cells are genetically identical.

31

All of the following are true concerning Down Syndrome EXCEPT A. it is caused by autosomal trisomy 21 B. in 23% of cases, the sperm contributes the extra chromosome C. it is the most common trisomy in humans

D. chances of a woman having a child with Down Syndrome decreases with her age

32

The correct number of chromosomes in a species is known as aneuploidy

33

A change in the chromosome number resulting from nondisjunction is called euploidy.

34

Monosomy occurs when an individual has only one of a particular type of chromosome.

35

Primary nondisjunction occurs during meiosis II when the sister chromatids fail to separate and both daughter chromosomes go in the same gamete.

36

Sex chromosome aneuploids are better tolerated and have a better chance of producing survivors.

37

The following picture depicts which of the following changes in chromosome structure. A. deletion B. duplication C. translocation

D. aneuploidy

38

Extra copies of sex chromosomes are more easily tolerated in humans than extra copies of autosomes.

39

The following picture depicts which of the following changes in chromosome structure. A. deletion B. duplication C. translocation

D. aneuploidy

40

Oogenesis always involves an equal division of cell contents in the formation of an egg and polar bodies.

41

A lattice holds the members of a bivalent together in such a way that the RNA of the non-sister chromatids is aligned.

42

It is estimated that an average of two or three cross-overs occur per human chromosome.

43

Aneuploidy is a change in the number of chromosomes resulting from nondisjunction during meiosis.

44

Only one of the four daughter cells becomes a functional gamete in spermatogenesis.

45

Jacobs Syndrome, XYY, results from nondisjunction during spermatogenesis.

46

Identify which event will occur during Prophase I of meiosis but does not occur during prophase of mitosis. A. bivalents will form B. nuclear envelope will dissolve C. centrioles will form in animal cells

D. chromatin will condense into chromosomes

47

Which of the following events occurs during prophase I but does not occur during prophase of mitosis? A. crossing over B. nuclear envelope dissolves C. chromatin condenses into chromosomes

D. centrioles appear in animal cells

48

During _______________ the homologous chromosome pairs separate in a random fashion leading to genetic diversity among the offspring. A. independent assortment B. metaphase C. anaphase II

D. mitosis

49

If the diploid number of chromosomes for an organism is 52, what will the haploid number of chromosomes be? A. 26 B. 48 C. 112

D. 24

50

Which of the following statements is correct about the chromosomal position during mitosis and meiosis? A. During metaphase of mitosis the duplicated chromosomes are at the metaphase plate while during metaphase I of meiosis the bivalents are present at the metaphase plate B. During metaphase I of meiosis the duplicated chromosomes are at the metaphase plate while during metaphase of mitosis the bivalents are present at the metaphase plate C. During metaphase of mitosis the homologues separate while during metaphase I of meiosis the sister chromatids separate

D. During metaphase I of meiosis the homologues separate while during metaphase of mitosis the sister chromatids separate

51

During which phase of mitosis and meiosis will the state of the chromosome be the same? A. Anaphase II and Anaphase B. Prophase II and Prophase C. Metaphase I and Metaphase

D. Telophase I and Telophase

52

Which of the following statements is true about the life cycle of animals? A. The gametes are the haploid phase of the animals life cycle. B. The gametes are the diploid phase of the life cycle. C. The products of the haploid phase of the life cycle are larger than those of the diploid phase.

D. Meiosis is not a necessary component of the animal life cycle.

53

Which statement is true about the life cycle of plants but not of animals? A. the gametophyte can be larger than the diploid generation B. they use sexual reproduction to produce new offspring C. they use meiosis in order to produce the gametes

D. they will have a haploid and diploid phase of the life cycle