12. Meiosis

Genetic Variation During Meiosis

Variation | Genetics |  Biology | FuseSchool

Animation: Linked Genes and Crossing Over

How many genetically unique gametes can be created in an organism with 4 chromosomes?a) 256.b) 23.c) 16.d) 1 million.e) 4.

Using the information from problem 4, scientists do a further testcross using a heterozygote for height and nose morphology. The offspring are tall upturned snout, 40; dwarf upturned snout, 9; dwarf downturned snout, 42; tall downturned snout, 9. Calculate the recombination frequency from these data, and then use your answer from problem 4 to determine the correct order of the three linked genes.

A fruit fly somatic cell contains 8 chromosomes. This means that ___________ different combinations of chromosomes are possible in its gametes.
a. 8
b. 16
c. 32
d. 64

Sources of genetic variation | Inheritance and variation | High school biology | Khan Academy

A wild-type fruit fly (heterozygous for gray body color and red eyes) is mated with a black fruit fly with purple eyes. The offspring are wild-type, 721; black purple, 751; gray purple, 49; black red, 45. What is the recombination frequency between these genes for body color and eye color? Using information from problem 3, what fruit flies (genotypes and phenotypes) would you mate to determine the order of the body color, wing size, and eye color genes on the chromosome?

Suppose you read in the newspaper that a genetic engineering laboratory has developed a procedure for fusing two gametes from the same person (two eggs or two sperm) to form a zygote. The article mentions that an early step in the procedure prevents crossing over from occurring during the formation of the gametes in the donor’s body. The researchers are in the process of determining the genetic makeup of one of their new zygotes. Which of the following predictions do you think they would make? Justify your choice, and explain why you rejected each of the other choices.
a. The zygote would have 46 chromosomes, all of which came from the gamete donor (its one parent), so the zygote would be genetically identical to the gamete donor.
b. The zygote could be genetically identical to the gamete donor, but it is much more likely that it would have an unpredictable mixture of chromosomes from the gamete donor’s parents.
c. The zygote would not be genetically identical to the gamete donor, but it would be genetically identical to one of the donor’s parents.
d. The zygote would not be genetically identical to the gamete donor, but it would be genetically identical to one of the donor’s grandparents.

During meiosis, if nondisjunction occurs during anaphase II (shown below in cell #3) what are the possible gametes that can be produced from cell #3?<img src="https://static.studychannel.pearsonprd.tech/courses/biology/thumbnails/25b5d114" class="fr-fic fr-dii">a) A only.b) B only.c) C only.d) A &amp; C.e) B &amp; C.f) A &amp; B.g) All three.

Would you describe the association between Pilobolus and cows as a symbiotic relationship? Explain your answer.

Explain the process of extracellular digestion that occurs during the growth of Pilobolus mycelia through dung.

MEIOSIS A-Level Biology - How CROSSING OVER and INDEPENDENT SEGREGATION introduce genetic variation

What is the physical basis for the independent assortment of alleles into offspring?
a. There are chromosome divisions during gamete production;
b. Homologous chromosome pairs are separated during gamete production;
c. Sperm and eggs are produced by different sexes;
d. Each gene codes for more than one protein;
e. The instruction manual for producing a human is incomplete.

Sketch a cell with three pairs of chromosomes undergoing meiosis, and show how nondisjunction can result in the production of gametes with extra or missing chromosomes.

Both asexual sporangia (shown in the photo) and zygosporangia can be found in cow dung. Make a simplified drawing that illustrates the events of plasmogamy, karyogamy, and meiosis that occur during sexual reproduction in Pilobolus. Be sure to indicate and label the zygosporangium.

________ cells have an extra or missing chromosomes after meiosis due to nondisjunction during Meiosis I.a) Somatic cells.b) Gametes.c) Diploid Cells.d) Aneuploid Cells.e) Haploid Cells.f) Sperm Cells.

No. The number of combinations possible when chromosomes assort independently into gametes during meiosis is 2n, where n is the haploid number of chromosomes.

Consider the rules of probability.

Ignoring crossover events, how many kinds of gametes can be produced by an organism with a diploid number of 8?

Nondisjunction

Animation: Origins of Genetic Variation 

During which of the following processes does independent assortment of chromosomes occur?a) In meiosis I only.b) In meiosis II only.c) In mitosis and meiosis I.d) In mitosis and meiosis II.e) In meiosis I and meiosis II.

Animation: Genetic Variation from Crossing Over

The transfer of DNA between two non-sister chromatids.

The transfer of DNA between two sister chromatids.

All of the above are involved in crossing over.

Crossing over involves each of the following EXCEPT:a) The transfer of DNA between two non-sister chromatids.b) The transfer of DNA between two sister chromatids.c) The formation of a synaptonemal complex.d) The alignment of homologous chromosomes.e) All of the above are involved in crossing over.

Linked Genes, Crossing Over and Genetic Recombination

No. A mutation is a change in the nucleotide sequence of DNA.

No. However, the recombining nonsister chromatids belong to homologous chromosomes.

No. These are the structures at centromeres where the spindle fibers attach to the chromosome.

No. However, crossing over occurs within tetrads, which consist of a pair of replicated homologous chromosomes.

What's the name of the Greek letter &Chi;?

Regions of chromosomes where nonsister chromatids cross over are called __________.

Animation: Genetic Variation from Independent Assortment of Chromosomes

All of the maternal chromosomes always move to one pole, and all the paternal chromosomes always move to the other pole.

No. Although the maternal and paternal chromosomes may segregate in this way, it represents only one of many possible outcomes.

No. This would fail to produce haploid gametes.

Exactly two maternal and two paternal chromosomes always move to each of the two poles.

The first to move influences all the others.

No. The first chromosomes to move do not influence the movement of the other chromosomes.

They align and assort independently to form any of 16 different combinations.

Consider independent assortment. Doing the math will help.

The diploid number of chromosomes in a certain animal is 8 (2n = 8). How do the four pairs of homologous chromosomes align and separate during meiosis?

The DNA in two nonsister chromatids is broken by specific proteins at exact points.

No. Crossing over involves the breakage and exchange of DNA between homologous pairs.

Chiasmata indicate where crossing over is occurring between homologs.

No. The presence of chiasmata provides evidence that crossing over is occurring between homologs.

Recombination leads to the formation of chromosomes containing paternal and maternal alleles.

No. Recombinant chromosomes such as these are the outcome of crossing over.

All of the listed responses pertain to the process of crossing over.

No. One of the answers is unrelated to crossing over.

Independent assortment results from the random alignment of homologous pairs of chromosomes at metaphase I.

Consider how recombinant chromosomes are formed.

Which of the following answers is neither involved with crossing over nor an outcome of crossing over?

No. During metaphase I, homologs align on the metaphase plate.

No. This phase is very similar to mitotic prophase.

No. During this phase chromosomes align on the metaphase plate.

Review the stages of meiosis.

Crossing over occurs during __________.

A wild-type fruit fly (heterozygous for gray body color and normal wings) is mated with a black fly with vestigial wings. The offspring have the following phenotypic distribution: wild-type, 778; black vestigial, 785; black normal, 158; gray vestigial, 162. What is the recombination frequency between these genes for body color and wing size? Is this consistent with the results of the experiment in Figure 15.9?

Assume that genes A and B are on the same chromosome and are 50 map units apart. An animal heterozygous at both loci is crossed with one that is homozygous recessive at both loci. What percentage of the offspring will show recombinant phenotypes resulting from crossovers? Without knowing these genes are on the same chromosome, how would you interpret the results of this cross?

What you think of as “a banana” is a Cavendish, one variety of the species Musa acuminate. It is a triploid organism (3n) with three sets of chromosomes in every somatic cell. The Cavendish cannot be naturally bred; it can only be reproduced by cloning. Explain how its triploid state accounts for its inability to form normal gametes. Discuss how the lack of sexual reproduction might make the species particularly vulnerable to a new pest.

Two genes of a flower, one controlling blue (B) versus white (b) petals and the other controlling round (R) versus oval (r) stamens, are linked and are 10 map units apart. You cross a homozygous blue oval plant with a homozygous white round plant. The resulting F1 progeny are crossed with homozygous white oval plants, and 1,000 offspring plants are obtained. How many plants of each of the four phenotypes do you expect?

Do not separate correctly from one another during anaphase.

Non-disjunction means that chromosomes:a) Do not cross over during synapsis.b) Do not replicate during interphase.c) Have serious mutations.d) Do not separate correctly from one another during anaphase.

Independent Assortment

Animation: Independent Assortment

No. The chance of any one paternal chromosome ending up in a particular gamete is 1/2, so the chance that all three would be of paternal origin would be (1/2)n, where n = 3.

The answer cannot be determined from these data.

In a diploid set of chromosomes, one member of each pair of homologous chromosomes is derived from the father (paternal), and the other comes from the mother (maternal). If 2n = 6, what is the probability that a particular gamete will contain only paternal chromosomes?

Two sister chromatids get tangled, resulting in one re-sequencing its DNA.

Two sister chromatids exchange identical pieces of DNA.

Maternal alleles are "corrected" to be like paternal alleles and vice versa.

Corresponding segments of non-sister chromatids from homologous chromosomes are exchanged.

Which of the following processes occurs when homologous chromosomes cross over in meiosis I?a) Two sister chromatids get tangled, resulting in one re-sequencing its DNA.b) Two sister chromatids exchange identical pieces of DNA.c) Maternal alleles are "corrected" to be like paternal alleles and vice versa.d) Corresponding segments of non-sister chromatids from homologous chromosomes are exchanged.

A planet is inhabited by creatures that reproduce with the same hereditary patterns seen in humans. Three phenotypic characters are height (T=tall, t=dwart), head appendages (A=antennae, a=no antennae), and nose morphology (S=upturned snout, s=downturned snout). Since the creatures are not “intelligent,” Earth scientists are able to do some controlled breeding experiments using various heterozygotes in testcrosses. For tall heterozygotes with antennae, the offspring are tall antennae, 46; dwarf antennae, 7; dwarf no antennae, 42; tall no antennae, 5. For heterozygotes with antennae and an upturned snout, the offspring are antennae upturned snout, 47; antennae downturned snout, 2; no antennae downturned snout, 48; no antennae upturned snout, 3. Calculate the recombination frequencies for both experiments.

Some fungi have elaborate mechanisms for dispersing spores. For example, the “squirt gun” fungus Pilobolus, which grows in cow dung, forcibly shoots off tiny black sporangia filled with spores. Any sporangia that are flung onto fresh grass are likely to be eaten by a grazing cow, passed through its digestive system unharmed, and deposited in a new batch of dung. The spores carried within the sporangia are perfectly positioned to grow into a new mycelium.

Pilobolus is a zygomycete, and the sporangia just described produce asexual spores. In contrast, which of the following best describes what happens during the sexual phase of its life cycle?
a. It produces eight ascospores in each ascus.
b. It produces basidiospores that grow into mushrooms.
c. It produces motile sperm and eggs.
d. Hyphae from two compatible mating types fuse and form a zygosporangium.

Animation: Genetic Variation from Random Fertilization

Briefly describe how three different processes that occur during a sexual life cycle increase the genetic diversity of offspring.

it is the only mechanism for species to reproduce

it provides a way in which somatic mutations can be inherited

No. Sex does not allow for the inheritance of somatic mutations.

it is the only mechanism for species to reproduce and it provides a method to increase genetic variation

it provides a method to increase genetic variation and it provides a way in which somatic mutations can be inherited

it provides a method to increase genetic variation

Consider what is required for evolution to occur.

The major contribution of sex to evolution is that __________.

Animation: The Chromosomal Basis of Independent Assortment

Why are individuals with an extra chromosome 21, which causes Down syndrome, more numerous than individuals with an extra chromosome 3 or chromosome 16?
a. There are probably more genes on chromosome 21 than on the others.
b. Chromosome 21 is a sex chromosome and chromosomes 3 and 16 are not.
c. Down syndrome is not more common, just more serious.
d. Extra copies of the other chromosomes are probably fatal.

The random way each pair of homologous chromosomes lines up at the metaphase plate.

The random combinations of eggs and sperm during fertilization.

The random distribution of the sister chromatids into the two daughter cells.

The diverse combination of alleles that may be found within any given chromosome.

Independent assortment of chromosomes is a result of which of the following processes?a) The random way each pair of homologous chromosomes lines up at the metaphase plate.b) The random combinations of eggs and sperm during fertilization.c) The random distribution of the sister chromatids into the two daughter cells.d) The diverse combination of alleles that may be found within any given chromosome.

You design Drosophila crosses to provide recombination data for gene a, which is located on the chromosome shown in Figure 15.12. Gene a has recombination frequencies of 14% with the vestigial wing locus and 26% with the brown eye locus. Approximately where is a located along the chromosome?

Crossing over, internal fertilization, independent assortment

No. Internal fertilization does not contribute to genetic variation. It is simply one way of getting gametes close to each other.

Independent assortment, spindle formation, random fertilization

No. Spindle formation occurs in mitosis as well as meiosis, so it is not necessarily involved in increasing genetic variation.

Random fertilization, DNA synthesis, independent assortment

No. DNA synthesis occurs before both mitotic and meiotic division and does not directly increase genetic variation.

Internal fertilization, spindle formation, crossing over

No. Internal fertilization and spindle formation do not increase genetic variation.

Random fertilization, independent assortment, crossing over

How does each of these affect chromosome number or complexity? All three have to be correct.

Which of the following contributes to genetic variation in sexually reproducing species?

Genetic Variation During Meiosis Example 1

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