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[h] Meiosis, Sex Determination, and Chromosomal Variation Flashcards.

[q] In the context of meiosis and inherited chromosomes, what does “homologous” mean? What does the information shown on the left say about the genetic information we inherit from our parents? Use the term “allele” in your answer.

[a]

In the context of meiosis (and genetics) “homologous” means “same kind of information.” We inherit chromosomes from our mothers and fathers with the same type of information (genes for proteins and RNAs), but the exact genetic information might differ. In other words, the genes are the same, but the alleles might be different.

[q] Use this diagram to define and explain such terms as somatic cell, germ cell, gamete, zygote, and fertilization.

[a] In this diagram, germ-line cells (A) in testes and ovaries (D and E) go through meiosis (F) to create haploid sperm and egg cells (G and H). Fertilization (I) creates a diploid zygote (J) which develops into the various somatic cells of the body (C).

[q] Use this diagram to explain the human sexual life cycle.

[a] Adult females and males (1a and 1b) have diploid germ-line cells in their ovaries and testes (2a and 2b). These germline cells undergo meiosis (3) to create haploid eggs and sperm (4a and 4b). Fertilization (5) restores the diploid number in a fertilized egg or zygote (6), which subsequently divides and develops (7) to create the next generation of adults.

[q] Using the diagram below, explain (on a very big picture level) the difference between meiosis 1 and meiosis 2.

[a] Meiosis starts with a round of DNA replication (1). In meiosis 1 (“2” in the diagram), homologous pairs are pulled apart, creating two haploid cells with doubled chromosomes. In meiosis 2 (at “3”) the sister chromatids are pulled apart, creating 4 haploid daughter cells, each with single chromosomes.

[q] Using the diagram below, explain (on a big-picture level) how mitosis and meiosis differ.

[a] In mitosis (image 1), a parent cell divides once to create two daughter cells, each of which is a clone of the parent. The number of chromosomes in the parent cell and daughter cells is the same.

In meiosis (image 2), a diploid germ cell (with two sets of homologous chromosomes) divides twice to create four genetically unique haploid daughter cells. In the daughter cells, because of crossing over, the chromosomes are recombinant. In addition, because of independent assortment, approximately half of the chromosomes are maternal in origin, and approximately half are paternal.

[q] Meiosis can be thought of as creating genetic variation along two axes. Explain the vertical and horizontal variation created by meiosis.

[a] Meiosis creates variation between siblings. It’s the reason why siblings, despite having the same parents, are genetically distinct from each other. That’s the horizontal dimension.

Meiosis also creates variation between the generations. It’s the reason why each generation of offspring is genetically different from the parent generation. That’s the vertical dimension.

[q] What are the two processes by which meiosis creates variation? What process follows meiosis that creates additional variation?

Note that your job in this card is to list these processes: you don’t have to explain them in depth.

[a] Meiosis creates variation through two processes.

1. Independent assortment
2. Crossing over between homologous chromosomes.

Following meiosis, fertilization creates additional variation by combining chromosomes from two different parents into one genetically unique zygote.

[q] How does independent assortment during gamete creation generate variation?

[a] During gamete creation, a diploid germ cell has to reduce its chromosome number by half to create a haploid gamete. What that means is that for each homologous chromosome pair, only one member of the pair will be sent to the haploid gamete. For each chromosome pair, the process is random and based on the orientation of the homologs as they are lined up by the spindle on the cell equator during metaphase 1. This process occurs independently in each pair.

Thus, in a cell where the number of homologous pairs = “n,” the number of possible gametes that can be made is 2ⁿ. In fruit flies, with four homologous pairs, that means that there are 2⁴, or 16 possible chromosomal arrangments in the gametes. In humans, with 23 homologous pairs, that means 2²³, or over 8 million possible chromosomal arrangments in the gametes.

[q] How does crossing over during gamete-creation generate variation?

[a] During crossing over, homologous pairs (1) form a tetrad (2). Within the tetrad, homologous chromosomes exchange pieces of DNA. That means that chromosomes that were originally maternal or paternal in origin recombine to become unique, never-before-seen recombinant sequences of DNA (3) that get passed to the offspring.

[q] Using the diagram below, explain what happens during meiosis 1.

[a] Meiosis 1 begins with interphase (1), during which DNA is replicated. During prophase 1 (2) the chromosomes condense. Homologous pairs pair up and, through crossing over, recombine. During metaphase 1 (3) homologous pairs are moved by the spindles so that they line up in the cell equator. Each pair’s orientation is random and independent, which is the basis of independent assortment. In anaphase 1 (4) homologs are pulled apart.

What follows varies by species, but the important thing is cytokinesis 1, which creates two haploid daughter cells, each with doubled chromosomes.

[q] Using the diagram below, explain what happens during meiosis 2.

[a] The main thing that happens during Meiosis 2 is the separation of sister chromatids. The sister chromatids are brought to the cell equator during metaphase 2. Note that this is another random process, and creates additional variation among the gametes.

During anaphase 2 (image 10) doubled chromosomes are pulled apart, separating the sister chromatids. This is followed by Telophase 2 and cytokinesis (combined into one image: 11/12).  The result is four unique haploid gametes (13) with single chromosomes.

[q]Explain how sex is determined in humans and other mammals.

[a]Humans and all other mammals, have an XX/XY sex-determination system. In males, the sex chromosomes are the X and Y chromosomes. These accompany autosomes, the number of which varies in different mammalian species. In females, the autosomes are accompanied by two X chromosomes.

During meiosis in a male, the X and Y chromosomes are brought together and separate. After their separation, half of the haploid male gametes have an X chromosome, and half have a Y chromosome. In females, where there are two X chromosomes, all the haploid gametes have an X chromosome.

If a Y-bearing sperm fertilizes the egg, the offspring will be an XY male. If an X-bearing sperm fertilizes the egg, the offspring will be an XX female.

[q]How does temperature-dependent sex determination work, and where is it found?

[a]In temperature-dependent sex determination the temperature at which the eggs develop determines the sex of the offspring. This system is found in certain reptiles which lay eggs in the soil or sand. The eggs closest to the surface experience warmer temperatures, while those deeper in the nest experience cooler temperatures. In sea turtles, temperatures above a pivot temperature result in male offspring. In tuataras, high temperatures lead to female offspring. In a third variation found in crocodiles and alligators, moderate temperatures result in males, while temperatures significantly above and below the pivot point result in females.

Note that memorizing which reptile uses which variation is not important: just the general principle.

[q]Explain how sex is determined in birds.

[a]Birds have a ZZ/ZW sex-determination system. In females, the sex chromosomes are the Z and W chromosomes. These accompany the autosomes, the number of which varies in different bird species. In males, the autosomes are accompanied by two Z chromosomes.

During meiosis in a female, the Z and W chromosomes are brought together and separate. After their separation, half of the haploid female gametes have a Z chromosome, and half have an X chromosome. In males, where there are two Z chromosomes, all the haploid gametes have a Z chromosome.

If a W-bearing egg is fertilized, the offspring will be a ZW female. If a Z-bearing egg is fertilized, the offspring will be a ZZ male.

[q]Define monosomy and trisomy, and describe two ways that these conditions can occur.

[a]A monosomy is when a diploid organism, in one of its chromosome pairs, has only one chromosome instead of a homologous pair. A trisomy is when, instead of a homologous pair, there are three chromosomes.

Monosomies and trisomies can be caused by nondisjunction during meiosis. Nondisjunction occurs when homologous pairs (b) or sister chromatids (h) fail to separate, resulting in gametes with extra chromosomes (e and j), or with missing chromosomes (f and k).

[q]In 95% of babies born with Down Syndrome, the trisomy can be traced to their mother’s egg. Explain.

[a]

1. A woman’s germ cells 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 (during prophase 1)
3. Meiosis resumes during a woman’s ovulatory cycle. As a result, a 25-year-old woman is creating haploid eggs from a germ cell that’s been paused in prophase 1 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, resulting in the extra chromosome 21 that leads to Down Syndrome.

[q]Explain how sex is determined in bees, ants, and wasps.

[a]Bees, ants, and wasps use a sex-determination system called haplodiploidy. In this system, the males are haploid and develop from unfertilized eggs. Fertilized eggs develop into female workers, or if fed with a secretion called Royal Jelly, into Queens. In honeybees, the queen is the sole reproductive female (but this varies in other haplodiploid species).

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