Chapter 19.1 - Sexual Reproduction and Genetics BIO 1703

What distinguishes a haploid cell from a diploid cell?

Haploid cells contain one complete set of chromosomes, whereas diploid cells have two sets – one inherited from each parent.

How are most human cells characterized in terms of chromosome number, and why?

Most human cells are diploid, meaning they contain two copies of each of the 23 chromosomes, with one copy coming from each parent.

What are gametes and what is their chromosomal status?

Gametes are the sex cells (eggs and sperm) produced by germ cells using meiosis, and they are haploid – containing only one copy of each chromosome.

What is the primary purpose of meiosis in sexual reproduction?

Meiosis produces haploid gametes from diploid germ-line cells and introduces genetic variation through processes like random segregation and crossing over.

How does mitosis differ from meiosis in terms of cell division and outcome?

Mitosis results in two genetically identical diploid cells for growth or repair, while meiosis involves two rounds of division (Meiosis I and II) to yield four genetically unique haploid cells that form gametes.

What is the difference between somatic cells and germ cells?

Somatic cells make up the body’s tissues and do not participate in sexual reproduction (they are diploid), whereas germ cells are found in the ovaries or testes and are dedicated to producing haploid gametes for reproduction.

What critical event takes place during the meiotic S phase?

During the meiotic S phase, all chromosomes are duplicated, creating two identical sister chromatids per chromosome in preparation for the subsequent divisions.

Why is chromosome duplication necessary before cell division?

Duplication ensures that each resulting cell—whether from mitosis or meiosis—receives a full set of genetic material needed for proper cell function.

What is crossing over, and at which stage of meiosis does it occur?

Crossing over is the process in which non-sister chromatids of homologous chromosomes exchange segments of DNA; it occurs during prophase I of meiosis, resulting in new genetic combinations.

What is a bivalent, and what is its importance during meiosis?

A bivalent is a paired set of homologous chromosomes (each with two sister chromatids) that come together during prophase I; it is essential for facilitating crossing over and proper chromosome segregation.

What are chiasmata and why are they significant?

Chiasmata are the physical sites where crossing over occurs between homologous chromosomes; they hold the bivalents together until the chromosomes are properly aligned and segregated during meiosis I.

How does random segregation of chromosomes contribute to genetic diversity?

Random segregation ensures that each gamete receives a random assortment of maternal and paternal chromosomes, which means that the possible chromosome combinations in offspring are extremely numerous, enhancing genetic diversity.

What is nondisjunction, and what potential outcomes can it cause?

Nondisjunction is the failure of chromosome pairs or sister chromatids to separate properly during meiosis, which can lead to gametes with too many or too few chromosomes—conditions that may result in disorders such as trisomy (e.g., Down syndrome) or monosomy.

How do the events in Meiosis I differ from those in Meiosis II?

In Meiosis I, homologous chromosomes are separated (reducing the cell from diploid to haploid with duplicated chromosomes), whereas in Meiosis II, the sister chromatids separate (similar to mitosis), resulting in four distinct haploid cells.

How does crossing over enhance the genetic diversity of gametes?

Crossing over shuffles genetic material between paired homologous chromosomes, creating new combinations of alleles that increase the genetic variability among gametes beyond what random segregation alone could achieve.

In what ways do random segregation and crossing over together ensure that siblings are genetically unique?

Random segregation assigns different combinations of maternal and paternal chromosomes to each gamete, and crossing over further mixes the genetic material within those chromosomes; as a result, even siblings with the same parents inherit a unique blend of alleles.

What are alleles, and what role do they play in heredity?

Alleles are different versions of a gene that can result in variation of a trait; the combination of alleles inherited from each parent determines the characteristics of an individual.

Why is sexual reproduction advantageous for populations facing changing environments?

Sexual reproduction creates genetic diversity through random chromosome segregation and crossing over, providing a population with a variety of traits that may be beneficial for adapting to environmental changes.

Who was Gregor Mendel, and why is his work significant in genetics?

Gregor Mendel was a scientist who conducted experiments with pea plants, leading to the discovery of the fundamental laws of inheritance that form the basis of modern genetics.

What did Mendel discover about dominant and recessive traits in pea plants?

Mendel observed that when true-breeding plants with different traits are crossed, only the dominant trait appears in the first generation (F1), while the recessive trait reappears in the second generation (F2), revealing a clear pattern of inheritance.

Define the terms “homozygous” and “heterozygous.”

An organism is homozygous if it has two identical alleles for a gene, and heterozygous if it has two different alleles for that gene.

What inheritance ratio did Mendel observe in the F2 generation, and what did it indicate?

Mendel observed a 3:1 ratio of dominant to recessive traits in the F2 generation, indicating that traits segregate in a predictable manner during gamete formation.

How do Mendel’s pea plant experiments illustrate the concept of true-breeding?

True-breeding plants consistently produce offspring with the same traits when self-fertilized, a phenomenon Mendel used to show that certain traits are reliably inherited, enabling clear examination of dominant and recessive patterns.

Why is the idea of “true-breeding” important in genetics research?

True-breeding individuals reliably pass on specific traits across generations, which makes it easier to predict inheritance patterns and understand the behavior of different alleles.

How is the heritability of traits ensured during gamete formation?

Heritability is ensured through meiosis because random segregation and crossing over produce gametes with unique combinations of alleles; when these fuse during fertilization, they determine the traits of the offspring.

According to the document, what is notable about nondisjunction in human oocytes?

The document reports that nondisjunction occurs frequently—about 10% of the time—in human oocytes, making errors in chromosome separation a relatively common event in female gamete formation.

How does generating a wide variety of gametes benefit a species?

A diverse genetic pool, achieved through mechanisms like random segregation and crossing over, provides a species with the ability to adapt to environmental challenges and improves overall survival.

What is a zygote and how is it formed?

A zygote is the fertilized diploid cell formed by the union of two haploid gametes (egg and sperm), combining genetic material from each parent to create a new organism.

Summarize the key processes that lead to the genetic uniqueness observed among individuals.

Genetic uniqueness arises from two main processes during meiosis: random segregation, which randomly distributes maternal and paternal chromosomes into gametes, and crossing over, which shuffles genetic material between homologous chromosomes. Together, these processes ensure that every zygote formed by fertilization has a unique combination of alleles.

Using random segregation alone, how many unique gamete combinations can a human produce, and why?

A human can produce 2²³ (approximately 8,388,608) unique gamete combinations because each of the 23 homologous chromosome pairs segregates independently during meiosis.

What do the designations “N”, “2N”, and “4N” indicate in the context of cell division?

“N” represents a haploid state (one copy of each chromosome), “2N” indicates a diploid state (two copies of each chromosome), and “4N” refers to the state immediately after DNA replication when each chromosome has duplicated, resulting in four copies present in the cell.

What role do recombination proteins play during meiotic prophase I?

Recombination proteins generate double-strand breaks, process the DNA ends, and facilitate the exchange of segments between non-sister chromatids; this initiates crossing over, which shuffles genetic material between homologous chromosomes.

Why can prophase I of meiosis last several days?

Prophase I is extended because it involves multiple complex processes such as homologous chromosome pairing, synapsis, and crossing over, all of which require ample time to ensure accurate alignment and genetic recombination.

What are cohesins, and how do they contribute to meiosis?

Cohesins are protein complexes that hold sister chromatids together, maintaining chromosome structure and ensuring proper alignment and segregation during both phases of meiosis.

How does nondisjunction in meiosis I differ from that in meiosis II in terms of impact on the resulting gametes?

In meiosis I, nondisjunction occurs when homologous chromosomes fail to separate, affecting all resulting gametes from that division; in meiosis II, the error happens as sister chromatids fail to separate, impacting only half of the gametes from the affected cell.

What are the potential outcomes of gametes that experience nondisjunction, and how might these outcomes affect the zygote formed after fertilization?

Gametes from nondisjunction may contain an extra chromosome (leading to trisomy) or be missing one (leading to monosomy); fertilization of such gametes can result in genetic disorders (like Down’s syndrome) or developmental lethality.

How does crossing over break genetic linkage between genes located near each other on a chromosome?

By exchanging segments between non-sister chromatids during crossing over, physically linked genes can be separated, allowing them to assort independently in gametes rather than being inherited together.

In what ways do the stages of meiosis (prophase, metaphase, anaphase, and telophase) work together to ensure accurate distribution of genetic material?

Prophase I allows homolog pairing and crossing over, metaphase I aligns the paired homologs, anaphase I separates them into different cells, and telophase (with subsequent meiosis II) completes the division into haploid cells, ensuring proper chromosome number and unique genetic combinations.

How does crossing over during meiosis influence the inheritance patterns of genes that are close together?

Crossing over can separate genes that are physically close on the same chromosome, reducing linkage and allowing them to be inherited independently, which increases the potential for new allele combinations in the offspring.