Chapter 6:

What are the two major types of cells in the human body?

The two major cell types are somatic cells (body cells), which make up most of the body's tissues and organs, and germ cells found in the reproductive organs that develop into gametes (egg and sperm cells).

How do somatic cells differ from germ cells in terms of genetics?

Somatic cells contain DNA that is not passed on to offspring. Germ cells develop into gametes, and the DNA in these cells can be passed on to children.

What are gametes, and why are they important?

Gametes are sex cells (egg in females and sperm in males). They are crucial for sexual reproduction, as they carry genetic information from each parent to offspring.

How many chromosomes are found in human body cells?

Human body cells each contain 46 chromosomes, which are organized into 23 pairs.

What are homologous chromosomes?

Homologous chromosomes are pairs of chromosomes (one from each parent) that have the same length and general appearance, and carry genes for the same traits, though the genes may differ.

Are homologous chromosomes identical to each other?

No, they are not identical. While homologous chromosomes have the same genes in the same order, each chromosome in the pair can have different versions (alleles) of those genes.

What are autosomes?

Autosomes are chromosomes that contain genes not directly related to the sex of an organism. In humans, chromosome pairs 1 through 22 are autosomes.

What determines an organism’s sex chromosomes?

In humans, the 23rd pair of chromosomes determines sex. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

What does diploid mean, and which cells in humans are diploid?

Diploid (2n) cells have two copies of each chromosome. All human body (somatic) cells are diploid, containing 46 chromosomes in total (23 pairs).

What are haploid cells, and which cells in humans are haploid?

Haploid (n) cells have only one copy of each chromosome. Human gametes (egg and sperm cells) are haploid, each containing 23 chromosomes.

Why must gametes be haploid cells?

Gametes must be haploid so that when they fuse during fertilization, they create a diploid zygote with the correct total number of chromosomes (46 in humans).

What is fertilization?

Fertilization is the process where two haploid gametes (egg and sperm) fuse, forming a diploid zygote with genetic material from both parents.

What is meiosis, and why is it important?

Meiosis is a special type of cell division that reduces the chromosome number by half, creating four haploid cells (gametes) from one diploid cell. This process is crucial for sexual reproduction and ensures genetic diversity in offspring.

How do mitosis and meiosis differ in purpose and outcome?

Mitosis creates genetically identical diploid body cells for growth and repair, while meiosis produces genetically diverse haploid gametes for sexual reproduction.

How many rounds of cell division occur during meiosis, and what are they called?

Meiosis involves two rounds of cell division: meiosis I and meiosis II. Each round has four phases—prophase, metaphase, anaphase, and telophase.

What is the difference between homologous chromosomes and sister chromatids?

Homologous chromosomes are a pair of chromosomes (one from each parent) that have the same length and genes but may have different alleles. Sister chromatids are identical copies of the same chromosome made during DNA replication and joined at the centromere.

Prophase I

Homologous chromosomes pair up, and crossing over can occur.

Metaphase I

Paired homologous chromosomes line up along the equator of the cell.

Anaphase I

Homologous chromosome pairs separate and move to opposite ends of the cell.

Telophase I

The cell divides into two haploid cells, each with duplicated chromosomes.

Prophase II

The nuclear membrane breaks down, and spindle fibers form in the two haploid cells.

Metaphase II

Chromosomes (made of sister chromatids) align at the cell equator.

Anaphase II

Sister chromatids separate and move to opposite sides of the cell.

Telophase II

Each of the two haploid cells divides, resulting in four haploid cells with unduplicated chromosomes.

What are the key differences between meiosis I and meiosis II?

• Meiosis I separates homologous chromosome pairs, resulting in two haploid cells with duplicated chromosomes.

• Meiosis II separates sister chromatids, producing four haploid cells with unduplicated chromosomes.

How does meiosis create genetic diversity?

Meiosis increases genetic diversity through:

1. Independent assortment: The random lining up and separation of homologous chromosomes during metaphase I.

2. Crossing over: The exchange of chromosome segments between homologous chromosomes during prophase I.

What is the difference in chromosome number between cells produced by mitosis and meiosis?

Mitosis produces two genetically identical diploid cells, each with the full set of chromosomes (46 in humans). Meiosis produces four genetically unique haploid cells, each with half the number of chromosomes (23 in humans).

What is gametogenesis, and how does it differ in males and females?

Gametogenesis is the process of forming mature gametes (egg and sperm). In males, it produces four sperm cells. In females, it produces one egg and three polar bodies, ensuring the egg has the nutrients and organelles needed for early development.

Why do sperm cells differ from egg cells in structure?

Sperm cells are small and motile, designed primarily to deliver DNA to the egg. They lose most of their cytoplasm and develop a tail for movement. Egg cells are larger, containing cytoplasm, organelles, and nutrients necessary to support early embryo development.

What are polar bodies, and what role do they play in egg formation?

Polar bodies are small cells produced during female meiosis that contain little more than DNA. They eventually break down. They ensure that the egg retains most of the cytoplasm and organelles to support the embryo's early development.

How do the outcomes of meiosis and mitosis differ in terms of cell type and genetic variability?

• Mitosis produces two genetically identical diploid somatic cells for growth and repair.

• Meiosis produces four genetically diverse haploid gametes for sexual reproduction, ensuring genetic variation in offspring.

Who is Gregor Mendel, and why is he significant in genetics?

Gregor Mendel was an Austrian monk considered the "father of genetics." In the mid-1800s, he conducted experiments with pea plants and discovered fundamental laws of inheritance, laying the groundwork for modern genetics.

What are traits, and how did Mendel study them?

Traits are distinguishing characteristics that are inherited, such as flower color or seed shape. Mendel studied simple, "either-or" traits in pea plants, which allowed him to observe clear patterns of inheritance.

What does Mendel’s “law of segregation” state?

Mendel’s Law of Segregation states that organisms inherit two copies of each gene (one from each parent) and that these copies segregate or separate during gamete formation, ensuring offspring receive only one copy from each parent.

How did Mendel’s findings about inheritance challenge the idea that traits are blended?

Mendel’s findings showed that traits are inherited as discrete units (genes), and traits can be hidden in one generation and reappear in another, contradicting the blending idea where traits would become uniform.

What conclusion did Mendel draw about how traits are passed on?

Mendel concluded that each parent passes on one allele (form of a gene) for every trait, and these alleles separate independently during gamete formation, leading to predictable inheritance patterns in offspring.

Why is Mendel considered the father of genetics?

Mendel established fundamental principles of inheritance, such as genes being discrete units and the law of segregation, which form the foundation of genetics despite his work preceding the discovery of chromosomes and DNA.

What is a gene, and what does it do?

A gene is a piece of DNA that provides instructions to a cell to make a specific protein. These proteins then contribute to a particular characteristic, or trait, in an organism.

What are alleles?

Alleles are different forms of the same gene. Each individual has two alleles for each gene, one inherited from each parent, which can be identical or different.

What is the difference between homozygous and heterozygous alleles?

• Homozygous alleles: Both alleles at a specific gene locus are the same (e.g., AA or aa).

• Heterozygous alleles: The two alleles at a specific gene locus are different (e.g., Aa).

What is a genome?

A genome is all of the genetic material in an organism. It includes all of the genes and can influence a wide range of traits in that organism.

Genotype

The genetic makeup of an organism, consisting of the specific alleles it carries (e.g., Aa, BB).

Phenotype

The physical characteristics or traits of an organism that are expressed (e.g., purple flowers, round seeds).

What is the difference between a dominant and a recessive allele?

• Dominant allele: An allele that is expressed when at least one copy is present. It masks the expression of a recessive allele.

• Recessive allele: An allele that is only expressed when two copies are present (no dominant allele is present).

How do alleles influence the phenotype of an organism?

The combination of alleles (genotype) determines which proteins are produced. Dominant alleles will show in the phenotype even if only one is present, while recessive alleles only show if the organism has two copies of that allele.

What does it mean if an organism is homozygous dominant, homozygous recessive, or heterozygous for a trait?

• Homozygous dominant: Two dominant alleles (e.g., AA).

• Homozygous recessive: Two recessive alleles (e.g., aa).

• Heterozygous: One dominant allele and one recessive allele (e.g., Aa).

Does a dominant allele mean it is the most common allele in a population?

Not necessarily. A dominant allele simply means it is expressed over a recessive allele. It does not indicate its frequency in a population. Some recessive alleles can be more common than dominant ones.

How do environmental factors influence gene expression and traits?

While genes provide the blueprint for traits, environmental factors such as temperature, nutrition, and sunlight can affect how genes are expressed, potentially altering an organism's phenotype.

What is the significance of Mendel’s law of segregation in genetics?

Mendel’s law of segregation states that organisms inherit two copies of each gene, and these copies separate during gamete formation. This explains how offspring can receive different allele combinations from their parents, contributing to genetic variation.

What is a Punnett square, and what is its purpose?

A Punnett square is a grid system used to predict all possible genotypes (genetic combinations) of offspring resulting from a cross between two parent organisms.

How are the axes of a Punnett square labeled, and what do they represent?

The axes represent the possible gamete genotypes of each parent. Each row and column corresponds to a parent's allele combinations for the gene(s) being examined.

What do the boxes inside a Punnett square represent?

Each box inside the Punnett square shows the possible genotype of an offspring resulting from the combination of one allele from each parent.

How do segregation and fertilization contribute to the predictions made by a Punnett square?

Segregation ensures each gamete carries only one allele for each gene, while fertilization randomly combines gametes. The Punnett square accounts for these events by showing all possible allele combinations for the offspring.

What is a monohybrid cross?

A monohybrid cross is a genetic cross that examines the inheritance of one specific trait (gene) between two organisms.

What is a testcross, and why is it used?

A testcross is a cross between an organism with an unknown genotype and an organism with a recessive phenotype. It helps determine the unknown genotype based on the appearance of offspring phenotypes.

What is a dihybrid cross?

A dihybrid cross is a genetic cross that examines the inheritance of two different traits simultaneously (e.g., seed color and seed shape in pea plants).

What is the law of independent assortment, and how does it relate to dihybrid crosses?

Mendel’s law of independent assortment states that allele pairs separate independently during gamete formation. In dihybrid crosses, this law explains how the inheritance of one trait generally does not affect the inheritance of another trait.

How does probability apply to genetics and Punnett squares?

Probability is used to predict the likelihood of inheriting certain genotypes or phenotypes. A Punnett square uses probability to show how likely each genetic combination is to occur in offspring.

How does sexual reproduction contribute to genetic variation?

Sexual reproduction generates genetic variation by combining genetic material from two parents. This variation results largely from independent assortment of chromosomes during meiosis and the random fertilization of gametes.

What is independent assortment, and how does it increase genetic diversity?

Independent assortment refers to the random distribution of homologous chromosomes during meiosis I. Each of the 23 pairs of human chromosomes can line up in 8 million different ways, leading to a huge number of possible genetic combinations in gametes.

How does random fertilization add to genetic variation?

Random fertilization means any one of the possible gametes from the father can fertilize any one of the possible gametes from the mother. In humans, this results in about 70 trillion genetically unique offspring combinations, vastly increasing genetic diversity.

What is crossing over, and when does it occur?

Crossing over is the exchange of chromosome segments between homologous chromosomes during prophase I of meiosis. This process increases genetic diversity by creating new combinations of alleles.

What is recombination in genetics?

Recombination refers to any mixing of parental alleles, including events such as crossing over during meiosis. It results in offspring with different genetic combinations than their parents.

What is genetic linkage?

Genetic linkage occurs when genes are located close together on the same chromosome and are inherited together. Because they are physically linked, these genes tend to be passed along to offspring as a group.

Summarize how independent assortment, random fertilization, and crossing over contribute to genetic diversity.

• Independent assortment randomizes allele distribution as homologous chromosomes separate.

• Random fertilization combines genetically unique sperm and egg, producing a random fusion of alleles.

• Crossing over swaps segments of DNA between homologous chromosomes, creating new allele combinations. All three processes result in increased genetic variation among offspring.