Genetics Key Terms and Concepts

Introduction to Variation, Pattern, and Process

• Unit 2 focuses on variation, pattern, and process, introducing key genetic concepts.

• Aims to build a deep understanding of fundamental concepts, minimizing memorization and focusing on connections between terms and concepts.

The Language of Genetics

• Reviews key terms in genetics to understand the role of genetics in explaining natural phenomena.

• Addresses common misconceptions about these terms.

Explaining Variation Among Living Organisms

• Biologists are interested in the diversity of biological form within and between species.

• Unit two focuses on downward explanations, which aim to answer the question, how does something work?

Learning Objectives

• Define: DNA, chromosome (copies vs. pairs), diploid, nucleotide, gene, allele, homozygous, heterozygous.

• Describe the relationship between a chromosome and DNA.

• Describe what a gene is in relation to a chromosome and DNA.

• Describe the relationship between a gene and an allele.

• Explain why all individuals have the same genes but different alleles.

Cells and Genomes

• All organisms have cells; large organisms have trillions.

• Inside each cell is the genome, consisting of DNA.

DNA and Chromosomes

• DNA carries genetic information for organism development and functioning.

• In most organisms, DNA exists as multiple subunits called chromosomes.

• Each chromosome contains a single, very long DNA molecule packaged with histone proteins.

• DNA winds around histone proteins for tight packing.

• The chromosome is one molecule of DNA in a condensed form.

Nucleotide Sequence

• DNA contains information in the form of its nucleotide sequence.

• Nucleotides: adenine (A), cytosine (C), guanine (G), and thymine (T).

• One DNA molecule has two strands connected by hydrogen bonding between paired bases: A binds with T, and C binds with G.

DNA Molecule Representation

• One DNA molecule consists of two strands bound to each other through hydrogen bonds in the base pairs.

Chromosome Appearance

• Chromosomes don't always look the same throughout the cell's life.

• Sometimes look like a chromosome, sometimes like an X.

• The X shape indicates the chromosome has been duplicated in preparation for cell division.

• After cell division, DNA returns to its unduplicated state.

Unduplicated vs. Duplicated Chromosomes

• An unduplicated chromosome contains one DNA molecule.

• A duplicated chromosome contains two DNA molecules, which are copies of each other.

• DNA replication is the process of duplicating one original chromosome to make an additional chromosome.

• The two duplicates forming the X shape are sister chromatids and are virtually identical in DNA sequence.

• Chromosome copies are connected by a centromere, which can be located anywhere along the chromosome.

• If the centromere is at the end, the duplicated chromosome looks like a V.

Summary

• All organisms have cells containing chromosomes.

• Chromosomes contain DNA.

• DNA is made of nucleotides.

• DNA and nucleotide sequences are information.

Definitions

• DNA: Molecule carrying genetic information for organism development and functioning (deoxyribonucleic acid).

• Chromosome: Subunit of the genome in the nucleus, consisting of a DNA molecule and proteins.

• Duplicated chromosomes: Two connected, duplicated DNA subunits in the nucleus that separate during cell division, often looking like an X or V.

• Nucleotides: Small molecules comprising the DNA sequence (adenine, cytosine, guanine, thymine).

Chromosome Pairs and Diploids

• In many organisms, including humans, chromosomes exist in pairs.

• Organisms with two of each chromosome are called diploids.

• Humans have 23 pairs of chromosomes in every body cell.

• One member of each pair comes from the mother's egg cell (orange).

• The other member comes from the father's sperm cell (green).

Chromosome Copies vs. Chromosome Pairs

• Chromosome Copies: Two connected subunits of DNA that are duplicates with the same genes and identical or nearly identical DNA sequences, separating during cell division and often look like an X.

• Chromosome Pairs: Two unconnected subunits of DNA that contain the same genes but are not copies, with one inherited from each parent, appearing as two separate eye shapes.

Venn Diagram: Chromosome Copies vs. Pairs

• Similarities:

  • Two molecules

  • Same genes present

  • Located in the cell nucleus

• Chromosome Copies (Only):

  • No or very few differences: duplicates

  • Connected at the centromere: looks like an X

  • Each duplicate from one parent

• Chromosome Pairs (Only):

  • Multiple differences: not duplicates

  • Not connected: distinct eye shapes

  • One from each parent

Practice Questions

• Examples to identify chromosome copies or chromosome pairs in images.
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Textbook Illustration

• Example of a typical illustration of the two members of one pair of chromosomes in a human cell.

• Blue X represents the member from one parent (mother).

• Red X represents the member from the other parent (father).

• The X shape indicates replication before mitosis or meiosis.

Questions

• Which of 1-4 have the same DNA sequence as each other?

  • 1 and 2 have the same DNA sequence because they are duplicate copies of each other. 3 and 4 have the same DNA sequence, again, they are duplicate copies of each other.

• Which of 1-4 have the same genes as each other?

  • All of 1-4 have the same genes in the same order on the chromosome because this is one chromosome pair representing that chromosome.

Additional Definitions

• Chromosome pairs: Unconnected DNA subunits with the same genes but not copies; one from each parent, causing differences.

• Diploid: Individual with two versions (one pair) of each chromosome.

Genes

• A gene is a segment of DNA on a chromosome that provides information for making certain molecules like mRNA and proteins.

• Each chromosome is typically millions of base pairs long.

• Each gene consists of hundreds to many thousands of nucleotides, and most chromosomes contain thousands of genes.

• Each chromosome in a pair has the same genes.

What Genes Do

• Genes do not make anything; they are information.

• Genes are like a recipe in a cookbook, providing guidance for the cell to make mRNA and proteins.

• Genes are the information that the cell uses to make mRNA and proteins.

Alleles

• Each chromosome in a chromosome pair has the same genes.

• Sometimes the nucleotide sequence of the gene is the same in the two chromosome versions.

• Often, the nucleotide sequence of each gene is different between the two chromosomes of the chromosome pair because the individual inherited a different version of the gene from its mother than it did from its father.

• Different versions of genes are called alleles.

• If a person's two alleles for a particular gene are the same, then the person is a homozygote for that gene or nucleotide position.

• If the two alleles of a particular gene are different, then the person is a heterozygote for that gene or nucleotide position.

Updated Flowchart

• A gene is a segment of nucleotides and DNA.

• Different versions of a gene are called alleles.

• Alleles like DNA and nucleotide sequences are information, and specifically, this is information about the building of mRNA and proteins.

• Each chromosome of a chromosome pair contains one allele of the genes that are on all versions of that chromosome.

Definitions

• Gene: DNA segment in a chromosome providing information for molecules like mRNAs and proteins.

• Allele: A version of a gene that is different from other versions based on nucleotide sequences.

• Most genes have more than one allele in a population, indicating genetic variation. Alleles are different versions of the same gene.

Relationship Between Alleles, Chromosome Pairs, and Copies

• Each chromosome in a chromosome pair contains one allele or version of each of the genes on that chromosome.

• When a chromosome is duplicated, each of the duplicates have the same alleles or versions of the genes because they are copies of each other.

• In a chromosome pair, remember that one member of the pair was inherited from the mother and the other member of the pair was inherited from the father.

• The two members of a pair may and often do have different alleles, that is different versions of the genes on them.

• Different alleles differ in their DNA sequences.

Back to Chromosome Diagram

• Which of 1-4 have the same alleles as each other?

  • 1 and 2 have the same alleles, they are duplicates. 3 and 4 have the same alleles, they are duplicates.

• Are the alleles in 1 and 2 different from the alleles in 3 and 4?

  • Yes, 1 and 2 have many different alleles of the genes than 3 and 4.

• These DNA sequence differences are the basis of the different alleles.

• It only takes one base pair difference to cause two DNA sequences to be different alleles.

• The blue chromosome (1 and 2) contains the maternal alleles.

• The red chromosome (3 and 4) contains the paternal alleles.

Homozygous and Heterozygous

• Homozygous: A diploid individual with the same allele in both versions of a gene or nucleotide position.

• Heterozygous: A diploid individual with different alleles of a given gene or nucleotide position.

Introduction to Variation, Pattern, and Process

• Unit 2 delves into the intricate aspects of variation, pattern, and process, laying a solid foundation with key genetic concepts. This unit emphasizes understanding over rote memorization, promoting a conceptual grasp of genetics.

• The primary goal is to foster a comprehensive understanding of fundamental genetic principles, encouraging students to connect terms and concepts in meaningful ways rather than simply memorizing definitions.

The Language of Genetics

• This section provides a thorough review of essential terms in genetics, ensuring a clear understanding of genetics' role in explaining natural phenomena. Terms such as DNA, genes, alleles, chromosomes, and genotypes are explored in depth.

• Common misconceptions related to these terms are addressed to clarify misunderstandings and reinforce accurate comprehension.

Explaining Variation Among Living Organisms

• Explores the diversity of biological forms within and between species, an area of significant interest for biologists. This includes understanding the genetic factors contributing to differences in traits, behaviors, and physical characteristics.

• Unit two adopts downward explanations to investigate how biological systems function at a detailed level, seeking to understand the underlying mechanisms driving biological processes.

Learning Objectives

• Define fundamental genetic terms: DNA, chromosome (copies vs. pairs), diploid, nucleotide, gene, allele, homozygous, heterozygous. Each definition should include examples and context for clarity.

• Describe the complex relationship between a chromosome and DNA, detailing how DNA is organized and packaged within chromosomes.

• Describe gene function in relation to chromosomes and DNA, clarifying how genes encode information for protein synthesis and cellular function.

• Describe the relationship between a gene and its alleles, explaining how different alleles arise and contribute to genetic variation.

• Explain the presence of identical genes but varied alleles among individuals, highlighting the role of inheritance and mutation in allele diversity.

Cells and Genomes

• All living organisms are composed of cells, with larger organisms consisting of trillions of cells, each performing specific functions. These cells coordinate to maintain the organism's life processes.

• Inside each cell is the genome, which comprises the organism’s complete set of DNA. This DNA contains all the genetic instructions needed for the organism to develop, function, and reproduce.

DNA and Chromosomes

• DNA, or deoxyribonucleic acid, serves as the carrier of genetic information, dictating the development and functioning of organisms. It contains the instructions for synthesizing proteins, which are essential for all biological processes.

• In most organisms, DNA is organized into multiple subunits known as chromosomes, which ensure efficient packaging and organization of the genetic material.

• Each chromosome consists of a single, very long DNA molecule meticulously packaged with histone proteins, forming a structure called chromatin.

• DNA is tightly wound around histone proteins, facilitating compact packaging and regulating gene expression. This ensures that the DNA fits within the cell nucleus and is accessible when needed.

• The chromosome represents a single molecule of DNA in a condensed form, making it easier to manage during cell division and other cellular processes.

Nucleotide Sequence

• DNA encodes information through its nucleotide sequence, which determines the genetic traits of an organism. The sequence governs protein synthesis and cellular functions.

• Nucleotides include adenine (A), cytosine (C), guanine (G), and thymine (T), which are the building blocks of DNA. These nucleotides pair in a specific manner to form the DNA double helix.

• A DNA molecule comprises two strands connected by hydrogen bonds between paired bases: adenine (A) bonds with thymine (T), and cytosine (C) bonds with guanine (G). This base pairing is fundamental to DNA's structure and function.

DNA Molecule Representation

• Each DNA molecule comprises two strands intertwined and bound by hydrogen bonds between specific base pairs. This structure ensures the stability and integrity of the genetic information.

Chromosome Appearance

• The visual appearance of chromosomes varies throughout the cell's life cycle, reflecting their dynamic role in cellular processes. The appearance changes based on activities such as DNA replication and cell division.

• Chromosomes may appear as a simple chromosome or as an X-shaped structure, depending on whether they have been duplicated in preparation for cell division.

• The X shape signifies that the chromosome has been duplicated in preparation for cell division, ensuring each daughter cell receives a complete set of genetic information.

• Following cell division, DNA returns to its unduplicated state, ready to perform its functions in the new cell.

Unduplicated vs. Duplicated Chromosomes

• An unduplicated chromosome consists of one DNA molecule, representing the basic unit of genetic information.

• A duplicated chromosome comprises two DNA molecules, which are identical copies of each other, ensuring accurate transmission of genetic information during cell division.

• DNA replication is the process of duplicating an original chromosome to produce an additional chromosome, ensuring that each daughter cell receives an identical copy of the genome.

• The two duplicates forming the X shape are known as sister chromatids, which are virtually identical in DNA sequence, ensuring genetic consistency.

• Chromosome copies are connected by a centromere, a specialized region that can be located anywhere along the chromosome. The centromere plays a crucial role in chromosome segregation during cell division.

• When the centromere is positioned at the end of the chromosome, the duplicated chromosome appears as a V-shaped structure.

Summary

• All organisms have cells, the fundamental units of life, which contain chromosomes that house the genetic material.

• Chromosomes are comprised of DNA, the molecule that carries genetic information and determines an organism's traits.

• DNA is constructed from nucleotides, the basic building blocks that form the genetic code. The sequence of these nucleotides encodes the instructions for building and maintaining an organism.

• DNA and nucleotide sequences are forms of information that dictate how cells function and how traits are inherited.

Definitions

• DNA: Deoxyribonucleic acid, the molecule containing genetic information essential for organism development, functioning, and reproduction.

• Chromosome: A structural subunit of the genome located in the nucleus, consisting of a DNA molecule and associated proteins that organize and protect the DNA.

• Duplicated chromosomes: Two connected, duplicated DNA subunits found in the nucleus that separate during cell division to ensure each daughter cell receives a complete set of genetic information. These often appear as an X or V shape.

• Nucleotides: Small molecules comprising the DNA sequence, including adenine, cytosine, guanine, and thymine, which form the building blocks of DNA.

Chromosome Pairs and Diploids

• In many organisms, including humans, chromosomes are organized into pairs, ensuring genetic diversity and proper segregation during cell division.

• Organisms possessing two of each chromosome are termed diploids, where one set is inherited from each parent.

• Humans have 23 pairs of chromosomes in every somatic (body) cell, totaling 46 chromosomes. These pairs include 22 pairs of autosomes and one pair of sex chromosomes.

• One member of each pair originates from the mother's egg cell, carrying her genetic contribution to the offspring.

• The other member is inherited from the father's sperm cell, contributing his genetic information to the offspring.

Chromosome Copies vs. Chromosome Pairs

• Chromosome Copies: Two connected subunits of DNA that are duplicates, containing the same genes and nearly identical DNA sequences, which separate during cell division and often appear as an X shape.

• Chromosome Pairs: Two unconnected subunits of DNA that contain the same genes but are not duplicates, with one inherited from each parent, appearing as two distinct eye shapes.

Venn Diagram: Chromosome Copies vs. Pairs

• Similarities:

  • Two molecules of DNA present in the cell nucleus.

  • Same genes present in the same order along the DNA.

• Chromosome Copies (Only):

  • Minimal to no differences; they are duplicates resulting from DNA replication.

  • Connected at the centromere, giving them an X-shaped appearance.

  • Each duplicate originates from a single parent chromosome.

• Chromosome Pairs (Only):

  • Multiple sequence differences; they are not duplicates and result from each parent.

  • Not connected; they appear as distinct eye shapes.

  • One chromosome is inherited from each parent, contributing to genetic diversity.

Practice Questions

• Examples to identify chromosome copies or chromosome pairs in images. Practice includes distinguishing between the structures and understanding their roles in cell division and inheritance.

Textbook Illustration

• Example of a typical illustration of the two members of one pair of chromosomes in a human cell, highlighting their structural differences and genetic content.

• A blue X represents the chromosome inherited from one parent (typically the mother), showcasing its genetic contribution.

• A red X represents the chromosome inherited from the other parent (typically the father), underscoring the biparental inheritance of genetic material.

• The X shape indicates that replication has occurred before mitosis or meiosis, ensuring each daughter cell receives a complete set of genetic information.

Questions

• Which of 1-4 have the same DNA sequence as each other? Chromosomes 1 and 2 have the same DNA sequence because they are duplicate copies of each other, ensuring genetic consistency. Chromosomes 3 and 4 also have the same DNA sequence as they are duplicate copies.

• Which of 1-4 have the same genes as each other? All of chromosomes 1-4 have the same genes in the same order on the chromosome because this represents one chromosome pair. Each chromosome carries the same set of genes arranged in the same order.

Additional Definitions

• Chromosome pairs: Unconnected DNA subunits that carry the same genes but are not identical copies; one is inherited from each parent, introducing genetic diversity.

• Diploid: An individual with two versions (one pair) of each chromosome, ensuring a complete set of genetic information.

Genes

• A gene is a specific segment of DNA located on a chromosome. It contains the instructions for synthesizing particular molecules, such as mRNA and proteins, which are essential for cellular functions.

• Each chromosome typically spans millions of base pairs in length, providing ample space for numerous genes and regulatory elements.

• Each gene comprises hundreds to thousands of nucleotides, and most chromosomes contain thousands of genes, each with a specific function.

• Each chromosome in a pair carries the same genes, ensuring that each cell can produce the necessary molecules; however, the alleles (versions) of these genes may differ between the chromosomes.

What Genes Do

• Genes serve as informational blueprints; they do not directly create anything but provide the necessary instructions. They are like recipes in a cookbook, guiding the cell to synthesize mRNA and proteins. These molecules perform various functions within the cell.

• Genes are the informational units that cells use to synthesize mRNA and proteins, which carry out a wide range of functions essential for life.

Alleles

• Each chromosome in a chromosome pair carries the same genes, ensuring that similar traits are heritable.

• Sometimes, the nucleotide sequence of a gene is identical in both chromosome versions; however, there are also many inherent differences.

• The nucleotide sequence of a gene often differs between the two chromosomes of a chromosome pair because individuals inherit different versions of the gene from their mother and father. These variations lead to genetic diversity.

• Different versions of genes are called alleles, which contribute to the variation in traits observed among individuals.

• If a person has two identical alleles for a particular gene, they are homozygous for that gene or nucleotide position, leading to specific trait expression.

• If a person has two different alleles for a particular gene, they are heterozygous for that gene or nucleotide position, potentially resulting in a different trait expression than homozygotes.

Updated Flowchart

• A gene is a segment of nucleotides and DNA that codes for specific traits.

• Different versions of a gene are called alleles, contributing to genetic variation.

• Alleles, like DNA and nucleotide sequences, are forms of information that guide the building of mRNA and proteins. These molecules are fundamental to cellular function.

• Each chromosome of a chromosome pair contains one allele of the genes present on all versions of that chromosome. This ensures that each individual has a complete set of genetic information.

Definitions

• Gene: A DNA segment within a chromosome that provides information for synthesizing molecules like mRNAs and proteins.

• Allele: A version of a gene that differs from other versions based on variations in nucleotide sequences. Alleles are the foundation of genetic diversity.

• Most genes have multiple alleles in a population, indicating genetic variation. Alleles are different versions of the same gene, each capable of producing slightly different traits.

Relationship Between Alleles, Chromosome Pairs, and Copies

• Each chromosome in a chromosome pair contains one allele or version of each of the genes on that chromosome. This ensures that each individual has a comprehensive set of genetic instructions.

• When a chromosome is duplicated, each of the duplicates carries the same alleles or versions of the genes. This ensures that the genetic information is faithfully copied during cell division.

• In a chromosome pair, one member is inherited from the mother, and the other from the father, contributing to genetic diversity.

• The two members of a pair may, and often do, have different alleles, reflecting the variations inherited from each parent.

• Different alleles differ in their DNA sequences. These differences are the basis of the genetic diversity observed in populations.

Back to Chromosome Diagram

• Which of 1-4 have the same alleles as each other? Chromosomes 1 and 2 have the same alleles because they are duplicates resulting from DNA replication. Similarly, chromosomes 3 and 4 have the same alleles because they are also duplicates.

• Are the alleles in 1 and 2 different from the alleles in 3 and 4? Yes, the alleles in chromosomes 1 and 2 often differ from those in chromosomes 3 and 4. These differences arise from the genetic contributions of each parent.

• These DNA sequence differences are the basis of the different alleles, leading to variations in traits.

• It only takes one base pair difference to cause two DNA sequences to be different alleles. This highlights the sensitivity of genetic variation to even small changes in DNA sequence.

• The blue chromosome (1 and 2) contains the maternal alleles, representing the genetic contribution from the mother.

• The red chromosome (3 and 4) contains the paternal alleles, representing the genetic contribution from the father.

Homozygous and Heterozygous

• Homozygous: A diploid individual carrying identical alleles in both versions of a gene or nucleotide position. Homozygosity leads to the expression of specific traits based on the identical alleles.

• Heterozygous: A diploid individual carrying different alleles of a given gene or nucleotide position. Heterozygosity can result in a variety of trait expressions, including intermediate or dominant/recessive inheritance patterns.