Introduction to Genetics Quiz
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Questions and Answers

What structure characterizes purines?

  • 5-carbon, triple-ring
  • 9-carbon, double-ring (correct)
  • 6-carbon, single-ring
  • 8-carbon, branched-chain

    • Which of the following pairs illustrates Chargaff’s rule?

  • C ≈ A
  • A ≈ G
  • C + G ≈ A + T
  • A ≈ T (correct)

    • What type of bond links two mononucleotides in DNA?

  • Peptide bond
  • Ionic bond
  • Phosphodiester bond (correct)
  • Hydrogen bond

    • What sugar is found in DNA?

    <p>Deoxyribose</p> Signup and view all the answers

    What describes the arrangement of the two polynucleotide chains in the Watson-Crick DNA model?

    <p>They run antiparallel to each other.</p> Signup and view all the answers

    Which nitrogenous base is found in RNA but not in DNA?

    <p>Uracil</p> Signup and view all the answers

    How far apart are the stacked nucleotide bases in the DNA structure according to the Watson-Crick model?

    <p>3.4 Å</p> Signup and view all the answers

    Which of the following statements about DNA and RNA stability is true?

    <p>DNA is more stable due to its deoxyribose structure.</p> Signup and view all the answers

    What is the length of a complete turn of the DNA double helix?

    <p>34 Å</p> Signup and view all the answers

    Watson and Crick's analysis of DNA structure was published in what year?

    <p>1953</p> Signup and view all the answers

    How many strands does a typical RNA molecule have?

    <p>One</p> Signup and view all the answers

    Which feature characterizes the hydrogen bond pairing in the DNA model?

    <p>They connect complementary nitrogenous bases.</p> Signup and view all the answers

    What is the significance of the major and minor grooves in DNA structure?

    <p>They are the sites of protein binding.</p> Signup and view all the answers

    Which component is attached to the 5' end of a DNA strand?

    <p>Phosphate group</p> Signup and view all the answers

    Which of the following statements about base-pair percentages in DNA is false?

    <p>A + T = C + G</p> Signup and view all the answers

    Which of the following accurately describes transcription?

    <p>It rewrites genetic information into RNA.</p> Signup and view all the answers

    What is the diameter of the DNA double helix?

    <p>20 Å</p> Signup and view all the answers

    During translation, what is synthesized from messenger RNA (mRNA)?

    <p>Polypeptides</p> Signup and view all the answers

    What is the primary flow of genetic information according to the Central Dogma of Molecular Biology?

    <p>DNA → RNA → Protein</p> Signup and view all the answers

    What structure indexes the process of DNA packaging into chromosomes?

    <p>Nucleosome</p> Signup and view all the answers

    Which of the following describes a chromatid?

    <p>A duplicated chromosome attached to centromeres</p> Signup and view all the answers

    Which molecule is considered to be a component of eukaryotic chromosomes that aids in packaging?

    <p>Histone</p> Signup and view all the answers

    At what stage are chromosomes formed during the cell cycle?

    <p>M phase</p> Signup and view all the answers

    What is the role of nucleosomes in chromatin structure?

    <p>To compact DNA for storage in the nucleus</p> Signup and view all the answers

    Which of the following best describes the organization of prokaryotic DNA?

    <p>Single circular DNA molecule without histones</p> Signup and view all the answers

    What is formed when six nucleosomes are bound together?

    <p>Chromatin fiber</p> Signup and view all the answers

    How is genetic information organized in eukaryotic chromosomes?

    <p>Linear sequence of genes</p> Signup and view all the answers

    What occurs to DNA before cell division?

    <p>It undergoes replication</p> Signup and view all the answers

    What part of the chromosome connects the sister chromatids?

    <p>Centromere</p> Signup and view all the answers

    How many total chromosomes are present in a duplicated chromosome?

    <p>92</p> Signup and view all the answers

    What information does a karyotype provide?

    <p>An individual’s complete set of chromosomes</p> Signup and view all the answers

    Which of the following correctly describes the arms of a chromosome?

    <p>P arm is the short arm; Q arm is the long arm.</p> Signup and view all the answers

    What is the primary function of telomeres in chromosomes?

    <p>Protecting the ends of chromosomes</p> Signup and view all the answers

    In a single chromosome, how many chromatids are present?

    <p>46</p> Signup and view all the answers

    What differentiates a sister chromatid from a chromatid?

    <p>A sister chromatid is part of a duplicated chromosome.</p> Signup and view all the answers

    What is the typical structure of a karyotype?

    <p>A complete set of chromosomes depicted in pairs</p> Signup and view all the answers

    What is the primary role of DNA?

    <p>To carry genetic information</p> Signup and view all the answers

    Which of the following correctly describes a gene?

    <p>A unit that codes for the production of RNA or functional polypeptides</p> Signup and view all the answers

    What do chromosomes primarily consist of?

    <p>DNA packed with proteins</p> Signup and view all the answers

    How is genetic variation introduced in populations?

    <p>Through mutation</p> Signup and view all the answers

    Which component is NOT part of a nucleotide?

    <p>Amino acid</p> Signup and view all the answers

    What characterizes homologous chromosomes?

    <p>They carry the same genes but may have different alleles</p> Signup and view all the answers

    Which description best fits the term 'nucleic acids'?

    <p>Biomolecules composed of nucleotide subunits</p> Signup and view all the answers

    What is the structure of DNA primarily comprised of?

    <p>Two antiparallel chains of deoxyribonucleotides</p> Signup and view all the answers

    What are the sex chromosomes for a normal human female?

    <p>XX</p> Signup and view all the answers

    What are the sex chromosomes for a normal human male?

    <p>XY</p> Signup and view all the answers

    Robertsonian translocation of chromosomes 14 and 21 causes familial Down syndrome.

    <p>True</p> Signup and view all the answers

    What determines the male phenotype in secondary sex determination?

    <p>Hormones produced by gonads</p> Signup and view all the answers

    What is the significance of gameotogenesis in sex determination?

    <p>Each gamete contains one chromosome.</p> Signup and view all the answers

    What type of sperm determines the sex of the offspring?

    <p>Both X and Y sperm</p> Signup and view all the answers

    Match the following animal groups with their sex determination system:

    <p>Mammals = XY (male) XX (female) Birds = ZZ (male) ZW (female) Bees = Haploid (male) Diploid (female) Drosophila = X:0 or XY (male) X:A = 1.0 (female)</p> Signup and view all the answers

    In turtles, sex determination can be temperature-dependent.

    <p>True</p> Signup and view all the answers

    What gene is responsible for activating male sex determination processes?

    <p>Sry gene</p> Signup and view all the answers

    All males have two X chromosomes.

    <p>False</p> Signup and view all the answers

    What does the presence of the Sry gene indicate?

    <p>It indicates male development.</p> Signup and view all the answers

    What is the role of testosterone in male development?

    <p>It stimulates the differentiation of male reproductive structures.</p> Signup and view all the answers

    What is aneuploidy?

    <p>Gain or loss of one or more chromosomes but not a complete set</p> Signup and view all the answers

    What can cause aneuploidy?

    <p>Nondisjunction</p> Signup and view all the answers

    Which syndrome is characterized by 47 XYY chromosomes?

    <p>Jacobs syndrome</p> Signup and view all the answers

    What is Turner syndrome?

    <p>Characterized by 45 chromosomes, X0</p> Signup and view all the answers

    What is the average lifespan of a person with Down syndrome?

    <p>50 years</p> Signup and view all the answers

    The probability of having a child with Down syndrome increases with the age of the mother.

    <p>True</p> Signup and view all the answers

    Klinefelter syndrome has a total number of chromosomes of ____.

    <p>47</p> Signup and view all the answers

    What is the primary cause of triploidy?

    <p>Nondisjunction of all chromosomes at meiosis I</p> Signup and view all the answers

    What is a common characterization of Cri du chat syndrome?

    <p>Small terminal deletion in chromosome 5</p> Signup and view all the answers

    Match the following chromosomal abnormalities with their descriptions:

    <p>Klinefelter syndrome = 47, XXY Turner syndrome = 45, XO Jacobs syndrome = 47, XYY Down syndrome = Trisomy 21</p> Signup and view all the answers

    The movement of a chromosome segment to a different location in the genome is known as _____.

    <p>translocation</p> Signup and view all the answers

    What are the sex chromosomes for a normal human female?

    <p>XX</p> Signup and view all the answers

    What are the sex chromosomes for a normal human male?

    <p>XY</p> Signup and view all the answers

    What determines the gonads in primary sex determination?

    <p>Sex chromosomes</p> Signup and view all the answers

    What is the result of Robertsonian translocation of chromosomes 14 and 21?

    <p>Familial Down syndrome</p> Signup and view all the answers

    What chromosome does a female gamete contain?

    <p>X chromosome</p> Signup and view all the answers

    What is the sex determined by in animals?

    <p>Type of sperm fertilizing the egg</p> Signup and view all the answers

    In birds, what are the sex chromosomes for males?

    <p>ZZ</p> Signup and view all the answers

    In bees, how is a male determined?

    <p>From an unfertilized egg</p> Signup and view all the answers

    The Sry gene is a testis-determining factor and acts as a _____ in differentiating bipotential gonads.

    <p>transcription factor</p> Signup and view all the answers

    What hormone do Leydig cells secrete?

    <p>Testosterone</p> Signup and view all the answers

    What happens to the Wolffian ducts in a developing female?

    <p>They degenerate.</p> Signup and view all the answers

    Which factor acts in the absence of the Y chromosome to promote ovarian development?

    <p>β-catenin</p> Signup and view all the answers

    Temperature influences the sex differentiation in some reptiles.

    <p>True</p> Signup and view all the answers

    What phenotype is associated with Androgen Insensitivity Syndrome in XY individuals?

    <p>Female external sex characteristics</p> Signup and view all the answers

    What is the term for the presence of complete haploid sets of chromosomes?

    <p>Euploidy</p> Signup and view all the answers

    What syndrome is characterized by the presence of an extra Y chromosome?

    <p>Jacobs syndrome</p> Signup and view all the answers

    Klinefelter syndrome involves the presence of more Y chromosomes than X chromosomes.

    <p>False</p> Signup and view all the answers

    Which of the following is an example of an autosomal aneuploid?

    <p>Down syndrome</p> Signup and view all the answers

    What is the general cause of aneuploidy?

    <p>Nondisjunction</p> Signup and view all the answers

    In Turner syndrome, there is an absence of one X chromosome, resulting in a total of ______ chromosomes.

    <p>45</p> Signup and view all the answers

    Trisomy 13 is also known as Patau syndrome.

    <p>True</p> Signup and view all the answers

    What is the probability of having a child with Down syndrome correlated with?

    <p>Maternal age</p> Signup and view all the answers

    Which chromosomal disorder is associated with underdeveloped secondary sex characteristics?

    <p>Triplo-X</p> Signup and view all the answers

    A segment of a chromosome turned 180° within itself is known as a ______.

    <p>inversion</p> Signup and view all the answers

    What are the two forms of deletion in chromosomal structure?

    <p>Terminal deletion and intercalary deletion</p> Signup and view all the answers

    What is a common result of nondisjunction in meiosis I?

    <p>Aneuploid gametes</p> Signup and view all the answers

    Robertsonian translocation involves the loss of genetic material.

    <p>True</p> Signup and view all the answers

    Study Notes

    Introduction to Genetics

    • DNA is a molecule that contains genetic information in the form of nucleotide sequences.
    • Genes are specific segments of DNA sequences that code for the production of functional polypeptides or RNA molecules.
    • Chromosomes are condensed forms of DNA molecules packed with proteins; they carry genetic information from one cell to the next.

    Nucleic Acids

    • Nucleic acids are biomolecules composed of nucleotide subunits responsible for carrying genetic information (except in some viruses).
    • There are two main nucleic acids: DNA and RNA.

    Nucleotides

    • Nucleotides are the building blocks of nucleic acids.
    • Each nucleotide is composed of three components: a nitrogenous base, a pentose sugar, and a phosphate group.

    Purines and Pyrimidines

    • Purines are nitrogenous bases with a 9-carbon, double-ring structure.
    • Pyrimidines are nitrogenous bases with a 6-carbon, single-ring structure.

    Chargaff’s Rule

    • The DNA nucleotide base composition varies from one species to another.
    • The percentages of specific bases in DNA are roughly the same. For example, the percentage of adenine (A) is approximately equal to the percentage of thymine (T), and the percentage of cytosine (C) is approximately equal to the percentage of guanine (G).
    • The percentages of base pairs are not necessarily the same. For example, the sum of the percentages of C and G does not necessarily equal the sum of the percentages of A and T.

    Phosphodiester Bond

    • A phosphodiester bond connects two mononucleotides through their 3' and 5' carbons.

    DNA vs. RNA

    • DNA and RNA differ in their pentose sugar, nitrogenous bases, number of strands, stability, and function.
    • DNA contains deoxyribose, while RNA contains ribose.
    • DNA contains thymine (T), while RNA contains uracil (U).
    • DNA is typically a double-stranded molecule, while RNA is typically single-stranded.
    • DNA is more stable than RNA.

    Watson-Crick DNA Model

    • This model proposed the 3D structure of DNA in 1953, based on Rosalind Franklin’s X-Ray diffraction photo of DNA.
    • Features of the model:
      • Two long polynucleotide chains coil around a central axis, forming a right-handed double helix.
      • The two chains are antiparallel (C-5' to C-3' orientation in opposite directions).
      • The nucleotide bases are flat, perpendicular to the central axis, and "stack" with each other (3.4 Å or 0.34 nm apart).
      • The nitrogenous bases are paired through hydrogen bonds.
      • One complete turn of the helix is 34 Å (or 3.4 nm) long and contains 10 nucleotide base pairs.
      • A larger major groove alternates with a smaller minor groove along the strand.
      • The double helix diameter is 20 Å (or 2.0 nm).

    Central Dogma of Molecular Biology

    • The flow of genetic information proceeds in one direction: from DNA to RNA to Protein (or RNA to protein).

    Transcription

    • Transcription is the process of synthesizing RNA from DNA. It involves the rewriting of genetic information.
    • The DNA strand serves as a template for the transcription of complementary RNA.

    Translation

    • Translation is the process of synthesizing a polypeptide (long chain of amino acids) from messenger RNA (mRNA).
    • Ribosomes, which are protein-synthesizing complexes, link amino acids together during translation.

    Genetic Code

    • The genetic code acts as a "translator" from RNA codons to proteins.

    Organization of DNA into Chromosomes

    • Histones: These are proteins that bind to DNA, helping to condense it into a compact structure.
    • Nucleosome: This is the basic unit of DNA packaging, consisting of eight histone protein subunits wrapped by a DNA molecule.
    • Chromatin: This is the general term for the complex of DNA and protein found in the nucleus.
    • Chromatin Fiber: This is a fiber of packed nucleosomes formed through coiling and stacking.
    • Chromatid: These are duplicated chromosomes that are attached to the centromere.
    • Chromosome: This is the highly condensed form of DNA that is present during cell division.

    Parts of a Replicated Chromosome

    • Chromatid: One-half of an identical replicated chromosome.
    • Sister Chromatid: Two parallel structures connected by the centromere that contain identical DNA sequences.
    • Centromere: The constricted region where the sister chromatids join together.
    • Telomere: The end or tip of the chromosome.
    • Arms: The two arms of a chromosome, designated as "p" (short arm) and "q" (long arm).

    Types of Chromosome According to Centromere Location

    • Metacentric: Centromere located at the center of the chromosome.
    • Submetacentric: Centromere located slightly off-center, resulting in a shorter "p" arm and a longer "q" arm.
    • Acrocentric: Centromere located near one end of the chromosome, resulting in a very short "p" arm.
    • Telocentric: Centromere located at the very end of the chromosome.

    Karyotypes

    • Refers to a complete set of an individual's chromosomes, consisting of 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes.

    Karyotyping

    • This is a method used to check an individual's karyotype.

    Chromosomal Disorders

    • Chromosomal disorders can be classified by variation in chromosome number or variation in chromosome structure.
    • Variation in Chromosome Number refers to the presence of complete haploid sets of chromosomes or multiples of haploid numbers.
      • Euploidy is the presence of complete haploid sets of chromosomes (multiples of haploid numbers, n).
        • Polyploidy is a condition where there are multiples of n chromosomes in a cell.
          • Triploidy is a condition where there are three sets of chromosomes, 3n.
          • Tetraploidy is a condition where there are four sets of chromosomes, 4n.
      • Aneuploidy is the gain or loss of one or more chromosomes but not an entire set (2n ± x chromosomes).
        • Aneuploidy is caused by nondisjunction.
          • Nondisjunction can occur during the first or second meiotic division.
          • First-division nondisjunction occurs during meiosis I.
          • Second-division nondisjunction occurs during meiosis II.
        • Sex Chromosome Aneuploids in Humans:
          • Klinefelter syndrome (47, XXY): The total number of chromosomes is 47 with more X chromosomes than Y chromosomes.
          • Turner syndrome (45, XO): The total number of chromosomes is 45 with the absence of an X chromosome in females.
          • Triplo-X Syndrome (47, XXX): The total number of chromosomes is 47 with an extra X chromosome in females.
          • Jacobs syndrome (47, XYY): The total number of chromosomes is 47 with an extra Y chromosome in males.
        • Autosomal Aneuploids in Humans:
          • Trisomy 21 (Down Syndrome): The total number of chromosomes is 47 with an extra chromosome 21. The extra chromosome 21 is typically inherited from the mother (~ 95% of the time).
          • Trisomy 13 (Patau Syndrome): The total number of chromosomes is 47 with an extra chromosome 13.
          • Trisomy 18 (Edwards Syndrome): The total number of chromosomes is 47 with an extra chromosome 18.
    • Variation in Chromosome Structure refers to alterations to the physical structure of chromosomes.
      • Deletion: A portion of the chromosome is missing.
        • Terminal Deletion: Deletion occurs at the end of a chromosome.
        • Intercalary Deletion: Deletion occurs within a chromosome.
        • Cri du chat syndrome is caused by a small terminal deletion at the end of the short arm (p arm) of chromosome 5.
      • Duplication: A portion of the chromosome is repeated.
        • 15q11-q13 duplication syndrome (Dup15q syndrome) is a condition characterized by a duplication on the long arm of chromosome 15.
      • Inversion: A portion of the chromosome is turned 180 degrees.
        • Paracentric Inversion does not include the centromere.
        • Pericentric Inversion includes the centromere.
      • Translocation: A portion of the chromosome moves to a different location.
        • Reciprocal Translocation: An exchange of parts between two nonhomologous chromosomes.
          • Chronic Myelogenous Leukemia (CML) is caused by a reciprocal translocation between chromosomes 9 and 22, leading to the formation of a Philadelphia chromosome.
        • Robertsonian Translocation: Fusion of the long arms of two nonhomologous acrocentric chromosomes.

    Sex Determination

    • Mammalian Pattern of Sex Determination: Sex is determined by the presence or absence of a Y chromosome.
      • Male Development: Requires the presence of a Y chromosome
      • Female Development: Occurs in the absence of a Y chromosome.
      • Androgen Insensitivity Syndrome: A condition where individuals with a Y chromosome (XY) develop as female.
        • Individuals with this condition lack functional androgen receptors which are necessary for male development.
        • Individuals with this condition may have external genitalia that resemble females, but their internal reproductive systems may be underdeveloped or absent.
    • Genetic Mechanisms of Primary Sex Determination:
      • Transcription Factors: Genes on the Y chromosome can stimulate or suppress the transcription of crucial genes.
      • Absence of Y Chromosome: Individuals develop as female if the Y chromosome is absent.
      • Presence of Y Chromosome: Individuals develop as male if the Y chromosome is present.
    • Importance of Timing: The timing of gene expression is crucial for the development of specific traits.
      • The presence of a Y chromosome is necessary for the activation of certain genes that control male development, which occur at specific points in time during development.
        • Sry (Sex-determining Region Y) gene on the Y chromosome controls the development of testes.
        • The Sry gene encodes a transcription factor that triggers genes for testicular development.
    • X-Chromosome Dosage Compensation:
      • The dosage of genes on the X chromosome must be equal in both males and females since females have two X chromosomes and males only have one.
        • Barr Bodies are inactivated X chromosomes in female cells.
          • This ensures that genes on the X chromosome are expressed at roughly the same level in males and females.
          • This is achieved by the inactivation of one X chromosome in female cells, thus preventing gene expression from being double what is needed.
    • Genetic Mechanisms of Secondary Sex Determination:
      • Sex hormones control secondary sex characteristics:
        • Androgens (testosterone) are produced by testes and influence secondary sex characteristics in males.
        • Estrogens (estradiol) and progestins (progesterone) are produced by ovaries and influence secondary sex characteristics in females.
      • These hormones will influence the development of physical traits including, but not limited to, hair growth, body fat distribution, and skeletal development.

    Sex Determination

    • Primary sex determination refers to the development of the gonads (ovaries or testes).
    • Secondary sex determination describes the development of male or female characteristics outside the gonads, influenced by gonadal hormone production.

    Mammalian Sex Determination

    • Mammals have an XY sex determination system.
    • Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
    • Sex is determined by the type of sperm that fertilizes the egg.

    Sex Determination in Other Animals

    • Birds have a ZW sex determination system.
    • Females (ZW) are heterogametic, while males (ZZ) are homogametic.
    • Bees, wasps, and ants: Females develop from fertilized eggs (diploid) while males develop from unfertilized eggs (haploid).
    • Drosophila sex determination is based on the ratio of X chromosomes to autosomes.

    Early Stages of Development in Mammals

    • Gonads are sexually indifferent during the first 4-7 weeks of human development.
    • Both Wolffian (male) and Müllerian (female) ducts are present.

    Male Development (XY Embryo)

    • The Y chromosome carries the Sry gene, which encodes the testis-determining factor (TDF).
    • Sertoli cells secrete anti-Müllerian hormone (AMH), which destroys the Müllerian ducts.
    • Leydig cells produce testosterone, which promotes the development of the Wolffian ducts into the epididymis, vas deferens, and seminal vesicle.

    Female Development (XX Embryo)

    • Ovaries develop in the absence of the Y chromosome.
    • Estrogen is produced by the ovaries, stimulating the development of the Müllerian ducts into the uterus, oviducts, cervix, and upper portion of the vagina.

    Genetic Mechanisms of Primary Sex Determination

    • Transcription factors in the bipotential gonad control gene expression.
    • In the absence of the Y chromosome: Wnt4 and R-spondin1 stimulate the Wnt pathway, activating β-catenin.
    • ** β-catenin** activates follistatin, which helps organize the ovary's granulosa cells and prevents Sox9 synthesis.
    • In the presence of the Y chromosome: Sry activates Sox9, which promotes testes formation.

    Androgen Insensitivity Syndrome (AIS)

    • Individuals with XY chromosomes can have defective testosterone receptors.
    • They experience female external genitalia but have undescended testes.

    Temperature-Dependent Sex Determination

    • Observed in turtles, crocodiles, and alligators.
    • Incubation temperature influences the expression of genes for male and female differentiation.

    Key Genes in Sex Determination:

    • Sry: Located on the Y chromosome, initiates male development.
    • Sox9: An autosomal gene, plays a crucial role in testes formation.
    • Dmrt1: A gene involved in maintaining testicular structure.
    • Fox12: A gene that contributes to ovarian development.

    Importance of Timing

    • Sry gene function is limited to a short time window in embryonic development.

    Important Genes and Their Roles:

    • Sry: Promotes testis development.
    • Sox9: Key for testis formation, inhibits female development.
    • Dmrt1: Maintains testis structure.
    • Fox12: Essential for ovarian development, inhibits testis development.
    • Wnt4: Promotes female development.
    • R-spondin1 (Rspo1): Stimulates the Wnt pathway, activates β-catenin.
    • β-catenin: Activates follistatin, helps organize the ovary's granulosa cells.
    • Follistatin: Inhibits Sox9 synthesis.
    • AMH (anti-Müllerian hormone): Destroys the Müllerian ducts.
    • Testosterone: Stimulates development of male genitalia.
    • Estrogen: Stimulates development of female genitalia.

    Chromosomal Disorders

    • Chromosomal disorders are caused by variation in chromosome number or structure.
    • Variation in chromosome number relates to the presence of more than one set of chromosomes.
    • Aneuploidy is the gain or loss of one or more chromosomes, but not a complete set.
      • Nondisjunction causes aneuploidy and happens when chromosomes fail to separate during meiosis.
      • First-division nondisjunction occurs during the first meiotic division.
      • Second-division nondisjunction occurs during the second meiotic division.

    Sex Chromosome Aneuploids in Humans

    • Klinefelter syndrome (47, XXY): 47 total chromosomes with more X than Y chromosomes in males.
    • Turner syndrome (45, XO): 45 total chromosomes with a missing X chromosome in females.
    • Triplo-X (47, XXX): 47 total chromosomes with an extra X chromosome in females.
    • Jacobs syndrome (47, XYY): 47 total chromosomes with an extra Y chromosome in males.

    Autosomal Aneuploids in Humans

    • Trisomy 21 (Down syndrome): Three copies of chromosome 21, leading to characteristic physical features, mental retardation, and a shorter lifespan.
    • Trisomy 13 (Patau syndrome): Three copies of chromosome 13, leading to severe birth defects and a short life expectancy.
    • Trisomy 18 (Edwards syndrome): Three copies of chromosome 18, leading to serious health issues and also very short life expectancy.

    Polyploidy

    • Polyploidy involves having more than two sets of chromosomes.
    • Triploidy is the presence of three sets of chromosomes, often caused by nondisjunction during meiosis I or fertilization by two sperm cells.
    • Tetraploidy is the presence of four sets of chromosomes, usually resulting in miscarriage or death in the early stages of development.

    Variation in Chromosome Structure

    • Deletion is the loss of a portion of a chromosome.
      • Terminal deletion occurs at the end of a chromosome.
      • Intercalary deletion occurs in the interior of a chromosome.
    • Cri du chat syndrome (46, 5p-) is caused by a small terminal deletion in the short arm of chromosome 5.
    • Duplication is the presence of a repeated segment of a chromosome.
      • 15q11-q13 duplication syndrome (dup15q syndrome) is a disorder caused by duplication in the q arm of chromosome 15.
    • Inversion is the reversal of a segment of a chromosome.
      • Paracentric inversion does not include the centromere.
      • Pericentric inversion includes the centromere.
    • Translocation is the movement of a chromosome segment to a different location in the genome.
      • Reciprocal translocation involves an exchange of parts between nonhomologous chromosomes.
        • Chronic myeloid leukemia (CML) is associated with a reciprocal translocation between chromosomes 9 and 22, creating the Philadelphia chromosome.
      • Robertsonian translocation is a fusion of the long arms of two acrocentric chromosomes.

    Sex Determination

    • The mammalian pattern of sex determination relies on the presence or absence of the Y chromosome.
      • Male development is determined by the presence of the Y chromosome.
      • Female development occurs in the absence of the Y chromosome.
      • Androgen insensitivity syndrome is a condition where individuals with XY chromosomes are resistant to androgens, resulting in female external genitalia despite possessing a Y chromosome.

    Genetic Mechanisms of Primary Sex Determination

    • Transcription factors play a critical role in sex determination.
    • The absence of the Y chromosome leads to a pathway promoting female development.
      • SRY gene on the Y chromosome is the primary determinant of male sex.
      • The presence of the Y chromosome activates the SRY gene.
    • The timing of sex determination is crucial. It occurs early in embryonic development.

    Chromosomal Disorders & Sex Determination

    • Sex determination is the process of determining the sex of an organism
    • Primary sex determination is the determination of the gonads (ovaries or testes) by the sex chromosomes
    • Secondary sex determination is the determination of the male or female phenotype outside the gonads by hormones produced by the gonads
    • Karyotype is a visualization of the complete set of chromosomes of an organism
    • Robertsonian translocation is a type of chromosomal translocation where one chromosome breaks and attaches to another
    • Down syndrome is a genetic disorder caused by a trisomy of chromosome 21
    • Chromosomal disorders occur when there is a change in the number or structure of chromosomes
    • Gametogenesis is the process of producing gametes (sperm or eggs)

    Sex Determination in Different Animals

    • Mammals have an XY sex determination system - females are XX and males are XY
    • Birds have a ZW sex determination system - females are ZW and males are ZZ
    • Bees, wasps, and ants have a haplodiploid sex determination system - females develop from fertilized eggs (diploid) and males develop from unfertilized eggs (haploid)
    • Drosophila (fruit fly) has a sex determination system based on the ratio of X chromosomes to autosomes (number of chromosome sets excluding sex chromosomes)
    • X:A Ratio: The ratio of X chromosomes to autosomes in Drosophila

    Mammalian Pattern of Sex Determination

    • The development of gonads in mammals begins with the presence of bipotential gonads - gonads that have the potential to develop into either testes or ovaries
    • By sixth week, germ cells migrate to the gonads and the Wolffian ducts (male reproductive ducts) and Müllerian ducts (female reproductive ducts) are present
    • By eighth week, the gonads differentiate into testes or ovaries based on the presence or absence of the Y chromosome
    • Sry gene is located on the Y chromosome and encodes the testis-determining factor (TDF)

    Male Development (XY Embryo)

    • The Sry gene triggers the development of testes
    • The testes secrete anti-Müllerian hormone (AMH) which destroys the Müllerian ducts
    • The testes also secrete testosterone which causes the differentiation of the Wolffian ducts into the epididymis, vas deferens, and seminal vesicles
    • Dihydrotestosterone (DHT), which is formed from testosterone, promotes the development of the penis, prostate gland, and scrotum
    • Sertoli cells surround incoming germ cells to form the testis cord and secrete AMH
    • Leydig cells produce testosterone

    Female Development (XX Embryo)

    • In the absence of the Y chromosome, the bipotential gonads differentiate into ovaries
    • Germ cells differentiate into eggs (ova)
    • Granulosa cells form follicles around germ cells
    • Thecal cells produce estrogen
    • Estrogen stimulates the differentiation of the Müllerian duct into the uterus, oviducts, cervix, and upper portion of the vagina
    • Wolffian duct degenerates due to insufficient testosterone

    Genetic Mechanisms of Primary Sex Determination

    • Transcription factors (TFs) regulate the transcription of genes involved in sex determination
    • Wnt4, Rspo1, and β-catenin promote the differentiation of the ovary
    • Sox9 promotes the differentiation of the testes
    • Dmrt1 gene maintains testicular structure

    Androgen Insensitivity Syndrome

    • Androgen insensitivity syndrome is a genetic condition where individuals, despite having an XY chromosome, are resistant to testosterone
    • The phenotype of these individuals tends to be female
    • Individuals with androgen insensitivity syndrome have undescended testes and female external sex characteristics, but no uterus or oviduct.

    Importance of Timing

    • The Sry gene only functions in a brief time window during embryonic development
    • Delay in Sry gene activation can lead to ovary development instead of testes

    Temperature-Dependent Sex Determination

    • Temperature-dependent sex determination is seen in some reptiles, such as turtles and crocodiles
    • Red-eared slider turtle: below 28℃ all males, above 31℃ all females, intermediate temperature males and females
    • Snapping turtle: at or below 22℃ all females, at or above 28℃ all males, intermediate temperature males

    Importance of TFs in Various Stages

    • Wnt4 promotes Mullerian duct development and inhibits Sry from activating Sox9
    • Rspo1 promotes Wnt pathway activation, resulting in the accumulation of β-catenin
    • β-catenin further activates genes for Rspo1 and Wnt4 and activates follistatin gene
    • Follistatin helps organize ovaries and prevents Sox9 synthesis
    • Sry and Sox9 activate testis-determining genes and block ovary formation
    • Dmrt1 maintains testicular structure and inhibits female development
    • SF1 activates transcription of anti-Müllerian hormone and promotes testosterone synthesis
    • Fox12 maintains ovarian structure and prevents testis development

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    Chromosomal Disorders & Sex Determination PDF

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    Test your knowledge on the fundamentals of genetics with this quiz. Explore concepts such as DNA, genes, nucleic acids, and the structures of nucleotides. Perfect for students beginning their journey in genetics!

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