MBG: BLOCK 2: REVIEW PACKETS FOR END OF BLOCK 2
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What is the expected frequency of phenotypes from a monohybrid cross between true-breeding parents with two different traits?

  • Phenotypes cannot be predicted
  • 1:1
  • 1:2:1
  • 3:1 (correct)

    • What is the most probable cause of a different karyotype found only in a growth on a healthy individual with a normal chromosome number?

  • Mosaicism (correct)
  • Environmental factors
  • Genetic inheritance
  • Aging

    • Which cross would yield a 1:1:1:1 ratio of four phenotypes in offspring?

  • AaBb x AaBb
  • Aa x AA
  • Aa x aa
  • AaBb x Aabb (correct)

    • What are the gametes produced by an individual who is heterozygous for two unlinked traits A and B?

    <p>AB, Ab, aB, ab</p> Signup and view all the answers

    In a cross between a short purple and a tall white plant yielding all tall purple offspring, what does this imply?

    <p>Tall phenotype is dominant</p> Signup and view all the answers

    If an individual with the genotype Aa is crossed with aa, what is the probability of obtaining recessive offspring?

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

    What type of inheritance pattern is most commonly associated with traits located on the X chromosome?

    <p>Sex-linked inheritance</p> Signup and view all the answers

    What is an example of aneuploidy related to chromosome abnormalities?

    <p>Having an extra copy of chromosome 21</p> Signup and view all the answers

    What is the expected phenotypic ratio from a dihybrid cross of Aa x Aa?

    <p>9:3:3:1</p> Signup and view all the answers

    In a test cross involving a recessive sex-linked trait, what is the probability of obtaining affected male offspring when crossing an affected female with an unaffected male?

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

    Which term describes the process involving changes in gene expression for sex determination?

    <p>Transcriptional Regulation</p> Signup and view all the answers

    What defines the bivalent in meiosis, particularly when there are three chromosomes?

    <p>A group of four chromatids</p> Signup and view all the answers

    Which of the following is the most likely cause of a trisomy during meiosis?

    <p>Meiotic nondisjunction</p> Signup and view all the answers

    What is the probability of producing carrier females when an unaffected female is crossed with an affected male for a recessive X-linked trait?

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

    How do meiotic errors differ from mitotic errors?

    <p>Meiotic errors can lead to aneuploidy</p> Signup and view all the answers

    What structure is necessary to fully condense a chromosome in its most duplicated form?

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

    What signifies the transition from euchromatin to heterochromatin?

    <p>Increased histone deacetylase activity</p> Signup and view all the answers

    What can result from nonallelic recombination of sister chromatids?

    <p>Structural rearrangements</p> Signup and view all the answers

    In terms of X-inactivation, what is the first step?

    <p>Selection of the inactive X</p> Signup and view all the answers

    What is the outcome when maternal imprinting does not occur in a differentially methylated region?

    <p>Increased gene expression</p> Signup and view all the answers

    Which type of inheritance pattern is displayed by mitochondrial genomes?

    <p>Exclusively maternal inheritance</p> Signup and view all the answers

    What is typically observed when paternal gene deletion occurs in chromosome 15’s differentially methylated region?

    <p>Prader-Willi syndrome</p> Signup and view all the answers

    The quadrivalent resolution with the best outcomes typically involves what?

    <p>Complete homologous pairs</p> Signup and view all the answers

    What effect will rapid demethylation of mono- or dimethylated histones have?

    <p>Enhancement of gene expression</p> Signup and view all the answers

    Which statement accurately describes the gene expression in Angelman syndrome?

    <p>UBE3A is expressed due to loss of the maternal copy.</p> Signup and view all the answers

    What characterizes pericentric inversions compared to paracentric inversions?

    <p>They change the chromosome arm ratio.</p> Signup and view all the answers

    What are the consequences of uniparental disomy for chromosome 15?

    <p>Loss of SNRPN and NDN expressed.</p> Signup and view all the answers

    Between isodisomy and heterodisomy in uniparental disomy, which best describes the origin of alleles?

    <p>Isodisomy has two alleles from one parent that are identical.</p> Signup and view all the answers

    Which condition is characterized by developmental and intellectual deficiencies along with epilepsy and tremors?

    <p>Angelman Syndrome.</p> Signup and view all the answers

    What alteration in the banding pattern is indicative of an inversion within chromosomes?

    <p>Breakpoints reflect altered orientations.</p> Signup and view all the answers

    What is the primary impact of loss of the maternal chromosome 15 in Prader-Willi syndrome?

    <p>Loss of both SNRPN and NDN.</p> Signup and view all the answers

    How does segmental mosaicism differ from classic mosaicism?

    <p>Multiple mutations occur but are confined to specific segments.</p> Signup and view all the answers

    What characterizes balanced chromosomal rearrangements?

    <p>They have no net loss or gain of genetic information.</p> Signup and view all the answers

    What best defines deletions in the context of genetic rearrangements?

    <p>Loss of segments that may affect adjacent genes.</p> Signup and view all the answers

    Which type of mosaicism involves the presence of two different sets of chromosomes from the same parent?

    <p>Uniparental disomy</p> Signup and view all the answers

    What distinguishes isodisomy from heterodisomy in the context of uniparental disomy?

    <p>Isodisomy consists of identical copies of a single parent's chromosome.</p> Signup and view all the answers

    What is a common consequence of unbalanced chromosomal rearrangements?

    <p>Clinically affected individuals depending on the extent of genetic material gained or lost.</p> Signup and view all the answers

    Which form of duplication occurs when a genomic segment is duplicated in the same orientation?

    <p>Tandem duplication</p> Signup and view all the answers

    How does segmental mosaicism differ from classic mosaicism?

    <p>It is characterized by the presence of different numbers of chromosomes in different cells.</p> Signup and view all the answers

    What is typically a detectable outcome of larger chromosomal changes?

    <p>Increased chances of detection due to their apparent nature.</p> Signup and view all the answers

    What is the main characteristic of segmental mosaicism?

    <p>It can affect specific regions of tissues without impacting others.</p> Signup and view all the answers

    Which type of uniparental disomy occurs when both chromosomes are identical?

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

    What is a potential consequence of uniparental disomy?

    <p>Homozygosity for recessive genes</p> Signup and view all the answers

    Which of the following best describes heterodisomy?

    <p>Two non-identical chromosomes from one parent.</p> Signup and view all the answers

    Trisomy in a chromosomal profile indicates which of the following?

    <p>Three copies of a chromosome</p> Signup and view all the answers

    Which trisomy is most commonly associated with live births?

    <p>Trisomy 21</p> Signup and view all the answers

    What is the typical survival probability for individuals with Trisomy 18?

    <p>Survival is highly variable, with many not surviving past a few days</p> Signup and view all the answers

    In what scenario is Turner syndrome observed?

    <p>Missing all or part of one X chromosome</p> Signup and view all the answers

    When does nondisjunction most commonly occur during meiosis?

    <p>During the separation of homologous chromosomes</p> Signup and view all the answers

    What distinguishes segmental mosaicism from other forms of mosaicism?

    <p>It manifests in isolated regions rather than uniformly across tissues.</p> Signup and view all the answers

    Which type of chromosome anomaly results from the presence of three copies of a chromosome?

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

    Which chromosomes are commonly affected in individuals with Turner syndrome?

    <p>Only one of the X chromosomes</p> Signup and view all the answers

    In cases of trisomic rescue, what is often the first step?

    <p>Fertilization with a trisomic conceptus</p> Signup and view all the answers

    Why might there be variable viability among various types of aneuploidies?

    <p>Functional redundancy in certain chromosomes</p> Signup and view all the answers

    Study Notes

    Autosomal Heredity

    • Aa x aa yields a 50% probability of recessive offspring
    • AA x aa yields a 0% probability of recessive offspring
    • A cross between a short purple and a tall white yields all tall purple. If several generations of self-breeding before this experiment never causes this mixed phenotype, this outcome tells you that:
      • Tall is dominant to short.
      • Purple is dominant to white.
      • Both traits are linked in cis.

    Autosomal Heredity and Probabilities

    • The dihybrid cross is most commonly used to study the independent assortment of two genes.
    • The backcross or testcross for a monohybrid would be a cross between the F1 generation and a homozygous recessive individual.
    • It is necessary to do the testcross for evaluating independent assortment because it allows you to determine the genotype of the F1 generation.
    • If a gene is linked in trans, we expect the progeny to inherit the dominant or recessive alleles together.

    Sex-linked Heredity

    • An affected female crossed with an unaffected male for a recessive sex-linked trait has a 50% probability of affected male offspring.
    • An unaffected female crossed with an affected male for a dominant X-linked trait would have a 50% chance of affected males.
    • An unaffected female crossed with an affected male for a recessive X-linked trait would have a 50% probability of producing carrier females.

    Sex Determination

    • All sex determination can be thought of as epigenetic because it involves changes in gene expression.
    • Sex determination that involves changes in conditions such as temperature is environmental sex determination.
    • Changes in the ratio of males to females expected can be caused by environmental factors like pollution, chemicals, or temperature.
    • Heterogametic sexes are those with two different sex chromosomes (e.g., XY in humans) and likely have genes for which they are hemizygous.

    Meiosis and Recombination

    • The order of meiotic prophase I is: leptotene, zygotene, pachytene, diplotene, diakinesis.
    • At the diplotene stage, chromosomes would exhibit the most force pushing them apart.
    • Homologous recombination occurs during the pachytene stage. The synaptonemal complex enables non-homologous chromosomes to align and exchange material.
    • Crossing-over begins during the pachytene stage of meiosis I.
    • The bivalent refers to paired homologous chromosomes, and if there are 3 chromosomes (a trisomy), a trivalent is formed.

    Meiosis

    • Meiotic errors are different from mitotic ones because they can result in aneuploidy (abnormal number of chromosomes), which is not possible in mitosis.
    • Proper segregation in meiosis I involves the separation of homologous chromosomes.
    • The two factors that lead to increased diversity in offspring are:
      • Independent assortment of chromosomes
      • Crossing over.

    Mitosis and Errors in Segregation

    • The most likely cause of a trisomy during meiosis is a nondisjunction event, where chromosomes fail to separate properly.
    • A mitotic error that generates mosaicism found ONLY in the placenta is known as confined placental mosaicism.
    • Two issues with mosaicism in terms of human testing, especially prenatally, are:
      • It is not always possible to determine the extent of mosaicism.
      • It is not always possible to predict the phenotypic effects of mosaicism.

    Aneuploidies

    • The only viable whole chromosome monosomy is 45,X (Turner syndrome).
    • The most likely cause of early pregnancy loss is aneuploidy.
    • Trisomy is worse than monosomy in terms of autosomes because it results in a greater imbalance of gene expression.
    • There is a phenotype associated with 47,XXY (Klinefelter syndrome) because the extra X chromosome disrupts the normal balance of gene expression.

    Chromosome Structures

    • Incorporation of CENP-A into the nucleosome promotes the formation of a centromere.
    • NORs (Nucleolar Organizing Regions) are located on the satellite regions of acrocentric chromosomes. They contain genes for ribosomal RNA.
    • In the most condensed form for duplicated chromosomes, cohesin is needed to fully condense the chromosome, while condensin holds sister chromatids together.
    • TTAGGG repeats in humans are most likely to be found at telomeres, which are protective caps at the ends of chromosomes.

    Chromosome Structures

    • When the sequence of TTAGGG in humans gets critically short, the cell is likely to enter senescence.
    • Lengthening or maintenance of telomeres should only occur in germ cells, stem cells, and some immune cells to ensure that they have the potential for continued division.

    Karyotyping

    • Normal Male: 46,XY
    • Normal Female: 46,XX
    • Trisomy of chromosome 21: 47,XX+21 or 47,XY+21
    • Karyotype of only known viable nonmosaic whole chromosome monosomy: 45,X (Turner Syndrome)
    • Klinefelter Syndrome: 47,XXY

    Chromatin

    • Histone deacetylase activity would be expected if transitioning from euchromatin to heterochromatin. Deacetylation of histones makes DNA less accessible, facilitating the formation of heterochromatin.
    • Chromatin remodeling using the Chromatin Remodeling Complex can reposition nucleosomes to regulate gene expression.
    • The addition of methyl groups is correlated with the repression of gene expression.
    • Histone 3 (H3) is expected to be trimethylated when the corresponding nucleosome is located within a region that is organized as heterochromatin.
    • Rapid demethylation of mono- or dimethylated histones would cause activation of gene expression.

    Structural Rearrangements

    • Nonallelic recombination of sister chromatids would cause gene duplication and deletion.
    • The larger the structural rearrangement in inversions and translocations, the more likely it is to be deleterious.
    • Interchromosomal exchanges occur between non-homologous chromosomes.
    • Allelic exchanges within the inversion loop of a paracentric inversion cause a deletion of genetic material.
    • The centromere is expected within an inversion loop when the inversion is pericentric.
    • The quadrivalent resolution with the best outcomes is alternate segregation, and it yields two normal chromosomes and two chromosomes with inversions.
    • Homologous centromeres will be found together under the alternate segregation pattern.

    Imprinting

    • If maternal imprinting of a differentially methylated region does NOT happen, we would expect the paternally inherited allele to be active.
    • Oogenesis ensures that only a maternal imprinting pattern is inherited by suppressing the paternal imprints during the process of oocyte development.

    Prader-Willi and Angelman

    • Maternal imprinting shuts off the UBE3A gene in the maternal allele of chromosome 15.
    • A paternal gene deletion would cause Prader-Willi syndrome if the gene loss is of chromosome 15’s differentially methylated region.
    • In addition to gene deletion, uniparental disomy (inheritance of both copies of a chromosome from one parent) can also cause the manifestation of the diseases associated with chromosome 15 anomalies.

    X-inactivation

    • The first step of X-inactivation is the recruitment of the XIST gene, which is transcribed into a long non-coding RNA that coats the inactive X chromosome.
    • The second step of X-inactivation is the silencing of genes on the inactive X chromosome.
    • PAR1 and PAR2 are pseudoautosomal regions on the X and Y chromosomes, they escape X inactivation and are expressed from both the active and inactive X chromosome.

    Non-mendelian Inheritance

    • Mitochondrial genomes are replicated independently of the nuclear genome.
    • Human mitochondria are inherited maternally.
    • The presence of paternal mitochondria when maternal inheritance is expected is termed heteroplasmy.

    A Review of Block 2

    • Remember, inheritance questions can be simplified by using Punnett squares.
    • If you get stuck, go back to the basics and change the terms to letters.
    • If there is a blank, then the question is leaning closer to a DEFINE objective.
    • If the question is shorter, it is meant to be a quick one that is based on a fact rather than something that should take a long time to work through.

    Basic Genetics

    • There are 46 human chromosomes, 22 pairs of autosomes and one pair of sex chromosomes.
    • A mosaic individual has two or more different cell lines with different karyotypes.
    • A chimera individual has cells derived from two or more different zygotes.
    • The most probable cause of this different karyotype is a mitotic error, such as nondisjunction.

    Making Gametes

    • The genotype of an individual who is heterozygous for two unlinked traits A and B is AaBb. Their possible gametes are AB, Ab, aB, ab.
    • The most probable gametes from an individual with the genotype AaBb linked in cis are AB and ab.
    • Gametes that represent recombinant gametes from an individual who is heterozygous for genes linked in trans are Ab and aB.

    Chromosomes and Meiosis

    • Chromosomes consist of linear sequences of genes which determine the physical expression of a phenotypic trait.
    • Multiple stages of chromosome condensation occur in Prophase I of meiosis: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
    • Homologous chromosomes are independent and can produce over 8.3 million unique combinations of chromosomes.
    • Premature separation of chromosomes is a major cause of aneuploidy.
    • Recombination between linked genes involves homologous crossing over, where alleles are exchanged between homologous chromosomes.
    • Recombination creates recombinant chromosomes, which have different combinations of alleles than the parental chromosomes.
    • In a double heterozygote, cis configuration means mutant alleles are on the same chromosome, while trans configuration means mutant alleles are on different homologues.
    • Homologous recombination repair uses similar mechanisms to traditional homologous recombination, such as homology search and strand invasion.
    • Inappropriate alignment during recombination can result in duplications and deletions.
    • Chromosome deletions are a common genetic abnormality.

    Cytogenetics and Chromosomes

    • The human karyotype consists of 22 pairs of autosomes and one pair of sex chromosomes.
    • Key features of a chromosome include: telomere, centromere, kinetochore, heterochromatin, and euchromatin.
    • FISH and microarray analysis are used for chromosome analysis.
    • The centromere's location determines a chromosome's shape:
      • Metacentric: middle of chromosome
      • Submetacentric: closer to one end
      • Acrocentric: near one end
      • Telocentric: at the telomere
    • Centromeres contain repetitive DNA and histone variants, including CENP-A, which is found exclusively at functional centromeres.
    • Neocentromeres form when centromeric histones inappropriately incorporate into non-centromeric regions.

    Kinetochore

    • The kinetochore is a conserved structure involved in microtubule attachment.
    • It contains regulatory components, including kinases and phosphatases, that play a critical role in microtubule attachment and regulation.
    • Aurora B kinase and cyclin-dependent kinase 1 (CDK1) are crucial for kinetochore function.

    Telomeres and Telomerase

    • Telomeres protect chromosome ends through capping.
    • Short telomeres can lead to reduced chromosome integrity, cellular senescence, and trigger DNA repair mechanisms in G1 phase.
    • Telomerase maintains telomere length in stem cells but should not be active in normal somatic cells.
    • Telomerase consists of two components: hTERT and hTR.

    Nucleolar Organizer Regions (NORs)

    • NORs are located on the satellite stalks of acrocentric chromosomes.
    • They play a critical role in nucleoli formation during interphase.
    • NORs contain ribosomal RNA genes and are involved in rRNA production.

    Aneuploidy

    • Trisomy: Three copies of a chromosome
    • Monosomy: One copy of a chromosome
    • Trisomy for all autosomes has been reported in spontaneous pregnancy losses, with varying frequencies.
    • Monosomies are extremely rare in both spontaneous losses and live births, suggesting lethality before sufficient tissue development.
    • Turner Syndrome (45,X) is the sole exception for adult viability in monosomy.

    Mechanisms of Meiosis I Nondisjunction and Mitotic Nondisjunction

    • Meiosis I nondisjunction occurs when homologous chromosomes fail to separate properly during meiosis I.
    • Both chromosomes can migrate to one pole, or premature separation can occur.
    • Mitotic nondisjunction occurs when sister chromatids fail to separate properly during mitosis, resulting in daughter cells with uneven genetic content.

    Timing and Consequences of Mitotic Nondisjunction

    • Mitotic nondisjunction can lead to varied outcomes depending on the timing and cell type affected.
    • Mosaicism is possible, with different cell populations having different genetic compositions.
    • Confined placental mosaicism (CPM) and fetal mosaicism can occur.

    Uniparental Disomy (UPD)

    • UPD occurs when both copies of a chromosome or chromosome region are derived from a single parent instead of one copy from each parent.
    • Most UPD cases have no phenotypic consequences but can have implications for differentially methylated regions and autosomal recessive gene homozygosity.
    • UPD can be either isodisomy (both chromosomes are identical) or heterodisomy (both chromosomes are different).
    • Mechanisms of UPD include:
      • Gametic complementation
      • Trisomic rescue
      • Mitotic error and rescue

    Chromosome Rearrangements

    • Nonallelic recombination due to regions of homology can lead to chromosome rearrangements.
    • Low copy repeats are frequently involved, while high copy repeats can also contribute.
    • Examples include Alu-mediated recombination and α-satellite recombination.

    Types of Recombination

    • Recombination can be interchromosomal (across homologs) or intrachromosomal (within or between sister chromatids).
    • Normal recombination occurs at the same loci or alleles on homologous chromosomes or sister chromatids.

    Balanced vs. Unbalanced Rearrangements

    • Balanced rearrangements involve no net loss or gain of genetic information.
    • They are generally phenotypically normal, although positional effects may cause observable phenotypes.
    • Unbalanced rearrangements involve additional or missing genetic material, leading to clinical consequences.
    • Molecular cytogenetic techniques are crucial for detecting chromosome rearrangements.

    Categories of Structural Rearrangements

    • Deletions (partial aneuploidies), Duplications, Inversions, Translocations

    Deletions

    • Deletions are losses of genetic material, also known as partial aneuploidies, segmental aneusomies, or contiguous gene disorders.
    • Deletions can affect adjacent material.
    • Small deletions can follow Mendelian inheritance, while most are de novo.
    • Expressivity and penetrance are important considerations for deletions.
    • Gonadal mosaicism in parents and dynamic mosaicism from postzygotic mitoses are possible.

    Duplications

    • Duplications are extra copies of a genomic segment, causing partial trisomy.
    • Pure duplications have no other imbalances, while combination with other rearrangements is also possible.
    • Tandem duplications involve a contiguous doubling of a segment:
      • Direct: Same orientation as original segment, most common in humans.
      • Inverted: Opposite orientation.
    • Duplication phenotypes are generally less severe than deletion phenotypes.

    Inversions

    • Inversions are intrachromosomal rearrangements where a segment is reversed.
    • Breakpoints occur at two locations on the same chromosome.
    • Two types:
      • Pericentric: Breakpoints on either side of the centromere, affecting arm ratio.
      • Paracentric: Breakpoints on the same side of the centromere, affecting only one arm.
    • Presence is indicated by altered banding pattern.

    Crossing Over in Inversions

    • Crossing over in inversions can create chromosomal loops.
    • This can lead to deletions, duplications, or other chromosomal abnormalities.

    Disease Examples: Prader-Willi vs. Angelman Syndrome

    • Both Prader-Willi and Angelman syndromes are caused by deletions or UPD in the 15q11-13 region.
    • The phenotypic consequences depend on whether the maternal or paternal copy of the region is affected.
    • Prader-Willi: Loss of paternal copy, leading to hypotonia, obesity, and hypogonadism.
    • Angelman: Loss of maternal copy, leading to developmental and intellectual deficiencies, epilepsy, and tremors.

    Translocations

    • Translocations are interchromosomal rearrangements where material is exchanged between nonhomologous chromosomes.
    • Can be reciprocal (exchange of material between two chromosomes) or Robertsonian (fusion of two acrocentric chromosomes).
    • Balanced translocations are generally phenotypically normal, while unbalanced translocations can cause significant problems.

    Common Examples of Human Chromosome Rearrangements

    • Inversions:
      • _Inversion 9:** A common inversion that may result in a slight risk of infertility or birth defects.
    • Deletions:
      • Cri-du-chat syndrome (5p-): Characterized by severe intellectual disability, a distinctive cat-like cry, and other physical abnormalities.
      • Wolf-Hirschhorn syndrome (4p-): A syndrome with a wide range of symptoms, including intellectual disability, facial dysmorphic features, and seizures.
    • Duplications:
      • Duplication 15q 11.2-13: A common duplication contributing to Angelman syndrome.
    • Translocations:
      • Philadelphia chromosome (t(9;22)): Associated with Chronic Myeloid Leukemia (CML).
      • Robertsonian translocation (t(13;14), t(14;21) or t(15;21): Can lead to recurrent miscarriages or the birth of infants with genetic disorders.

    Applications of Cytogenetics

    • Paternity testing: Identifying the father of a child.
    • Prenatal diagnosis: Screening for chromosomal abnormalities in a fetus before birth.
    • Cancer diagnosis and prognosis: Identifying chromosomal abnormalities that can indicate the presence of cancer or predict its course.
    • Human evolution: Studying chromosomal rearrangements that have occurred over time to help understand the evolution of the human genome

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