Aneuploidy and Polyploidy in Humans
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Questions and Answers

Which term describes cells with complete sets of chromosomes?

  • Euploid (correct)
  • Trisomic
  • Aneuploid
  • Monosomic

    • What results from nondisjunction during meiosis?

  • Diploid gametes
  • Haploid gametes
  • Normal gametes
  • Aneuploid gametes (correct)

    • How does aneuploidy usually affect survival in humans?

  • Only sex-chromosome aneuploidies affect survival
  • Aneuploid individuals typically have enhanced survival
  • Most aneuploid conditions are compatible with life
  • Aneuploidy often results in nonviable embryos or newborns (correct)

    • What mechanism leads to an imbalance of gene products in aneuploid individuals?

    <p>Gene dosage changes</p> Signup and view all the answers

    Which of the following describes the consequence of chromosome nondisjunction in somatic cells?

    <p>One daughter cell has an extra chromosome and the other is missing one</p> Signup and view all the answers

    Which type of offspring can result from the fusion of aneuploid gametes with normal gametes?

    <p>Trisomic and monosomic offspring</p> Signup and view all the answers

    What is the significance of chromosome breakage mechanisms?

    <p>They can lead to chromosomal abnormalities</p> Signup and view all the answers

    Which of the following statements is true regarding human trisomies?

    <p>Only trisomy 21 has a life expectancy greater than six months</p> Signup and view all the answers

    Which type of plants are known to tolerate changes in gene dosage better than animals?

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

    What is a common outcome of aneuploid gametes in plants compared to animals?

    <p>They are more frequently viable</p> Signup and view all the answers

    What is the primary consequence of polyploidy in organisms?

    <p>It leads to a doubling of the entire genome.</p> Signup and view all the answers

    Which statement accurately describes fetal hemoglobin in comparison to adult hemoglobin?

    <p>Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin.</p> Signup and view all the answers

    What can result from unequal crossing over during meiosis?

    <p>It can result in duplication on one chromosome and deletion on another.</p> Signup and view all the answers

    What is a characteristic of a pericentric inversion?

    <p>It occurs when the centromere is within the inverted region.</p> Signup and view all the answers

    Which mechanism leads to gene duplication and potentially contributes to evolutionary complexity?

    <p>Unequal crossing over.</p> Signup and view all the answers

    Which type of chromosome translocation arises with no reciprocal exchange of genetic material?

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

    What can result from a terminal deletion of a chromosome?

    <p>The chromosome loses a segment, reducing its total gene dosage.</p> Signup and view all the answers

    What is the outcome of chromosome breakage followed by misattachment of the broken ends?

    <p>Translocation and potential gene dosage imbalance.</p> Signup and view all the answers

    In terms of evolutionary implications, what is one consequence of gene duplications?

    <p>They serve as a source for new functions through divergence.</p> Signup and view all the answers

    What can chromosome inversions result in if no critical genes are mutated?

    <p>No phenotypic consequences if gene dosage remains balanced.</p> Signup and view all the answers

    Which type of polyploidy results from the combination of chromosome sets from different species?

    <p>Allopolyploidy.</p> Signup and view all the answers

    What is a common reason for aneuploidy in human sperm?

    <p>Exposure to environmental pollutants.</p> Signup and view all the answers

    How do paralogs form after genome duplication events?

    <p>Via relaxation of natural selection on duplicated genes.</p> Signup and view all the answers

    What is one potential outcome of large chromosomal changes detected microscopically?

    <p>They can produce severe abnormalities due to gene dosage imbalances.</p> Signup and view all the answers

    What is a key characteristic of euploid cells?

    <p>They have complete sets of chromosomes.</p> Signup and view all the answers

    What typically results from nondisjunction in germ-line cells?

    <p>Production of aneuploid offspring.</p> Signup and view all the answers

    Why do most animals have a limited survival rate for aneuploid cells?

    <p>They are highly sensitive to changes in gene dosage.</p> Signup and view all the answers

    What consequence can arise from changes in gene dosage due to aneuploidy?

    <p>Imbalance of gene products from the affected chromosome.</p> Signup and view all the answers

    Which of the following is characteristic of trisomy in human newborns?

    <p>Only specific trisomies are viable at birth.</p> Signup and view all the answers

    Which statement reflects the nature of aneuploidy in human conditions?

    <p>Trisomy 21 is the only form of aneuploidy with extended life expectancy.</p> Signup and view all the answers

    What occurs during chromosome nondisjunction in somatic cells?

    <p>One daughter cell gains an extra chromosome while the other loses one.</p> Signup and view all the answers

    How do plants differ from animals in terms of gene dosage changes?

    <p>Plants are less sensitive to gene dosage changes.</p> Signup and view all the answers

    What type of offspring results from the fusion of aneuploid gametes with normal gametes?

    <p>Trisomic or monosomic offspring.</p> Signup and view all the answers

    What is the result of autopolyploidy in an organism?

    <p>Duplicated chromosome sets within a species</p> Signup and view all the answers

    What is one proposed advantage of having duplicate genes in polyploid organisms?

    <p>They allow for gene divergence and potentially new functions.</p> Signup and view all the answers

    How do reciprocal balanced translocations differ from unbalanced translocations?

    <p>Unbalanced translocations include gene duplications in addition to gene loss.</p> Signup and view all the answers

    What typically leads to interstitial deletions of chromosomes?

    <p>Two breaks on the same chromosome and reattachment of broken ends</p> Signup and view all the answers

    Which scenario is most likely to occur during unequal crossing over?

    <p>A complete duplication of genes on one chromosome and deletion on another</p> Signup and view all the answers

    What is the key factor that influences the frequency of aneuploidy in male sperm?

    <p>Consumption of alcohol and caffeine</p> Signup and view all the answers

    In what type of inversion is the centromere found outside the inverted region?

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

    Which statement best describes Robertsonian translocations?

    <p>They result in the fusion of two non-homologous chromosomes.</p> Signup and view all the answers

    How does maternal age affect the frequency of aneuploidy in human pregnancies?

    <p>Older mothers are more likely to have pregnancies with chromosomal abnormalities.</p> Signup and view all the answers

    What are paralogs in genetics?

    <p>Genes that evolve from a common ancestral gene through duplication.</p> Signup and view all the answers

    What can larger chromosome deletions lead to in terms of gene function?

    <p>They may cause a loss of multiple gene functions simultaneously.</p> Signup and view all the answers

    What process is likely to produce a gene family?

    <p>Gene duplication followed by divergence</p> Signup and view all the answers

    Which type of chromosomal change does not typically cause phenotypic consequences?

    <p>Balanced reciprocal translocations</p> Signup and view all the answers

    What is the primary effect of chromosome breakage on the genome?

    <p>It leads to various types of chromosomal rearrangements.</p> Signup and view all the answers

    Study Notes

    Aneuploidy in Humans

    • About half of all conceptions spontaneously abort during the first trimester of pregnancy.
    • Approximately half of these aborted pregnancies (~25% of all conceptions) have abnormalities in chromosome number or structure.
    • 2-13% of male sperm are aneuploid.
    • The frequency of aneuploidy increases with the age of the female.

    Polyploidy

    • Polyploidy is the presence of more than two sets of chromosomes in the nucleus of an organism.
    • Polyploidy can result from the duplication of chromosome sets within a species (autopolyploidy) via multiple fertilization or non-disjunction.
    • Polyploidy can occur from combining chromosome sets of different species (allopolyploidy) via hybridization, which is usually infertile.

    Polyploidy and Evolution

    • Polyploidy results in a doubling of the entire genome, duplicating every gene.
    • Natural selection is relaxed on duplicated copies of genes, allowing them to accumulate mutations.
    • Over time, duplicated genes can become inactive and degrade, but they may also take on new functions.
    • Genes originating from genome duplication events retain similarities in sequence, structure, and location relative to other syntenic genes.
    • These genes are called paralogs and form gene families.

    Hemoglobin Families

    • Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin, helping the fetus harvest oxygen from maternal blood.

    Chromosome Breakage

    • Mutations that result in the loss or gain of chromosome segments can cause severe abnormalities due to gene dosage imbalances.
    • Changes can be large enough to detect microscopically (>100 kb) or small enough that only molecular methods can detect.
    • Large changes affect many genes, while small changes affect only a few.

    Partial Chromosome Deletion

    • When a chromosome breaks, both DNA strands are severed at a chromosome break point.
    • Broken chromosome ends can adhere to other broken ends or the ends of intact chromosomes.
    • An acentric fragment (lacking a centromere) of a broken chromosome can be lost during cell division because there is no kinetochore for microtubules to attach to during mitosis.

    Larger Chromosome Deletions

    • Detachment of one chromosome arm leads to a terminal deletion; the broken fragment is lost during cell division.
    • An interstitial deletion is the loss of an internal portion of a chromosome.

    Unequal Crossing Over

    • Occasionally, unequal crossover occurs between two homologous chromosomes during recombination in meiosis.
    • This can result in partial duplication on one chromosome and partial deletion on the other.

    Gene Duplications

    • Gene dosage imbalance due to duplication may result in abnormal development.
    • Duplication & divergence is a major factor in evolution, leading to larger and more complex genomes.

    Chromosome Inversion and Translocation

    • Reattachment of broken chromosome ends in the wrong orientation leads to chromosome inversion.
    • Reattachment to a non-homologous chromosome leads to chromosome translocation.

    Chromosome Inversion

    • In a paracentric inversion, the centromere is outside of the inverted region.
    • In a pericentric inversion, the centromere is within the inverted region.

    Chromosome Translocation

    • Unbalanced translocations arise when a piece of one chromosome is translocated to a non-homolog.
    • Reciprocal balanced translocations occur when pieces of two non-homologs switch places.
    • Robertsonian translocations, also called chromosome fusions, involve the fusion of two non-homologous chromosomes.

    Robertsonian Translocations

    • Robertsonian translocations are chromosomal fusions that involve the fusion of two non-homologous chromosomes.
    • For example, the fusion of human chromosomes 12 and 13 is the result of a Robertsonian translocation.

    Euploidy

    • Cells are euploid when they have complete sets of chromosomes, e.g., n, 2n, 3n.
    • The total number of chromosomes is an exact multiple of the number of chromosomes in one set.

    Aneuploidy

    • Cells are aneuploid if they contain a number of chromosomes that is not euploid.
    • Aneuploidy is due to failed chromosome and sister chromatid segregation, called chromosome nondisjunction.

    Chromosome Nondisjunction

    • Chromosome nondisjunction is the failure of homologous chromosomes or sister chromatids to separate normally during cell division.
    • In somatic cells, nondisjunction results in one daughter cell with an extra chromosome (2n+1) and the other missing one chromosome (2n-1).
    • In germ-line cells, nondisjunction results in aneuploid gametes that are either n+1 or n-1.

    Gene Dosage & Balance

    • Changes in gene dosage lead to an imbalance of gene products from the affected chromosome relative to the unaffected chromosomes.
    • Most animals are highly sensitive to changes in gene dosage.
    • Plants tolerate gene dosage changes more readily.

    Aneuploidy in Humans

    • Humans are very sensitive to gene dosage changes, and aneuploids usually do not survive.
    • Trisomies of chromosomes 8, 13, 18 & 21 are seen in newborns, and no autosomal monosomies (only trisomy 21 has life expectancy >6 months).
    • Multiple forms of sex-chromosome trisomies and one type of sex-chromosome monosomy occur.

    Euploidy

    • The number of chromosomes in a nucleus, and the relative size and shape of each chromosome, are species-specific characteristics.
    • Cells are euploid when they have complete sets of chromosomes (e.g., n, 2n, 3n)
    • The total chromosome count is an exact multiple of the number of chromosomes in one set.

    Aneuploidy

    • Cells are aneuploid if they contain a number of chromosomes that is not euploid (i.e., not an exact multiple of the number of chromosomes in one set).
    • This means one or more chromosomes are either missing or there are extra copies of them.
    • Aneuploidy is caused by failed chromosome and sister chromatid segregation during cell division.
    • This is referred to as chromosome nondisjunction, and it results in abnormalities in chromosome number.

    Chromosome Nondisjunction

    • Chromosome nondisjunction is the failure of homologous chromosomes or sister chromatids to separate normally during cell division.
    • In somatic cells, nondisjunction results in one daughter cell with an extra chromosome (2n+1) and the other daughter cell missing one chromosome (2n-1).
    • Usually, the poor survival of these cells limits their number in animals; however, some can lead to cancer.
    • In germ-line cells, nondisjunction results in aneuploid gametes that are either n+1 or n-1.

    Nondisjunction in Germ-Line Cells

    • The fusion of aneuploid gametes with normal (n) gametes produces trisomic (2n+1) or monosomic (2n-1) offspring.
    • Changes in gene dosage lead to an imbalance of gene products from the affected chromosome relative to the unaffected chromosomes.

    Gene Dosage & Balance

    • Most animals are highly sensitive to changes in gene dosage.
    • Conversely, plants tolerate gene dosage changes more readily.

    Aneuploidy in Humans

    • Humans are very sensitive to gene dosage changes, and aneuploids usually do not survive.
    • Only trisomies of chromosomes 8, 13, 18 & 21 are seen in newborns, and no autosomal monosomies (only trisomy 21 has life expectancy > 6 months).
    • Multiple forms of sex-chromosome trisomies and one type of sex-chromosome monosomy occur.

    Aneuploidy In Humans cont.

    • Trisomies and monosomies other than those found in newborn infants are known to occur.
    • Studies monitoring human pregnancies indicate that about half of all conceptions spontaneously abort during the first trimester.
    • About half of these (~ 25% of all) had abnormalities in chromosome number or structure.

    Aneuploidy in Humans cont.

    • 2-13% of male sperm are aneuploid - the frequency increases with age, smoking, alcohol, caffeine, pollution, etc.
    • The frequency of aneuploidy increases with female age.

    Changes in Euploidy Result in Various Kinds of Polyploidy

    • Polyploidy is the presence of > 2 sets of chromosomes in the nucleus of an organism.
    • Various types of polyploidy are possible e.g., triploid, tetraploid, pentaploid, hexaploid, etc.

    Changes in Euploidy

    • Polyploidy can result from duplication of chromosome sets within a species (autopolyploidy).
      • This can occur via multiple fertilization or non-disjunction.
    • Polyploidy can occur from combining chromosome sets of different species (allopolyploidy).
      • This happens via hybridization, which is usually infertile.

    Polyploidy and Evolution

    • Polyploidy results in a doubling of the entire genome, which means every gene is duplicated.
    • This allows natural selection to be relaxed on duplicated copies of genes, and they are free to accumulate mutations.
    • Over time, duplicated genes become inactive and degrade; however, sometimes they may take on new functions.
    • Genes originating from genome duplication events retain many similarities in sequence and structure as well as their relative location to other syntenic genes.
    • Such genes are called paralogs and form gene families.

    Hemoglobin Families

    • Hemoglobin is a protein in the blood that carries oxygen to the cells.
    • The family of genes encoding globin proteins demonstrates the importance of mutations in evolution.
    • Genes can duplicate and diverge, sometimes resulting in changes in gene function.

    Functions and Timing of Hemoglobin Family

    • Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin.
    • This helps the fetus harvest oxygen from maternal blood.

    Chromosome Breakage Causes Loss, Gain, and Rearrangement of Chromosomes

    • Mutations that result in loss or gain of chromosome segments can produce severe abnormalities due to gene dosage imbalances.
    • Changes may be large enough to detect microscopically (> 100kb) or small enough that only molecular methods can detect them.
    • Large changes affect many genes, and small changes affect few.

    Partial Chromosome Deletion

    • When a chromosome breaks, both DNA strands are severed at a chromosome break point.
    • The broken chromosome ends can adhere to other broken ends or the ends of intact chromosomes.
    • Because part of the broken chromosome is acentric (lacks a centromere), it may be lost during cell division. This is because there is no kinetochore for microtubules to attach to during mitosis.

    Larger Chromosome Deletions

    • Detachment of one chromosome arm leads to a terminal deletion.
    • This occurs when the broken fragment lacks a centromere and is lost during cell division.
    • An interstitial deletion is the loss of an internal portion of a chromosome and results from two chromosome breaks.

    Unequal Crossing Over

    • Occasionally, unequal crossover occurs between two homologous chromosomes during recombination in meiosis.
    • This can result in partial duplication on one chromosome and partial deletion on the other.
    • Although uncommon, this occurs when the repetitive regions of homologous chromosomes misalign.

    Gene Duplications

    • Gene dosage imbalance due to duplication may result in abnormal development.

    Gene Duplications

    • However, duplication & divergence is a major factor in evolution.
      • Genomes become larger and more complex.

    ### Chromosome Breakage Leads to Inversion and Translocation of Chromosomes

    • Sometimes chromosome breakage leads to reattachment of the wrong broken ends.
    • Reattachment in the wrong orientation leads to chromosome inversion.
    • Reattachment to a non-homologous chromosome leads to chromosome translocation.
    • If no critical genes are mutated and dose-sensitive genes remain in balance, there may be no phenotypic consequences.

    Chromosome Inversion

    • Two types of chromosome inversion can occur:
      • In a paracentric inversion, the centromere is outside of the inverted region.
      • In a pericentric inversion, the centromere is within the inverted region.

    Chromosome Translocation

    • Translocations occur when the broken ends of non-homologous chromosomes are reattached.
    • There are three types of translocations:
      • Unbalanced translocations arise when a piece of one chromosome is translocated to a non-homolog and there is no reciprocal event.

    Types of Translocations

    • Reciprocal balanced translocations occur when pieces of two non-homologs switch places.
    • Robertsonian translocations, also called chromosome fusions, involve the fusion of two non-homologous chromosomes.

    Robertsonian Translocations (Chromosomal Fusions)

    • These involve the fusion of two acrocentric chromosomes (chromosomes with a centromere close to one end) to form a single metacentric chromosome (centromere in the middle)
    • This process can involve the loss of the short arms of the acrocentric chromosomes.
    • Robertsonian translocations are common in humans and other animals.

    Chromosome 2 of Sp. 1

    • This example is a comparison of the number of chromosomes in four different species.

    Chromosome# (haploid n)

    • Sp. 4, 3, 2, and 1 are four different species.
    • The chromosome number for each species (haploid n) is 24, except for Sp. 1, which has 23.
    • The ancestral state had 24 chromosomes.

    Sp. 1

    • This diagram shows the chromosomes of species 1.
    • It illustrates that two of the chromosomes in species 1 were fused to form a single chromosome in species 2.

    Head-to-Head Telomere Fusion Site

    • This shows the point where two chromosomes fused together to form chromosome 2.

    2.6 Mb Degenerate Centromeric Region

    • This diagram shows the location of the degenerate centromere on chromosome 2.

    Orangutan, Gorilla, & Chimpanzee Human

    Chromosomes 12 & 13 Chromosome 2

    • This diagram compares the chromosomes of humans versus orangutans, gorillas, and chimpanzees.
    • It shows how human chromosome 2 is homologous to a pair of chromosomes in other great apes.
    • The orangutan, gorilla, and chimpanzee chromosomes 12 & 13 are the homologs of human chromosome 2.

    Chromosome# (haploid n)

    • The orangutan, gorilla, and chimpanzee have 24 chromosomes.

    • Humans have 23 chromosomes.

    • The diagram above illustrates that the fusion of two acrocentric chromosomes in the ancestor of humans gave rise to human chromosome 2.

    • This fusion event is a key difference in the karyotypes (chromosome set) between humans and our closest living relatives.

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    BIOL 23373 - General Genetics Lecture 15 Fall 2024 PDF

    Description

    Explore the concepts of aneuploidy and polyploidy, focusing on their significance in human reproduction and evolution. Understand the impact of chromosome abnormalities in pregnancies and the role of polyploidy in genetic diversity and adaptation. This quiz covers essential topics in genetics and reproductive biology.

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