Lec 54: Medical Genetics I: Introduction to Medical Genetics Flashcards Preview

Fundamentals > Lec 54: Medical Genetics I: Introduction to Medical Genetics > Flashcards

Flashcards in Lec 54: Medical Genetics I: Introduction to Medical Genetics Deck (31):

Chromosomal disorders:

Chromosomal disorders: Due to gain or loss of segments or entire chromosomes


Single gene disorders:

Single gene disorders: Due to individual mutant genes


Multifactorial disorders:

Multifactorial disorders: Due to multiple genes, often acting in concert with environmental factors



Allele: one of the different forms (variants) of a gene within a population



Homozygote: An individual with two identical alleles



Heterozygote: An individual with two different alleles



Hemizygote: An individual with only one allele, chromosome or chromosome segment, instead of the usual two


Dominant trait (or allele):

Dominant trait (or allele): one that is expressed in the heterozygote


Recessive trait (or allele):

Recessive trait (or allele): one that is expressed only in homozygotes or hemizygotes



Genotype: genetic constitution; the genotypic ratio in the offspring is 1CC : 2Cc : 1cc.



Phenotype: observable expression of genes; if c is recessive, the phenotypic ratio in the offspring is 3 normal : 1 affected.



Carrier: heterozygote for a mutant allele; generally used for a recessive disorder.


Explain Mendel’s insights into the nature of inheritance and how traits are transmitted from parent to offspring.

Each physical trait (such as plant height and flower color in peas) is controlled by a pair of hereditary determinants (genes). For each trait, an individual inherits one gene from the father and one from the mother. Law of segregation: Each gamete (oocyte or sperm) receives only one allele of a pair, and alleles segregate equally. That is, if an individual is heterozygous (Cc), half the gametes will receive the C allele, and half will receive the c allele.


How many stages of meiosis are there and generally happens?

Meiosis I: Chromosomes divide (diploid) Meisosi II: Chromatids divide (haploid)


Relate meiosis and Mendel’s laws of segregation and independent assortment of genes:

Accounts for Mendel’s laws of segregation and independent assortment of genes. During meiosis, the homologs segregate away from each other. Each gamete receives only one chromosome from each homologous pair and thus only one allele of each pair. This accounts for equal segregation of alleles, and allows one to predict the types and frequencies of offspring of a particular mating. The independent assortment of genes arises from the fact that each pair of chromosomes segregates independently from the others. If two genes controlling different traits are located on different chromosomes, all combinations of alleles are possible, depending on how the chromosomes line up on the equator at metaphase in meiosis I. Thus, an individual who is heterozygous for each of two gene pairs on different chromosomes (Aa Bb) would produce four kinds of gametes: AB, Ab, aB, and ab


Describe the basic organization of chromosomes and list the major components and their characteristics.

Basic organization: chromosome have a telomere with a short and long arm divide by a centromere. The structure and number of chromosomes (karyotype) is characteristic for each species. A standard description of a karyotype lists the total number of chromosomes, followed by the sex chromosome constitution and a description of any chromosomal abnormalities that may be present; 46,XY indicates a normal male karyotype.


Describe the normal human karyotype:

Normal karyotype: Male46, XY Female46, XX



Telomere: chromosome end Centromere: heterochromatin to which the spindle fibers attach during cell division Metacentric: having the centromere in the middle Submetacentric: having the centromere somewhat distant from the middle Acrocentric: having the centromere near the telomere



Centromere: heterochromatin to which the spindle fibers attach during cell division



Metacentric: having the centromere in the middle



Submetacentric: having the centromere somewhat distant from the middle



Acrocentric: having the centromere near the telomere


Describe chromosomal numerical abnormalities:

Numerical abnormalities: The terms euploid and aneuploid are frequently used to refer to numerical abnormalities.

  • Euploid: describes an exact multiple of the number of chromosomes in a normal gamete
  • Aneuploidy usually refers to the gain or loss of a single chromosome. Aneuploidy arises during mitosis or meiosis by a failure of chromosomes to separate normally (nondisjunction). If one set of chromatids fails to separate, one of the daughter cells will be trisomic and the other will be monosomic for that particular chromosome.
    • If nondisjunction occurs during embryonic development, the individual will be a mosaic of normal and abnormal cells, unless the nondisjunction occurs at the first mitotic division in which case all cells will be aneuploid.
      • Aneuploidy for one or more chromosomes is also a common feature of human cancers.
        • Cancer cells can be also polyploid, i.e., have more than two complete sets of chromosomes.


What is a chromosomal structural abnormalities?

Structural abnormalities: Chromosome structural abnormalities can result in the gain or loss of portions of chromosomes, chromosome fusions, inversions of portions of chromosomes or exchange of material between chromosomes.


What is a translocation abnormality? Describe two types:

Translocation: exchange of chromosome segments between non-homologous chromosomes; two main types are reciprocal translocations and Robertsonian translocations.

  • Reciprocal translocations: material is simply exchanged between two chromosomes.
  • Robertsonian translocation: essentially a fusion at the centromere between two acrocentric chromosomes, with the loss of the short arms and the satellites.
    • Common Robertsonian translocations are confined to the acrocentric chromosomes 13, 14, 15, 21 and 22, because the short arms of these chromosomes contain no essential genetic material.


Describe the following structural abnormalities: Duplication

Duplication: extra copy of part of a chromosome, leading to partial trisomy



Describe the following structural abnormalities: Inversion

Inversion: breakage and rejoining of a chromosome segment in reverse order, pericentric if the centromere is included, paracentric if not



Describe the following structural abnormalities: ring chromosome

Ring chromosome: deletion of telomeres and fusion of the broken ends to form a ring; very unstable during mitosis and frequently lost



Describe the following structural abnormalities: insertion

Insertion: segment of one chromosome inserted into a non-homologous chromosome



Describe the following structural abnormalities: isochromosome

Isochromosome: chromosome having one arm duplicated and the other arm lost; arises when the centromere divides abnormally and can involve either the short (ISO p) or long (ISO q) arm


Describe the most common chromosome aberration syndromes in humans and discuss associated risk factors:


  • Down’s syndrome: trisomy 21
    • risk factor (maternal age)
  • Trisomy 13 (Patau syndrome): typically very short life span
  • Trisomy 18 (Edwards syndrome): typically very short life span


Most common aberration syndromes are on the sex chromosome:

  • Turner Syndrome: Paternal non-disjunction accounts for the majority (70-80%) of Turner syndrome cases.
  • Kleinfelter:
  • Triple X:
  • XYY:

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