Genetics - Lecture 6

• •Although most diploid species normally contain precisely two haploid chromosome sets, there are many known variations:

  –a change in the total number of chromosomes

  –the deletion or duplication of genes or segments of a chromosome

  –rearrangements of the genetic material either within or among chromosomes
• •Such changes are called chromosome mutations or chromosome aberrations

  •This is to distinguish them from gene mutations

  • Variations in chromosome number are known as aneuploidy when an organism gains or loses one or more chromosomes and has other than an exact multiple of the haploid set
• •In euploidy, complete haploid sets of chromosomes are present

  •Polyploidy occurs when more than two sets of chromosomes are present

  Chromosomal variation can arise from nondisjunction, in which chromosomes or chromatids fail to disjoin and move to opposite poles during meiosis I or II
• • Variations in chromosome number are known as aneuploidy when an organism gains or loses one or more chromosomes and has other than an exact multiple of the haploid set

  • Aneuploidy commonly causes an abnormal phenotype

  • It leads to an imbalance in the amount of gene products
• •The loss of one chromosome to produce a 2n – 1 complement is called monosomy

  •The loss of a single chromosome may have severe phenotypic effects

  •Monosomy for the X chromosome occurs in humans

  •Monosomy for any of the autosomes is usually not tolerated in humans and other animals
• •Trisomy (2n + 1 chromosomes)

  •Trisomies for autosomes are often lethal

  • For the sex chromosomes has a less dramatic impact on the phenotype

  •In trisomy, three copies of one chromosome are present, so pairing configurations are usually irregular

  •At any particular region along the chromosome length, only two of the three homologs may synapse, though different regions of the trio may be paired
• •When three copies of a chromosome are synapsed, the configuration is called a trivalent

  •In some cases, prior to the first meiotic division, one bivalent and one univalent (an unpaired chromosome) may be present instead of a trivalent
• Down syndrome results from trisomy of chromosome 21

  Down syndrome has 12 to 14 characteristics, and affected individuals express 6 to 8 on average
• •Down syndrome is usually a result of nondisjunction of the maternal chromosome 21 during meiosis

  It shows an increased incidence with increasing maternal age
• Down syndrome occasionally runs in families

  This is referred to as familial Down syndrome

  These instances involve a translocation of chromosome 21
• •Genetic counseling is recommended for women who become pregnant late in their reproductive years

  •Diagnostic testing—amniocentesis or chorionic villus sampling (CVS)—may be recommended

  • CVS involves taking tissue from the placenta, growing it, and taking a karyotype from it
• •Trisomies are often found in spontaneously aborted fetuses, but monosomies are not

  •This suggests that monosomic gametes may be very functionally impaired
• •Chromosomal (or Centric) fusion

  • Chromosomal (or Centric) fission

  Turner Syndrome:

      - XO condition

      - sterile females

      - short in stature

      - neck webbing, broad chest,

        underdeveloped breasts

      - 1/2000 births

      - abortion – 1/18
•  Klinefelter Syndrome;

      - sterile males

      - rudimentary testes, tall, long legs and 

        hands, enlargement of breasts, rounded

        hips, abnormal social development.

      - some female characteristics

      - XXY or XXXY or XXXXY or XXYY

      - 2/1000 births
• • Meta males:

      - XYY condition,20/15000

      - suggested – have criminal tendencies

      - Frequency of XYY in prison is high 

        than normal

      - studies show minimum correlation

      - less than 5% are institutionalized
• •XXX syndrome – metafemale

  •1/1200 births

  •Super or metafemale characteristics

  •Not normal

  •Sterility

  •Mental retardation

  •Other examples of trisomy syndromes are Patau syndrome (trisomy 13) and Edwards syndrome (trisomy 18)
• Patau syndrome

  •1960 – Klaus Patau

  •Karyotype of 47 chromosomes

  •Trisomy of chromosome #13 in newborns

        Affected children are:

    - mentally not alert

    - thought to be deaf

    - have harelip, cleft palate

      and polydactyly

    - malformation of most organ systems

    - abnormal developmental stages occur during the first 

      5-6 weeks of gestation

    - survival rate -  3 months

    - 1/19,000 – higher age group parents
• Edwards syndrome

  •1960 John H. Edwards

  •Trisomy of chromosome # 18

  •Similar to down syndrome child symptoms

  •Reduced life expectancy

  •Elongated skulls

  •Webbed neck

  •Survival time is less than 4 months

  •Death is usually caused by pneumonia or heart failure

  •Maternal age 34 and above

  •1/8000
• Partial monosomy

  •The cri-du-chat syndrome.

  • Part of the chromosome is lost.

  • P-arm (petit or shot arm)

      of chromosome # 5

  • 1/50,000 births

  • mental retardation, poorly developed glottis and larynx cry – similar to that of the mewing of cat
• Fragile sties in the chromosomes

  •Fail to stain certain areas

  •Gaps – fragile, weak sites

  •Susceptible to breakage

  •Chromatin is not tightly coiled
• Fragile X/Martin-Bell syndrome

  Sensitive fragile site on X chromosome causes - fragile X syndrome

  or Martin –Bell Syndrome

  Mental retardation

  1/4000 males; 1/8000 females

  Gene is not fully penetrant

  FMR1 (fragile X mental retardation protein) gene is responsible for fragile sites – several repeats of CGG

  Normal individuals have 6 - 54 repeats

  Carriers have 54 - 230 repeats
• Extra-nuclear inheritance

  1. Organelle heredity – mitochondria or 

      chloroplast with DNA

     - Uni-parental  transmission; Mother to 

       children

  2. Infectious heredity – ex. Microorganism in 

      the cytoplasm

  3. Maternal effect – nuclear gene products

      stored in the egg – transmitted to the child
• Polyploidy, in which more than 2 haploid sets of chromosomes are present, is prevalent in plants

  •The naming of polyploids is based on the number of sets of chromosomes found:–a triploid has 3n chromosomes–a tetraploid has 4n chromosomes–a pentaploid has 5n chromosomes–and so forth
• •Polyploidy can originate by

  –the addition of one or more sets of chromosomes identical to the haploid complement of the same species (autopolyploidy) or

  –the combination of chromosome sets from different species as a consequence of interspecific matings (allopolyploidy)
• •Each identical set of chromosomes is identical to parent species

  • Autotriploids can arise in several ways

  • Autotetraploids are more prevalent in a natural population as they produce balanced gametes
• •An allotetraploid arises from hybridization of two closely related species

  •If the sterile hybrid undergoes a natural chromosomal doubling, a fertile amphidiploid is produced

  •Amphidiploid plants can also be produced by somatic cell hybridization
• •Endopolyploidy is the condition in which only certain cells in an otherwise diploid organism are polyploid

  •In these cells, replication and segregation of chromosomes occur without nuclear division
• Variation occurs in the internal composition and arrangement of chromosomes

  •Rearrangements of chromosome segments include:

  –deletions

  –duplications

  –inversions

  –nonreciprocal translocations

  • reciprocal translocations
• •There are two primary ways in which the structure of chromosomes can be altered

  –The total amount of genetic information in the chromosome can change

  •Deletions

  •Duplications

  –The genetic material remains the same, but is rearranged

  •Inversions

  • Translocations