02 Chromosomes, Genes & Alleles

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  • 1. Chromosomes, Genes & Alleles
  • 2. What Are Chromosomes?
    • Chargaff’s Ratios
    • 3. Every species contains identical quantities of complimentary base pairs
    • 4. But different species can have differing ratios of GC to TA
    • 5. DNAi ANIMATION
    • 6. Packaging
    • 7. DNA is made up of 2 parallel strands of complimentary nucleotides attached to a deoxyribose-phosphate backbone
    • 8. Negatively charged DNA is wrapped around positively charged histone proteins to form a nucleosome
    • 9. Histones in adjacent nucleosomes interact to form chromatin
    • 10. Strands of chromatin are folded around a protein scaffold to form chromosomes (this only occurs during cell division)
    • 11. DNAi ANIMATIONS x 2
  • Prokaryotes vs Eukaryotes
    DNA is packaged in to structures called chromosomes
    Eukaryotic chromosomes are linear strands
    1 set = haploid
    2 sets = diploid
    >2 sets = polyploid
    Prokaryotic chromosomes are circular
    Prokaryotic DNA considered to be “naked” as there is very little packaging
  • 12. Chromosomes in Humans
    • A Karyotype is the full complement of chromosomes in a particular species.
    • 13. Humans have 22 pairs of homologous (matched) autosomes
    • 14. We also have one pair of non-homologous (unmatched) sex chromosomes
  • Human Karyotype
    • Chromosomes differ with regard to their size, banding pattern and location of centromere.
    • 15. The position of a particular gene on one of these chromosomes is referred to as its locus (plural: loci)
  • Other species
    Chromosome number is not an indication of the complexity of a species.
  • 16. Sex Chromosomes
    The X chromosome may carry up to 1500 genes, the Y, only 300 and may be degenerating
    In humans the default sex is female
    Sex is determined by the male, via expression of the SRY gene located on the Y chromosome
  • 17. Kleinfelter’s Syndrome
    Due to a mistake in meiosis, an individual inherits an extra X chromosome (47 XXY)
    Occurs in 1/650 males but is asymptomatic in 500 of these
    Extremely variable symptoms
    Small testes and reduced fertility
    Lanky build / youthful features
    Reading / language impairment
    Motor impairment
    Rounded body type
    Gynecomastia (increased breast tissue)
  • 18. Turner’s Syndrome
    Due to a mistake in meiosis, an individual inherits an only one sex chromosome (45 X)
    Occurs in 1 in 2000-5000
    Phenotypically female
    Short stature, broad chest
    Low hairline, low-set ears
    Webbed neck
    Undeveloped ovaries
    Diabetes, heart disease
    Vision problems
    Learning difficulties
  • 19. Sex chromosomes in other species
    • All mammals operate using the XX/XY system
    • 20. The system in birds and some reptiles is WZ/ZZ
    • 21. Males have a homologous ZZ genotype
    • 22. Females determine the sex of offspring due to being WZ
    • 23. In reptiles such as green sea turtles and crocodiles, sex is determined by incubation temperature.
    • 24. Turtles:>31◦ = male, <28◦ = female, 29-30◦ = male/female
    • 25. The karyotype of an individual can be represented symbolically
    • 26. Human female = 46 XX
    • 27. Female tiger snake = 34 WZ
  • Genes
    The number of genes varies between species, and as can be seen is not indicative of species complexity
    Other than humans, the above species are very useful for genetic studies, WHY?
    Answer = very short generational life cycles
  • 28. Genome
    • The genome of a species is not just the number of genes, but the entire genetic complement
    • 29. Amoeba are rumored to be genetic hoarders, their junk (non-coding) DNA is retained to provide them with greater adaptability to a changing environment
  • Genes, genome, genotype
    • Only a small percentage of our DNA actually codes for a particular function
    • 30. The remainder, non-coding DNA, is slowly being discovered not to be all junk (more on this later)
    • 31. The position of a gene on a chromosome can be mapped:
    • 32. The position of a gene on a chromosome is identical in both maternal and paternal copies
    LDLR gene
  • 33. Naming Genes
    Genes are usually named after the functions that they control.
  • 34. Alleles
    • Functions can be controlled by one or many genes
    • 35. These genes can exist in different forms, called alleles
    • 36. Alleles will code for a specific phenotype (outward expression of genotype)
    • 37. Alleles are usually represented by letters of the alphabet
    • 38. Capital letters are used to express whether the allele is dominant or recessive
    • 39. eg. Genetic instructions for making a chocolate cake could be:
    • 40. Allele 1 = put cherries on top (C)
    • 41. Allele 2 = put strawberries on top (c)
    • 42. There can be many more than 2 alleles for a particular gene, but an individual can only possess 2 of these.
  • Genotype
    Your genotype is a way of expressing the two alleles that you hold for a particular gene
    Human eye colour is controlled by one gene in particular, for which there are only 2 available alleles
    B – codes for phenotypically blue eyes (dominant)
    b – codes for phenotypically brown eyes (recessive)
    You need only 1 copy of a dominant allele for it to be expressed
    You need 2 copies of a recessive allele for it to be expressed
    BB =
    bb =
    Bb =
    Brown eyes
    blue eyes
    Brown eyes
  • 43. Genotype - vocabulary
    When one possesses identical alleles on the maternal and paternal chromosome, this is referred to as a homozygous genotype.
    eg BB = homozygous dominant
    eg bb = homozygous recessive
    Having two different alleles is a heterozygous genotype.
    If you only have one copy of the chromosome in question (eg X or Y in males) and therefore only one copy of the gene, this is a hemizygous genotype
    eg SRY gene / male pattern baldness
    Some alleles can be co-dominant or display incomplete dominance
  • 44. Carriers
    An individual with an unexpressed recessive allele is said to be a carrier for a trait.
    Even though they do not express this trait, if they have offspring with another individual who is also recessive for the trait, they may produces homozygous recessive offspring.
    eg. Albanism is a recessive trait, two parents who are carriers for the trait (Aa x Aa) can have an albino child (aa)
  • 45. Multiple alleles & dominance
    There can be many more than 2 alleles for a particular gene, but an individual can only possess 2 of these.
    A good example is the ABO blood type system in humans. If blood type is represented as the letter i, capitalisation will represent dominance and lower case represents recessiveness.
    IA = type A blood
    IB = type B blood
    i = type O blood
    The following represents all combinations and results:
    Blood type A = genotypes IAIA & IAi
    Blood type B = genotypes IBIB & IBi
    Blood type AB = genotype IAIB
    Blood type O = genotype ii
    ?
    ?
    ?
    ?
  • 46. Phenotype
    The expression of a gene is determined by the combination of dominant and recessive alleles possessed by the individual
    The ABO blood group system represents not only a gene with multiple alleles, but also a system of codominance (IA & IB are codominant and i is recessive)
    Phenotypic expression is not always visible, it can be physical, biochemical or physiological
  • 47. Incomplete dominance
    Incomplete dominance is when two alleles are neither dominant nor recessive to each other
    The result is a phenotype that is a blend of the parents.
    Red flowers (RR) and white flowers (WW) will produce pink flowers (RW)
    RR
    RW
    WW
  • 48. Co-dominance
    Co-dominance is when two equally dominant alleles are both expressed in the phenotype.
    eg. Some cattle can have red hair (RR) or white hair (WW), if these are crossed, the produce roan (RW) offspring.
    Roan is not a blend, these cattle have both completely red and completely white hairs
  • 49. One or many genes
    Not all traits are controlled by a single gene, many are controlled by multiple genes.
    B
    O
    AB
    A
    Height in cm
    ABO Blood groups are controlled by
    a single gene and show discontinuous
    variation
    Height is controlled by multiple genes
    and shows continuous variation
  • 50. Sex-linked traits
    Traits governed by genes that are located on a sex chromosome are said to be sex-linked, or more specifically x-linked or y-linked.
    An example is male pattern baldness (x-linked, recessive)
    This trait is quite uncommon in females as two copies of the allele are required (XbXb)
    With only one copy, they are an asymptomatic carrier (XbX)
    As males are hemizygous for genes on the X chromosome, a single copy results in expression of the trait (XbY)
  • 51. Relationship between genotype & phenotype
    Blue and pink hydrangeas are genetically identical.
    In acidic soils (low pH) the flowers are blue
    In alkaline soils (high pH) the flowers are pink
  • 52. PKU & Phenotype
    Phenylketonuria is a serious genetic disorder in humans resulting in very low production of the enzyme phe hydroxylase.
    Phenylketonurics cannot metabolise large quantities of the amino acid phenylalanine (phe).
    In their developmental years, if fed food containing large amounts of phe, this will result in mental impairment.
    If put on a low phe diet, development will be normal
    So same genotype, different phenotype
    Genotype: pp, Phenotype: PKU with mental impairment
    Genotype: pp, Phenotype: PKU with normal development
  • 53. Siamese cats and temperature
  • 54. Siamese cats and temperature
    Precursor -> pigment
    Tyrosinase will only catalyse this reaction when temperature is lower than body temperature
    These cats are born white
    Pigmentation occurs in extremities due to lower temperature
    tyrosinase
  • 55. Phenotypic Complexity
    Female cats and other species will sometimes display evidence of X-inactivation
    This is where phenotype is determined by the allele on one X in some cells and the other X in other cells.
    The tortoiseshell colouring is a product of the different expression within the follicle-producing cells in a XoX+ cat (Xo = orange, X+ = black).
    Male tortoiseshell cats (rare) will usually be XXY.
  • 56. A new layer to phenotypic complexity
    The internal structure and organisation of DNA can also effect phenotypic expression.
    Eg. methyl groups or chromatin remodelling
    An identical mutation can have very different effects dependent on whether is on the maternal or paternal copy of the chromosome.
    Eg. Mutation on chromosome 15 at the q11.2-13 locus
  • 57. Praeder-Willi Syndrome
    If mutation occurs on the paternal chromosome …
    The result is Praeder-Willi Syndrome
    Learning difficulties
    Insatiable hunger
    Behavioural problems
  • 58. Angelman Syndrome
    If mutation occurs on the maternal chromosome …
    The result is Angelman Syndrome
    Severe learning difficulties
    Jerky movements
    Behavioural problems