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02 Chromosomes, Genes & Alleles
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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)
    • 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.
      Height in cm
      ABO Blood groups are controlled by
      a single gene and show discontinuous
      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
    • 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