1. Chromosomes, Genes & Alleles

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)

13. Humans have 22 pairs of homologous (matched) autosomes

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

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

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 copiesLDLR gene

33. Naming Genes Genes are usually named after the functions that they control.

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)

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