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Dwarfism

Dwarfism. Can be caused by over 300 different things! A form of dwarfism called Achondroplasia is caused by a mutation to a gene (FGFR3) that regulates bone growth. The mutation makes the FGFR3 gene too aggressive, which stops cartilage being turned to bone so limbs don’t grow properly.

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Dwarfism

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  1. Dwarfism Can be caused by over 300 different things! A form of dwarfism called Achondroplasia is caused by a mutation to a gene (FGFR3) that regulates bone growth. The mutation makes the FGFR3 gene too aggressive, which stops cartilage being turned to bone so limbs don’t grow properly.

  2. Gene-Gene interactions

  3. Gene-gene Interactions Most traits are actually controlled by more than one gene Epistasis: when the expression of one gene affects the expression of another E.g: Biochemical pathway with 2 steps controlled by 2 genes A B C Gene 1 Gene 2 Enzyme 1 Enzyme 2

  4. Epistasis Collaboration Complementary Genes Supplementary Genes

  5. Collaboration Two genes influence the same trait, and interact to produce phenotypes that could not result from the action of either gene alone The genes interact to produce FOUR possible phenotypes Combs in chickens: Genes P and R control comb P_R_ Walnut ppR_ Rose P_rrPea pprrSingle

  6. Complete a punnet square for a cross between two chickens PpRr x PpRr • How many offspring have Walnut combs? 9 • How many have pea? 3 • How many have rose? 3 • How many have single 1 So the typical ratio for Collaborative Genes is 9:3:3:1

  7. Epistasis Collaboration Complementary Genes Supplementary Genes

  8. Supplementary Genes When one gene masks or alters the effect of another gene There are THREE possible phenotypes In mice, coat colour is controlled by gene A and gene B Gene A makes melanin for coat colour Gene B deposits the melanin For black coat colour, a dominant A allele is needed (causes lots of melanin produced) AND a dominant B is needed to deposit the melanin (A_B_) For brown coat colour, no dominant A allele is present (causes only a small amount of melanin produced) AND a dominant B is needed to deposit the melanin (aaB_) If no dominant B allele is present, the melanin is not deposited and so the mouse will be white (_ _ bb)

  9. Complete a punnet square for a cross between two mice AaBb x AaBb • How many offspring are Black? 9 • How many are Brown? 3 • How many are White? 4 So the typical ratio for Supplementary Genes is 9:3:4

  10. Epistasis Collaboration Complementary Genes Supplementary Genes

  11. Complimentary Genes Some genes can only be expressed in the presence of other genes. These genes are said to be complimentary; both genes need a dominant allele for the final phenotype to be produced. There are TWO possible phenotypes Sweet pea flower colour involves gene P and gene C. For the flower to be purple, dominant alleles for BOTH genes must be present. Otherwise the flower will be white.

  12. Flower Colour Colourless starting chemical Gene P Gene C Colourless (white) intermediate PURPLE PRODUCT Enzyme A Enzyme B Purple: White: P_C_ P_cc, ppC_, ppcc

  13. Complete a punnet square for a cross between two flowers PpCc x PpCc • How many offspring are Purple? 9 • How many are White? 7 So the typical ratio for Complementary Genes is 9:7

  14. Worksheet

  15. RECAP Match the definition with the type of gene interaction…

  16. Sex Linked Genes

  17. What do you remember? • The human sex chromosomes are…. • The genotype of a female is….. • The genotype of a male is…. • Is it the same for all species?? X and Y XX << This is called Homogametic (2 of the same) XY << This is called heterogametic (2 different) Nope!! In birds, butterflies and moths, the female is heterogametic and the male is homogametic. And in some insects (grasshoppers) there is no Y chromosome at all, so: XO = male and XX = female.

  18. A sex linked gene is…. A gene that is only found on only one of the sex chromosomes – either the X or the Y

  19. These are your sex chromosomes: Which sex chromosome (X or Y) do you think has the most sex linked genes and why?

  20. A good way to visualise sex-linked genes is to think of the Y as being like an X with on bottom cut off. Imagine the sex-linked genes are the ones that are carried on the the bottom right hand bar of the X, which the Y is missing. X

  21. Sex linked genes generally show up more in one sex than the other What are some examples of conditions or characteristics that show up more regularly in females or males? • Red-green colour blindness • Haemophilia(blood disorder where clotting factors aren’t produced so blood can’t clot properly) • All tortoiseshell cats are female!

  22. Red Green Colour Blindness X – Linked Recessive Disorder. XcXc or XcY to have the disorder

  23. Haemophillia A disease that impairs the body’s ability to form blood clots X-linked recessive disorder. XhXhor XhY to have the disorder

  24. Tortoiseshell Cats – They’re all Girls!

  25. Red-Green Colour Blindness An X-linked RECESSIVE disorder Who gets it more, men or women? Colour blindness is an X-linked recessivedisorder. If it’s found on the X chromosome, why do you think that more males get it than females? If the trait is recessive, males only need to receive one recessive allele to show the trait because they have nothing to mask it. Females need two recessive alleles to receive the trait, so they are less likely to show it than males.

  26. Checkpoint: If an X-linked gene for a disorder is recessive, why is it more common for males to get it than females? Either write in in your book or explain it to the person next to you.

  27. In humans, red-green colour blindness is a sex linked trait. Having the disorder (Xc) is recessive to not having the disorder (XC) What are the sexes and phenotypes (colour blind, normal, normal but carrier) of humans with with following genotypes? XCXc XCY XcXc XCXC XcY Female, Normal but carrier Male, Normal Female, colour blind Female, Normal Male, colour blind

  28. Red-Green colour blindness is an X linked recessive trait (c) A woman with normal vision but who is a carrier of the red-green colour blindness gene marries a man with normal vision. Give the predicted genotype and phenotype of their children. Woman (carrier) x Man (normal) XCXcXCY Gametes: Don’t put anything above the Y! It’s genetically empty! Genotype: XC Xc XC Y Genotype: 25% XCXC 25% XCXc, 25% XCY, 25% XcY Phenotype: 25% girl normal vision 25% girl normal but carrier 25% boy normal vision 25% boy with colour blindness XCXC XCXc XCY XcY

  29. Haemophillia is an X linked recessive trait (h) A woman with normal blood but who is a carrier of the haemophillia gene marries a man who has haemophillia. Give the predicted genotype and phenotype of their children. Woman (carrier) x Man (has haemophillia) XHXhXhY Gametes: Genotype: Don’t put anything above the Y! It’s genetically empty! XH Xh Xh Y Genotype: 25% XHXh, 25% XHXh, 25% XHY, 25% XhY Phenotype: 25% girl normal but carrier 25% girl with haemophillia 25% boy with haemophillia 25% boy normal

  30. Worksheet… If you’ve finished.. A normal sighted woman whose father was colour blind marries a man with normal vision. Give the genotypes and phenotypes, plus their ratios, that could be expected among the children of such a marriage

  31. Test Yourself: • What is a sex-linked Gene? A gene found on only ONE of sex chromosomes • Why can’t a male be a carrier for an X-linked recessive trait like colour blindness? Because he only has ONE copy of the allele, so he will either HAVE the disorder, or NOT HAVE the disorder

  32. Linked Genes

  33. Where are genes located? CHROMOSOMES!

  34. How are genes arranged on a chromosome?? Genes are arranged in a linear order on the chromosome, so one chromosome has many genes.

  35. Linked Genes are: Genes that are located on the same chromosome. A B written AB a b a b OR A b written Ab a B aB Genes A and B are LINKED

  36. Linked Genes result in fewer possible combinations of alleles in offspring…

  37. If genes are located on different chromosomes, each allele will separate during meiosis (in this case, the A allele will be able to separate from the B allele) so that new combinations can be produced. Normally…

  38. But if the genes are linked, then the alleles don’t separate – and so the traits are inheritedtogether.

  39. Checkpoint: Why do linked-genes result in fewer combinations of alleles in offspring? Because when genes are linked, the alleles aren’t separated during meiosis so they move into the gametes together.

  40. This means that the inheritance patterns we would expect to see aren’t always seen…. Flies have different phenotypes for wings and body colour. They can have: Grey body (B) Normal wings (W) Black body (b) Curly wings (w) What genotype and phenotype ratios would you expect if a fly heterozygous for body colour and wing type mated with a fly with a black body and curly wings? 1 BbWw : 1 bbWw : 1 Bbww : 1 bbww 1 Grey body, normal wings : 1 black body normal wings : 1 grey body curly wings : 1 black body curly wings. Bw BW bW bw bw

  41. BUT when the cross was actually done, the ratios that were observed were very different…. What connection can you make between the parental phenotypes and the phenotype ratio of the offspring?? BW bW bw Bw bw Much higher ratio of offspring that are of the parental type Grey body, curly wings Black Body, Curly wings Grey Body, Normal wings Black body, normal wings

  42. Why are most offspring of the parental type?? Think back to this…. If the genes are linked, then the alleles don’t separate – and so the traits are inheritedtogether.

  43. What can you remember about linked genes?

  44. A recap: Linked genes are genes that are found on the same chromosome. Linkage results in fewer genetic combinations of alleles in offspring, because traits that are linked tend to be inherited together Most offspring are of the parental type

  45. But why were there some flies that were a mixture of the parents phenotypes?? • What happens during meiosis to reshuffle alleles? • CROSSING OVER!! • When crossing over occurs between linked alleles, then they will be separated into different gametes and so you get different combinations of alleles together (recombinants)

  46. Practice… Red Hair - b Brown hair - B Freckles - f No Freckles - F A woman with red hair and freckles (bbff) marries a man with brown hair and no freckles (BbFf). They have 12 children. 6 have red hair and freckles, 5 have brown hair and no freckles and 1 has red hair and no freckles. • Is this the ratio you would expect from this cross? Support your answer with a punnet square, and state the expected ratio. • Explain in detail a possible reason why the expected ratio is not seen. • Explain how one of the children had red hair and no freckles

  47. Nature vs Nurture One of the great debates in life is whether you are the way you are because of your genes or because of the environment you were brought up in. What do you think?? Does nature or nurture have the biggest effect?? Its very difficult to determine this in reality, we would need about 1000 sets of identical twins, separated at birth, exposed to different environments and then compared later in life.

  48. Research… There are many things in the environment that can influence genes, such as: • Drugs (alcohol, cigarettes, medications) • Nutrition • Temperature • Ionising Radiation • Polluting Chemicals

  49. Drugs When taken by pregnant woman these can affect the unborn child. Thalidomide • This was a mild sedative, given to mothers for morning sickness. • This drug caused severe deformities, particularly unformed limbs, in unborn babies.

  50. Thalidomide Babies

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