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GENE INTERACTIONS

GENE INTERACTIONS. Slide 2 Meiosis Slide 3 Crossing Over Slide 4 Mendel Slide 5 Punnet squares Slide 6 Bears Slide 7 Dihybrid Crosses Slide 8 Incomplete/Codominant Slide 9 Multiple Alleles and Lethal Genes Slide 10 Linked Genes. MEIOSIS. Meiosis is sex cell division.

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GENE INTERACTIONS

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  1. GENE INTERACTIONS Slide 2 Meiosis Slide 3 Crossing Over Slide 4 Mendel Slide 5 Punnet squares Slide 6 Bears Slide 7 Dihybrid Crosses Slide 8 Incomplete/Codominant Slide 9 Multiple Alleles and Lethal Genes Slide 10 Linked Genes

  2. MEIOSIS • Meiosis is sex cell division. • It consists of: • the DNA replicating normally • 2. Homologous chromosomes line up independently (and may cross over). • 3. A meiotic cell division. • 4. A mitotic cell division. • This has the effect of halving the chromosome number and forming gametes. • Sexual reproduction is important as it greatly increases variation in a species.

  3. CROSSING OVER During meiosis, as the homologous chromosomes line up before the first cell division, part of the neighbouring homologues may swap. The point at which the crossing over occurs is called the chiasma. Instead of two possible gametes, there are four produced. Lab manual pages 105/6, (107 opt)

  4. BASIC GENETIC CROSSES Remember Mendel? And Peas? And Punnet squares? He found that traits were inherited in chunks, called genes. • Simple monohybrid (one trait) crosses: • A purple pea is crossed with a white flowered plant (P generation). All of the offspring (F1 generation) are purple. • When the resulting plants were crossed he found that there was always a 3:1 ratio in the offspring (F2 generation). • He correctly deduced that: • the parents are separately donating their information to the offspring • the purple colour is dominant to the white flower (recessive)

  5. The cross of the F1 generation: Gametes Offspring PP is homozygous dominant pp is homozygous recessive Pp is heterozygous Also called “pure breeding” The genotype is a description of the genes contained in the individual The phenotype is a description of its physical appearance (e.g. Purple)

  6. BEARS In bears, white ears are recessive to black Momma bear (white) What is the genotype of Momma Bear? Momma bear=__________  Poppa bear (black) What genotypes could Poppa Bear have? Poppa bear=________ or _______ This is baby bear (white eared). What is his genotype? Baby bear = Can we say something more about Poppa’s genotype?

  7. Incomplete Dominance and Codominance Some alleles (gene forms) are not simply dominant or recessive. In Incomplete Dominance an intermediate phenotype is produced: In Codominance both alleles are expressed at the same time: Lab manual pages 114 and 115

  8. CwCw CRCw CRCR Incomplete Dominance • In cases of incomplete dominance, neither allele dominates and the heterozygote is intermediate in phenotype between the two homozygotes. • In crosses involving incomplete dominance, the genotype and phenotype ratios are identical. • Examples of incomplete dominance include flower color in snapdragons (right) and sweetpeas, where red and white flowered plants cross to produce pink flowered plants.

  9. CW CR CR CW Possiblefertilizations Red flower White flower Parents CRCR X CWCW Gametes F1 offspring CRCW CRCW CRCW CRCW Pink Flower Color in Snapdragons • Flower color in snapdragons exhibits incomplete dominance, with red flowered and white flowered plants crossing to produce offspring with pink flowers. Pink Pink Pink

  10. CRCR CRCw CR CR Cw CR CRCR CRCw CRCR CRCw Red Red Pink Pink Snapdragon Backcross • Determine the phenotype and genotype ratios of the offspring resulting from a backcross of the F1 heterozygote to the red parent. • 50% of the offspring are red (RR) and 50% are pink (Rr). Pink flower Red flower Parents X Gametes Possiblefertilizations Offspring

  11. CR CW CW CR CRCW CRCW CRCW CRCW Possiblefertilizations F1 offspring Roan Roan Roan Roan Gametes Red bull White cow CWCW CRCR Parents X Codominance • In cases of codominance, both alleles are independently and equallyexpressed in the heterozygote. Examples include: • Roan (stippled red and white) coat color in cattle. A cross between a red bull and a white cow produces all roan offspring. • ABO human blood groups.

  12. CW CW CR CR CRCW CRCW Possiblefertilizations Gametes Roan bull Roan cow X Parents CRCW CWCW CRCR CRCW Offspring Red Roan Roan White Codominance in Cattle • In a cross between two heterozygous (roan) shorthorn cattle, red, roan, and white offspring are produced in a 1:2:1 ratio.

  13. CR CW CR CR CRCW CRCR CRCW CRCR CRCW CRCR Possiblefertilizations Offspring Roan Roan Red Red Gametes Roan cow Red bull Parents X Crosses Involving Codominance • In examples of codominance where a true breeding red or white parent is crossed with a roan parent, the offspring will occur in a 1 : 1 ratio of the parental types (i.e. roan and red, or roan and white)

  14. CW CW CR CW CWCW CRCW CWCW CWCW CRCW CRCW Possiblefertilizations Offspring White White Roan Roan Gametes White bull Roan cow Parents X Crosses Involving Codominance

  15. Multiple alleles It is possible to have more than 2 alleles for a particular trait. A common example is the ABO blood groups in humans: O is non-functional A forms a protein with A antigen B forms a protein with B antigen A and B are codominant Lethal Genes Lethal genes are ones that cause death in the individual. The lethal gene may be dominant or recessive. In the heterozygous individual there may be some observed difference, e.g. Manx (tailless) cats. Even when dominant the lethal gene may be passed on if it does not have onset until after reproductive age (e.g. Huntington’s). Lab manual pages 116/117 and 120

  16. Lethal Alleles • Lethal alleles are gene mutations that result in a gene product which is not only non-functional, but affects organism survival. Some lethal alleles are fully dominant and are therefore lethal in the heterozygote. Dominant lethal alleles are usually eliminated rapidly, because their expression is fatal. • Exceptions occur when the expression of the allele is delayed until after reproductive maturity, as occurs in Huntington disease. • In other cases (e.g. Manx cats), the allelemutation results in a viable heterozygotewith a recognizable phenotype. • Recessive lethal alleles are fatal only inthe homozygote since their effect ismasked in the heterozygote carrier.

  17. yy YY y Y y Y Yy Yr Yy Yr Possiblefertilizations Parents Gametes X Not viable Offspring Examples of Lethal Alleles • When lethal genes prevent full term development of the embryo, offspring are produced in a 2:1 ratio (2 heterozygotes to one normal). • In the inheritance of coat color in yellow mice, offspring phenotype ratios depart from the expected Mendelian 3:1 when yellow mice are mated together. • About two thirds of the offspring are yellow, and one third are non-yellow (right). A testcross reveals the yellow colored mice to be heterozygotes.

  18. X-ray of a normal hand Brachydactyly: note the shortened bones Incidence of Lethal Alleles • The average human is heterozygous for 3 to 5 lethal recessive genes. • Example: brachydactyly in humans • Shortening of the finger bones is caused by a lethal allele; heterozygotes have shorter fingers, but homozygotes for the lethal allele die in infancy from other skeletal defects. • Of the offspring of two brachydactylic people, one in four will die in infancy, one half will show brachydactyly, and one in four will be normal (1:2:1 ratio).

  19. The Manx Mutation MML MM MML MML MLML Parents Possiblefertilizations MML Normal Manx Manx Not viable X Offspring M ML M ML Gametes • Cats produce a gene controlling spine length and therefore production of a tail. • The allele for taillessness (ML) is incompletely dominant over the allele for a normal tail (M). • The Manx allele ML interferes with spinal development and heterozygotes (ML M) have no tail (the Manx phenotype). • In ML ML homozygotes, the double dose of the allele produces an abnormal embryo, which does not survive.

  20. Multiple Alleles in Blood • The four common blood groups of the human ABO blood group system are determined by three alleles: A, B, and O (also represented in some texts as IA, IB, IO or just i). This is an example of a multiple allele system for a gene. • ABO antigens consist of sugars attached to the red blood cell surface. These sugars provide the individual antigenic properties. The alleles code for enzymes that join thesesugars together. • Allele O produces a non-functional enzymethat is unable to make changes to the basicantigen (sugar) molecule. • The other two alleles (A, B) each producea different enzyme that adds a different,specific sugar to the basic antigen. • Any one individual possesses only twoalleles and they are expressed equally. RBC RBC

  21. Multiple Alleles in Blood

  22. A B AB O Multiple Alleles in Blood • Blood donors must be compatible otherwise the red blood cells of the donated blood will clump together (agglutinate) and block capillaries.

  23. AB BB AB AB AA AB Possible fertilizations Children's genotypes Blood group: AB Blood group: AB Parent genotypes X A B A B Gametes Blood groups A AB AB B Multiple Alleles in Blood • EXAMPLE 1:For both parents with AB blood type, half of the offspring will be the same as the parents (AB), one quarter will be type A and one quarter will be type B.

  24. AO OO AO BO BO AB Possible fertilizations Children's' genotypes Blood group: B Blood group: A Parent genotypes X B O A O Gametes Blood groups B A AB O Multiple Alleles in Blood • EXAMPLE 2:Two parents with blood groups A and B respectively, may produce offspring with all four possible blood groups: AB, A, B and O. • This may only occur if both parents are carrying the allele for group O.

  25. DIHYBRID CROSSES This involves two traits that are not linked (not on the same chromosome). Each of the traits are inherited independently. In “Quarks” Two eyes (E) is dominant to one eye and Triangular shape (T) is dominant to Pentagonal. 2 Quarks both EeTt are crossed: Lab manual page 113

  26. LINKED GENES Linked genes are on the same chromosome. This means that when cell division occurs the 2 genes are very likely to stay together. So where we might expect a offspring phenotype ratio of 1:1:1:1, we actually get something else. Two genes B (Bent) and D (Dark) are linked. For a cross between BbDd and bbdd… Draw the gametes each could form. Draw a punnet square for the cross. Explain these results: Bent Dark: Bent Light: Straight Dark: Straight Light 24 1 3 22 B and D (and b and d) are linked. The 1 Bbdd and 3 bbDd individuals are due to crossing over. The different numbers are due to random chance. Lab manual page 108

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