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Genetics after Mendel

Explore the concepts of polygene inheritance, multiple alleles, and the role of chromosomes in genetic inheritance. Understand how these discoveries have shaped our understanding of genetics after Mendel.

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Genetics after Mendel

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  1. Genetics after Mendel

  2. http://www.youtube.com/watch?v=2xpTz7SUbnc Mendel’s song

  3. Santhi Soundarajan was born in 1981 in the village of Kathakkurichi, India. She holds the Indian national record for the women’s 3000 meters steeplechase clocking 10:44.65 seconds. At a national meet in Bangalore in July 2005 she won the 800m, 1,500m and 3000m. She won the silver medal in 800 m at the Asian Championships in Incheon, South Korea in 2005. Santhi Soundarajan is a gifted athlete

  4. Santhi Soundarajan She won the silver medal for 800 meters at the 2006 Asian Games in Doha, Qatar. Following her silver medal performance, her sex was officially questioned for the first time.

  5. How could this happen? She looked like a woman. She grew up as a woman. Her story could have never happened if all we knew was Mendelian genetics. Santhi’s story is linked to key genetic discoveries of the 20th century.

  6. Mendel Assumed • Traits are largely due to the action of a single factor (gene) • BUT most traits are polygenic • Height • Skin color • Disease predisposition • Heart disease • High blood pressure • Diabetes • Cancer

  7. 1) Concept of Polygene Inheritance • Most traits are determined by several genes at different places on the chromosomes. • e.g. Human height

  8. Mendel Assumed Traits are determined by only two possible alleles, e.g. tall vs. short But many traits have more than two possibilities

  9. 2) Concept of Multiple Alleles • Many traits have multiple alleles not just two, e.g. ABO blood type there are 3 alleles = A, B, O So these are the possible 6 genotypes: AA, AB, AO, BB, BO, OO AA & AO are blood type “A” AB is blood type “AB” BB & BO are blood type “B” OO is blood type “O”

  10. Mendel assumed that his factors only affected one trait • But we now know that some genes affect many traits. • 3) Concept of pleiotropy • e.g. Marfan’s syndrome

  11. Marfan’s Syndrome Long limbs m& fingers Flexible joints Tall and skinny Near sighted Defects in heart valves & aorta Misfolding of fibrillin-1 which forms elastic fibers in connective tissue. Mutation in gene on 15th chromosome. Autosomal dominant

  12. And Mendel never knew where his factors were located. WE DO NOW4) Chromosomes discovered in 1875

  13. We know his factors (now called genes) are arranged in a linear array along the chromosomes. This concept of the chromosome theory of inheritance was proposed in 1903. We now know where all of the major genes are located on the human chromosomes.

  14. Human x chromosome has 1098 genes. The banding is due to staining and can be seen with a microscope The 59 diseases shown here can be traced to defective (mutant) alleles

  15. 0 When do you see chromosomes? • You have to use stains to see them • Chromosomes are seen only in condensed form when preparing for nuclear division • Otherwise the genetic material is unraveled & called chromatin

  16. 5) Chromosome Numbers Vary Among Species • Ant 2 • Mosquito 6 • Orange 18, 27, 36 • Frog 26 • Humans 46 • Cattle 60 • Spanish Butterfly 380 • Adders Tongue (plant) 1020

  17. 6. Cromosomes Come In PairsHuman Karyotype= Chromosome Pattern Humans have 46 chrosomes----23 pairs

  18. Humans Have 46 Chromosomes in most cells.23 Pairs • Diploid Number = 46 (2n) • Haploid Number = 23 (n) • Polyploid Numbers = 3n, 4n, 5n,…….etc. can occur in some intestinal and liver cells

  19. Humans Have 46 Chromosomes 23 Pairs • One set of 23 from father • One set of 23 from mother 46

  20. Humans Have 46 Chromosomes 23 Pairs • 22 Pairs are autosomes • 1 Pair are sex chromosomes ♀ = xx ♂ = xy

  21. 7) Mendel’s Factors (Genes) occur in a particular place= locus Remember this?

  22. Sister chromosomes carry their alleles in the same position 2 Alleles Homozygous Dominant T T T t Heterozygous t t Homozygous Recessive

  23. 8) Many Genes Exist on Each Chromosome • Viruses = 5  10 genes • Bacteria = 1,000  6,000 genes • Human ~ 21,000 genes

  24. 9) Concept of sex-linked traits = Traits carried on the sex chromosomes Duchenne muscular dystrophy X chromosome Pattern baldness Color blindness Hemophilia A

  25. 9) Concept of sex-linked traits = Traits carried on the sex chromosomes ♂ have only one X chromosome. So if they have the allele for the trait, they will show it. ♀ have 2 X’s and so if one is defective, the other can compensate.

  26. 10) Concept of Linkage • Genes on the same chromosome are linked: Example: 2 sister chromosomes where Y y R r Y & R are linked They do not sort independently y & r are linked

  27. Which mating combinations would be possible if RY were linked & ry were linked? 1) RRYY 2) RRYy Not possible 3) RrYy 4) rryy RRYY RrYy RrYy rryy Only these are possible

  28. 11) Cell Division Occurs

  29. Cell division in the body cells 46 46 46 Mitosis Daughter cells have same # chromosomes as parent cell Mitosis Before this happens, the chromosomes must duplicate

  30. 11) Cell Division Occurs Each arm is called a chromatid Step 1: The chromosomes duplicate Producing two identical arms on each one, each with the same alleles

  31. 11) Cell Division Occurs Each arm is called a chromatid Step 1: The chromosomes duplicate Producing two identical arms on each one, each with the same alleles

  32. 1. Chromosomes are duplicated Mitosis 2. The chromosomes line up 3.The chromatids are pulled apart 46 chromosomes in each cell

  33. Cell Division in the Sex Organs (In the Testes) 46 Diploid Meiosis = Reduction Division Daughter cells with ½ # Chromosomes As parent cell 23 23 23 23 23 23 Haploid 4 Sperm

  34. Cell Division in the Sex Organs (In the Testes) Notice: 2 rounds of cell division 46 Meiosis I: cells become haploid 23 23 23 23 23 23 Meiosis II: The chromatids are split apart 4 Sperm

  35. 1. Chromosomes are duplicated Meiosis I 2. Sister chromosomes line up side by side 3. Sister chromosomes separate—going to different daughter cells Meiosis II 4. Chromosomes line up again 5. Chromatids are pulled apart 6. 23 chromosomes are in each cell

  36. Division in Sex Organs (In the Ovary) 46 Diploid 23 Meiosis 23 23 Egg + Haploid 3 Polar Bodies

  37. Division in Sex Organs ) In Testes In Ovary 46 46 Diploid 23 23 23 23 23 23 23 23 Egg + Haploid 3 Polar Bodies 4 Sperm

  38. Distribution of Chromosomes in gametes The gametes end up with only one set of chromosomes. It is random which chromosome of a pair goes into which gamete. • Each gamete has a mixture of the mother & father’s chromosomes. 223 = ~ 8 million combinations Sexual Recombination= Variety

  39. 12) Concept of Crossover = Sister chromosomes swap pieces during meiosis. This occurs when the sister chromosomes line up during Meiosis I Genetic Recombination= Variety

  40. Remember this? Sister chromosomes line up side by side This is when crossover occurs Meiosis

  41. Crossover breaks up linkages on the same chromosome So assume RY are linked and ry are linked. Independent assortment wasn’t possible. The only gametes that could be formed are RY and ry. Crossover can break these linkages producing new gamete possibilities: RY, ry, Ry, and rY Genetic Recombination= Variety

  42. Now back to our story

  43. Indian athlete Santhi Soundararajan fails gender test • Sports writer KP Mohan said that a team of doctors, including a gynecologist, endocrinologist and psychologist examined the woman athletes and put them through physical and clinical examinations. • Santhi Soundararajan's test was done soon after she came second in the women's 800m race on 9 December, 2006. She was disqualified soon afterwards. It is not clear how she failed the test at the Asian Games in Doha.

  44. How is Sex Characterized in Humans? In addition to anatomy, physiology, and chromosomes, there is another answer: Genes:Specific genes determine whether an embryo will develop as a male or female.

  45. SRY (Sex- Determining Region of the Y chromosome) Gene • Early in development, the immature gonads of males and females are indistinguishable. • Males: In the 7th week of development, the SRY gene on the Y chromosome activates a number of genes, and the gonads develop as testes. • Females: lack the SRY gene; gonads develop as ovaries by default.

  46. What about Santhi Soundararajan? An anonymous official reported that Ms. Soundarajan's tests revealed more Y chromosomes than are usually present in the genetic make-up of a female. Yet she has female anatomy.

  47. How could this occur? Crossing overcan occur anywhere along the autosomes. The sex chromosomes (X and Y chromosomes) normally cross over only at their tips. Sometimes the SRY gene is involved

  48. What about Santhi Soundararajan? If she has female genitalia, which condition is most likely? • She is XY and SRY- • She is XX and SRY+ • She is XX and SRY- • She is XY and SRY+ 30

  49. If you were a member of the Asian Games medal committee, and Santhi’s karyotype revealed that she is XY and SRY-, what would you do? • She has female genitalia, allow her to keep her medal • She is genetically male, take her medal away • She is genetically female, allow her to keep her medal • She is a hermaphrodite and should not be allowed to compete in women’s events. 30

  50. What about Santhi Soundararajan? In September2007, Santhi Soundarajan was reported to have attempted suicide, reportedly by consuming pesticide at her residence. The attempt was blamed on gender, economic, and sports pressure in India. Santhi is now a successful track coach in India.

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