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Chapter 11

Chapter 11. Observing Patterns in Inherited Traits. Early Ideas about Heredity. People did not know that sperm and eggs transmitted information about traits Blending Theory – fluids at fertilization mixed to give rise to offspring- later disapproved Problem:

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Chapter 11

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  1. Chapter 11 Observing Patterns in Inherited Traits

  2. Early Ideas about Heredity People did not know that sperm and eggs transmitted information about traits Blending Theory – fluids at fertilization mixed to give rise to offspring- later disapproved Problem: Would expect variation to disappear Variation in traits persists 2

  3. Cont. Darwin’s Theory of Natural Selection seemed to support this Mendel studies on pea plants showed observable evidence of how parents transmit genes to offspring 3

  4. Mendel’s experimental approach Monk from Czech Republic Raised on farm Developed improved varieties of vegetables & fruits Strong background in plant breeding and mathematics Using pea plants (Pisum sativum), found indirect but observable evidence of how parents transmit genes to offspring 4

  5. For thousands of years farmers and herders have been selectively breeding their plants and animals to produce more useful hybrids It was somewhat of a hit or miss process since the actual mechanisms governing inheritance were unknown Mendel discovered patterns of dominance and recessiveness in certain traits, which were later connected with pairs of alleles on pairs of homologous chromosomes. 5

  6. Cont. Mendel's research was with plants, the basic underlying principles of heredity that he discovered also apply to people and other animals because the mechanisms of heredity are essentially the same for all complex life forms. Mendel discovered that certain traits show up in offspring without any blending of parent characteristics. 6

  7. Mendel used pea plants partly because they are self-fertilizing and can “breed true” for certain traits. • The plants can be cross-fertilized artificially by a breeder.

  8. Terms in modern genetics • Genes- • unit of hereditary trait info • parents transmit to offspring • have specific locus on chromosomal DNA • Diploid- • 2n • pr genes on homologous chromosomes • Mutation – • alteration of gene’s molecular structure • Can change trait • Molecular forms of same gene is called an allele 8

  9. cont • Hybrids- cross between two individuals that breed true for different forms of a trait • Homologous – pr identical alleles • Heterozygous – pr of non-identical alleles • Dominant – mask traits in control • Recessive – only in control if dominant absent • Heterozygous – Aa • Pr non identical alleles • Homologous dominant – AA • pr dominant alleles • Homologous recessive – aa • pr recessive alleles 9

  10. cont Genotype – alleles carried represented by letters Phenotype – observable traits words to describe P -stands for true breeding parents F1 – first generation F2 – crossed F1 organisms (second generation) 10

  11. Tracking Generations Parental generation P mates to produce First-generation offspring F1 mate to produce Second-generation offspring F2 11

  12. Monohybrid experiment Mendel used monohybrid experiments to test a hypothesis: Pea plants inherit two units of information (genes) for a trait, one from each parent. • Two homologous parents with different traits crossed • AA (P)x aa(P) Aa (F1) • Probability-The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached • Punnett Square method –possible outcomes of offspring 12

  13. http://learning.mgccc.cc.ms.us/jd/science/carter/chapter11/animations/cross_pollination.htmlhttp://learning.mgccc.cc.ms.us/jd/science/carter/chapter11/animations/cross_pollination.html Mendel observed seven traits that are easily recognized and apparently only occur in one of two forms: 1.  flower color is purple or white 2.  seed color is yellow or green 3 . flower position is axil or terminal  4. pod shape is inflated or constricted 5. stem length is long or short 6. pod color is yellow or green 7. seed shape is round or wrinkled 13

  14. Mendel’s Monohybrid Cross Results 5,474 round 1,850 wrinkled 6,022 yellow 2,001 green 882 inflated 299 wrinkled 428 green 152 yellow F2 plants showed dominant-to-recessive ratio that averaged 3:1 705 purple 224 white 651 long stem 207 at tip 787 tall 277 dwarf 14

  15. Test Cross Individual that shows dominant phenotype is crossed with individual with recessive phenotype Examining offspring allows you to determine the genotype of the dominant individual http://learning.mgccc.cc.ms.us/jd/science/carter/chapter11/animations/test_cross.html 15

  16. Homozygous recessive Homozygous recessive a a a a A A Aa Aa Aa Aa a A aa Aa aa Aa Punnett Squares of Test Crosses Two phenotypes All dominant phenotype 16

  17. Mendel’s Theory of Segregation An individual inherits a unit of information (allele) about a trait from each parent During gamete formation, the alleles segregate from each other 17

  18. According to the principle of segregation, for any particular trait, the pair of alleles of each parent separate and only one allele passes from each parent on to an offspring.  Which allele in a parent's pair of alleles is inherited is a matter of chance.  We now know that this segregation of alleles occurs during the process of sex cell formation (i.e., meiosis 18

  19. Homozygous dominant parent Homozygous recessive parent Mendel’s Theory of Segregation (chromosomes duplicated before meiosis) meiosis I meiosis II (gametes) (gametes) fertilization produces heterozygous offspring

  20. Summary Mendel’s Theory of Segregation- Diploid cells have pairs of genes, on pairs of homologous chromosomes. The two genes of each pair are separated from each other during meiosis, so they end up in different gametes. 20

  21. Dihybrid Cross Experimental cross between individuals that are homozygous for different versions of two traits AABB x aabb -- Pure breed parents AB AB ab ab– Gametes AaBb ---------- F1 generation FIRST, OUTER, INNER, LAST (FOIL) 21

  22. Dihybrid Cross: F1 Results purple flowers, tall white flowers, dwarf TRUE- BREEDING PARENTS: AABB x aabb GAMETES: AB AB ab ab AaBb F1 HYBRID OFFSPRING: All purple-flowered, tall 22

  23. Dihybrid Cross: F2 Results http://learning.mgccc.cc.ms.us/jd/science/carter/chapter11/animations/dihybrid_v2.html X AaBb AaBb 1/4 AB 1/4 Ab 1/4 aB 1/4 ab 9/16 purple-flowered, tall 1/4 AB 1/16 AABB 1/16 AABb 1/16 AaBB 1/16 AaBb 3/16 purple-flowered, dwarf 3/16 white-flowered, tall 1/16 AaBb 1/16 AAbb 1/16 Aabb 1/4 Ab 1/16 AABb 1/16 white-flowered, dwarf 1/16 AaBB 1/16 aaBB 1/16 aaBb 1/4 aB 1/16 AaBb 1/16 Aabb 1/16 aaBb 1/16 aabb 1/16 AaBb 1/4 ab 23

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  25. Principle of Independent assortment -different pairs of alleles are passed to offspring independently of each other.  • The result is that new combinations of genes present in neither parent are possible.  • Ex. a pea plant's inheritance of the ability to produce purple flowers instead of white ones does not make it more likely that it will also inherit the ability to produce yellow pea seeds in contrast to green ones.  25

  26. The principle of independent assortment explains why the human inheritance of a particular eye color does not increase or decrease the likelihood of having 6 fingers on each hand.  • Today, we know this is due to the fact that the genes for independently assorted traits are located on different chromosomes

  27. Independent Assortment Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis 27

  28. Section 11.4More Patterns than Mendel thought Key Concepts: Not all traits clearly have dominant or recessive forms. One allele of a pair may be fully or partially dominant over its partner or codominant with its partner. Two or more gene pairs often influence the same trait, and some single genes influence many genes.

  29. Cont. Mendel happened to focus on traits that have clearly dominant or recessive forms. However, expression of genes for some traits is not as straight forward.

  30. Dominance Relations three types Complete dominance Incomplete dominance Codominance 30

  31. Codominance in ABO Blood Types • A pair of nonidentical alleles affecting two phenotypes are expressed at the same time • Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells • Three different alleles in humans • Two alleles (IA and IB) are codominant when paired • Third allele (i) is recessive to others

  32. ABO Blood Type:Allele Combinations Range of genotypes: IA IA IB IB or or IA i IA IB IB i ii Blood types: A AB B O 32

  33. ABO and Transfusions Recipient’s immune system will attack blood cells that have an unfamiliar glycolipid (antigen) on surface Type O is universal donor because it has neither type A nor type B glycolipid Type A can receive from – A & O Type can receive from – B & O Type AB can receive from -A, B, AB & O Type O can receive from - O 33

  34. Incomplete dominance http://learning.mgccc.cc.ms.us/jd/science/carter/chapter11/animations/abo_genetics.html One allele of a pair is not fully dominant over the other allele in the pair resulting in the phenotype a mix of the two

  35. Incomplete Dominance Incomplete Dominance Homozygous parent X Homozygous parent F1 shows all offspring are of same phenotypes F2 shows three phenotypes in 1:2:1 ratio (one each like parents and then 2-mixed) All F1 are heterozygous X 35

  36. Comb Shape in Poultry RRpp (rose comb) rrPP (pea comb) P: X RrPp (all walnut comb) F1: F2: 9/16 walnut 3/16 rose 3/16 pea 1/16 single rrpp rrPP rrPp RRpp Rrpp RRPP RRPp RrPP RrPp 36

  37. Comb Shape in Poultry http://learning.mgccc.cc.ms.us/jd/science/carter/chapter11/animations/chicken_combs.html rose comb pea comb walnut comb single comb X rose RRpp pea rrPP F1 all walnut RrPp RrPp RrPp F2 9/16 walnut RRPP, RRPp,RrPP, or RrPp 3/16 rose RRpp or Rrpp 3/16 pea rrPP or rrPp 1/16 single rrpp

  38. Co-dominance Rhododendron

  39. Epistasis Interaction between the products of two or more gene pairs Common among genes for hair color in mammals 39

  40. Coat Color inRetrievers BBEE X bbee F1puppies are all BbEe F2puppies BE Be bE be BE black BBEE BBEe BbEE BbEe Be BBee BbEe Bbee BBEe brown bE BbEe bbEE bbEe BbEE yellow be Bbee bbEe bbee BbEe 40

  41. In horses brown coat color (B) is dominant over tan (b). Gene expression is dependent on a second gene that controls the deposition of pigment in hair. The dominant gene (C) codes for the presence of pigment in hair The recessive gene (c) codes for the absence of pigment. Horse is homozygous recessive for the second gene (cc), it will have a white coat regardless of the genetically programmed coat color (B gene) because pigment is not deposited in the hair.

  42. Single Gene With A Wide reach Alleles at a single locus may have effects on two or more traits (good & bad). This is referred to as pleitropy. Disorders: Cystic Fibrosis, Sickle cell Anemia & Marfan syndrome Marfan syndrome – Autosomal dominant mutation; Mutation in gene for fibrillin affects skeleton, cardiovascular system, lungs, eyes, and skin

  43. Summary An allele at a given gene locus may be fully dominant, incompletely dominant, or codominant with its partner on a homologous chromosome. Some gene products may interact with each other and influence the same trait through epistasis. A single gene’s product may have pleiotropic effects, or positive or negative impact on two or more traits.

  44. Temperature Effects on Phenotype Himalayan Rabbit is homozygous for an allele that specifies a heat-sensitive version of an enzyme in melanin-producing pathway Melanin is produced in cooler areas of body (above 91F color is not produced) 45

  45. Environmental Effects on Plant Phenotype Hydrangea macrophylla Action of gene responsible for floral color is influenced by soil acidity Flower color ranges from pink to blue 46

  46. Color is due to pH of soil blue acid soil pink basic soil

  47. Serotonin transport in humans • Serotonin is a chemical in our bodies that helps relieve emotional stress. • One of our genes codes for a “serotonin transporter” (a protein transporter) that transports the serotonin across the membrane of brain cells. • A mutation in this gene disrupts our ability to handle stress and recover when bad things happen.

  48. Summary Variation in traits arise not only from gene mutations and interactions, but also in response to variation in environmental conditions that each individual faces.

  49. Regarding the unexpected phenotype • Comptodactyly- rare abnormality –Effect shape and movement of fingers: • Bent fingers on both hands & immoveable • Bent fingers on one hand & moveable • No effect • Many factors affect gene expression

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