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Introduction to Genetics. Chapter 11 (M). Genetics and Inheritance. Genetics  the study of heredity that deals with the transmission of traits or characteristics from one generation to another Inheritance  The reception of traits by transmission from parent to offspring. Prehistoric Times.

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Introduction to Genetics


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    1. Introduction to Genetics Chapter 11 (M)

    2. Genetics and Inheritance • Genetics the study of heredity that deals with the transmission of traits or characteristics from one generation to another • InheritanceThe reception of traits by transmission from parent to offspring

    3. Prehistoric Times • Little is known when humans first recognized the importance of genetics • The breeding of horses, cattle, and various breeds of dogs began around 8000 B.C. and 1000 B.C. • Plants such as corn, wheat, and rice was cultivated in Mexico around 5000 B.C.

    4. The History Of Genetics Pythagoras 500 B.C., a Greek philosopher, stated that human life began with male and female fluids

    5. The History Of Genetics Aristotle furthered this idea and suggested that these fluids, or “semens,” were actually purified blood—therefore, blood must be part of heredity.

    6. One Bizarre Theory • The theory of Homunculus -17th century • sex cells contained a complete miniature adult, perfect in form • This statement was popular way into the18th century Small individual

    7. Blending Hypothesis 1800’s • This stated that both the genetic material from the mother and father were blended in order to produce an offspring Parents Red Flower X Yellow FlowerOffspring Orange (offspring all orange) • Exceptions Red yellow ,etc • Theory discarded, If blending occurred  all extreme characteristics would disappear from the population

    8. Gregor Mendel-1800’s • Considered to be the “father of genetics • Austrian monk, teacher and mathematician • Expt. approach to genetics • Particulate Hypothesis

    9. Particulate Hypothesis • Parents pass on to their offspring separate and distinct factors (genes) that are responsible for inherited traits • Used pea plants to study this

    10. Rapid reproduction rate Presence of distinctive traits Closed structure of flowers (each pea plant has male (stamens) and female (carpal) sexual organs) allows self-fertilization Why peas?

    11. Cross-fertilization • One plant is fertilized by another

    12. P Generation  parents F1 Generation  first filial F2 Generation second filial P X P F1 F1 F1 F2 Terminology

    13. Allele Alternative forms of a gene which determines a trait.

    14. Alleles cont. • Uppercase (Capital) letters for dominant traits • Lowercase letters for recessive traits • Ex: Tall = T short = t, expressed in pairs TT, Tt, tt

    15. Phenotype Physical appearance • Genotype Genetic makeup • Dominant trait that is easily observed • Recessive  trait that is often masked • Homozygous2 allelesfor a trait are identical TT or tt • Heterozygous – 2 alleles for a trait are not identical Tt

    16. Inheritance Follows Rules of Chance • Mendel began experiments to track the inheritance of characters in pea plants • Results led him to formulate several hypotheses

    17. Seven Traits Studied by Mendel

    18. P Generation  parents F1 Generation  first filial F2 Generation second filial P X P F1 F1 F1 F2 Terminology

    19. Mendel’s Experiment • Crossed pure purple and a pure white flower (P generation) =F1 generation • All F1 plants (purple) are crossed by self pollination = F2 generation yields ¾ purple and ¼ yellow

    20. Mendel’s Hypotheses • There are alternate forms of genes • For each character an organism has two alleles • Alleles are either dominant or recessive • Alleles segregate during formation of gametes

    21. Law of Dominance When organisms pure for contrasting traits are crossed, all their offspring will show the dominant trait

    22. Probability • Fractions or ratios that will predict that an event will occur

    23. Punnett Square • Diagram which shows the possible outcome of a cross

    24. Monohybrid Cross Using a single trait – crossing a pure bred Tall (TT) with a pure bred short (tt) plant

    25. Mating 2 heterozygous black (Bb) rabbits

    26. Test Cross • Individual with dominant phenotype  not possible to predict the genotype run a test cross with individual with recessive phenotype to determine the allele Dominant Phenotype purple flower (genotype PP or Pp) Recessive Phenotype  white flower ( genotype pp)

    27. Test Cross

    28. Law of Independent Assortment Each pair of alleles segregates into gametes independently

    29. Independent Assortment • Mendel followed the inheritance of two different characters ( dihybrid cross) • The allele for yellow seeds (Y) is dominant to the allele for green seeds (y). • The allele for round seeds (R) is dominant to the allele for wrinkled seeds (r). • He crossed true-breeding plants that had yellow, round seeds (YYRR) with true-breeding plants that had green, wrinkled seeds (yyrr).

    30. Dihybrid CrossCrosses Involving Two Traits: Color: Yellow, Green Shape: Round, Wrinkled Yellow-Round Yellow-Wrinkled Green-Round Green-Wrinkled

    31. The Y and R alleles and y and r alleles stay together F1 offspring would produce yellow, round seeds. The F2 offspring would produce two phenotypes in a 3:1 ratio, just like a monohybrid cross.

    32. If the two pairs of alleles segregate independently of each other Four classes of gametes (YR, Yr, yR, and yr) would be produced in equal amounts. These combinations produce four distinct phenotypes in a 9:3:3:1 ratio

    33. Practice problems • Cross a homozygous yellow, homozygous roundplant with a green, wrinkled plant • Cross a homozygous yellow, homozygous round plant with a heterozygous yellow, wrinkled plant

    34. Other Patterns of Inheritance 11.3

    35. Variation in Inheritance • Incomplete Dominance • Codominance (multiple alleles) • Polygenic inheritance

    36. Incomplete Dominance • Both alleles contribute to a phenotype of a heterozygous individuals to produce a trait not like either parent. • Phenotype intermediate between two pure traits Ex: Snapdragons, Andalusian chick.

    37. Snapdragon Flower Color • Alleles often written as capital letters with superscripts ex. CRCR (red) x CWCW (white) • Incomplete Dominance revealed in Heterozygous Individual • A cross between a white-flowered plant and a red-flowered plant will produce all pink F1 offspring • Self-pollination of the F1 offspring produces 25% white, 25% red, and 50% pink offspring.

    38. Codominance (multiple alleles) • Two dominant alleles are expressed at the same time • Ex: Roan coat color in horses CRCR, CWCW, CRCW

    39. CRCR (Red) CWCW (White) CRCW (Roan)

    40. If a roan cow (RW) is mated to a roan bull (RW), what are the phenotypes of the offspring?

    41. Multiple Alleles • Most genes have more than two alleles in a population • The ABO blood groups in humans are determined by three alleles, IA, IB, and i. • Both the IA and IB alleles are dominant to the i allele • The IA and IB alleles are codominant to each other • Because each individual carries two alleles, there are six possible genotypes and four possible blood types.

    42. ABO Blood Group System • Type A IA IA or IA i • Type B IB IB or IB i • Type AB IA IB • Type O ii

    43. Problems • A man homozygous for type A blood marries a woman who is heterozygous for type B blood. What are the possible genotypes and phenotypes of their children? • A couple has a child with type O blood. If one parent is type O, what are the possible genotypes of the other parent?

    44. Polygenic inheritance • When two or more genes effect a single characteristic • For example, skin color in humans is controlled by at least three different genes. • Imagine that each gene has two alleles, one light and one dark, that demonstrate incomplete dominance. • An AABBCC individual is dark and aabbcc is light

    45. A cross between two AaBbCc individuals (intermediate skin shade) would produce offspring covering a wide range of shades The range of phenotypes forms a normal distribution.