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Fundamentals of genetics

Fundamentals of genetics. Chapter 9. The velociraptor said that they were going to play a game of chicken, but in the end they played a game of horse.

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Fundamentals of genetics

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  1. Fundamentals of genetics Chapter 9 The velociraptorsaid that they were going to play a game of chicken, but in the end they played a game of horse...

  2. Genetics is the filed of biology devoted to understanding how characteristics are transmitted from parents to offspring. Genetics was founded with the work of Gregor Johann Mendel. This section describes Mendel’s experiments and the principles of genetics that resulted from them. 9.1 Mendel’s Legacy

  3. 9.1 Objectives • Describe how Mendel was able to control how his pea plants were pollinated. • Describe the steps in Mendel’s experiments on true-breeding garden peas. • Distinguish between dominant and recessive traits. • State two laws of heredity that were developed from Mendel’s work. • Describe how Mendel’s results can be explained by scientific knowledge of genes and chromosomes.

  4. Mendel’s Study of Traits Mendel= “father of GENETICS”: STUDY OF HEREDITY Your traits resemble your PARENTS • Hair, color/eye shape, height, weight, etc… This is due to HEREDITY: Passage of traits from parents to offspring. TRAIT: GENETICALLY DETERMINED VARIANT OF A CHARACTERISTIC. • Ex. Yellow, white, pink, etc… flower color.

  5. Mendel’s Experiments: Peas Chose peas because: • Have two distinct forms. • Flowers are both male and female. • Grows easily and quickly. Mendel studied 7 traits: • Flower color • Seed color • Seed texture • Pod color • Pod appearance • Flower position along stem • Plant height

  6. Mendel’s Experiments Mendel initially studied each characteristic and contrasting traits individually. • Done using plants that were TRUE-BREEDING: BREEDING FOR A TRAIT THAT IS ALWAYS PRODUCED WHEN OFFSPRING ARE SELF-POLLINATED. Mendel initially did MONOHYBRID CROSSES: CROSS INVOLVING ONE PAIR OF CONTRASTING TRAITS. • Ex. Crossing purple flowered plants with white flowered plants

  7. Mendel’s Experiments Mendel's experiments followed 3 steps: • Forming the P (PARENT) Generation • F₁ (FIRST FILIAL) Generation • F₂ (SECOND FILIAL) Generation

  8. Step 1: Forming P generation Peas SELF-POLLINATEDfor several generations to form the P GENERATION: PARENT GENERATION FOR START OF A BREEDING EXPERIMENT. All plants in the P generation are TRUE-BRED, so that they only produce offspring like themselves. • Ex. Purple flowers should only produce purple flowers.

  9. Step 2: F₁ Generation CROSS-POLLINATIONof 2 P generation plants with contrasting traits. • Ex. Purple and white flowers The generation that follows is called the F₁ GENERATION: FIRST FILIAL GENERATION.

  10. Step 3: F₂ Generation Self-pollination of F₁ GENERATION plants to form the F₂ GENERATION: SECOND FILIAL GENERATION.

  11. Mendel’s Results: All F₁ plants had only 1 form of the trait (ex. either all purple or all white flowers), but when self-pollinated, the missing trait reappeared in the F₂ generation. • Figure 9.3 pg. 175; Table 9-1 pg. 176 Also, found consistent mathematical RATIOS applicable to the traits shown in offspring.

  12. Recessive and Dominant Traits Mendel found that crossing of two P generation plants always forms a 3:1 ratio in the F₂ generation: • 3 dominant trait plants • 1 recessive trait plants DOMINANT TRAIT: Trait or allele that is fully expressed; masks the recessive allele. RECESSIVE TRAIT: Trait or allele that is expressed only when 2 recessive alleles for the same characteristic are inherited.

  13. LAW OF SEGREGATION: A pair of alleles is separated when gametes are formed. • One chromatid in each of 4 gametes formed. Alleles separate independently when gametes are formed. • Traits are not inherited based on other characteristics. LAW OF INDEPENDENT ASSORTMENT:

  14. Modern Genetics Mendel’s findings were the start of modern genetics. MOLECULAR GENETICS: STUDY OF THE STRUCTURE AND FUNCTION OF CHROMOSOMES AND GENES. ALLELE: Two or more alternative forms of a gene. • Represented by letters: • Capital letters = DOMINANT ALLELE • Ex. Purple flowers are dominant; allele for purple is “P” • Lower-case letters = RECESSIVE ALLELE • Ex. White flowers are recessive; allele for white is “p”

  15. Today, geneticists rely on Mendel’s work to predict the likely outcome of genetic crosses. In this section, you will learn how to predict the probable genetic makeup and appearance of offspring resulting from specified crosses. 9.2 Genetic Crosses

  16. 9.2 Objectives • Differentiate between the genotype and the phenotype of an organism. • Explain how probability is used to predict the results of genetic crosses. • Explain how a test cross is used to show the genotype of an individual whose phenotype expresses the dominant trait. • Differentiate a monohybrid cross from a dihybrid cross.

  17. Genotype and Phenotype Basics GENOTYPE:Organism’s genetic makeup inherited from each parent. • Ex. Purple-flowered plant, PP or Pp PHENOTYPE: Organisms appearance based on presence of dominant and/or recessive genes. • Ex. A: PP or Pp; Plants show purple flowers. • Ex. B: pp; Plants show white flowers.

  18. Genotype and Phenotype Basics HOMOZYGOUS:When both paired alleles are the same. • HOMOZYGOUS DOMINANT: Both alleles are the dominant trait; PP. • HOMOZYGOUS RECESSIVE: Both alleles are the recessive trait; pp. HETEROZYGOUS:When paired alleles are different from each other. • Ex. Pp

  19. Genotype and Phenotype Basics GENOTYPIC RATIO: Ratio of genotypes that appear in offspring of a monohybrid cross. PHENOTYPIC RATIO: Ratio of phenotypes that appear in offspring of a monohybrid cross. Ratios are written in simplest form with colons between each, and follow a specific order: • HOMOZYGOUS DOMINANT • HETEROZYGOUS • HOMOZYGOUS RECESSIVE

  20. Probability in Genetics PROBABILITY: Likelihood that a specific event will occur. Probability = NUMBER OF TIMES AN EVENT IS EXPECTED TO HAPPEN NUMBER OF TIMES AN EVENT COULD HAPPEN Used to predict what genotypes and phenotypes will be produced from a cross. • Ex. Mendel produced a total of 8,023 plants. 6,022 plants showed yellow seed color. 2,001 plants showed green seed color. • Probability of producing yellow seed color = 6,022/8,023= 0.75 = 75% • Probability of producing green seed color = 2,001/8,023= 0.25 = 25%

  21. Predicting Results of Monohybrid Crosses MONOHYBRID CROSS: Cross in which one trait is tracked. • Ex. Crossing a true-breeding purple pea plant with a true-breeding white pea plant. Biologists use a PUNNETT SQUARE to determine the products of monohybrid cross. FEMALE MALE

  22. Examples of Monohybrid Crosses:HOMOZYGOUS Do. x HOMOZYGOUS Re. PARENT #1: Pea plant homozygous dominant for purple flower: PP PARENT #2: Pea plant homozygous recessive for white flower: pp ** Predicted genotype(s): 4:0 ratio; Pp ** Predicted phenotype(s): 4:0 ratio; All purple flowers WHITE PURPLE

  23. Examples of Monohybrid Crosses:HOMOZYGOUS Do. x HETEROZYGOUS PARENT #1: Guinea pig homozygous dominant for black fur: BB PARENT #2: Guinea pig heterozygous for black dominant and brown fur recessive: Bb ** Predicted genotype(s): 1:1 ratio; BB and Bb ** Predicted phenotype(s): 4:0 ratio; All black fur BLACK D./BROWN R. BLACK

  24. Examples of Monohybrid Crosses:HETEROZYGOUS x HETEROZYGOUS PARENT #1: Rabbit heterozygous for black fur dominant and brown fur recessive: Bb PARENT #2: Rabbit heterozygous for black fur dominant and brown fur recessive: Bb ** Predicted genotype(s): 1:2:1 ratio; BB, Bb and bb ** Predicted phenotype(s): 3:1 ratio; 75% Black fur and 25% brown fur. BLACK D./ BROWN R. BLACK D./ BROWN R.

  25. Testcross TESTCROSS: An individual of a unknown dominant genotype (DD or Dd) is crossed with a homozygous recessive (dd) individual. OFFSPRING RATIOSdetermine the unknown genotype. • Ex. DD and dd would produce only dominant traits; Dd and dd would produce a 1:1 ratio dominant traits and recessive traits.

  26. Dihybrid Crosses DIHYBRID CROSS: Cross in which two traits are tracked. • Ex. Both seed texture and seed color of one seed. More complicated!! More ALLELES to combine.

  27. Examples of Dihybrid Crosses:Homozygous D. x Homozygous R. rryy= Wrinkled, green seeds RRYY = Round, yellow seeds ** ALL WILL HAVE ROUND, YELLOW SEED PHENOTYPES!

  28. Examples of Dihybrid Crosses:Heterozygous x Heterozygous RrYy = Round, yellow seeds RrYy = Round, yellow seeds * OFFSPRING WILL HAVE 9 DIFFERENT GENOTYPES; 4 DIFFERENT PHENOTYPES. ** 9:3:3:1 RATIO!

  29. Traits Influenced by Several Genes POLYGENIC INHERITENCE: SEVERAL GENES INFLUENCE A TRAIT. • Due to crossing-over and independent assortment, many INTERMEDIATES (blends) of traits appear in offspring. • Ex. Eye color, hair color, skin color, height, weight, etc…

  30. Intermediate Characteristics INCOMPLETE DOMINANCE: OFFSPRING PHENOTYPE THAT IS INTERMEDIATE OF THE PARENT’S PHENOTYPES. • Ex 1. Snapdragon with red flowers is crossed with a snapdragon that has white flowers and produces a snapdragon with PINK flowers; neither the red allele (R) or the white allele (r) is completely dominant over the other. • Ex2.Child of a straight haired parent and a curly-haired parent has wavy hair.

  31. Codominance CODOMINANCE: Both alleles for a gene are expressed at the same time in a heterozygous offspring. • Neither allele is dominant or recessive, nor do the blend. • Ex. AB blood groups; both A and B are dominant, but a person can have blood type AB.

  32. Sex-linked Genes: SEX-LINKED GENE:Trait found only on the X or Y chromosome. Most sex-linked genes are carried on female chromosome and recessive, therefore… Usually seen only in MALES because they only possess one X chromosome.

  33. Inheritance of Traits PEDIGREE: Family history showing how a trait is inherited over several generations. Used to determine if a person is a carrier for a GENETIC DISORDERand/or to determine if their children will be affected by the disorder.

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