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Genetics: an Introduction

Genetics: an Introduction. Genetics: the study of Heredity. Heredity: transmission of traits from one generation to the next Gregor Mendel: father of modern genetics worked in 1860’s bred garden peas used applied mathematics: statistics!!!!. Genetics.

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Genetics: an Introduction

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  1. Genetics: an Introduction

  2. Genetics: the study of Heredity • Heredity: transmission of traits from one generation to the next • Gregor Mendel: father of modern genetics • worked in 1860’s • bred garden peas • used applied mathematics: statistics!!!!

  3. Genetics • character: observable physical feature • flower color, eye color, etc • trait: form of character • purple or white flowers; brown or blue eyes • heritable trait: passed from parent to offspring

  4. Garden Peas: who knew? Petal Carpel Stamen

  5. Figure 9.2C_s3 2 3 1 4 White Removal ofstamens Stamens Carpel Transferof pollen Parents(P) Purple Carpel maturesinto pea pod Seeds frompod planted Offspring(F1)

  6. Genetics • True-breeding varieties result when self-fertilization produces offspring all identical to the parent. • The offspring of two different varieties are hybrids. • The cross-fertilization is a hybridization, or genetic cross. • True-breeding parental plants are the P generation. • Hybrid offspring are the F1 generation. • A cross of F1 plants produces an F2 generation.

  7. Genetics: Mendel (monohybrid cross) • The all-purple F1 generation did not produce light purple flowers, as predicted by the blending hypothesis. • Mendel needed to explain why • white color seemed to disappear in the F1 generation and • white color reappeared in one quarter of the F2 offspring. • Mendel observed the same patterns of inheritance for six other pea plant characters.

  8. Genetics: Mendel (monohybrid cross) Mendel developed four hypotheses, described below using modern terminology. 1. Alleles are alternative versions of genes that account for variations in inherited characters. 2. For each characteristic, an organism inherits two alleles, one from each parent. The alleles can be the same or different. • A homozygous genotype has identical alleles. • A heterozygous genotype has two different alleles.

  9. Genetics: Mendel (monohybrid cross) • If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the organism’s appearance and is called the recessive allele. • The phenotype is the appearance or expression of a trait. • The genotype is the genetic makeup of a trait. • The same phenotype may be determined by more than one genotype.

  10. Genetics: Mendel (monohybrid cross) • A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation. Mendel’s hypotheses also explain the 3:1 ratio in the F2 generation. • The F1 hybrids all have a Pp genotype. • A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine

  11. Figure 9.3B_4 F2 generation Sperm from F1 plant p P Phenotypic ratio3 purple : 1 white PP Pp P Eggsfrom F1plant Genotypic ratio1 PP : 2 Pp : 1 pp Pp pp p

  12. Genetics: Test Cross • if you have an individual with the dominant phenotype: you CANNOT know its genotype ( Black = B, b = brown) • Black phenotype could be BB or Bb genotype • How do you find out (without DNA testing)? • do a TEST CROSS • an y individual with the recessive phenotypeMUST have the homozygous recessive genotype

  13. Figure 9.6 What is the genotype of the black dog?  Testcross Genotypes B_? bb Two possibilities for the black dog: Bb BB or Gametes B B b b Bb bb b Bb Offspring 1 black : 1 chocolate All black

  14. Genetics: Test Cross • if the Black dog is BB B B phenotype of ALL offspring = Black genotype of ALL offspring = heterozygous b Bb Bb b Bb Bb

  15. Genetics: Test Cross • if Black dog is Bb b B phenotype of HALF offspring = Black phenotype of HALF offspring = brown b Bb bb Bb bb b

  16. Genetics: Dihybrid Cross • looking at two traits at the same time • start with ‘true breeding’ plants for BOTH traits • R = round; r = wrinkled • Y = yellow; y = green • RRYY (homozygous Dominant for both traits) crossed with rryy (homozygous recessive for both traits) = R generation • F1 generation ALL = RrYy (round and yellow) • cross F1 with F1 to get F2 generation

  17. Genetics: Dihybrid Cross F2 generation: NEW COMBINATIONS!! • 9/16 had round yellow seeds • 3/16 had wrinkled yellow seeds • 3/16 had round green seeds • 1/16 had wrinkled green seeds Independent assortment occurred (genes assort independently into gametes)

  18. F2 Generation Genotypes RRYY = 1/16 round yellow RRYy = 2/16 round yellow Rryy = 1/16 round green RrYY = 2/16 round yellow RrYy = 4/16 round yellow Rryy = 2/16 round green rrYY = 1/16 wrinkled yellow rrYy = 2/16 wrinkled yellow rrYy = 1/16 wrinkled green

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