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Exploring Genetics: Mendel's Pea Plant Experiments

Discover the basis of genetics through Gregor Mendel's groundbreaking studies with pea plants, exploring inheritance, dominance, generations, alleles, and genetic crosses. Learn how traits are passed down and expressed, unraveling the mysteries of heredity.

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Exploring Genetics: Mendel's Pea Plant Experiments

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  1. Genetics Ch. 11 pgs. 263-274

  2. What is Genetics? • The study of heredity • Passing of traits from one generation to the next.

  3. Gregor Mendel (1822) • Monk who worked in garden of monastery in Czech Republic. • Used pea plants to study heredity • Known as the “father of modern genetics”

  4. Why Pea Plants? • Many contrasting traits • Reproduce sexually • Crosses can be controlled • Large # of offspring • Short lifecycles

  5. A little more about pea plants… • Normally self pollinating • Pollen (sperm) fertilizes the ovule (egg) to produce offspring. • Produce genetically identical offspring. • Mendel used cross-pollination in his experiments. • Used pollen from one plant to fertilize another.

  6. Genes and Dominance • Mendel studied 7 traits of pea plants • Traits are specific characteristics • For each trait there are 2 contrasting characteristics.

  7. Generations • P- parental generation; original pair • F1- first filial generation; offspring from P generation • F2- second filial generation; offspring from F1

  8. Mendel’s Findings • When he crossed P generation, F1 showed only one trait from the 2 parents. • Ex. Crossed Tall x Short short or tall

  9. Mendel’s Conclusions • Biological inheritance is determined by factors passed from one generation to the next. • Factors are called genes. • 2 contrasting forms of the same trait are called alleles. • Ex. Trait is height, alleles are tall and short

  10. Mendel’s Conclusions • Principal of dominance- Some alleles are dominant and some are recessive. • Dominant- when allele is present organism will always exhibit the dominant trait. • Recessive- only exhibit recessive trait if dominant allele is not present.

  11. Alleles • Symbols for alleles: • A letter symbol is used for a specific trait. • Ex- height, Tt • Dominant- symbol is uppercase letter, T • Recessive- symbol is lowercase letter, t

  12. Segregation • When Mendel crossed plants from the F1 generation traits from the P generation reappeared. P Tall x Short all tall plants F1 Tall x Tall some tall some short

  13. Law of Segregation • The 2 alleles for each trait must separate when gametes form. • Parent only passes one allele for each trait to its offspring.

  14. Probability and Punnett Squares • The principles of probability can be used to predict outcomes of genetic crosses. • Punnett square- diagram which shows possible Results of genetic crosses

  15. Heterozygous vs. Homozgous • Hetero different • Homo same • Homozygous dominant- 2 identical dominant alleles (upper case). Expresses the dominant trait. • Ex- TT, tall

  16. Heterozygous vs. Homozygous • Homozygous recessive- 2 identical recessive alleles (lower case). Expresses the recessive trait. • Ex- tt, short • Heterozygous- Have 2 different alleles for the same trait. ( upper and lower case) Express the dominant trait. • Ex. Tt, tall

  17. Genotype vs Phenotype • Genotype- genetic make-up, letters • Tt vs tt • Phenotype- physical expression of the trait • Tall vs short

  18. Sample Problem • In summer squash, white fruit color (W) is dominant over yellow fruit color (w). If a squash plant homozygous for white is crossed with a plant homozygous for yellow what will the phenotypic and genotypic ratios be?

  19. Sample Problem • If 2 heterozygous white plants are crossed what will be the phenotypic and genotypic ratios?

  20. Types of Genetic Crosses • Monohybrid one trait • Ex. Tt x tt • Dihybrid 2 traits • Ex- Hair and height • B= black, b=blonde, H=tall, h=short • What would be the phenotypes of a cross between the following parents? HhBb x HHbb

  21. Types of genetic crosses cont. • Ex 2- Shape and color of pea plants R= round, r=wrinkled, Y= yellow, y=white What would be the phenotypes of a cross between a plant that is RRYY and a plant that is rrYy?

  22. Genetics Review • If normal vision (N) is dominant to colorblindness (n) what are the chances that a heterozygous normal man and a colorblind woman will have a child with colorblindness?

  23. Genetics Review • In some flowers red flowers (R) are dominant to white flowers (r) and tall stems (T) are dominant to short stems (t). What is the genotypic ratio for a cross between the following plants: RrTT x rrTT DD:DR:RD:RR

  24. Types of genetic crosses cont. • Incomplete dominance • One allele is not completely dominant over the other • Heterozyous phenotype is somewhere between the 2 homozygous phenotypes. • 3 phenotypes instead of 2 • Use all capitol letters • Ex; RR= red, WW= white, RW= pink

  25. Types of genetic crosses cont. • Inc. Dom. Example: In snapdragons the combination of red and white flowered plants can produce a pink flowered plant. RR= red WW= white RW= pink What would be the phenotypes of the offspring if a red flower were crossed with a pink flower?

  26. Types of genetic crosses cont. • Co-dominance • Both alleles contribute to the phenotype • Ex- BB= black feather, WW= white feather, BW= black and white spotted feathers. • Same rules as inc. dom. but both phenotypes appear separately

  27. Types of genetic crosses cont. • Co-dominance sample problem We have 2 fuzzy bunnies in class. One has black and white fur, the other is pure white. What are the genotypes of both rabbits? Black and White= White= What would be the phenotypes of the offspring if these rabbits mated?

  28. Blood Type • Human blood type is determined by the type of protein found on the red blood cells (A or B). • Antigen- protein located on blood cell • Antibody- protein found in plasma, prevent foreign particles

  29. Blood type

  30. Blood Type Example • What are the chances of a mother with A type blood and a father with O type blood having a child with A type blood?

  31. Polygenic Inheritance • Some traits are polygenic- have more than one gene coding for the trait. • Ex- skin color, eye color, height

  32. Polygenic Inheritance cont • 1. The weight of a fruit in a certain variety of squash is determined by two pairs of genes: AABB produces fruits weighing 4 lbs each and aabb produces fruits weighing 2 lbs each. A. How many pounds does each dominant allele add to the total weight of an individual squash?

  33. Human Genetics • Chromosomes and Sex Determination • Humans have 23 pairs of chromosomes in autosomal cells. (diploid) • In sex cells there are only 23. (haploid) • Sex chromsomomes are the only pair that are non-homologous. • XX female • XY Male

  34. Human Genetics • Ex- Cross between male and female for sex chromosomes: • Chance of boy: • Chance of girl:

  35. Sex-Linked Inheritance • Some traits are linked to the X chromosome. • Most genetic disorders are X-linked. • Most X-linked disorders are expressed in males. WHY???

  36. Sex- Linked Inheritance • Females usually act as carriers, they carry the recessive trait on one of their X chromosomes but the trait is not expressed.

  37. Sex-Linked Inheritance • Ex- Normal colored vision (N) is dominant to colorblindness (n). Colorblindness is an X-linked trait. • Males are colorblind more then women and the gene is only found on the X chromosome. What is the chance that a man with normal vision and a woman who is a carrier will have a child that is colorblind?

  38. Autosomal Traits • Genetic traits that are carried on the autosomal chromosomes.

  39. Autosomal Recessive • Only expressed with homozygous recessive genotype. • Heterozygous genotype is carrier. • If trait present usually lethal disorder. • Ex- cystic fibrosis

  40. Autosomal Recessive • Ex- C= no cystic fibrosis c= cystic fibrosis A female homozygous dominant for cystic fibrosis marries a man heterozygous for cystic fibrosis. What is the probability the child will have cystic fibrosis?

  41. Autosomal Dominant • Expressed in homozygous dominant and heterozygous genotype. • No carrier (Either you have it or you don’t) • If trait is present usually lethal. • Ex- Huntington’s Disease

  42. Autosomal Dominant • Ex H= Huntington’s disease h= normal A female who has Huntington’s disease and is heterozygous for the disorder marries a man who does not have the disorder. What is the probability that their child will have the disorder?

  43. Pedigrees • Chart which shows the relationships within a family. • Shows the expression of genetic traits. • Symbols used: • Circle=female • Square=male • Half-shaded=carrier • Completely shaded=expresses trait

  44. Pedigree

  45. Pedigree • Used to determine if disorder is autosomal dominant, autosomal recessive or X-linked. • If only expressed in males and carried in female X-linked • If there are many carriers of both sexes autosomal recessive. • If no carriers and expressed in both sexes autosomal dominant

  46. Pedigree examples

  47. Pedigree Examples

  48. Pedigree Examples

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