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BASIC GENETICS

BASIC GENETICS. 1. Gregor Mendel. (1822-1884). Austrian monk Studied science and math High school teacher and gardener Experimented on pea plants. Gregor Mendel. Father of Genetics. We credit Mendel for forming the basis of genetics. A zygote inherits traits from both parents.

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BASIC GENETICS

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  1. BASIC GENETICS 1

  2. Gregor Mendel (1822-1884) • Austrian monk • Studied science and math • High school teacher and gardener • Experimented on pea plants Gregor Mendel Father of Genetics • We credit Mendel for forming the basis of genetics.

  3. A zygote inherits traits from both parents. Heredity is the passing of traits or characteristics from parents to offspring.

  4. Sexual Reproduction Haploid egg (gamete) Haploid sperm (gamete) Diploid zygote + = 1n 1n 2n

  5. ? So how can we predict what the offspring will look like?

  6. Through Probability!!! Probability: • is the likelihood that a specific event will happen. • helps understand past events and the possibilityof future events.

  7. number of times one outcome is likely to occur Probability = total number of all possible outcomes To find the probability of an event… • compare the number of times a certain outcome can occur to the total number of possible outcomes. • write it as a fraction. What are the possible outcomes when you flip a coin?

  8. 1 2 Probability = What is the likelihood that a flipped coin will land heads up? • Equal chance of 2 possible outcomes: “heads” or “tails” • “Heads” is one possible outcome out of a total of 2 possible outcomes. So… What is the probability of flipping a coin and it landing heads up? What does this have to do with genetics?

  9. P Pp p pp We use probability to predict possible outcomes of genetic crosses. • Genetic crosses involve 2 independent events Because: Alleles contributed by one parent Do not depend on P p Alleles contributed by the other So…

  10. 1 1 1 4 4 4 1 2 1 2 1 2 1 2 1 2 2 1 We combine both probabilities. • We multiply the separate probabilities of the two events. What is the probability of two heterozygous purple individuals (Pp x Pp) producing a white offspring (pp)? ? ? “p” from one parent = p “p” from other parent = ? Pp p p X = p Pp The probability of these parents producing a white offspring ? One chance in four

  11. P p P P P p P p P p p p Probability using Pedigrees and Punnett Squares • Pedigrees and Punnett squares are scientific tools used to… • predict possible outcomes from genetic crosses. • simplify analysis of genetic probabilities.

  12. Unknown Gender Pedigree • A chart of a family's history showing relationships and how a trait or disease has been inherited over many generations

  13. Pedigree Analysis Circles represent Females. Squares represent Males. Vertical lines and brackets show birth relationships. Horizontal lines show mating relationships. Half-shaded circle or square represents a carrier of the trait. Shaded circles or squares show a person expressing the trait. Unshaded circles or squares show a person that does not express the trait.

  14. P p PP Pp P Pp pp p Punnett Squares A diagram that shows the possible gene combinations that might result from a genetic cross

  15. P p PP Pp P Pp pp p Punnett Squares The letters in a Punnett square stand for alleles (one of a number of different forms of a gene).

  16. We call the actual genetic make-up of an organism its genotype. Genotype: PP Genotype: pp Genotype: Pp

  17. We call the appearance of an organism its phenotype. Genotype: PP Phenotype: Purple Genotype: pp Phenotype: White Genotype: Pp Phenotype: Purple

  18. P p PP Pp P Pp pp p The genotype (the genes) is represented by letters such as Pp. The phenotype (like a photograph) is represented by a description such as purple.

  19. Offspring can be: Homozygous for a trait or Heterozygous for a trait PP Pp • An organism that has two identical alleles for a particular trait • An organism that has two different alleles for the same trait

  20. More Examples… Homozygous Dominant Genotype: PP Phenotype: Purple flowers Heterozygous Genotype: Pp Phenotype: Purple flowers Homozygous Recessive Genotype: pp Phenotype: White flowers

  21. Dominant and Recessive Alleles When 2 different alleles for the same trait occur together, one may be expressed while the other is not expressed. Some traits are… while others are dominant recessive • Homozygous Dominant • Genotype: PP • Phenotype: Purple flowers • Homozygous Recessive • Genotype: pp • Phenotype: White flowers

  22. Dominant Alleles The dominant trait will be expressed if a dominant allele is present. • Dominant alleles “overpower” recessive alleles. • If there is a dominant allele and a recessive allele, the dominant trait will be the one that is expressed. • Example: A genotype of Pp (Purple/white) shows a phenotype of Purple.

  23. Recessive Alleles The recessive trait is expressed only when the dominant allele is not present. • The allele that is overshadowed by a dominant allele • Expressed only if both alleles are recessive • Example: A genotype of pp (white/white) shows a phenotype of white.

  24. Dominant and Recessive Alleles • Pea plants have purple and white alleles for flower color. • The allele for purple flowers is dominant and the allele for white flowers is recessive. • If the allele for purple flowers is present, the plant will produce purple flowers.

  25. P p PP Pp P Pp pp p Complete Dominance 2 Heterozygous Parents (Genotype Pp) (Phenotype Purple) would produce 1/4 (25%) Homozygous Offspring (Genotype PP) (PhenotypePurple) 2/4 (50%) Heterozygous Offspring (Genotype Pp) (PhenotypePurple) 1/4 (25%) Homozygous Offspring (Genotype pp) (PhenotypeWhite)

  26. P p PP Pp P Pp pp p Complete Dominance 2 Heterozygous Parents (Genotype Pp) (Phenotype Purple) would produce 1/4 (25%) Homozygous Offspring (Genotype PP) (PhenotypePurple) 2/4 (50%) Heterozygous Offspring (Genotype Pp) (PhenotypePurple) 1/4 (25%) Homozygous Offspring (Genotype pp) (PhenotypeWhite)

  27. P p PP Pp P Pp pp p Complete Dominance 2 Heterozygous Parents (Genotype Pp) (Phenotype Purple) would produce 1/4 (25%) Homozygous Offspring (Genotype PP) (PhenotypePurple) 2/4 (50%) Heterozygous Offspring (Genotype Pp) (PhenotypePurple) 1/4 (25%) Homozygous Offspring (Genotype pp) (PhenotypeWhite)

  28. P p PP Pp P Pp pp p Complete Dominance 2 Heterozygous Parents Genotype Pp Phenotype Purple would produce 1/4 (25%) Homozygous Offspring (Genotype PP) (PhenotypePurple) 2/4 (50%) Heterozygous Offspring (Genotype Pp) (PhenotypePurple) 1/4 (25%) Homozygous Offspring (Genotype pp) (PhenotypeWhite)

  29. Let’s try some examples with Mendel’s pea plants. Pea pod color Genotype Phenotype GG Green Gg Green gg Yellow This is complete dominance because yellow is hidden in a heterozygous genotype.

  30. A cross of a homozygous green pea plant and a heterozygous green pea plant would yield: g G GG Gg G GG Gg G all offspring with a phenotype of green, but ½ (50%) heterozygous (Gg) and ½ (50%) homozygous (GG).

  31. A cross of a homozygous yellow pea plant and a heterozygous green pea plant would yield: g G Gg gg g Gg gg g ½ (50%) offspring with a phenotype of green, genotype Gg and ½ (50%) with a phenotype of yellow, genotype gg.

  32. Let’s add another trait. Pea seed shape Genotype Phenotype RR Round Rr Round rr Wrinkled This is complete dominance because “wrinkled” is hidden in a heterozygous genotype.

  33. A cross of two pea plants, both with heterozygous green seed pods and heterozygous round seeds would yield: GgRr GR Gr gR gr GGRr GgRR GgRr GGRR GR GgRr GGrr GgRr Ggrr GGRr Gr GgRr ggRR ggRr GgRR gR Ggrr ggRr ggrr GgRr gr There is only one chance in 16 that both recessive traits will be expressed.

  34. Complex Patterns of Heredity

  35. Incomplete Dominance Produce a heterozygous offspring with a blended phenotype Two homozygous parents with different phenotypes red + white = pink r r + w w = r w BUT THE ALLELES REMAIN DISTINCT; ONLY THE PHENOTYPE APPEARS BLENDED.

  36. Incomplete Dominance r r w rw rw w rw rw Crossing homozygous parents to produce F1 generation THE ALLELES REMAIN DISTINCT; ONLY THE PHENOTYPE APPEARS BLENDED.

  37. Incomplete Dominance r w r rr rw w rw ww Crossing heterozygous F1 generation to produce F2 generation THE ALLELES REMAINED DISTINCT; THE ORIGINAL PHENOTYPES REAPPEAR IN THE F2 GENERATION

  38. Codominance Both alleles in the heterozygote express themselves fully. Example: Blood types

  39. Codominance And a person homozygous for Type B blood A person homozygous for Type A blood Will produce a child that will demonstrate both Type A and Type B blood (Type AB)

  40. In summary:

  41. Pleiotropy A single gene affects more than one trait. For example, sickle-cell disease results from one gene, but it has numerous effects on the body.

  42. Polygenic Traits A trait controlled by more than one gene. Eye color is an example of a polygenic trait.

  43. In summary:

  44. Carrier An individual who carries a recessive trait… • is heterozygous for the trait. • does not express the trait. • can pass the trait to offspring. For Example: • This female is heterozygous for colorblindness. • She has normal vision. • She can pass the trait on to her son who will be colorblind. XC Xc

  45. Without sexual reproduction,we would have very little genetic variation.

  46. With sexual reproduction, there is great variety in the appearance of offspring.

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