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Genetics

Genetics. The study of heredity. Genetics. Genetics is the scientific study of heredity - how traits are passed from generation to generation. The characteristics that are inherited are called traits . Genes. Humans have 23 homologous pairs of chromosomes.

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Genetics

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  1. Genetics The study of heredity

  2. Genetics • Genetics is the scientific study of heredity - how traits are passed from generation to generation. • The characteristics that are inherited are called traits.

  3. Genes • Humans have 23 homologous pairs of chromosomes. • On each chromosomes, there are sections called genes that code for traits.

  4. Genes

  5. Alleles • An allele is a distinct form of a gene. • Every person has 2 alleles for a gene  1 from the father and 1 from the mother

  6. Alleles • Letters of the alphabet are used to represent an allele of interest. • Every person has two copies of an allele, so they will have two letters. T P Y A R

  7. Alleles • Dominantalleles are symbolized with a capital letter. Dominant alleles will mask a recessive alleles in cases of simple dominance/recessiveness. • Recessive alleles are symbolized with lower case letters. A a

  8. Homozygous Alleles • If an organism has two like copies of an allele, it is homozygous (homo = same). • If the two alleles are dominant, the organism is homozygous dominant. • If the two alleles are recessive, the organism is homozygous recessive. AA aa

  9. Heterozygous Alleles • If an organism has two different copies of an allele, it is heterozygous (hetero = different). Aa

  10. Genotype and Phenotype • The letters an organism has represent the organism’s genotype - what alleles the organism has. • As a result of the alleles present, a trait is expressed. The phenotype is the expressed trait.

  11. Genotype and Phenotype • Example: In a plant species, there are two alleles for flower color: R and r. • R is dominant, and codes for red flowers • r is recessive and codes for white flowers

  12. Genotype and Phenotype • The genotype is the combination of alleles: either RR, Rr, or rr. • The phenotype is what is expressed: either red or white flowers.

  13. Genotype and Phenotype • RR - • Homozygous dominant • Red flowers • Rr - • Heterozygous dominant • Red flowers • rr - • Homozygous recessive • White flowers In cases of simple dominance, an organism must have two copies of the recessive alleles to express the recessive trait.

  14. Purebreds and Hybrids • Purebred - an organism that receives the same genetic traits from both of its parents • Hybrid - an organism that receives different forms of a genetic trait (different alleles) from each parent

  15. Mendel’s Laws Contributions of Gregor Mendel

  16. Law of Dominance • The dominant alleles is expressed and may mask a recessive allele. The recessive form of a trait is only shown in a homozygous recessive organism. • Ex. R is allele for round, r is allele for square. • RR - round • Rr - round • rr - square

  17. Law of Segregation • Gene pairs separate when gametes are formed. Parent: Dd Parent: dd D d d d Gametes Gametes

  18. Law of Independent Assortment • Genes segregate randomly and independently. This means that if there are 2 or more traits, every combination of those traits is possible. AbC Abc abC abc AabbCc

  19. Probability and Punnett Squares Predicting the genotypes and phenotypes of offspring

  20. Probability • Probability - the likelihood that a particular event will occur (what are the odds?) • What is the probability that a single coin flip comes up heads? • 50% or 1/2

  21. Probability • True or False? The past outcomes of coin flips greatly affects the outcomes of future coin flips. • False. • There’s still a 50% chance of heads and 50% chance of tails!

  22. Probability • The way in which alleles separate is random, like a coin flip. (Mendel’s Law of Segregation) • From a mother who is heterozygous for an allele, there is a 50% chance she passes on the dominant allele and a 50% chance she passes on the recessive allele.

  23. Punnett Squares • Punnett squares show probabilities for genotypes and phenotypes of offspring of two parent organisms. • Example: • In Mendel’s pea plants, the plants had either purple (P) or white (p) flowers.

  24. Punnett Squares • Step 1. Make the grid. • If there is 1 trait, it is a 2x2 grid. • If there are 2 traits, it is a 4x4 grid. • Because we are only looking at 1 trait (flower color), a 2x2 grid is needed.

  25. Punnett Squares Pp • Step 2: Determine the parents’ genotypes and possible gametes. • Example: a heterozygous pea plant and a homozygous dominant pea plant. P p P PP P

  26. Punnett Squares Pp • Step 3: Fill in the squares by combining what is on top of the column and to the left of the row. P p P PP Pp PP PP Pp P

  27. Punnett Squares Pp • Step 4: Use the Punnett square to determine probabilities and ratios. P p P PP Pp PP PP Pp P

  28. Punnett Squares • What is the probability of an offspring plant having purple flowers? • 100% • What is the probability of an offpsring plant being heterozygous? • 2/4 = 1/2 = 50% PP Pp PP Pp

  29. Punnett Squares • If there are 2 traits, the Punnett square will be a 4x4 grid. • Example: Cross a pea plant that is heterozygous for both flower color and seed shape with a plant that has white flowers and is heterozygous for seed shape P - purple; p - white R - round, r - wrinkled

  30. Punnett Squares • Cross a pea plant that is heterozygous for both flower color and seed shape with a plant that has white flowers and is heterozygous for seed shape PpRr ppRr PR, Pr, pR, pr pR, pr, pR, pr

  31. Punnett Squares ppRr pR pR pr pr PR Pr PpRr pR pr

  32. Punnett Squares ppRr pR pR pr pr PR Pr PpRr pR pr

  33. Punnett Squares ppRr pR pR pr pr PR Pr PpRr pR pr

  34. Punnett Squares pR pR pr pr • What is the probability of an offspring having white flowers and wrinkled seeds? PR 2 / 16 = 1 / 8 or 12.5% Pr pR pr

  35. Punnett Squares pR pR pr pr • What is the probability of an offspring having purple flowers and round seeds? PR 6 / 16 = 3 / 8 or 37.5% Pr pR pr

  36. Punnett Squares • Write the probable genotypic ratio. • 2 PpRR : 4 PpRr : 2 Pprr : 2 ppRR : 4 ppRr : 2 pprr • 1 PpRR : 2 PpRr : 1 Pprr : 1 ppRR : 2 ppRr : 1 pprr

  37. Intermediate Inheritance Beyond Simple Dominance

  38. Intermediate Inheritance • There are 3 types of intermediate inheritance, genetic patterns that don’t follow the simple dominant-recessive rules. • Incomplete dominance • Codominance • Multiple alleles

  39. Incomplete Dominance • Incomplete dominance - neither allele is completely dominant over the other • The heterozygous form is a “blended” form of the two alleles.

  40. Incomplete Dominance • Example: In snapdragon flowers, there is an allele that codes for red (r), and allele that codes for white (w). • rr - red • ww - white • rw - pink

  41. Incomplete Dominance • Ex. Cross a red and a pink snapdragon. r w r r

  42. Incomplete Dominance • Sometimes two like capital letters are used, but one gets a prime sign (‘). • Ex: Human hair • Curly hair HH • Straight hair H’H’ • Wavy hair HH’

  43. Codominance • Codominance - both alleles are dominant and get expressed equally • In the heterozygous has some of each phenotype, but they are not blended.

  44. Codominance • Example - in a type of cattle, red hair (R) and white hair (W) are codominant. • RR - red • WW - white • RW - roan • Some red, some white, but not pink!

  45. Codominance • Ex. Cross a red parent and a white parent. R R W W

  46. Multiple Alleles • Multiple alleles - there are more than 2 alleles for a trait. • Ex. Fur color - gray, black, striped • Ex. Human blood types

  47. Sex Linkage Sex-linked, sex-limited, and sex-influenced traits

  48. Human Chromosomes • Humans have 23 homologous pairs of chromosomes, for a total of 46. • 22 pairs are called autosomes , which are all of the non-sex chromosomes • The 23rd pair is the sex chromosomes - X and Y.

  49. Sex Chromosomes • X and Y • Females - XX • All eggs have an X • Males - XY • Sperm have either an X or Y

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