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Chapter 4

Chapter 4. Heredity and Genes. 4.1 Basic Principles of Heredity. Heredity. The passing of traits from one generation to the next. Trait: Characteristic, such as eye color or nose shape. Genetics: The field of biology related to studying heredity. Gametes: sex cells Sperm: male gamete

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Chapter 4

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  1. Chapter 4 Heredity and Genes

  2. 4.1 Basic Principles of Heredity

  3. Heredity • The passing of traits from one generation to the next. • Trait: Characteristic, such as eye color or nose shape. • Genetics: The field of biology related to studying heredity.

  4. Gametes: sex cells • Sperm: male gamete • Ovum: female gamete • Fertilization: the process by which the male gamete unites with the female gamete. • Zygote: The cell formed after the male and female gamete unite.

  5. Monohybrid Cross • A cross between two parents that differ only in one trait. • Hybrid: The offspring of parents that have different forms of a trait.

  6. Dominant and Recessive Traits • Every trait is controlled by two factors. • Dominant trait: Trait that is expressed whenever at least one factor for that form of the characteristic is present. • Recessive trait: Trait that is not seen if a factor for the dominant form of the characteristic is present.

  7. Alleles • Gene: The “factors” described earlier. A trait that can be inherited. • Alleles: Different forms of a gene.

  8. Law of Dominance • Each organism receives two alleles for a trait (one from each parent). • The dominant trait appears if one or both alleles are for that trait. • The recessive trait appears ONLY if both alleles are for that recessive trait.

  9. Law of Segregation • Gametes randomly receive one of the parent’s alleles for a trait.

  10. Genotype • The combination of alleles in an organism. • Homozygous: The two alleles for the trait are the same. • Heterozygous: The two alleles for the trait are different.

  11. Phenotype • The trait that the organism displays.

  12. Incomplete dominance: The offspring show a blend of their parents’ traits. • Codominance: Both traits are displayed in the organism.

  13. 4.2 Punnett SquaresA.K.A “The Alien Lesson” • Punnettsquare: A chart used to predict the possible genotypes of the offspring of a cross between two parent organisms. • A capital letter is used to represent a DOMINANT allele, and a lower-case letter is used to represent a recessive allele.

  14. Remember that homozygous means that the two alleles are the same, and heterozygous means that the two alleles are different.

  15. Our first example involves alien skin color. • Blue skin color is dominant, and expressed with a capital B. • Green skin color is recessive, and expressed with a lower-case b. • The first parent is a homozygous, blue-skinned alien. Her genotype will be _______. • The second parent is a homozygous, green-skinned alien. His genotype will be ______.

  16. Our first example involves alien skin color. • Blue skin color is dominant, and expressed with a capital B. • Green skin color is recessive, and expressed with a lower-case b. • The first parent is a homozygous, blue-skinned alien. Her genotype will be BB. • The second parent is a homozygous, green-skinned alien. His genotype will be bb.

  17. A Punnett Square for one trait is a box divided into four squares:

  18. We write the genotype for one parent across the top, and the other along the left side (Law of Segregation). It doesn’t matter which parent you write where. B B b b

  19. Now we start the cross. In each box, write the letter from above and the letter from the side. It is customary to write capital letter first in a heterozygote. B B b b

  20. Continue to fill in the other three boxes of the Punnett square. B B b b

  21. The genotypes (combinations of alleles) from this cross are all Bb. All of the offspring (4/4 = 1) can be expected to have this genotype. B B b b

  22. The phenotypes (expressed traits) from this cross are all blue skin. All of the offspring (4/4 = 1) can be expected to have this phenotype. B B b b

  23. Now, let’s cross two heterozygous blue-skinned aliens. • The genotype for each of these aliens is _______.

  24. Now, let’s cross two heterozygous blue-skinned aliens. • The genotype for each of these aliens is Bb.

  25. Now let’s write the alleles for each parent along the top and the side of the Punnett’s Square. B b B b

  26. And now we fill in the four boxes. B b B b

  27. The possible genotypes of this cross are BB (1/4), Bb (2/4 = 1/2), and bb (1/4). B b B b

  28. The possible phenotypes of this cross are blue skin (3/4) and green skin (1/4). B b B b

  29. Alien Eye Number • The three-eyes trait is dominant, represented with a capital E. • The two-eyes trait is recessive, represented with a lowercase e. • Genotype for homozygous two-eyed is _____. • Genotype for heterozygous three-eyed is ______.

  30. Alien Eye Number • The three-eyes trait is dominant, represented with a capital E. • The two-eyes trait is recessive, represented with a lowercase e. • Genotype for homozygous two-eyed is ee. • Genotype for heterozygous three-eyed is Ee.

  31. Now let’s write the alleles for each parent along the top and the side of the Punnett’s Square. e e E e

  32. And now fill in the boxes. e e E e

  33. The possible genotypes for this cross are Ee (2/4 = 1/2)and ee (2/4 = 1/2). e e E e

  34. The possible phenotypes for this cross are three-eyed (2/4 = 1/2) and two-eyed (2/4 = 1/2). e e E e

  35. Dihybrid cross • A cross that looks at two different traits. • Generally, traits segregate independently, so for two traits, there are four different combinations of alleles that the parent can pass on to its offspring.

  36. A heterozygous blue-skinned, heterozygous three-eyed alien’s genotype is BbEe. • The possible combinations of alleles from this parent are: • BE • Be • bE • be

  37. To do a dihybrid cross, we need a Punnett’s square divided into 16 boxes.

  38. Each possible allele combination goes across the top and along the side. BE BebEbe BE Be bE be

  39. And now we fill in the boxes. BE BebEbe BE Be bE be

  40. Possible Genotypes: BBEE 1/ 16 BBEe 2/16 = 1/8 BBee 1/16 BbEE 2/16 = 1/8 BbEe 4/16 = 1/4 Bbee 2/16 = 1/8 bbEE 1/16 bbEe 2/16 = 1/8 bbee 1/16

  41. Phenotypes: BE BebEbe BE Be bE be

  42. Possible phenotypes: Blue skin and three eyes: 9/16 Blue skin and two eyes: 3/16 Green skin and three eyes: 3/16 Green skin and two eyes: 1/16

  43. What is the phenotype for Dermed, our Intro to Health Care alien?

  44. What is the phenotype for Dermed, our Intro to Health Care alien? He is a green-skinned, three-eyed alien!

  45. What are the possible genotypes for Dermed’s skin color?

  46. What are the possible genotypes for Dermed’s skin color? He has green skin, which is a recessive trait. Therefore, the only possible genotype is bb.

  47. What are the possible genotypes for Dermed’s eye number? He has three eyes, which is a dominant trait. Therefore, the possible genotypes are EE or Ee.

  48. 4.3 Chromosomes • Chromosome: The structure that contains genes. • Made of DNA (and some proteins to help it hold shape). • Found in the nucleus of eukaryotic cells.

  49. Chromosomes look like thin threads until the cell is almost ready to divide. • Just before cell division, the chromosome coils itself into an X-shape.

  50. Chromatid: Each side of X-shaped chromosome. Sister chromatids: The pair of chromatids that make up one chromosome. They are exact copies of each other (so that when the cell divides, each new cell gets a copy). Centromere: The point at which the two sister chromatids are held together. Sister chromatids

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