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Section 10.1 Summary – pages 253-262

Section 10.1 Summary – pages 253-262. Punnett Squares. In 1905, Reginald Punnett , an English biologist, devised a shorthand way of finding the expected proportions of possible genotypes in the offspring of a cross. . This method is called a Punnett square . .

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Section 10.1 Summary – pages 253-262

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  1. Section 10.1 Summary – pages 253-262 Punnett Squares In 1905, Reginald Punnett, an English biologist, devised a shorthand way of finding the expected proportions of possible genotypes in the offspring of a cross. This method is called a Punnett square.

  2. Section 10.1 Summary – pages 253-262 Punnett Squares If you know the genotypes of the parents, you can use a Punnett square to predict the possible genotypes of their offspring.

  3. Probability and Punnett Squares • Genetics and Probability • If you flip a coin three times in a row what is the probability that it will land on heads up every time? • PunnettSquares • Monohybrid Cross • Genotype Ratio • Phenotype Ratio

  4. PunnettSquare-Monohybrid Monohybrid Cross

  5. Punnett Square-Monohybrid Punnett Squares – Ratios

  6. Section 10.1 Summary – pages 253-262 Probability In reality you don’t get the exact ratio of results shown in the square. That’s because, in some ways, genetics is like flipping a coin—it follows the rules of chance. The probability or chance that an event will occur can be determined by dividing the number of desired outcomes by the total number of possible outcomes.

  7. Section 10.1 Summary – pages 253-262 Probability A Punnett square can be used to determine the probability of getting a pea plant that produces round seeds when two plants that are heterozygous (Rr) are crossed.

  8. Section 10.1 Summary – pages 253-262 Probability r R The Punnett square shows three plants with round seeds out of four total plants, so the probability is 3/4. RR Rr R Rr rr r

  9. Section 10.1 Summary – pages 253-262 Probability r R It is important to remember that the results predicted by probability are more likely to be seen when there is a large number of offspring. RR Rr R Rr rr r

  10. Section 1 Check Question 1 The passing on of characteristics from parents to offspring is __________. A. genetics B. heredity C. pollination D. allelic frequency

  11. Section 1 Check Question 2 What are traits? • Traits are characteristics that are inherited. • Height, hair color and eye color are examples. C. Different gene forms D. Both A and B

  12. Section 1 Check Question 3 Gametes are __________. A. male sex cells B. female sex cells C. both male and female sex cells • fertilized cells that develop into adult organisms

  13. Section 1 Check Question 4 Which of the following genotypes represents a plant that is homozygous for height? A. Tt B. Hh C. tT D. tt

  14. Answers Section 1 Check • B • D • C • D

  15. Section 12.2 Summary – pages 315 - 322 Complex Patterns of Inheritance • Patterns of inheritance that are explained by Mendel’s experiments are often referred to as simple. • However, many inheritance patterns are more complex than those studied by Mendel.

  16. Incomplete dominance: Appearance of a third phenotype Section 12.2 Summary – pages 315 - 322 • When inheritance follows a pattern of dominance, heterozygous and homozygous dominant individuals both have the same phenotype. • When traits are inherited in an incomplete dominancepattern, however, the phenotype of heterozygous individuals is intermediate between those of the two homozygotes.

  17. Incomplete dominance: Appearance of a third phenotype Section 12.2 Summary – pages 315 - 322 • For example, if a homozygous red-flowered snapdragon plant (RR) is crossed with a homozygous white-flowered snapdragon plant (R′ R′), all of the F1 offspring will have pink flowers.

  18. Incomplete dominance: Appearance of a third phenotype Section 12.2 Summary – pages 315 - 322 White Red All pink Red (RR) Pink (RR’) White (R’R’) Pink (RR’) All pink flowers 1 red: 2 pink: 1 white

  19. Incomplete dominance: Appearance of a third phenotype Section 12.2 Summary – pages 315 - 322 • The new phenotype occurs because the flowers contain enzymes that control pigment production. • The R allele codes for an enzyme that produces a red pigment. The R’ allele codes for a defective enzyme that makes no pigment.

  20. Incomplete dominance: Appearance of a third phenotype Section 12.2 Summary – pages 315 - 322 • Because the heterozygote has only one copy of the R allele, its flowers appear pink because they produce only half the amount of red pigment that red homozygote flowers produce.

  21. Section 12.2 Summary – pages 315 - 322 White Red All pink Red (RR) Pink (RR’) White (R’R’) Pink (RR’) All pink flowers 1 red: 2 pink: 1 white

  22. Codominance: Expression of both alleles Section 12.2 Summary – pages 315 - 322 • Codominant allelescause the phenotypes of both homozygotes to be produced in heterozygous individuals. In codominance, both alleles are expressed equally.

  23. Multiple phenotypes from multiple alleles Section 12.2 Summary – pages 315 - 322 • Although each trait has only two alleles in the patterns of heredity you have studied thus far, it is common for more than two alleles to control a trait in a population. • Traits controlled by more than two alleles have multiple alleles.

  24. Sex determination Section 12.2 Summary – pages 315 - 322 • In humans the diploid number of chromosomes is 46, or 23 pairs. • There are 22 pairs of homologous chromosomes called autosomes. Homologous autosomes look alike. • The 23rd pair of chromosomes differs in males and females.

  25. Sex determination Section 12.2 Summary – pages 315 - 322 • These two chromosomes, which determine the sex of an individual, are called sex chromosomesand are indicated by the letters X and Y.

  26. Sex determination Section 12.2 Summary – pages 315 - 322 • If you are female, your 23rdpair of chromosomes are homologous, XX. X X Female • If you are male, your 23rd pair of chromosomes XY, look different. X Y Male

  27. Sex determination Section 12.2 Summary – pages 315 - 322 • Males usually have one X and one Y chromosome and produce two kinds of gametes, X and Y. • Females usually have two X chromosomes and produce only X gametes. • It is the male gamete that determines the sex of the offspring.

  28. Sex determination Section 12.2 Summary – pages 315 - 322 XY Male X Y X XX Female XY Male XX Female X XY Male XX Female

  29. Sex-linked inheritance Section 12.2 Summary – pages 315 - 322 • Traits controlled by genes located on sex chromosomes are called sex-linked traits. • The alleles for sex-linked traits are written as superscripts of the X or Y chromosomes. • Because the X and Y chromosomes are not homologous, the Y chromosome has no corresponding allele to one on the X chromosome and no superscript is used.

  30. Sex-linked inheritance Section 12.2 Summary – pages 315 - 322 • Also remember that any recessive allele on the X chromosome of a male will not be masked by a corresponding dominant allele on the Y chromosome.

  31. Sex-linked inheritance Section 12.2 Summary – pages 315 - 322 White-eyed male (XrY) F2 Red-eyed female (XRXR) Females: all red eyed Males: 1/2red eyed 1/2white eyed F1 All red eyed

  32. Sex-linked inheritance Section 12.2 Summary – pages 315 - 322 • The genes that govern sex-linked traits follow the inheritance pattern of the sex chromosome on which they are found.

  33. Polygenic inheritance Section 12.2 Summary – pages 315 - 322 • Polygenic inheritance is the inheritance pattern of a trait that is controlled by two or more genes. • The genes may be on the same chromosome or on different chromosomes, and each gene may have two or more alleles. • Uppercase and lowercase letters are used to represent the alleles.

  34. Polygenic inheritance Section 12.2 Summary – pages 315 - 322 • However, the allele represented by an uppercase letter is not dominant. All heterozygotes are intermediate in phenotype. • In polygenic inheritance, each allele represented by an uppercase letter contributes a small, but equal, portion to the trait being expressed.

  35. Polygenic inheritance Section 12.2 Summary – pages 315 - 322 • The result is that the phenotypes usually show a continuous range of variability from the minimum value of the trait to the maximum value. • Examples in humans: height, eye color, intelligence, skin color

  36. Environmental Influences Section 12.2 Summary – pages 315 - 322 • The genetic makeup of an organism at fertilization determines only the organism’s potential to develop and function. • As the organism develops, many factors can influence how the gene is expressed, or even whether the gene is expressed at all. • Two such influences are the organism’s external and internal environments.

  37. Influence of external environment Section 12.2 Summary – pages 315 - 322 • Temperature, nutrition, light, chemicals, and infectious agents all can influence gene expression.

  38. Influence of external environment Section 12.2 Summary – pages 315 - 322 • In arctic foxes temperature has an effect on the expression of coat color.

  39. Influence of external environment Section 12.2 Summary – pages 315 - 322 • External influences can also be seen in leaves. Leaves can have different sizes, thicknesses, and shapes depending on the amount of light they receive.

  40. Influence of internal environment Section 12.2 Summary – pages 315 - 322 • The internal environments of males and females are different because of hormones and structural differences. • An organism’s age can also affect gene function.

  41. Section 2 Check Question 1 Which of the following does NOT have an effect on male-pattern baldness? A. hormones B. internal environment C. sex-linked inheritance D. incomplete dominance

  42. Section 2 Check Answer D. incomplete dominance

  43. Codominance in Humans Section 12.3 Summary – pages 323 - 329 • Remember that in codominance, the phenotypes of both homozygotes are produced in the heterozygote. • One example of this in humans is a group of inherited red blood cell disorders called sickle-cell disease.

  44. Sickle-cell disease Section 12.3 Summary – pages 323 - 329 • In an individual who is homozygous for the sickle-cell allele, the oxygen-carrying protein hemoglobin differs by one amino acid from normal hemoglobin. • This defective hemoglobin forms crystal-like structures that change the shape of the red blood cells. Normal red blood cells are disc-shaped, but abnormal red blood cells are shaped like a sickle, or half-moon.

  45. Sickle-cell disease Section 12.3 Summary – pages 323 - 329 • The change in shape occurs in the body’s narrow capillaries after the hemoglobin delivers oxygen to the cells. Normal red blood cell Sickle cell

  46. Sickle-cell disease Section 12.3 Summary – pages 323 - 329 • Abnormally shaped blood cells, slow blood flow, block small vessels, and result in tissue damage and pain. Normal red blood cell Sickle cell

  47. Sickle-cell disease Section 12.3 Summary – pages 323 - 329 • Individuals who are heterozygous for the allele produce both normal and sickled hemoglobin, an example of codominance. • Individuals who are heterozygous are said to have the sickle-cell trait because they can show some signs of sickle-cell-related disorders if the availability of oxygen is reduced.

  48. Multiple Alleles Govern Blood Type Section 12.3 Summary – pages 323 - 329 • Mendel’s laws of heredity also can be applied to traits that have more than two alleles. • The ABO blood group is a classic example of a single gene that has multiple alleles in humans.

  49. Multiple Alleles Govern Blood Type Section 12.3 Summary – pages 323 - 329 Human Blood Types Genotypes Phenotypes Surface Molecules A A lAlA or lAli B B lBlB or lBi lAlB A and B AB None ii O

  50. The importance of blood typing Section 12.3 Summary – pages 323 - 329 • Determining blood type is necessary before a person can receive a blood transfusion because the red blood cells of incompatible blood types could clump together, causing death.

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