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Chapter 5 Genetics: The Science of Heredity Section 1: Mendel’s Work • • What were the results of Mendel’s experiments, or crosses? What controls the inheritance of traits in organisms? Chapter 5 Genetics: The Science of Heredity What is Genetics? Genetics: the study of heredity Heredity: the passing of physical characteristics from parents to offspring Chapter 5 Genetics: The Science of Heredity The Father of Genetics The field of Genetics was founded by Gregor Mendel, an Augustinian priest. Between 1856 and 1863, Mendel cultivated and tested almost 30,000 pea plants. The importance of Mendel's work was not discovered until almost 30 years after Mendel died. Chapter 5 Genetics: The Science of Heredity Crossing Pea Plants Gregor Mendel crossed pea plants that had different traits. The illustrations show how he did this. Chapter 5 Genetics: The Science of Heredity Mendel’s Experiments In all of Mendel’s crosses, only one form of the trait appeared in the F1 generation. However, in the F2 generation, the “lost” form of the trait always reappeared in about one fourth of the plants. Chapter 5 Genetics: The Science of Heredity Dominant and Recessive Alleles Mendel studied several traits in pea plants. Chapter 5 Genetics: The Science of Heredity Dominant and Recessive Alleles Today, scientists use the word “gene” to describe a piece of DNA that controls a trait. Chapter 5 Genetics: The Science of Heredity Dominant and Recessive Alleles The traits that Mendel studied in his pea plant experiments are controlled by different genes: GENE Seed Shape Seed color Seed coat color Pod shape Pod color Flower position Stem height Chapter 5 Genetics: The Science of Heredity Dominant and Recessive Alleles These genes usually have 2 or more alleles, or different forms of the gene: GENE Seed Shape Seed color Seed coat color Pod shape Pod color Flower position Stem height ALLELE round yellow gray smooth green side tall ALLELE wrinkled green white pinched yellow end short Chapter 5 Genetics: The Science of Heredity Dominant and Recessive Alleles Some of these alleles are known as dominant. Others are known as recessive: DOMINANT RECESSIVE GENE ALLELE ALLELE Seed Shape round wrinkled Seed color yellow green Seed coat color gray white Pod shape smooth pinched Pod color green yellow Flower position side end Stem height tall short Chapter 5 Genetics: The Science of Heredity Dominant and Recessive Alleles In a dominant allele, the trait always shows up as long as there is at least one dominant allele. Key T = tall t = short TT “pure tall” Tt “hybrid tall” Chapter 5 Genetics: The Science of Heredity Dominant and Recessive Alleles In a recessive allele, the trait only shows up if both alleles are recessive. Key T = tall t = short tt “pure short” Chapter 5 Genetics: The Science of Heredity Dominant and Recessive Alleles Dominant alleles are always symbolized with capital letters. Recessive alleles are always symbolized with lower-case letters. Key for Height Key for Seed Color T = tall t = short Y = yellow seed color y = green seed color Key for Pod Color Key for Coat Color G = green pod color g = yellow pod color A = gray coat color a = white coat color Chapter 5 Genetics: The Science of Heredity End of Section: Mendel’s Work Chapter 5 Genetics: The Science of Heredity Section 2: Probability and Heredity What is probability and how does it help explain the results of genetic crosses? What is meant by genotype and phenotype? What is codominance? Chapter 5 Genetics: The Science of Heredity A Punnett Square The diagrams show how to make a Punnett square. In this cross, both parents are heterozygous for the trait of seed shape. R represents the dominant round allele, and r represents the recessive wrinkled allele. Chapter 5 Genetics: The Science of Heredity Probability and Genetics In a genetic cross, the allele that each parent will pass on to its offspring is based on probability. Chapter 5 Genetics: The Science of Heredity Phenotypes and Genotypes An organism’s phenotype is its physical appearance, or visible traits. An organism’s genotype is its genetic makeup, or allele combinations. Chapter 5 Genetics: The Science of Heredity Practicing Punnett Squares Key for Height T = tall t = short Key for Seed Color Y = yellow seed color y = green seed color Key for Pod Color G = green pod color g = yellow pod color 1) T T x T T 2) t t x t t 3) T t x T t Chapter 5 Genetics: The Science of Heredity Practicing Punnett Squares Key for Height T = tall t = short Key for Seed Color Y = yellow seed color y = green seed color Key for Pod Color G = green pod color g = yellow pod color 1) 2) 3) 4) Y Y g G y Y g g x x x x y y y y G g G g Chapter 5 Genetics: The Science of Heredity Homozygous vs. Heterozygous Homozygous = 2 identical alleles also called “pure” or “purebred” Examples: T T tt Heterozygous = 2 different alleles also called “hybrid” Examples: T t Chapter 5 Genetics: The Science of Heredity Codominance In codominance, the alleles are neither dominant nor recessive. As a result, both phenotypes are expressed in the offspring. Chapter 5 Genetics: The Science of Heredity Incomplete Dominance In incomplete dominance, the contributions of both alleles are visible and do not overpower each other in the phenotype. As a result, both phenotypes look “mixed”. Chapter 5 Genetics: The Science of Heredity Dihybrid Cross Chapter 5 Genetics: The Science of Heredity Dihybrid Cross Key for Height T = tall t = short Key for Seed Color Y = yellow seed color y = green seed color Key for Pod Color G = green pod color g = yellow pod color Chapter 5 Genetics: The Science of Heredity Dihybrid Cross Chapter 5 Genetics: The Science of Heredity End of Section: Probability and Heredity Chapter 5 Genetics: The Science of Heredity Section 3: The Cell and Inheritance What role do chromosomes play in inheritance? What events occur during meiosis? What is the relationship between chromosomes and genes? Chapter 5 Genetics: The Science of Heredity Meiosis During meiosis, the chromosome pairs separate and are distributed to two different cells. The resulting sex cells have only half as many chromosomes as the other cells in the organism. Chapter 5 Genetics: The Science of Heredity Punnett Square A Punnett square is actually a way to show the events that occur at meiosis. Chapter 5 Genetics: The Science of Heredity A Lineup of Genes Chromosomes are made up of many genes joined together like beads on a string. The chromosomes in a pair may have different alleles for some genes and the same allele for others. Chapter 5 Genetics: The Science of Heredity Human Chromosomes Humans have 23 pairs of chromosomes: 23 from their mother, and 23 from their father. Chapter 5 Genetics: The Science of Heredity Human Chromosomes The first 22 pairs are organized and named according to their size: Chromosomes #1 are the largest, #2 are the second largest, etc. Chapter 5 Genetics: The Science of Heredity Human Chromosomes The final pair of chromosomes (“X” and “Y”) are the sex chromosomes because they determine the gender of the person: XX = girl XY = boy Chapter 5 Genetics: The Science of Heredity Sex Chromosomes The father is who determines the gender of the child since males need a “Y” chromosome, and only males have “Y” chromosomes. Mother The mother can only give out an “X”, and both boys and girls have at least 1 “X” chromosome. Father Chapter 5 Genetics: The Science of Heredity End of Section: The Cell and Inheritance Chapter 5 Genetics: The Science of Heredity Section 4: Genes, DNA, and Proteins • • • What forms the genetic code? How does a cell produce proteins? How can mutations affect an organism? Chapter 5 Genetics: The Science of Heredity The DNA Code Chromosomes are made of DNA. Each chromosome contains thousands of genes. The sequence of bases in a gene forms a code that tells the cell what protein to produce. Chapter 5 Genetics: The Science of Heredity How Cells Make Proteins During protein synthesis, the cell uses information from a gene on a chromosome to produce a specific protein. Chapter 5 Genetics: The Science of Heredity Mutations Mutations can cause a cell to produce an incorrect protein during protein synthesis. As a result, the organism’s trait, or phenotype, may be different from what it normally would have been. Chapter 5 Genetics: The Science of Heredity Damages Made by Mutation THEBIGBADCATATETHEBIGREDRAT Chapter 5 Genetics: The Science of Heredity End of Section: Genes, DNA, and Proteins