Chapter 10What Is the Physical Basis of Inheritance?In the ordinary body cells (not sperm or eggs) of many organisms the chromosomes occur in pairs.Slide 4Slide 5Slide 6Slide 7The Relationship Among Genes, Alleles, and ChromosomesSlide 9Slide 10Slide 11How Were the Principles of Inheritance Discovered?Gregor MendelSlide 14Flowers of the Edible Pea PlantSlide 16Slide 17Slide 18Slide 19How Are Single Traits Inherited?Slide 21Slide 22Cross of Pea Plants True-Breeding for White or Purple FlowersSlide 24Self-Fertilization of F1 Pea Plants with Purple FlowersSlide 26Slide 27Slide 28Slide 29Slide 30The Distribution of Alleles in GametesSlide 32Slide 33Slide 34Slide 35Slide 36Segregation of Alleles and Fusion of GametesSlide 38Slide 39Determining the Outcome of a Single-Trait CrossSlide 41Punnett Square of a Test CrossSlide 43Slide 44Sex Determination in MammalsSlide 46Slide 47Slide 48Slide 49Incomplete DominanceSlide 51Slide 52Slide 53Slide 54Slide 55Slide 56Slide 57Slide 58Slide 59Environmental Influence on PhenotypeSlide 61Slide 62Slide 63Trisomy 21, or Down SyndromeChapter 10Patterns of InheritanceWhat Is the Physical Basis of Inheritance? –A gene is a unit of DNA that encodes information needed to produce proteins, cells, and entire organisms –Can be a few hundred to many thousands of nucleotides in length –The location of a gene on a chromosome is called its locus (plural, loci)In the ordinary body cells (not sperm or eggs) of many organisms the chromosomes occur in pairs. -Each species has a characteristic number of chromosomes -Human cells contain 46 chromosomes, 23 pairsHumans: Autosomes (22 pairs) + 1 pair of sex chromosomes---Autosomes are the non-sex chromosomes.Genes are sequences of nucleotides at specific locations on chromosomes–Homologous chromosomes carry the same kinds of genes for the same characteristics–Genes for the same characteristic are found at the same loci on both homologous chromosomesEach chromosome with oneidentical DNAdouble helix---Homologous chromosomes (homologues) are a pair of chromosomes that contain similar genes.–Genes for a characteristic found on homologous chromosomes may not be identical–Alternative versions of genes found at the same gene locus are called alleles.–Alleles are formed by mutations.An organism’s two alleles may be the same or different–Each cell carries two alleles per characteristic, one on each of the two homologous chromosomes–If both homologous chromosomes carry the same allele (gene form) at a given gene locus, the organism is homozygous at that locus-- If two homologous chromosomes carry different alleles at a given locus, the organism is heterozygous at that locus, also called hybrid.Both chromosomes carry the same alleleof the gene at this locus; the organism ishomozygous at this locusThis locus contains another gene for whichthe organism is homozygousEach chromosome carries a different alleleof this gene, so the organism is heterozygous at this locusa pair of homologouschromosomesgene locithe chromosomefrom the maleparentthe chromosomefrom the femaleparentThe Relationship Among Genes, Alleles, and Chromosomes Fig. 10-1–The Punnett square method predicts offspring genotypes and phenotypes from combinations of parental gametes1. Assign letters to the different alleles of the characteristic under consideration (uppercase for dominant, lowercase for recessive)2. Determine the possible gametes from both parents3. Write the gametes from each parent, along each side of a 2 x 2 grid (Punnett square)4. Fill in the genotypes of each pair of combined gametes in the grid.5. Determine the genotypic ratio based on the outcome of the Punnett Square.6. Based on the which traits are dominant, and which are recessive, calculate the predicted phenotypic ratio.How Were the Principles of Inheritance Discovered? – Gregor Mendel - Austrian monk in a monastery in the late 1800s--Discovered patterns of inheritance and many essential facts about genes, alleles, and the distribution of alleles in gametes during sexual reproduction –He chose edible pea plant for his experiments –Allowed him to see patterns in the way plant characteristics were inheritedGregor MendelFig. 10-2–Why Pea plants? –Pea flowers have stamens, the male structures that produce pollen. Pollen contains the sperm (male gametes); sperm are gametes and pollen is the vehicle–Pea flowers have carpels, female structures housing the ovaries, which produce the eggs (female gametes) –Pea flower petals enclose both male and female flower parts and prevent entry of pollen from another pea plantintact pea flower flower dissected to showits reproductive structuresCarpel (female,produces eggs)Stamen (male, producespollen that contain sperm)Flowers of the Edible Pea Plant Fig. 10-3–Because of their structure, pea flowers naturally self-fertilize–Pollen from the stamen of a plant transfers to the carpel of the same plant, where the sperm then fertilizes the plant’s eggs–Mendel was able to mate two different plants by (cross-fertilization)–Female parts (carpels) were dusted with pollen from other selected plants–The key to Mendel’s success:He chose a simple experimental design–He chose to study individual characteristics (called traits) that had unmistakably different forms, such as white versus purple flowers–He started out by studying only one trait at a time–He followed the inheritance of these traits for several generations, counting the numbers of offspring with each type of trait–By analyzing these numbers, he saw the basic patterns of inheritance emergeHow Are Single Traits Inherited?Pea plants that are homozygous for a particular characteristic always produce the same phenotype–If a plant is homozygous for purple flowers, it will always produce offspring with purple flowers–Plants that are homozygous for a characteristic are described as true-breeding–A genetic cross is the mating of pollen and eggs (from same or different parents)–The parents used in a cross are part of the parental generation (known as P)–The offspring of the P generation are members of the first filial generation (F1)–Offspring of the F1 generation are members of the F2 generationMendel’s flower color experiments:–Crossed a true-breeding (homozygous) purple flower plant with a true-breeding (homozygous) white-flower plant (the P generation)–The F1 generation consisted of all purple-flowered plants–What had happened to the
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