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Lecture 12students(2) - Lecture 12 Inheritance, genes...

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Unformatted text preview: Lecture 12 Inheritance, genes and chromosomes What Are the Mendelian Laws of Inheritance? Early study of inheritance worked under two assump@ons about how inheritance works: •  Each parent contributes equally to offspring in reciprocal crosses (supported by experiments) •  Hereditary determinants blend in offspring (not supported by experiments) What Are the Mendelian Laws of Inheritance? •  Each parent contributes equally to offspring. (Correct.) Supported by reciprocal crosses, 1770s, by Kölreuter •  Hereditary determinants blend in the offspring. (Incorrect.) It was thought that once hereditary elements had blended they could never be separated Gregor Mendel’s studies refuted this. What Are the Mendelian Laws of Inheritance? Mendel’s new theory of inheritance was published in 1866, but was largely ignored. Most biologists at the @me were not used to thinking in mathema@cal terms. Even Darwin missed the significance of Mendel’s work. What Are the Mendelian Laws of Inheritance? Mendel worked with the garden pea. He could control pollina@on and fer@liza@on —and be sure of the parents of offspring. What Are the Mendelian Laws of Inheritance? Character: observable physical feature (e.g., flower color) Trait: form of a character (e.g., purple flowers or white flowers) A heritable trait is passed from parent to offspring What Are the Mendelian Laws of Inheritance? Mendel looked for well ­defined, true ­ breeding traits—the observed trait is the only one present for many genera@ons. True ­breeding strains were isolated by inbreeding and selec@on. He concentrated on seven traits. What Are the Mendelian Laws of Inheritance? Mendel’s crosses: •  Pollen from one parent was transferred to the s@gma of the other parent. Parental genera6on = P •  Resul@ng offspring = first filial genera6on or F1 •  If F1 plants self pollinate, produce second filial genera6on or F2 What Are the Mendelian Laws of Inheritance? Mendel’s first experiment: Crossed plants differing in just one trait (P). F1 genera@on are monohybrids. The monohybrids were then allowed to self pollinate to form the F2 genera@on: A monohybrid cross. Mendel repeated this for all seven traits. What Are the Mendelian Laws of Inheritance? One trait of each pair disappeared in the F1 genera@on and reappeared in the F2— these traits are recessive. The trait that appears in the F1 is the dominant trait. The ra@o of dominant to recessive in the F2 was about 3:1. What Are the Mendelian Laws of Inheritance? Reciprocal crosses yielded the same results: it made no difference which parent contributed pollen. The idea that each parent contributes equally was supported. What Are the Mendelian Laws of Inheritance?? The blending theory was not supported by Mendel’s crosses. Mendel proposed that the heritable units were discrete par@cles—the par6culate theory. Each plant has two par@cles for each character, one from each parent. What Are the Mendelian Laws of Inheritance? Diploid: The two copies of heritable unit in an organism. During gamete produc@on, only one copy is given to the gamete̶this single set is called haploid. What Are the Mendelian Laws of Inheritance? Mendel also concluded that each gamete contains only one par@cle (or unit), but the zygote contains two—because it is produced from the fusion of two gametes. The “par@cles” are now called genes. The totality of all genes in an organism is the genome. What Are the Mendelian Laws of Inheritance? The true ­breeding plants in the P genera@on had two iden@cal copies of the par@cle (gene) for each character. Example: Spherical SS; wrinkled ss gametes from SS will have one S gametes from ss will have one s offspring (F1) will be Ss S is dominant; s is not expressed in F1. What Are the Mendelian Laws of Inheritance? Alleles: Different forms of a gene True ­breeding individuals have two copies of the same allele—they are homozygous for the allele (e.g., ss). Heterozygous individuals have two different alleles (e.g., Ss). What Are the Mendelian Laws of Inheritance? Phenotype: Physical appearance of an organism (e.g., spherical seeds). Genotype: The gene@c makeup (e.g., Ss). Spherical seeds can be the result of two different genotypes—SS or Ss. What Are the Mendelian Laws of Inheritance? Mendel’s first law The law of segrega6on: The two copies of a gene separate when an individual makes gametes. Figure 12.4 Mendel’s Explanation of Inheritance (Part 1) What Are the Mendelian Laws of Inheritance? When the F1 self ­pollinates, there are three ways to get the dominant trait (e.g., spherical), but only one way to get the recessive (wrinkled)—resul@ng in the 3:1 ra@o. Allele combina@ons can be predicted using a PunneE square. Figure 12.4 Mendel’s Explanation of Inheritance (Part 2) What Are the Mendelian Laws of Inheritance? A gene is a sequence on a DNA molecule that resides at a par@cular site on a chromosome—the locus—and encodes a par@cular character. Different alleles of a gene separate during meiosis. Figure 12.5 Meiosis Accounts for the Segrega@on of Alleles (Part 1) Figure 12.5 Meiosis Accounts for the Segrega@on of Alleles (Part 2) What Are the Mendelian Laws of Inheritance? Mendel tested his hypothesis by doing test crosses: •  Determines whether an individual is homozygous or heterozygous for a trait by crossing it with the homozygous recessive Mendel crossed the F1 with known homozygotes (e.g., wrinkled or ss) Figure 12.6 Homozygous or Heterozygous? (Part 1) Figure 12.6 Homozygous or Heterozygous? (Part 2) What Are the Mendelian Laws of Inheritance? Mendel’s next experiment: Crossing peas that differed in two characters— seed shape and seed color. True ­breeding parents: SSYY—spherical yellow seeds ssyy—wrinkled green seeds What Are the Mendelian Laws of Inheritance? F1 genera@on is SsYy—all spherical yellow. Crossing the F1 genera@on (all iden@cal double heterozygotes) is a dihybrid cross. Mendel asked whether, in the gametes produced by SsYy, the traits would be linked, or segregate independently. What Are the Mendelian Laws of Inheritance? If linked, gametes would be SY or sy; F2 would have three @mes more spherical yellow than wrinkled green. If independent, gametes could be SY, sy, Sy, or sY. F2 would have nine different genotypes; phenotypes would be in 9:3:3:1 ra@o. Results indicated new combina@ons called recombinant phenotypes. Figure 12.7 Independent Assortment (Part 1) Figure 12.7 Independent Assortment (Part 2) What Are the Mendelian Laws of Inheritance? Mendel’s second law The law of independent assortment: Alleles of different genes assort independently during gamete forma@on. Doesn’t always apply to genes on the same chromosome; but chromosomes do segregate independently. Figure 12.8 Meiosis Accounts for Independent Assortment of Alleles (Part 1) Figure 12.8 Meiosis Accounts for Independent Assortment of Alleles (Part 2) What Are the Mendelian Laws of Inheritance? Probability: •  If an event is certain to happen, probability = 1 •  If an event cannot possibly happen, probability = 0 •  All other events have a probability between 0 and 1 What Are the Mendelian Laws of Inheritance? Probability of two independent events happening together: Mul@ply the probabili@es of the individual events. Tossing two coins—probability that both will come up heads: ½ x ½ = ¼ The mul6plica6on rule. What Are the Mendelian Laws of Inheritance? Dihybrid crosses: Probability that F2 seed will be spherical is ¾: probability of heterozygote + probability of homozygote or ½ + ¼ = ¾ Joint probability that a seed will be spherical and yellow: ¾ × ¾ = 9/16 What Are the Mendelian Laws of Inheritance? Human pedigrees can show Mendel’s laws. Humans have few offspring; pedigrees do not show the clear propor@ons that the pea plants showed. Gene@cists use pedigrees to determine whether a rare allele is dominant or recessive. Figure 12.10 Pedigree Analysis and Inheritance (Part 1) Figure 12.10 Pedigree Analysis and Inheritance (Part 2) What Are the Mendelian Laws of Inheritance? When there is a rare recessive phenotype in a family, there is usually marriage of rela@ves. If a recessive allele is rare in the general popula@on, it is unlikely that two people that marry will both carry it unless they are related (e.g., cousins). How Do Alleles Interact? Different alleles arise through muta6on: rare, stable, inherited changes in the gene@c material. Wild type: allele present in most of the popula@on. Other alleles are mutant alleles. Locus with wild ­type allele present less than 99 percent of the @me is polymorphic. How Do Alleles Interact? A given gene may have more than two alleles. Example: Coat color in rabbits Mul@ple alleles increase the number of possible phenotypes. Figure 12.11 Inheritance of Coat Color in Rabbits How Do Alleles Interact? Some alleles are neither dominant nor recessive—a heterozygote has an intermediate phenotype: Incomplete dominance. Example: Snapdragons. Figure 12.12 Incomplete Dominance Follows Mendel’s Laws How Do Alleles Interact? Codominance: Two alleles at one locus produce phenotypes that are both present in the heterozygote. Example: ABO blood group system—three alleles at one locus. Figure 12.13 ABO Blood Reac@ons Are Important in Transfusions How Do Alleles Interact? A single allele can have mul@ple phenotypic effects: Pleiotropic. Example: Allele for colora@on papern in Siamese cats; the same allele results in crossed eyes—both result from the same protein. How Do Alleles Interact? Epistasis: Phenotypic expression of one gene is influenced by another gene. Example: Coat color in Labrador retrievers Allele B (black) dominant to b (brown) Allele E (pigment deposi@on) is dominant to e (no pigment deposi@on—yellow). Figure 12.14 Genes May Interact Epista@cally (Part 1) Figure 12.14 Genes May Interact Epista@cally (Part 2) How Do Genes Interact? Inbreeding: Ma@ng among close rela@ves; can result in offspring of low quality. Close rela@ves tend to have the same recessive alleles. How Do Genes Interact? A cross between two different true ­breeding homozygotes can result in offspring with stronger, larger phenotypes: “Hybrid vigor” or heterosis. First discovered with corn by G.H. Shull. Figure 12.15 Hybrid Vigor in Corn How Do Genes Interact? Environment also affects phenotype. Light, temperature, nutri@on, etc., can affect expression of the genotype. Siamese cats and certain rabbit breeds—the enzyme that produces dark fur is inac@ve at higher temperatures. Figure 12.16 The Environment Influences Gene Expression How Do Genes Interact? Effects of genes and environment on phenotype: •  Penetrance: Propor@on of individuals with a certain genotype that show the phenotype •  Expressivity: Degree to which genotype is expressed in an individual How Do Genes Interact? Mendel’s characters were discrete and qualita6ve. For more complex characters, phenotypes vary con@nuously over a range— quan6ta6ve, or con@nuous, varia@on. Quan@ta@ve varia@on is usually due to both genes and environment. What Is the Rela@onship between Genes and Chromosomes? Some crosses performed with Drosophila did not yield expected ra@os according to the law of independent assortment. Some genes were inherited together; the two loci were on the same chromosome, or linked. All of the loci on a chromosome form a linkage group. Figure 12.19 Crossing Over Results in Gene@c Recombina@on (Part 1) Figure 12.19 Crossing Over Results in Gene@c Recombina@on (Part 2) What Is the Rela@onship between Genes and Chromosomes? Recombinant offspring phenotypes (non ­ parental) appear in recombinant frequencies: Divide number of recombinant offspring by total number of offspring. Recombinant frequencies are greater for loci that are farther apart. Figure 12.20 Recombinant Frequencies What Is the Rela@onship between Genes and Chromosomes? Recombinant frequencies can be used to make gene6c maps showing the arrangement of genes along a chromosome. Distance between genes = map unit = recombinant frequency of 0.01. Map unit also called a cen6morgan (cM). Figure 12.21 Steps toward a Gene@c Map Figure 12.22 Map These Genes (Part 1) Figure 12.22 Map These Genes (Part 2) Figure 12.22 Map These Genes (Part 3) Figure 12.22 Map These Genes (Part 4) Figure 12.22 Map These Genes (Part 5) What Is the Rela@onship between Genes and Chromosomes? Mammals: Female has two X chromosomes (XX). Male has one X and one Y (XY). Male mammals produce two kinds of gametes—half carry a Y and half carry an X. The sex of the offspring depends on which chromosome fer@lizes the egg. 12.4 What Is the Rela@onship between Genes and Chromosomes? In other animals, sex determina@on by chromosomes is different from mammals. Insert Table 12 .2 What Is the Rela@onship between Genes and Chromosomes? The SRY gene (sex ­determining region on the Y chromosome) encodes a protein involved in primary sex determina6on. If SRY protein is present, the embryo develops testes. If there is no SRY, the embryo develops ovaries. What Is the Rela@onship between Genes and Chromosomes? A gene on the X chromosome, DAX1, produces an an@ ­tes@s factor. In males, SRY inhibits the DAX1 maleness inhibitor, DAX1 protein. In females, who lack SRY, DAX1 func@ons to inhibit maleness. What Is the Rela@onship between Genes and Chromosomes? Secondary sex determina6on results in outward characteris@cs of each sex—not determined directly by presence or absence of Y chromosome. These characteris@cs are determined by genes on autosomes and the X chromosomes that control hormonal ac@on. What Is the Rela@onship between Genes and Chromosomes? Genes on sex chromosomes don’t follow Mendelian paperns. The Y chromosome carries few genes; the X chromosome carries many. Thus, males have only one copy of these genes—hemizygous. What Is the Rela@onship between Genes and Chromosomes? Sex ­linked inheritance—a character is governed by loci on the sex chromosomes. Example: Eye color in Drosophila. What Are the Effects of Genes Outside the Nucleus? Mitochondria and plas@ds contain small numbers of genes—important in organelle assembly and func@on. Mitochondria and plas@ds are inherited from the mother. There may be hundreds of mitochondria or plas@ds in a cell. What Are the Effects of Genes Outside the Nucleus? Organelle genes tend to mutate faster than nuclear genes—mul@ple alleles. Some plas@d gene muta@ons affect chlorophyll synthesis, resul@ng in a white phenotype. Mitochondrial gene muta@ons may affect ATP produc@on, no@ceably in @ssues with high energy requirements. How Do Prokaryotes Transmit Genes? Bacteria exchange genes by conjuga@on: •  Sex pilus—a projec@on that ini@ates contact between bacterial cells •  Conjuga6on tube—cytoplasmic bridge that forms between cells The donor chromosome fragments and some material enters the recipient cell. Figure 12.26 Bacterial Conjuga@on and Recombina@on (Part 1) Figure 12.26 Bacterial Conjuga@on and Recombina@on (Part 2) How Do Prokaryotes Transmit Genes? Bacteria have plasmids—small circular chromosomes—besides the main one. Genes on the plasmids are in categories: •  Metabolic tasks, breaking down hydrocarbons •  Involved in conjuga@on •  An@bio@c resistance How Do Prokaryotes Transmit Genes? Plasmids can move between the cells during conjuga@on. They can: •  Replicate independently of the main chromosome •  Add their genes to the recipient cell’s genome Figure 12.27 Gene Transfer by Plasmids (Part 1) Figure 12.27 Gene Transfer by Plasmids (Part 2) ...
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