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Chapter 14 Mendel and the Gene Idea discoveries/videos/100-greatest-discoveries- shorts-genetics.htm
One possible explanation of heredity is a “blending” hypothesis ▫The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea ▫Parents pass on discrete heritable units, genes What genetic principles account for the transmission of traits from parents to offspring?
Gregor Mendel ▫Mendel used the scientific approach to identify two laws of inheritance Mendel discovered the basic principles of heredity by breeding garden peas Vocabulary ▫Character: a heritable feature, such as flower color ▫Trait: a variant of a character, such as purple or white flowers
Mendel chose to track ▫Only those characters that varied in an “either-or” manner Ex: Flower color trait is either purple or white, there is no intermediate Mendel also made sure that ▫He started his experiments with varieties that were “true-breeding” all successive generations display only the desired trait Ex: A purple-flowered plant is self-pollinated and all the offspring have purple flowers
Pollen transferred from white flower to stigma of purple flower anthers removed all purple flowers result Mendel’s work F1F1 P F2F2 self-pollinate Bred pea plants ▫cross-pollinate two true breeding parents (P) hybridization P = parental ▫raised seed & then observed traits (F 1 ) Hybrid offspring F = filial ▫allowed offspring to self-pollinate & observed next generation (F 2 )
F 2 generation 3:1 75% purple-flower peas 25% white-flower peas Looking closer at Mendel’s work P 100% F 1 generation (hybrids) 100% purple-flower peas X true-breeding purple-flower peas true-breeding white-flower peas self-pollinate Where did the white flowers go? White flowers came back!
Mendel reasoned that ▫In the F 1 plants, only the purple flower factor was affecting flower color in these hybrids ▫Purple flower color was dominant, and white flower color was recessive Mendel observed the same pattern ▫In many other pea plant characters Table 14.1
Mendel’s Experiments and Observations Allowed Mendel to deduce two fundamental laws of heredity: 1.Law of Segregation 2.Law of Independent Assortment
Mendel’s Model Mendel developed a hypothesis ▫To explain the 3:1 inheritance pattern that he observed among the F 2 offspring Four related concepts make up this model 1.Alternative versions of genes (alleles) 2.Each Allele is represented twice 3.If two alleles differ, the dominant one is expressed 4.Two alleles segregate during meiosis
What did Mendel’s findings mean? Traits come in alternative versions ▫purple vs. white flower color ▫alleles different alleles vary in the sequence of nucleotides at the specific locus of a gene some difference in sequence of A, T, C, G purple-flower allele & white-flower allele are two DNA variations at flower-color locus different versions of gene at same location on homologous chromosomes
Traits are inherited as discrete units For each characteristic, an organism inherits 2 alleles, 1 from each parent ▫diploid organism inherits 2 sets of chromosomes, 1 from each parent ▫homologous chromosomes ▫A genetic locus is actually represented twice, one on each homolog of a pair of chromosomes Two alleles may be identical or different
What did Mendel’s findings mean? Some traits mask others ▫purple & white flower colors are separate traits that do not blend purple x white ≠ light purple purple masked white ▫dominant allele functional protein masks other alleles ▫recessive allele allele makes a malfunctioning protein homologous chromosomes I’ll speak for both of us! wild type allele producing functional protein mutant allele producing malfunctioning protein
Fourth, the law of segregation Law of segregation ▫during meiosis, alleles segregate homologous chromosomes separate ▫each allele for a trait is packaged into a separate gamete ▫An egg or sperm only receives one of the two alleles present in the somatic cell PP P P pp p p PpPp P p
Law of Segregation Which stage of meiosis creates the law of segregation? Whoa! And Mendel didn’t even know DNA or genes existed! Metaphase 1
Genotype vs. phenotype Difference between how an organism “looks” & its genetics ▫phenotype description of an organism’s trait the “physical” ▫genotype description of an organism’s genetic makeup Explain Mendel’s results using …dominant & recessive …phenotype & genotype F1F1 P X purplewhite all purple
Making crosses Can represent alleles as letters ▫flower color alleles P or p ▫true-breeding purple-flower peas PP ▫true-breeding white-flower peas pp PP x pp PpPp F1F1 P X purplewhite all purple
Mendel’s law of segregation, probability and the Punnett square Try a cross: Pp x Pp P Generation F 1 Generation F 2 Generation P p P p P p P p PpPp PP pp Pp Appearance: Genetic makeup: Purple flowers PP White flowers pp Purple flowers Pp Appearance: Genetic makeup: Gametes: F 1 sperm F 1 eggs 1/21/2 1/21/2 Each true-breeding plant of the parental generation has identical alleles, PP or pp. Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele. Union of the parental gametes produces F 1 hybrids having a Pp combination. Because the purple- flower allele is dominant, all these hybrids have purple flowers. When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele. 3 : 1 Random combination of the gametes results in the 3:1 ratio that Mendel observed in the F 2 generation. This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F 1 F 1 (Pp Pp) cross. Each square represents an equally probable product of fertilization. For example, the bottom left box shows the genetic combination resulting from a p egg fertilized by a P sperm.
Genotypes Homozygous = same alleles = PP, pp ▫True-breeding, all sperm/egg contain P Heterozygous = different alleles = Pp ▫½ sperm/egg contain P other ½ contains p homozygous dominant homozygous recessive heterozygous How do you determine the genotype of an individual with with a dominant phenotype? Can’t tell by lookin’ at ya!
Test cross Breed the dominant phenotype — the unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele pp is it PP or Pp? x How does that work?
PPpp How does a Test cross work? pp P P pp P p PpPppp xx PpPp PpPpPpPp PpPp 100% purple PpPp pp PpPp 50% purple:50% white or 1:1 pp Am I this? Or am I this?
The Law of Independent Assortment Mendel derived the law of segregation ▫By following a single trait The F 1 offspring produced in this cross ▫Were monohybrids, heterozygous for one character Crossing two heterozygotes is a monohybrid cross x Pp x F1F1
The Law of Independent Assortment Mendel identified his second law of inheritance ▫By following two characters at the same time See color & seed shape Crossing two, true-breeding parents differing in two characters ▫Produces dihybrids in the F 1 generation, heterozygous for both characters x YYRR yyrr P Y = yellow R = round y = green r = wrinkled
How are two characters transmitted from parents to offspring? ▫As a package? ▫Independently? A dihybrid cross ▫Illustrates the inheritance of two characters Produces four phenotypes in the F 2 generation YYRR P Generation GametesYRyr yyrr YyRr Hypothesis of dependent assortment Hypothesis of independent assortment F 2 Generation (predicted offspring) 1⁄21⁄2 YR yr 1 ⁄ 2 1⁄21⁄2 yr YYRRYyRr yyrr YyRr 3 ⁄ 4 1 ⁄ 4 Sperm Eggs Phenotypic ratio 3:1 YR 1 ⁄ 4 Yr 1 ⁄ 4 yR 1 ⁄ 4 yr 1 ⁄ 4 9 ⁄ 16 3 ⁄ 16 1 ⁄ 16 YYRR YYRr YyRR YyRr YyrrYyRr YYrr YyRR YyRr yyRRyyRr yyrr yyRr Yyrr YyRr Phenotypic ratio 9:3:3: Phenotypic ratio approximately 9:3:3:1 F 1 Generation Eggs YR Yr yRyr 1 ⁄ 4 Sperm RESULTS CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. EXPERIMENT Two true-breeding pea plants— one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F 1 plants. Self-pollination of the F 1 dihybrids, which are heterozygous for both characters, produced the F 2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant. Figure :3:3:1
What’s going on here? If genes are on different chromosomes… ▫how do they assort in the gametes? ▫together or independently? YyRr YRyr YyRr YryRYRyr Is it this?Or this? Which system explains the data?
9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled Is this the way it works? YyRr YRyr YR yr x YyRr YryRYR yr YyRr YRyr or YYRRYyRr yyrr Well, that’s NOT right!
Dihybrid cross YyRr YRYryR yr YR Yr yR yr YYRR x YYRrYyRRYyRr YYRrYYrrYyRrYyrr YyRRYyRryyRRyyRr YyRrYyrryyRryyrr 9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled YyRr YryRYR yr YyRr YRyr or BINGO!
Using the information from a dihybrid cross, Mendel developed the law of independent assortment ▫Each pair of alleles segregates independently during gamete formation ▫Works for alleles on different chromosomes (chromosomes that are not homologous) Or genes far apart from each other on the same chromosome that frequently cross over Mendel’s 2 nd law of heredity
Law of Independent Assortment Which stage of meiosis creates the law of independent assortment? Metaphase 1 EXCEPTION If genes are on same chromosome & close together will usually be inherited together rarely crossover separately “linked”
The chromosomal basis of Mendel’s laws…
Review: Mendel’s laws of heredity Law of segregation ▫monohybrid cross single trait ▫each allele segregates into separate gametes established by Metaphase 1 Law of independent assortment ▫dihybrid (or more) cross 2 or more traits ▫genes on separate chromosomes assort into gametes independently established by Metaphase 1 EXCEPTION linked genes
Concept Check 14.1 A pea plant heterozygous for inflated pods (Ii) is crossed with a plant homozygous for constricted pods (ii). Draw a Punnett square for this cross. Pea plants heterozygous for flower position and stem length (AaTt) are allowed to self pollinate, and 400 of the resulting seeds are plants. How many offspring would be predicted to have terminal flowers and be dwarf?
Concept 14.2: The laws of probability govern Mendelian inheritance Mendel’s laws of segregation and independent assortment ▫Reflect the rules of probability The multiplication rule ▫Finding the probability that two or more independent events will occur together: Multiply the probability of one event by the probability of the other even Ex: Probability of 2 offspring from the same parents are both homozygous recessive?
Probability in a monohybrid cross ▫Can be determined using this rule The rule of addition ▫States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities One or more possibilities that can occur in the same event Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm R r r R R R R 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄21⁄2 r r R r r Sperm Eggs Figure 14.9 What is the likelihood that an offspring is heterozygote? ¼ + ¼ = ½ What is the likelihood two offspring from the same parents are both homozygous recessive? ¼ x ¼ = 1/16
Solving Complex Genetics Problems with the Rules of Probability We can apply the rules of probability ▫To predict the outcome of crosses involving multiple characters A dihybrid or other multicharacter cross ▫Is equivalent to two or more independent monohybrid crosses occurring simultaneously In calculating the chances for various genotypes from such crosses ▫Each character first is considered separately and then the individual probabilities are multiplied together
Concept Check 14.2 For any gene with a dominant allele C and recessive allele c, what proportions of the offspring from a CC x Cc cross are expected to be homozygous dominant, homozygous recessive and heterozygous? An organism with the genotype BbDD is mated to one with the genotype BBDd. Assuming independent assortment of these two genes, write the genotypes of all possible offspring from this cross and use the rules of probability to calculate the chance of each type occurring.
Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics The relationship between genotype and phenotype is rarely simple The inheritance of characters by a single gene ▫May deviate from simple Mendelian patterns
The Spectrum of Dominance Complete dominance ▫Occurs when the phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance ▫The phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties P Generation F 1 Generation F 2 Generation Red C R Gametes CRCR CWCW White C W Pink C R C W Sperm CRCR CRCR CRCR CwCw CRCR CRCR Gametes 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 Eggs 1⁄21⁄2 C R C R C W C W C R C W RRWWRW
Co-dominance 2 alleles affect the phenotype equally & separately ▫not blended phenotype ▫human ABO blood groups ▫Multiple Alleles: 3 alleles I A, I B, i I A & I B alleles are co-dominant glycoprotein antigens on RBC I A I B = both antigens are produced i allele recessive to both
The Relation Between Dominance and Phenotype Dominant and recessive alleles ▫Do not really “interact” Dominant alleles do not “subdue” recessive alleles ▫Lead to synthesis of different proteins that produce a phenotype Ex: Tay Sachs Disease: autosomal recessive inheritance pattern Frequency of Dominant Alleles Dominant alleles ▫Are not necessarily more common in populations than recessive alleles Ex: Polydactyly: occurs in 1 in 400 births; autosomal dominant
Pleiotropy In pleiotropy ▫A gene has multiple phenotypic effects ▫Most genes are pleiotrophic Ex: A genetic disease caused by a single allele has many symptoms associated with it One gene can affect many characteristics in an organism
Extending Mendelian Genetics for Two or More Genes Some traits ▫May be determined by two or more genes ▫This type of expression includes: 1.Epistasis 2.Polygenic Inheritance
Epistasis B_C_ bbC_ _ _cc How would you know that difference wasn’t random chance? Chi-square test! One gene completely masks another gene ▫coat color in mice = 2 separate genes C,c: pigment (C) or no pigment (c) B,b: more pigment (black=B) or less (brown=b) cc = albino, no matter B allele 9:3:3:1 becomes 9:3:4
Epistasis in Labrador retrievers 2 genes: (E,e) & (B,b) ▫pigment (E) or no pigment (e) ▫pigment concentration: black (B) to brown (b) E–B–E–bbeeB–eebb
Polygenic inheritance Some phenotypes determined by additive effects of 2 or more genes on a single character ▫phenotypes on a continuum ▫human traits skin color height weight intelligence behaviors
enzyme Skin color: Albinism However albinism can be inherited as a single gene trait ▫aa = albino tyrosine melanin albinism
Environmental effects Phenotype is controlled by both environment & genes ▫Multifactorial characters Color of Hydrangea flowers is influenced by soil pH Human skin color is influenced by both genetics & environmental conditions Coat color in arctic fox influenced by heat sensitive alleles
Concept Check 14.3 If a man with type AB blood marries a woman with type O blood, what blood types would you expect in their children? A rooster with gray feathers is mated with a hen of the same phenotype. Among their offspring, 15 chicks are gray, 6 are black and 8 are white. What is the simplest explanation for the inheritance of these colors in chickens? What phenotypes would you expect in the offspring of a cross between a gray rooster and a black hen?
Pedigree analysis Pedigree analysis reveals Mendelian patterns in human inheritance ▫data mapped on a family tree = male= female= male w/ trait = female w/ trait
Simple pedigree analysis What’s the likely inheritance pattern?
Genetic counseling Pedigree can help us understand the past & predict the future Thousands of genetic disorders are inherited as simple recessive traits ▫from benign conditions to deadly diseases albinism cystic fibrosis Tay sachs sickle cell anemia PKU
Recessive diseases The diseases are recessive because the allele codes for either a malfunctioning protein or no protein at all ▫Heterozygotes (Aa) carriers have a normal phenotype because one “normal” allele produces enough of the required protein Aa male / sperm A a female / eggs AAAaaaAa carrier disease
Cystic fibrosis (recessive) Primarily whites of European descent ▫strikes 1 in 2500 births 1 in 25 whites is a carrier (Aa) ▫normal allele codes for a membrane protein that transports Cl - across cell membrane defective or absent channels limit transport of Cl - & H 2 O across cell membrane thicker & stickier mucus coats around cells mucus build-up in the pancreas, lungs, digestive tract & causes bacterial infections ▫without treatment children die before 5; with treatment can live past their late 20s normal lung tissue
Effect on Lungs Chloride channel transports salt through protein channel out of cell Osmosis: H 2 O follows Cl – airway Cl – H2OH2O H2OH2O mucus secreting glands bacteria & mucus build up thickened mucus hard to secrete normal lungs cystic fibrosis cells lining lungs Cl – channel
Tay-Sachs (recessive) Primarily Jews of eastern European (Ashkenazi) descent & Cajuns (Louisiana) ▫strikes 1 in 3600 births 100 times greater than incidence among non-Jews ▫non-functional enzyme fails to breakdown lipids in brain cells fats collect in cells destroying their function symptoms begin few months after birth seizures, blindness & degeneration of muscle & mental performance child usually dies before 5yo
Sickle cell anemia (recessive) Primarily Africans ▫strikes 1 out of 400 African Americans high frequency ▫caused by substitution of a single amino acid in hemoglobin ▫when oxygen levels are low, sickle-cell hemoglobin crystallizes into long rods deforms red blood cells into sickle shape sickling creates pleiotropic effects = cascade of other symptoms
Sickle cell phenotype 2 alleles are codominant ▫both normal & mutant hemoglobins are synthesized in heterozygote (Aa) ▫50% cells sickle; 50% cells normal ▫carriers usually healthy ▫sickle-cell disease triggered under blood oxygen stress exercise
Dominantly Inherited Disorders Some human disorders ▫Are due to dominant alleles ▫Dominant alleles that cause lethal disease are much less common ▫Ex: achondroplasia: a form of dwarfism that is lethal when homozygous for the dominant allele ▫What is the chance that two married dwarves with achondroplasia would have a child who was of normal height?
Heterozygote advantage Malaria ▫single-celled eukaryote parasite spends part of its life cycle in red blood cells In tropical Africa, where malaria is common: ▫homozygous dominant individuals die of malaria ▫homozygous recessive individuals die of sickle cell anemia ▫heterozygote carriers are relatively free of both reproductive advantage High frequency of sickle cell allele in African Americans is vestige of African roots
Aa x aa Inheritance pattern of Achondroplasia aa A a Aa A a Aa x Aa Aa aa Aa 50% dwarf:50% normal or 1:1 AA aa Aa 67% dwarf:33% normal or 2:1 Aa lethal dominant inheritance dwarf
Huntington’s chorea (dominant) Dominant inheritance ▫repeated mutation on end of chromosome 4 mutation = CAG repeats glutamine amino acid repeats in protein one of 1 st genes to be identified ▫build up of “huntingtin” protein in brain causing cell death memory loss muscle tremors, jerky movements “chorea” starts at age early death years after start
Multifactorial Disorders Many human diseases ▫Have both genetic and environment components Examples include ▫Heart disease, cancer, diabetes, and alcoholism ▫The hereditary component of these diseases is polygenic
Genetic Testing and Counseling Based on Mendelian Genetics and Probability Rules Genetic counselors ▫Can provide information to prospective parents concerned about a family history for a specific disease Using family histories ▫Genetic counselors help couples determine the odds that their children will have genetic disorders
Tests for Identifying Carriers For a growing number of diseases ▫Tests are available that identify carriers and help define the odds more accurately ▫Tests are available for Tay-Sachs, Sickle Cell Anemia, and Cystic Fibrosis sequence individual genes
Fetal & Newborn Testing In amniocentesis ▫The liquid that bathes the fetus is removed and tested In chorionic villus sampling (CVS) ▫A sample of the placenta is removed and tested (a) Amniocentesis Amniotic fluid withdrawn Fetus PlacentaUterus Cervix Centrifugation A sample of amniotic fluid can be taken starting at the 14th to 16th week of pregnancy. (b) Chorionic villus sampling (CVS) Fluid Fetal cells Biochemical tests can be Performed immediately on the amniotic fluid or later on the cultured cells. Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping. Several weeks Biochemical tests Several hours Fetal cells Placenta Chorionic viIIi A sample of chorionic villus tissue can be taken as early as the 8th to 10th week of pregnancy. Suction tube Inserted through cervix Fetus Karyotyping and biochemical tests can be performed on the fetal cells immediately, providing results within a day or so. Karyotyping