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Chapter p 14 Mendel and the Gene Idea Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance • Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments LE 14-2 Removed stamens from purple flower Transferred sperm spermbearing pollen from stamens of white flower to eggbearing carpel of purple flower Parental generation (P) Carpel Stamens Pollinated carpel matured into pod Planted seeds from pod First generation offspring (F1) Examined offspring: all purple flowers • Mendel chose to track only those characters that varied in an “either-or” either-or manner • He also used varieties that were “true-breeding” ( l t th (plants thatt produce d offspring ff i off the th same variety i t when they self-pollinate) The Law of Segregation • When any individual produces gametes, the copies of a gene separate, so that each gamete receives only one copy. PP x pp Pp x Pp P Generation (true-breeding parents) F1 Generation (hybrids) F2 Generation Purple White flowers flowers All p plants had purple flowers • Mendel called the purple flower color a dominant trait and white flower color a recessive trait • “heritable factor” is what we now call a gene Mendel’s Model • Mendel developed four related concepts to explain the 3:1 inheritance pattern he observed in F2 offspring. • First Concept – alternate versions of genes, alleles, exist and explain for inherited variations. Allele for purple flowers Locus for flower-color g gene Allele for white flowers Homologous pair of chromosomes • Second concept - for each character an organism inherits two alleles alleles, one from each parent P Generation (true-breeding parents) F1 Generation (hybrids) F2 Generation Purple White flowers flowers All p plants had purple flowers • Third concept - if the two alleles at a locus differ, then the dominant allele determines the organism’s i ’ appearance, and d th the recessive i allele ll l has no noticeable effect on appearance P Generation (true-breeding parents) F1 Generation (hybrids) F2 Generation Purple White flowers flowers All plants l t had h d purple flowers • Fourth concept – The Law of Segregation states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes. P Generation (true-breeding parents) F1 Generation (hybrids) F2 Generation Purple White flowers flowers All plants l t had h d purple flowers LE 14-5_2 P Generation Appearance: Genetic makeup: Purple flowers PP White flowers pp P p Gametes F1 Generation Appearance: Genetic makeup: Purple flowers Pp 1 Gametes: 2 1 P p 2 F1 sperm P p PP Pp Pp pp F2 Generation P F1 eggs p 3 :1 Useful Genetic Vocabulary • Homozygous - An organism with two identical alleles for a character character. • Heterozygous - An organism that has two diff different t alleles ll l ffor a gene. – Unlike homozygotes, yg , heterozygotes yg are not true-breeding LE 14-6 3 Phenotype Genotype Purple PP (homozygous Purple Pp (heterozygous 1 2 1 Purple Pp p (heterozygous White pp (homozygous Ratio 3:1 Ratio 1:2:1 1 LE 14-7 Dominant phenotype, unknown genotype: PP or Pp? Test Cross Recessive phenotype, known genotype: pp If Pp, then 2 offspring purple and 1 2 offspring white: If PP, then all offspring purple: p 1 p P p p Pp Pp pp pp P Pp Pp P P Pp Pp The Law of Independent Assortment • Monohybrids - individuals that are heterozygous for one character • A cross between such heterozygotes is called a monohybrid h b id cross Yy x Yy • Dihybrids - heterozygous for two characters • A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are t transmitted itt d to t offspring ff i as a package k or independently YyRr x YyRr ? LE 14-8 P Generation YYRR yyrr Gametes YR yr YyRr F1 Generation Hypothesis of dependent assortment Hypothesis of independent assortment Sperm 1 Sperm 1 2 YR 1 2 yr 1 1 2 2 1 4 Yr 1 4 yR 1 4 yr YR 4 YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr YR YYRR 1 YR Eggs Eggs F2 Generation (predicted offspring) 4 YyRr 1 Yr 4 yr YyRr 3 4 yyrr 1 1 yR y 4 4 1 Phenotypic ratio 3:1 yr 4 9 16 3 16 3 16 3 16 Phenotypic ratio 9:3:3:1 • Law of Independent Assortment - each pair of alleles segregates independently of other pairs of alleles during gamete formation – This law applies only to genes on different, non-homologous chromosomes Concept 14.2: The laws of probability govern Mendelian inheritance • Mendel’s laws of segregation and independent assortment reflect the rules of probability • The alleles of one gene segregate into gametes independently of another gene’s gene s alleles The Multiplication and Addition Rules Applied to Monohybrid Crosses • Multiplication rule - the probability that two or more independent events will occur together is the product of their individual probabilities • Rr x Rr – Probability of gamete having: • RR? • rr? LE 14-9 Rr Rr Segregation of gg alleles into eggs Segregation of alleles into sperm Sperm 1 R 2 R 1 2 1 r 2 R R R 1 r 1 4 4 Eggs r r 1 2 R r 1 4 r 1 4 • Rule of addition - the probability that any one of two or more exclusive events will occur is calculated by adding together their individual p probabilities • Rr x Rr – Probability of gamete having: • Rr? Solving Complex Genetics Problems with the Rules of Probability • A dihybrid cross is equivalent to two or more independent monohybrid crosses occurring simultaneously • Cross: YyRr x YyRr • What is the probability of YYRR? – of Yyrr? Genetics Problems • PPYyRrTt x PpyyRrtt • What is the probability of PpyyrrTt? Genetics Problems • PpYyRr x Ppyyrr • What fraction of the offspring from this cross would exhibit the recessive phenotypes for at least two of the three characteristics? Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics • Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: – Wh When alleles ll l are nott completely l t l d dominant i t or recessive – When a gene has more than two alleles – When a gene produces multiple phenotypes The Spectrum of Dominance • Complete dominance – PP=Purple Pp=Purple • Codominance – BB=Black Fur WW=White Fur BW=Black & White Fur • Incomplete dominance – RR=Red WW=White RW=Pink P Generation Red CRCR White CWCW CR Gametes CW Pink CRCW F1 Generation Gametes 1 1 F2 Generation 2 CR 2 CR 1 2 1 CW Sperm 2 CW Eggs 1 1 2 2 CR CRCR CRCW CRCW CWCW CW Frequency of Dominant Alleles • Dominant alleles are not necessarily more common in populations than recessive alleles • For example, one baby out of 400 in the United St t is States i born b with ith extra t fi fingers or ttoes – this unusual trait is dominant • Known as polydactyly • Natural Selection at work! Multiple Alleles • Most genes exist in populations in more than two allelic forms • Four phenotypes of the ABO blood group – three alleles for the enzyme (I): IA, IB, and i Pleiotropy • Most genes have multiple phenotypic effects, a property called pleiotropy • For example, the multiple symptoms of – cystic fibrosis and sickle-cell disease Epistasis • Epistasis - a gene at one locus alters the phenotypic expression of a gene at a second locus • IIn mice i and d many other th mammals, l coatt color l depends on two genes – One gene determines the pigment color – The other gene determines whether the pigment will be deposited in the hair LE 14-11 BbCc BbCc Sperm 1 1 1 1 1 4 BC 1 4 bC 1 4 1 Bc 4 bc 4 BC BBCC BbCC BBCc BbCc 4 bC BbCC bbCC BbCc bbCc 4 Bc BBCc BbCc BBcc Bbcc 4 bc BbCc bbCc Bbcc bbcc 9 16 3 16 4 16 Polygenic Inheritance • Quantitative characters are those th t vary in that i th the population along a continuum • Skin color in humans is an example of polygenic inheritance AaBbCc aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCcAABBCC 20/64 15/64 Fraction n of progeny • usually indicates polygenic l i inheritance i h it AaBbCc 6/64 1/64 Nature and Nurture: The Environmental Impact on Phenotype • Norm of reaction - the phenotypic range of a genotype influenced by the environment • Hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity Concept 14.4: Many human traits follow Mendelian patterns of inheritance • Pedigree - a family tree that describes the interrelationships of parents and children across generations • Inheritance patterns of particular traits can be traced and described using pedigrees LE 14-14a Ww ww ww Ww ww ww Ww WW or Ww Ww Ww ww Dominant trait (widow’s peak) Second generation (parents plus aunts and uncles) Third generation (two sisters) ww Widow’s peak First generation (grandparents) No widow’s peak LE 14-14b First generation (grandparents) Second generation (parents plus aunts and uncles) Ff FF or Ff ff Third Thi d generation (two sisters) Attached earlobe Recessive trait (attached earlobe) Ff ff ff Ff Ff ff FF or Ff Ff ff Free earlobe • Pedigrees can also be used to make predictions about future offspring Dominantly Inheritance Recessively Inherited Disorders • Recessively inherited disorders show up only in individuals homozygous for the allele – Carriers are heterozygous individuals who carryy the recessive allele but appear normal Recessively Inheritance X-Linked Recessive Inheritance X-Linked Dominant Inheritance Mitochondrial Inheritance Cystic Fibrosis • Cystic fibrosis is the most common lethal genetic disease in the United States States, striking one out of every 2,500 people of European descent – Common C symptom t - abnormal b l mucus b buildup ild Sickle-Cell Disease • Sickle-cell disease affects one out of 400 AfricanAmericans • Symptoms include physical p y weakness,, pain, organ damage, and even paralysis l i Dominantly Inherited Disorders • Some human disorders are due to dominant alleles • One example is achondroplasia, a form of d dwarfism fi th thatt iis llethal th l when h h homozygous ffor th the dominant allele Fetal Testing • In amniocentesis, the liquid that bathes the fetus is removed and tested • In chorionic villus sampling (CVS), a sample of th placenta the l t is i removed d and d ttested t d Video: Ultrasound of Human Fetus I LE 14-17a Amniocentesis Amniotic fluid withdrawn Fetus A sample of amniotic fluid can b taken be t k starting t ti att the 14th to 16th week of pregnancy. C t if Centrifugation ti Placenta Uterus Cervix 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. Biochemical tests Several weeks k Karyotyping LE 14-17b Chorionic villus sampling (CVS) A sample of chorionic villus tissue can be taken as early as the 8th to 10th week of pregnancy. Fetus Suction tube inserted through cervix Placenta Chorionic villi Fetal cells Biochemical tests Several h hours Karyotyping Karyotyping and biochemical tests can be performed on the fetal cells immediately, providing results within a day or so.