"112 Chapter 14 Mendelian genetics"
Mendelian Genetics Fig. 14-1 • Blending theory versus Particulate theory of heredity Definitions • Character = detectable inheritable feature of an organism • Trait = a variant of an inheritable character • True Breeding = Always producing offspring with the same traits as the parents when the parents are self-fertilized Definitions • P generation = parental • F1 generation = first filial (offspring of P generation) • F2 generation = second filial (offspring of the first filial generation) Definitions • Alleles = alternate forms of a gene • Monohybrid cross = a mating between parents that differ in a single character • Dihybrid cross = a mating between parental varieties that differ in 2 characters Definitions • Homozygous = 2 identical alleles for a given trait (PP or pp) • Heterozygous = 2 different alleles for a trait (Pp) • Phenotype = An organism’s expressed traits (purple or white flowers) • Genotype = An organism’s genetic makeup (PP or Pp or pp) Fig. 14-2 TECHNIQUE How Mendel 1 made his crosses 2 Parental generation (P) Stamens Carpel 3 4 RESULTS First 5 filial gener- ation offspring (F1) Mendel’s Monohybrid Crosses Fig. 14-3-2 EXPERIMENT P Generation (true-breeding parents) Purple White flowers flowers F1 Generation (hybrids) All plants had purple flowers Fig. 14-3-3 EXPERIMENT P Generation (true-breeding parents) Purple White flowers flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation 705 purple-flowered 224 white-flowered plants plants F2 generation showed 3:1 ratio (no blending) • From this he reasoned: – Alternate forms of genes are responsible for variations in inherited characters – For each character, an organism inherits two alleles, one from each parent – If the 2 alleles differ, one is fully expressed (dominant allele) and the other is masked, with no noticeable effect on the organism’s appearance (recessive allele) – The 2 alleles for each character segregate during gamete production Fig. 14-4 Allele for purple flowers Homologous Locus for flower-color gene pair of chromosomes Allele for white flowers Law of segregation • Allele pairs segregate during gamete formation and the paired condition is restored by the random fusion of gametes at fertilization Fig. 14-5-1 P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p Fig. 14-5-2 P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p F1 Generation Appearance: Purple flowers Genetic makeup: Pp Gametes: 1/ 2 P 1/ 2 p Punnett Square • Possible gametes from one parent are listed on one side of the square • Possible gametes from the other parent are listed on the other side of the square Fig. 14-5-3 P Generation Mendel’s Appearance: Purple flowers White flowers Law of Genetic makeup: PP pp Segregation Gametes: P p F1 Generation Appearance: Purple flowers Genetic makeup: Pp Gametes: 1/ 2 P 1/ 2 p Sperm F2 Generation P p P PP Pp Eggs p Pp pp 3 1 Fig. 14-6 Phenotype Genotype Purple PP 1 (homozygous) 3 Purple Pp (heterozygous) 2 Purple Pp (heterozygous) White pp 1 1 (homozygous) Ratio 3:1 Ratio 1:2:1 Fig. 14-7 TECHNIQUE Testcross: breeding of an organism of Dominant phenotype, Recessive phenotype, unknown unknown genotype: known genotype: PP or Pp? pp genotype with a homozygous Predictions recessive. If PP If Pp or Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs P p Pp Pp pp pp RESULTS or All offspring purple 1/2 offspring purple and 1/2offspring white Mendel’s Dihybrid Crosses Fig. 14-8 EXPERIMENT P Generation YYRR yyrr Gametes YR yr F1 Generation YyRr Hypothesis of Hypothesis of dependent independent Predictions assortment assortment or Sperm Predicted 1/ 1/ 1/ 1/ 4 YR 4 Yr 4 yR 4 yr offspring of Sperm F2 generation 1/ YR 1/ 2 2 yr 1/ 4 YR YYRR YYRr YyRR YyRr 1/ 2 YR YYRR YyRr 1/ 4 Yr Eggs YYRr YYrr Yyrr YyRr 1/ 2 yr Eggs YyRr yyrr 1/ 4 yR YyRR YyRr yyRR yyRr 3/ 1/ 4 4 1/ 4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9/ 3/ 3/ 1/ 16 16 16 16 Phenotypic ratio 9:3:3:1 RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 Mendel’s Dihybrid Crosses • F2 generation showed 9:3:3:1 ratio (not 3:1) • From this he reasoned: – Law of independent assortment = each allele pair segregates independently of other gene pairs during gamete formation. Probability Segregation and independent assortment of alleles are random events. • Random events are independent of each other. Probability • Rule of multiplication = the probability that independent events will occur simultaneously is the product of their individual probabilities. • Rule of Addition = The probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways. Fig. 14-9 Rr Rr Segregation of Segregation of alleles into eggs alleles into sperm Sperm 1/ 1/ 2 R 2 r R R 1/ R r 2 R 1/ 1/ 4 4 Eggs r r 1/ R r 2 r 1/ 1/ 4 4 Variations on Mendelian Genetics Dominance • Incomplete dominance • Complete dominance • Codominance Fig. 14-10-1 P Generation Red White CRCR CWCW Gametes CR CW Fig. 14-10-2 P Generation Red White CRCR CWCW Gametes CR CW Pink F1 Generation CRCW Gametes 1/2 CR 1/ 2 CW Fig. 14-10-3 P Generation Red White CRCR CWCW Gametes CR CW Pink F1 Generation CRCW Gametes 1/2 CR 1/ 2 CW Sperm 1/ 1/ 2 CR 2 CW F2 Generation 1/ 2 CR Eggs CRCR CRCW 1/ 2 CW CRCW CWCW ABO Blood Group • An example of Codominance and Multiple alleles Fig. 14-11 Allele Carbohydrate IA A IB B i none (a) The three alleles for the ABO blood groups and their associated carbohydrates Red blood cell Phenotype Genotype appearance (blood group) IAIA or IA i A IBIB or IB i B IAIB AB ii O (b) Blood group genotypes and phenotypes Pleiotropy • Most genes have multiple phenotypic effects, a property called pleiotropy • For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease Epistasis • In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus • For example, in mice and many other mammals, coat color depends on two genes • One gene determines the pigment color (with alleles B for black and b for brown) • The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair Fig. 14-12 BbCc BbCc Sperm 1/ 1/ 1/ 1/ 4 BC 4 bC 4 Bc 4 bc Eggs 1/ 4 BC BBCC BbCC BBCc BbCc 1/ 4 bC BbCC bbCC BbCc bbCc 1/ 4 Bc BBCc BbCc BBcc Bbcc 1/ 4 bc BbCc bbCc Bbcc bbcc 9 : 3 : 4 Polygenic inheritance • Additive effect of 2 or more genes determines a single phenotypic character • Quantitative characters = characters that vary by degree in a continuous distribution rather than by discrete (either/or) qualitative differences Fig. 14-13 AaBbCc AaBbCc Sperm 1/ 1/ 1/ 1/ 1/ 1/ 1/ 1/ 8 8 8 8 8 8 8 8 1/ 8 1/ 8 1/ 8 1/ 8 Eggs 1/ 8 1/ 8 1/ 8 1/ 8 Phenotypes: 1/ 64 6/ 64 15/ 64 20/ 64 15/ 64 6/ 64 1/ 64 Number of dark-skin alleles: 0 1 2 3 4 5 6 The Environmental Impact on Phenotype • Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype • The norm of reaction is the phenotypic range of a genotype influenced by the environment • For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity Fig. 14-14 Norms of reaction are generally broadest for polygenic characters Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype Pedigree Analysis Key Male Affected Mating male Female Offspring, in Affected birth order female (first-born on left) Fig. 14-15b 1st generation (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, and uncles) Ww ww ww Ww Ww ww 3rd generation (two sisters) WW ww or Ww Widow’s peak No widow’s peak (a) Is a widow’s peak a dominant or recessive trait? Fig. 14-15c 1st generation (grandparents) Ff Ff ff Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff 3rd generation (two sisters) ff FF or Ff Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait? Some Human Disorders The Behavior of Recessive Alleles • Recessively inherited disorders show up only in individuals homozygous for the allele • Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal • Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair Fig. 14-16 Parents Normal Normal Aa Aa Sperm A a Eggs Aa AA A Normal Normal (carrier) Aa Normal aa a Albino (carrier) Some Human Disorders Recessively inherited disorders – Cystic fibrosis – Tay-Sachs disease – Sickle-cell disease Cystic Fibrosis • Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent – The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes – Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine Sickle-Cell Disease • Sickle-cell disease affects one out of 400 African- Americans – The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells – Symptoms include physical weakness, pain, organ damage, paralysis, etc, etc. Dominantly Inherited Disorders • Some human disorders are caused by dominant alleles • Dominant alleles that cause a lethal disease are rare and arise by mutation • Achondroplasia is a form of dwarfism caused by a rare dominant allele Fig. 14-17 Parents Dwarf Normal Dd dd Sperm D d Eggs Dd dd d Dwarf Normal Dd dd d Normal Dwarf Some Human Disorders Dominantly inherited disorders – Huntington’s disease (late-acting lethal dominants) • Huntington’s disease is a degenerative disease of the nervous system • The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age Multifactorial Disorders • Many diseases, such as heart disease and cancer, have both genetic and environmental components • Little is understood about the genetic contribution to most multifactorial diseases Genetic Testing and Counseling • 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 – For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately Fig. 14-18 Amniotic fluid withdrawn Fetus Centrifugation Fetus Suction tube inserted Placenta through Placenta Chorionic Uterus Cervix villi cervix Fluid Bio- Fetal Several chemical cells hours tests Fetal Several cells hours Several weeks Several Several weeks Karyotyping hours (a) Amniocentesis (b) Chorionic villus sampling (CVS) Fig. 14-UN2 Degree of dominance Description Example Complete dominance Heterozygous phenotype of one allele same as that of homo- PP Pp zygous dominant Incomplete dominance Heterozygous phenotype of either allele intermediate between the two homozygous phenotypes C RC R C RC W C WC W Codominance Heterozygotes: Both phenotypes expressed IAIB Multiple alleles In the whole population, ABO blood group alleles some genes have more than two alleles IA , IB , i Pleiotropy One gene is able to Sickle-cell disease affect multiple phenotypic characters Fig. 14-UN3 Relationship among genes Description Example Epistasis One gene affects BbCc BbCc the expression of another BC bC Bc bc BC bC Bc bc 9 :3 :4 Polygenic A single phenotypic inheritance character is AaBbCc AaBbCc affected by two or more genes