Chapter 9 Patterns of Inheritance PDF

Chapter 9 Patterns of Inheritance  Mendel: father of modern genetics Modern genetics began with Gregor Mendel’s quantitative experiments with pea plants  White...

Chapter 9 Patterns of Inheritance  Mendel: father of modern genetics Modern genetics began with Gregor Mendel’s quantitative experiments with pea plants  White 1 Removed Mendel crossed pea plants stamens from purple flower that differed in certain characteristics and traced Stamens the traits from generation to Carpel 2 Transferred generation PARENTS Purple pollen from stamens of white flower to carpel (P) of purple flower 3 Pollinated carpel matured into pod cross-fertilization = fertilization of one plant with pollen from another plant 4 Planted seeds from pod OFF-SP RING (F1) Figure 9.2C  Mendel studied seven pea FLOWER COLOR Purple White characteristics FLOWER POSITION Axial Terminal SEED COLOR Yellow Green SEED He studied characteristics SHAPE Round Wrinkled that occur in only two POD SHAPE Inflated Constricted distinct forms POD COLOR Green Yellow STEM Figure 9.2D LENGTH Tall Dwarf Terminology True-breeding = varieties for which self-fertilization produce offspring all identical to the parents Hybrids = offspring of two different true-breeding varieties P generation = parents in a cross between two true-breeding varieties F1 generation = hybrid offspring resulting from mating 2 true-breeding variations F2 generation = offspring resulting from F1 that self-fertilize Terminology continued….. Trait = each variant for a particular character (ex: petal color) Monohybrid cross = a genetic cross that tracks the inheritance of a single character Dihybrid cross = a genetic cross that tracks the inheritance of 2 characters simultaneously  Monohybrid cross and Mendel’s principle of segregation Mendel crossed two P GENERATION (true-breeding parents) different true-breeding Purple flowers White flowers varieties Mendel deduced that an All plants have organism has two genes F1 generation purple flowers (alleles) for each inherited Fertilization characteristic among F1 plants (F1 x F1) One characteristic comes from each parent F2 generation 3 1 /4 of plants /4 of plants Figure 9.3A have purple flowers have white flowers Mendel used gene symbols P generation: PP x pp (each parent is homozygous) F1 all Pp (heterozygous) F2 ¼ PP (purple), ½ Pp (purple), ¼ pp (white) Dominant allele is capitalized, recessive allele is lower case Phenotype = an organism’s expressed, or physical trait Genotype = organism’s genetic makeup Homologous chromosomes Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes GENE LOCI DOMINANT allele P a B P a b RECESSIVE allele GENOTYPE: PP aa Bb HOMOZYGOUS HOMOZYGOUS HETEROZYGOUS for the for the dominant allele recessive allele Figure 9.4  The principle of independent assortment By looking at two characteristics at once, Mendel found that the alleles of a pair segregate independently of other allele pairs during gamete formation This is known as the principle of independent assortment  HYPOTHESIS: HYPOTHESIS: DEPENDENT ASSORTMENT INDEPENDENT ASSORTMENT RRYY rryy RRYY rryy P GENERATION Gametes RY ry Gametes RY ry F1 RrYy RrYy GENERATION 1 1 1 1 Eggs /2 RY /2 RY Sperm Eggs /4 RY /4 RY 1 1 1 1 /2 ry /2 ry /4 rY /4 rY RRYY 1 1 /4 Ry RrYY RrYY /4 Ry F2 1 1 GENERATION /4 ry /4 ry RRYy rrYY RrYy RrYy RrYy RrYy RrYy 9 Yellow /16 round Actual results 3 Green rrYy RRyy rrYy /16 contradict round hypothesis ACTUAL Rryy Rryy 3 Yellow RESULTS /16 wrinkled SUPPORT HYPOTHESIS 1 Yellow rryy /16 wrinkled Figure 9.5A  Independent assortment of two genes in the Labrador retriever Blind Blind PHENOTYPES Black coat, Black coat, Chocolate coat, Chocolate coat, normal vision blind (PRA) normal vision blind (PRA) GENOTYPES B_N_ B_nn bbN_ bbnn MATING OF HETEROZYOTES BbNn BbNn (black, normal vision) PHENOTYPIC RATIO 9 black coat, 3 black coat, 3 chocolate coat, 1 chocolate coat, OF OFFSPRING normal vision blind (PRA) normal vision blind (PRA) Figure 9.5B  Geneticists use the testcross to determine unknown genotypes The offspring of a testcross often reveal the genotype of an individual when it is unknown TESTCROSS: Cross dominant phenotype with GENOTYPES B_ bb homozygous recessive Two possibilities for the black dog: BB or Bb B B b GAMETES b Bb b Bb bb OFFSPRING All black 1 black : 1 chocolate  Mendel’s principles reflect the rules of probability Inheritance follows the rules F1 GENOTYPES of probability Bb female Bb male Formation of eggs Formation of sperm The rule of multiplication and the rule of addition 1 /2 B B can be used to determine 1 /2 the probability of certain b B B 1 /2 b events occurring 1 /2 b B 1 /4 B b 1 1 /4 /4 b b F2 GENOTYPES 1 /4 Figure 9.7  Genetic traits in humans can be tracked through family pedigrees The inheritance of many human traits follows Mendel’s principles and the rules of probability Figure 9.8A  Family pedigrees are used to determine patterns of inheritance and individual genotypes Dd Dd D_? D_? Joshua Abigail John Hepzibah Lambert Linnell Eddy Daggett D_? dd Dd Abigail Jonathan Elizabeth Lambert Lambert Eddy Dd Dd dd Dd Dd Dd dd Female Male Deaf Hearing  Many inherited disorders in humans are controlled by a single gene Most disorders are caused PARENTS Normal Dd Normal Dd by autosomal recessive alleles Eggs D D Sperm DD Normal d d Examples: OFFSPRING Dd Normal (carrier) Dd Normal (carrier) cystic fibrosis, sickle-cell anemia, Tay dd Deaf Sachs disease  A few are caused by dominant alleles – Examples: achondroplasia, Huntington’s disease Figure 9.9B Fetal testing can spot many inherited disorders early in pregnancy Amniocentesis – sample of amniotic fluid is taken Amniotic Centrifugation Amniotic fluid fluid withdrawn Fluid Fetal Fetus cells (14-20 weeks) Biochemical tests Placenta Several Figure 9.10A Uterus Cervix weeks later Karyotyping Cell culture  Chorionic villus sampling = small sample of placenta is taken (can be performed earlier than amniocentesis) Fetus Several hours (10-12 later weeks) Placenta Suction Fetal cells Karyotyping (from chorionic villi) Some Chorionic villi biochemical tests Figure 9.10B  Examination of the fetus with ultrasound is another helpful technique Figure 9.10C, D VARIATIONS ON MENDEL’S PRINCIPLES  Incomplete dominance P GENERATION When an offspring’s Red White phenotype—such RR rr as flower color— is Gametes R r in between the phenotypes of its parents, it exhibits F1 GENERATION Pink Rr incomplete dominance 1 1 /2 R /2 r 1 1 /2 R /2 R Eggs Sperm Red 1 1 /2 r RR /2 r Pink Pink F2 GENERATION Rr rR White rr Figure 9.12A  Many genes have more than two alleles in the population In a population, multiple alleles often exist for a characteristic The three alleles for ABO blood type in humans is an example  The alleles for A and B blood types are codominant, and both are expressed in the phenotype Blood Antibodies Reaction When Blood from Groups Below Is Mixed with Group Present in Antibodies from Groups at Left (Phenotype) Genotypes Blood O A B AB Anti-A O ii Anti-B IA IA A or Anti-B IA i IB IB B or Anti-A IB i AB IA IB  A single gene may affect many phenotypic characteristics A single gene may affect phenotype in many ways This is called pleiotropy The allele for sickle-cell disease is an example  Individual homozygous for sickle-cell allele Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickle cells Clumping of cells Breakdown of red Accumulation of and clogging of blood cells sickled cells in spleen small blood vessels Physical Heart Pain and Brain Damage to Spleen Anemia weakness failure fever damage other organs damage Impaired Pneumonia Kidney mental Paralysis and other Rheumatism failure function infections Figure 9.14  Genetic testing can detect disease-causing alleles Genetic testing can be of value to those at risk of Figure 9.15B developing a genetic disorder or of passing it on to offspring Genes on the same chromosome tend to be inherited together Certain genes are linked They tend to be inherited together because they reside close together on the same chromosome  Crossing over produces new combinations of alleles This produces gametes with recombinant chromosomes The fruit fly Drosophila melanogaster was used in the first experiments to demonstrate the effects of crossing over A B a b A B A b a B Tetra Crossing over d Gametes  A partial genetic map of a fruit fly chromosome Mutant phenotypes Short Black Cinnabar Vestigial Brown aristae body eyes wings eyes (g) (c) (l) Long aristae Gray Red Normal Red (appendages body eyes wings eyes on head) (G) (C) (L) Wild-type phenotypes Figure 9.20C  Sex-linked disorders affect mostly males Most sex-linked human disorders are due to recessive alleles Examples: hemophilia, red-green color blindness These are mostly seen in males Figure 9.23A A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected  A high incidence of hemophilia has plagued the royal families of Europe Queen Albert Victoria Alice Louis Alexandra Czar Nicholas II of Russia Alexis Figure 9.23B