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Introduction: Mendelian Genetics
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Introduction: Mendelian Genetics

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  • 1. Introduction: Mendelian Genetics Chapter 9 Copyright © 2009 Pearson Education, Inc. The science of genetics has ancient roots  Pangenesis was an early explanation for inheritance – It was proposed by Hippocrates – Particles called pangenes came from all parts of the organism to be incorporated into eggs or sperm – Characteristics acquired during the parents’ lifetime could be transferred to the offspring – Aristotle rejected pangenesis and argued that instead of particles, the potential to produce the traits was inherited  Blending was another idea, based on plant breeding – Hereditary material from parents mixes together to form an intermediate trait, like mixing paint Copyright © 2009 Pearson Education, Inc. Experimental genetics began in an abbey garden  Gregor Mendel discovered principles of genetics in experiments with the garden pea – Mendel showed that parents pass heritable factors to offspring (heritable factors are now called genes) – Advantages of using pea plants – Controlled matings – Self-fertilization or cross-fertilization – Observable characteristics with two distinct forms – True-breeding strains Copyright © 2009 Pearson Education, Inc.
  • 2. Flower color Purple White Flower position Axial Terminal Seed color Yellow Green Seed shape Round Wrinkled Pod shape Inflated Constricted Pod color Green Yellow Stem length Tall Dwarf MENDEL’S LAWS Mendel’s first law describes the inheritance of a single character  Example of a monohybrid cross – Parental generation: purple flowers × white flowers – F1 generation: all plants with purple flowers 3/4 – F2 generation: of plants with purple flowers 1/4 of plants with white flowers  Mendel needed to explain – Why one trait seemed to disappear in the F 1 generation – Why that trait reappeared in one quarter of the F 2 offspring Copyright © 2009 Pearson Education, Inc.
  • 3. P generation (true-breeding parents) Purple flowers White flowers P generation (true-breeding parents) Purple flowers White flowers F1 generation All plants have purple flowers P generation (true-breeding parents) Purple flowers White flowers F1 generation All plants have purple flowers Fertilization among F 1 plants (F1 X F1) F2 generation 3 – of plants 1 – of plants 4 4 have purple flowers have white flowers
  • 4. Mendel’s law of segregation describes the inheritance of a single character  Four Hypotheses 1. Genes are found in alternative versions called alleles; a genotype is the listing of alleles an individual carries for a specific gene 2. For each characteristic, an organism inherits two alleles, one from each parent; the alleles can be the same or different – A homozygous genotype has identical alleles – A heterozygous genotype has two different alleles Copyright © 2009 Pearson Education, Inc. Mendel’s law of segregation describes the inheritance of a single character  Four Hypotheses 3. If the alleles differ, the dominant allele determines the organism’s appearance, and the recessive allele has no noticeable effect – The phenotype is the appearance or expression of a trait – The same phenotype may be determined by more than one genotype 4. Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes so that a sperm or egg carries only one allele for each gene Copyright © 2009 Pearson Education, Inc. Genetic makeup (alleles)⇣ P plants PP pp Gametes All P All p F1 plants All Pp (hybrids) 1 1 Gametes – 2 P – 2 p Sperm P p F2 plants Phenotypic ratio P PP Pp 3 purple : 1 white Eggs Genotypic ratio 1 PP : 2 Pp : 1 pp p Pp pp
  • 5. Homologous chromosomes bear the alleles for each character  For a pair of homologous chromosomes, alleles of a gene reside at the same locus – Homozygous individuals have the same allele on both homologues – Heterozygous individuals have a different allele on each homologue 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 Mendel’s second law is revealed by tracking two characters at once  Example of a dihybrid cross – Parental generation: round yellow seeds × wrinkled green seeds – F1 generation: all plants with round yellow seeds – F2 generation: 9/16 of plants with round yellow seeds 3/16 of plants with round green seeds 3/16 of plants with wrinkled yellow seeds 1/16 of plants with wrinkled green seeds Mendel needed to explain –Why nonparental combinations were observed –Why a 9:3:3:1 ratio was observed among the F 2 offspring Copyright © 2009 Pearson Education, Inc.
  • 6. The law of independent assortment is revealed by tracking two characters at once  Law of independent assortment – Each pair of alleles segregates independently of the other pairs of alleles during gamete formation – For genotype RrYy , four gamete types are possible: RY, Ry , rY , and ry Hypothesis: Dependent assortment Hypothesis: Independent assortment P RRYY rryy RRYY rryy generation Gametes RY ry Gametes RY ry F1 RrYy RrYy generation Sperm Sperm 1 1 1 1 – 4 RY – 4 rY – 4 Ry – 4 ry 1 1 – 2 RY – 2 ry 1 F2 – 4 RY 1 generation – 2 RY RRYY RrYY RRYy RrYy Eggs 1 – rY 1 4 – 2 ry RrYY rrYY RrYy rrYy Eggs 1 9 Yellow –– – 4 Ry 16 round RRYy RrYy RRyy Rryy 3 –– Green 16 round 1 Hypothesized – 4 ry 3 Yellow (not actually seen) RrYy rrYy Rryy rryy –– 16 wrinkled Actual results 1 –– Green (support hypothesis) 16 wrinkled Blind Blind Phenotypes Black coat, normal vision Black coat, blind (PRA) Chocolate coat, normal vision Chocolate coat, blind (PRA) Genotypes B_N_ B_nn bbN_ bbnn Mating of heterozygotes 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)
  • 7. Mendel’s laws reflect the rules of probability  The probability of a specific event is the number of ways that event can occur out of the total possible outcomes.  Rule of multiplication – Multiply the probabilities of events that must occur together  Rule of addition – Add probabilities of events that can happen in alternate ways Copyright © 2009 Pearson Education, Inc. F1 genotypes Bb male Formation of sperm Bb female Formation of eggs 1 1 – 2 B – 2 b B B B b 1 – 2 B 1 – 1 – 4 4 1 b B b b – 2 b 1 – 1 – 4 4 F2 genotypes Parents Normal Normal × Dd Dd Sperm D d Dd DD D Normal Normal (carrier) Offspring Eggs Dd dd d Normal Deaf (carrier)
  • 8. VARIATIONS ON MENDEL’S LAWS Copyright © 2009 Pearson Education, Inc. Incomplete dominance results in intermediate phenotypes  Incomplete dominance – Neither allele is dominant over the other – Expression of both alleles is observed as an intermediate phenotype in the heterozygous individual Copyright © 2009 Pearson Education, Inc. P generation Red White RR rr Gametes R r F1 generation Pink Rr 1 1 Gametes – R – r 2 2 Sperm 1 1 – 2 R – 2 r F2 generation 1 RR rR – 2 R Eggs 1 Rr rr – 2 r
  • 9. Codominance –Neither allele is dominant over the other –Expression of both alleles is observed as a distinct phenotype in the heterozygous individual Genotypes: HH Hh hh Homozygous Heterozygous Homozygous for ability to make for inability to make Phenotypes: LDL receptors LDL receptors LDL LDL receptor Normal Cell Mild disease Severe disease Many genes have more than two alleles in the population  Multiple alleles – More than two alleles are found in the population – A diploid individual can carry any two of these alleles – The ABO blood group has three alleles, leading to four phenotypes: type A, type B, type AB, and type O blood Copyright © 2009 Pearson Education, Inc. Blood Group (Phenotype) Genotypes Red Blood Cells O ii IAIA A or Carbohydrate A IAi IBIB B or Carbohydrate B IBi Codominance AB IAIB observed for type AB blood
  • 10. A single gene may affect many phenotypic characters  Pleiotropy – One gene influencing many characteristics – The gene for sickle cell disease – Affects the type of hemoglobin produced – Affects the shape of red blood cells – Causes anemia – Causes organ damage – Is related to susceptibility to malaria Copyright © 2009 Pearson Education, Inc. 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 Accumulation of and clogging of red 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 A single character may be influenced by many genes  Polygenic inheritance - Many genes influence one trait – Skin color is affected by at least three genes – Height – Intelligence Copyright © 2009 Pearson Education, Inc.
  • 11. P generation aabbcc AABBCC (very light) (very dark) F1 generation AaBbCc AaBbCc Sperm 1 1 1 1 1 1 1 1 – 8 – – – – – – – 8 8 8 8 8 8 8 F2 generation 1 – 8 1 – 8 1 – 8 20 –– 64 1 – 8 Eggs 1 Fraction of population – 15 8 –– 64 1 – 8 1 – 8 1 – 8 6 –– 64 1 –– 64 1 6 15 20 15 6 1 –– –– –– –– –– –– –– Skin color 64 64 64 64 64 64 64 The environment affects many characters  Phenotypic variations are influenced by the environment – Skin color is affected by exposure to sunlight – Susceptibility to diseases, such as cancer, has hereditary and environmental components Copyright © 2009 Pearson Education, Inc.
  • 12. Incomplete dominance Red White Pink RR rr Rr Pleiotropy Single Multiple characters gene Polygenic Multiple inheritance Single characters genes (such as skin color) THE CHROMOSOMAL BASIS OF INHERITANCE Copyright © 2009 Pearson Education, Inc. Sex-linked genes exhibit a unique pattern of inheritance  Sex-linked genes are located on either of the sex chromosomes – Reciprocal crosses show different results – White-eyed female × red-eyed male red-eyed females and white-eyed males – Red-eyed female × white-eyed male red-eyed females and red-eyed males – X-linked genes are passed from mother to son and mother to daughter – X-linked genes are passed from father to daughter – Y-linked genes are passed from father to son Copyright © 2009 Pearson Education, Inc.
  • 13. Female Male XR XR Xr Y Sperm Xr Y Eggs XR XR Xr XR Y All RED means Red is ? R = red-eye allele r = white-eye allele
  • 14. Female Male XR Xr XR Y Sperm XR Y 3 RED : 1 white XR XR XR XR Y means? Eggs Xr Xr XR Xr Y Sex-linked disorders affect mostly males  Males express X-linked disorders such as the following when recessive alleles are present in one copy – Hemophilia – Red/Green Colorblindness – Duchenne muscular dystrophy Copyright © 2009 Pearson Education, Inc. Queen Albert Victoria Alice Louis Alexandra Czar Nicholas II of Russia Alexis
  • 15. SEX CHROMOSOMES AND SEX-LINKED GENES Copyright © 2009 Pearson Education, Inc. Chromosomes determine sex in many species  X-Y system in mammals, fruit flies – XX = female; XY = male Copyright © 2009 Pearson Education, Inc. (male) (female) 44 Parents’ 44 + + XY diploid XX cells 22 22 22 + + + X Y X Sperm Egg 44 Offspring 44 + + XX (diploid) XY
  • 16. Chromosomes determine sex in many species Other Systems 1. X-O system in grasshoppers and roaches 1. XX = female; XO = male 22 22 + + XX X 2. Z-W in system in birds, butterflies, and some fishes – ZW = female, ZZ = male 76 76 + + ZW ZZ Copyright © 2009 Pearson Education, Inc. Chromosomes determine sex in many species Other Systems 3. Chromosome number in ants and bees – Diploid = female; haploid = male 32 16 Copyright © 2009 Pearson Education, Inc. Nonchromosomal Sex Determination Reptiles - temperature at a stage of egg development Shifts dependent of ratio of the sexes in a population Both sexes in same organism – Earthworms, some plants
  • 17. Genes located alternative on versions called chromosomes (a) alleles at specific if both same, if different, locations called genotype called genotype called (b) loci (c) homozygous heterozygous expressed unexpressed allele called allele called (d) dominant (e) recessive inheritance when phenotype In between called (f) incomplete dominance A few Non-Sex Linked Alleles 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 You should now be able to 1. Explain and apply Mendel’s laws of segregation and independent assortment 2. Distinguish between terms in the following groups: allele—gene; dominant—recessive; genotype—phenotype; F1—F2; heterozygous—homozygous; incomplete dominance—codominance 3. Explain the meaning of the terms locus, multiple alleles, pedigree, pleiotropy, polygenic inheritance Copyright © 2009 Pearson Education, Inc.
  • 18. You should now be able to 4. Describe the difference in inheritance patterns for linked genes and explain how recombination can be used to estimate gene distances 5. Describe how sex is inherited in humans and identify the pattern of inheritance observed for sex-linked genes 6. Solve genetics problems involving monohybrid and dihybrid crosses for autosomal and sex- linked traits, with variations on Mendel’s laws Copyright © 2009 Pearson Education, Inc.