Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The science of heredity dates back to ancient attempts at selective breeding Parents pass their traits (color of hair, color of eyes etc.) to children- How? MENDEL’S PRINCIPLES 9.1 The science of genetics has ancient roots

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Modern genetics began with Gregor Mendel’s quantitative experiments with pea plants 9.2 Experimental genetics began in an abbey garden Figure 9.2A, B Stamen Carpel

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Mendel crossed pea plants that differed in certain characteristics and traced the traits from generation to generation Figure 9.2C This illustration shows his technique for cross-fertilization 1 Removed stamens from purple flower White Stamens Carpel Purple PARENTS (P) OFF- SPRING (F 1 ) 2 Transferred pollen from stamens of white flower to carpel of purple flower 3 Pollinated carpel matured into pod 4 Planted seeds from pod

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Mendel studied seven pea characteristics/traits Figure 9.2D He hypothesized that there are alternative forms of genes (although he did not use that term), that is traits were governed by some heredity units FLOWER COLOR FLOWER POSITION SEED COLOR SEED SHAPE POD SHAPE POD COLOR STEM LENGTH PurpleWhite AxialTerminal YellowGreen RoundWrinkled InflatedConstricted GreenYellow TallDwarf

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic –One characteristic comes from each parent 9.3 Mendel’s principle of segregation describes the inheritance of a single characteristic P GENERATION (true-breeding parents) F 1 generation F 2 generation Purple flowersWhite flowers All plants have purple flowers Fertilization among F1 plants (F 1 x F 1 ) 3 / 4 of plants have purple flowers 1 / 4 of plants have white flowers Figure 9.3A

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings A sperm or egg carries only one allele of each pair –The pairs of alleles separate when gametes form –This process describes Mendel’s law of segregation –Alleles can be dominant or recessive GENETIC MAKEUP (ALLELES) P PLANTS F 1 PLANTS (hybrids) F 2 PLANTS PPpp All PAll p All Pp 1/2 P1/2 P 1/2 p1/2 p Eggs P p P PP p Sperm Pp pp Gametes Phenotypic ratio 3 purple : 1 white Genotypic ratio 1 PP : 2 Pp : 1 pp Figure 9.3B

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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 9.5 The principle of independent assortment is revealed by tracking two characteristics at once

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 9.5A HYPOTHESIS: DEPENDENT ASSORTMENT HYPOTHESIS: INDEPENDENT ASSORTMENT P GENERATION F 1 GENERATION F 2 GENERATION RRYYrryy GametesRY Yellow round ry RrYy EggsSpermRY ry RY ry 1/21/2 1/21/2 1/21/2 1/21/2 Actual results contradict hypothesis RRYYrryy RY ry Gametes RrYy EggsRY rY 1/41/4 1/41/4 Ry ry 1/41/4 1/41/4 RY rY Ry ry 1/41/4 1/41/4 1/41/4 1/41/4 RRYY RrYY RRYyrrYYRrYy rrYyRRyyrrYy Rryy rryy 9 / 16 3 / 16 1 / 16 Green round Yellow wrinkled Yellow wrinkled ACTUAL RESULTS SUPPORT HYPOTHESIS

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Independent assortment of two genes in the Labrador retriever Figure 9.5B PHENOTYPES Black coat, normal vision B_N_ Blind GENOTYPES MATING OF HETEROZYOTES (black, normal vision) PHENOTYPIC RATIO OF OFFSPRING Black coat, blind (PRA) B_nn Chocolate coat, normal vision bbN_ Chocolate coat, blind (PRA) bbnn 9 black coat, normal vision 3 black coat, blind (PRA) 3 chocolate coat, normal vision 1 chocolate coat, blind (PRA) Blind BbNn

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The offspring of a testcross often reveal the genotype of an individual when it is unknown 9.6 Geneticists use the testcross to determine unknown genotypes TESTCROSS: B_GENOTYPESbb BBBbor Two possibilities for the black dog: GAMETES OFFSPRING All black1 black : 1 chocolate B b B b b Bb bb Figure 9.6

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance follows the rules of probability –The rule of multiplication and the rule of addition can be used to determine the probability of certain events occurring 9.7 Mendel’s principles reflect the rules of probability F 1 GENOTYPES Bb female F 2 GENOTYPES Formation of eggs Bb male Formation of sperm 1/21/2 1/21/2 1/21/2 1/21/2 1/41/4 1/41/4 1/41/4 1/41/4 BB BB B B b b b b bb Figure 9.7

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The inheritance of many human traits follows Mendel’s principles and the rules of probability 9.8 Connection: Genetic traits in humans can be tracked through family pedigrees Figure 9.8A

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Family pedigrees are used to determine patterns of inheritance and individual genotypes Figure 9.8B Dd Joshua Lambert Dd Abigail Linnell D_ Abigail Lambert Female Dd Elizabeth Eddy D_ John Eddy ?D_ Hepzibah Daggett ? ? ddDd ddDd Male Deaf Hearing dd Jonathan Lambert

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Mendel’s principles are valid for all sexually reproducing species –However, often the genotype does not dictate the phenotype in the simple way his principles describe VARIATIONS ON MENDEL’S PRINCIPLES 9.11 The relationship of genotype to phenotype is rarely simple

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings When an offspring’s phenotype—such as flower color— is in between the phenotypes of its parents, it exhibits incomplete dominance 9.12 Incomplete dominance results in intermediate phenotypes P GENERATION F 1 GENERATION F 2 GENERATION Red RR GametesRr White rr Pink Rr Rr RR rr 1/21/2 1/21/2 1/21/2 1/21/2 1/21/2 1/21/2 SpermEggs Pink Rr Pink rR White rr Red RR Figure 9.12A

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Incomplete dominance in human hypercholesterolemia Figure 9.12B GENOTYPES: HH Homozygous for ability to make LDL receptors Hh Heterozygous hh Homozygous for inability to make LDL receptors PHENOTYPES: LDL LDL receptor Cell NormalMild diseaseSevere disease

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings In a population, multiple alleles often exist for a characteristic –The three alleles for ABO blood type in humans is an example 9.13 Many genes have more than two alleles in the population

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 9.13 –The alleles for A and B blood types are codominant, and both are expressed in the phenotype Blood Group (Phenotype) O Genotypes Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left OABAB A B ii I A or I A i I B or I B i I A I B Anti-A Anti-B Anti-A

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 9.14 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

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Individual homozygous for sickle-cell allele Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickle cells Breakdown of red blood cells Clumping of cells and clogging of small blood vessels Accumulation of sickled cells in spleen Physical weakness Anemia Heart failure Pain and fever Brain damage Damage to other organs Spleen damage Kidney failure Rheumatism Pneumonia and other infections Paralysis Impaired mental function Figure 9.14

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings This situation creates a continuum of phenotypes –Example: skin color 9.16 A single characteristic may be influenced by many genes

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 9.16 P GENERATION F 1 GENERATION F 2 GENERATION aabbcc (very light) AABBCC (very dark) AaBbCc EggsSperm Fraction of population Skin pigmentation

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Most such disorders are caused by autosomal recessive alleles –Examples: cystic fibrosis, sickle-cell disease 9.9 Connection: Many inherited disorders in humans are controlled by a single gene Figure 9.9A DD dd Normal Dd Normal Dd DD Normal Dd Normal (carrier) Dd Normal (carrier) dd Deaf EggsSperm PARENTS OFFSPRING

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings A few are caused by dominant alleles Figure 9.9B –Examples: achondroplasia, Huntington’s disease

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Table 9.9

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Karyotyping and biochemical tests of fetal cells and molecules can help people make reproductive decisions –Fetal cells can be obtained through amniocentesis 9.10 Connection: Fetal testing can spot many inherited disorders early in pregnancy Figure 9.10A Amniotic fluid Fetus (14-20 weeks) Placenta Amniotic fluid withdrawn Centrifugation Fetal cells Fluid UterusCervix Cell culture Several weeks later Karyotyping Biochemical tests

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Chorionic villus sampling is another procedure that obtains fetal cells for karyotyping Figure 9.10B Fetus (10-12 weeks) Placenta Chorionic villi Suction Several hours later Fetal cells (from chorionic villi) Karyotyping Some biochemical tests

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Examination of the fetus with ultrasound is another helpful technique Figure 9.10C, D

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Genetic testing can be of value to those at risk of developing a genetic disorder or of passing it on to offspring 9.15 Connection: Genetic testing can detect disease-causing alleles Figure 9.15B Figure 9.15A Dr. David Satcher, former U.S. surgeon general, pioneered screening for sickle-cell disease