Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Genetics is the science of heredity These black Labrador puppies are purebred— their parents and grandparents were black Labs with very similar genetic makeups –Purebreds often suffer from serious genetic defects Purebreds and Mutts — A Difference of Heredity

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The parents of these puppies were a mixture of different breeds –Their behavior and appearance is more varied as a result of their diverse genetic inheritance

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 Figure 9.2D He hypothesized that there are alternative forms of genes (although he did not use that term), the units that determine heredity 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 Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes 9.4 Homologous chromosomes bear the two alleles for each characteristic GENE LOCI Figure 9.4 PaB DOMINANT allele RECESSIVE allele Pab GENOTYPE:PPaaBb HOMOZYGOUS for the dominant allele HOMOZYGOUS for the recessive allele HETEROZYGOUS

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 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 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 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 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 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 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 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 A single gene may affect phenotype in many ways, a phenomenon called pleiotropy –The allele for sickle-cell disease is an example Figure 9.14

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 The chromosomal basis of Mendel’s principles Figure 9.17

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 9.21A XY Male (male) Parents’ diploid cells (female) Sperm Offspring (diploid) Egg

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Other systems of sex determination exist in other animals and plants Figure 9.21B-D –The X-O system –The Z-W system –Chromosome number

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings –Their inheritance pattern reflects the fact that males have one X chromosome and females have two Figure 9.22B-D –These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait FemaleMaleFemaleMaleFemaleMale XrYXrYXRXRXRXR XRXrXRXr XRYXRY XRXR XrXr Y XRXrXRXr XRXR XrXr XRXRXRXR XRXR Y XRYXRY XrXRXrXR XRYXRY XrYXrY XRXrXRXr XRXR XrXr XrXr Y XRXrXRXr XrXrXrXr XRYXRY XrYXrY XrYXrY R = red-eye allele r = white-eye allele

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Most sex-linked human disorders are due to recessive alleles –Examples: hemophilia, red-green color blindness –These are mostly seen in males –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 9.23 Connection: Sex-linked disorders affect mostly males Figure 9.23A