Chapter 9 S10

Chapter 9 Patterns of Inheritance Dogs are one of man’s longest genetics experiments Dog breeds are the result of artificial selection Populations of dogs became isolated from each other Humans chose dogs with specific traits for breeding Each breed has physical and behavioral traits due to a unique genetic makeup Sequencing of the dog’s genome shows evolutionary relationships between breeds 0 Introduction: Barking Up the Genetic Tree Copyright © 2009 Education, Inc. 0 Ancestral canine Chinese Shar-Pei Akita Basenji Siberian Husky Alaskan Malamute Rottweiler Sheepdog Retriever Afghan hound Saluki Wolf MENDEL’S LAWS Copyright © 2009 Education, Inc. 9.1 The science of genetics has ancient roots Pangenesis was an early explanation for inheritance Proposed by Hippocrates Principles: Particles called pangenes came from all parts of the organism and were incorporated into eggs or sperm Characteristics acquired during the parents’ lifetime could be transferred to the offspring Rejected by Aristotle Argued for the inheritance of the potential to produce certain traits, not that particles of the features themselves congealed Blending was another idea in 19th Century based on plant breeding Hereditary material from parents mixes together to form an intermediate trait, like mixing paint How do traits disappear one generation and return subsequently? Chocolate lab has black lab puppies 0 Copyright © 2009 Education, Inc. 9.2 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 0 Copyright © 2009 Education, Inc. 0 Transferred pollen from stamens of white flower to carpel of purple flower Stamens Carpel Parents (P) Purple 2 White Removed stamens from purple flower 1 0 Transferred pollen from stamens of white flower to carpel of purple flower Stamens Carpel Parents (P) Purple 2 White Removed stamens from purple flower 1 Pollinated carpel matured into pod 3 0 Transferred pollen from stamens of white flower to carpel of purple flower Stamens Carpel Parents (P) Purple 2 White Removed stamens from purple flower 1 Pollinated carpel matured into pod 3 Offspring (F1) Planted seeds from pod 4 0 Flower color White Axial Purple Flower position Terminal Yellow Seed color Green Round Seed shape Wrinkled Inflated Pod shape Constricted Green Pod color Yellow Tall Stem length Dwarf Some characteristics Mendel studied 9.3 Mendel’s law of segregation 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 F2 generation: of plants with purple flowers of plants with white flowers Mendel needed to explain Why one trait seemed to disappear in the F1 generation Why that trait reappeared in one quarter of the F2 offspring 0 Copyright © 2009 Education, Inc. 0 P generation (true-breeding parents) Purple flowers White flowers 0 P generation (true-breeding parents) Purple flowers White flowers F1 generation All plants have purple flowers 0 P generation (true-breeding parents) Purple flowers White flowers F1 generation All plants have purple flowers F2 generation Fertilization among F1 plants (F1 ´ F1) of plants have purple flowers 3 – 4 of plants have white flowers 1 – 4 Video 9.3 Mendel’s law of segregation describes the inheritance of a single character Four Hypotheses Genes are found in alternative versions called alleles; a genotype is the listing of alleles an individual carries for a specific gene 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 0 Copyright © 2009 Education, Inc. 9.3 Mendel’s law of segregation describes the inheritance of a single character Four Hypotheses 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 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 0 Copyright © 2009 Education, Inc. 0 P plants 1 – 2 1 – 2 Genotypic ratio 1 PP : 2 Pp : 1 pp Phenotypic ratio 3 purple : 1 white F1 plants (hybrids) Gametes Genetic makeup (alleles) All All Pp Sperm Eggs PP p pp Pp Pp P p P p P P p PP pp All Gametes F2 plants 9.4 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 0 0 Gene loci Homozygous for the dominant allele Dominant allele Homozygous for the recessive allele Heterozygous Recessive allele Genotype: P B a P PP a aa b Bb 9.5 The law of independent assortment 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: of plants with round yellow seeds of plants with round green seeds of plants with wrinkled yellow seeds 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 F2 offspring 0 Copyright © 2009 Education, Inc. 9.5 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 Genetic Variation Video Two-Cross Video 0 Copyright © 2009 Education, Inc. 0 P generation 1 – 2 Hypothesis: Dependent assortment Hypothesis: Independent assortment 1 – 2 1 – 2 1 – 2 1 – 4 1 – 4 1 – 4 1 – 4 1 – 4 1 – 4 1 – 4 1 – 4 9 –– 16 3 –– 16 3 –– 16 1 –– 16 RRYY Gametes Eggs F1 generation Sperm Sperm F2 generation Eggs Gametes rryy RrYy ry RY ry RY ry RY Hypothesized (not actually seen) Actual results (support hypothesis) RRYY rryy RrYy ry RY RRYY rryy RrYy ry RY RrYy RrYy RrYy rrYY RrYY RRYy RrYY RRYy rrYy rrYy Rryy Rryy RRyy rY Ry ry Yellow round Green round Green wrinkled Yellow wrinkled RY rY Ry 0 Phenotypes Genotypes Mating of heterozygotes (black, normal vision) Phenotypic ratio of offspring Black coat, normal vision B_N_ 9 black coat, normal vision Black coat, blind (PRA) B_nn 3 black coat, blind (PRA) Chocolate coat, normal vision bbN_ 3 chocolate coat, normal vision Chocolate coat, blind (PRA) bbnn 1 chocolate coat, blind (PRA) Blind Blind BbNn BbNn 9.6 Geneticists use the testcross to determine unknown genotypes Testcross Mating between an individual of unknown genotype and a homozygous recessive individual Will show whether the unknown genotype includes a recessive allele Used by Mendel to confirm true-breeding genotypes 0 Copyright © 2009 Education, Inc. 0 B_ or Two possibilities for the black dog: Testcross: Genotypes Gametes Offspring 1 black : 1 chocolate All black Bb bb BB Bb bb B b Bb b b B 9.7 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 0 Copyright © 2009 Education, Inc. 0 F1 genotypes 1 – 2 1 – 2 1 – 2 1 – 2 1 – 4 1 – 4 1 – 4 1 – 4 Formation of eggs Bb female F2 genotypes Formation of sperm Bb male B B B B B B b b b b b b 9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees A pedigree Shows the inheritance of a trait in a family through multiple generations Demonstrates dominant or recessive inheritance Can also be used to deduce genotypes of family members 0 Copyright © 2009 Education, Inc. Freckles Free earlobe No freckles Straight hairline Attached earlobe Widow’s peak 0 Ff Female Male Affected Unaffected First generation (grandparents) Second generation (parents, aunts, and uncles) Third generation (two sisters) Ff Ff Ff Ff Ff Ff ff ff ff ff ff FF FF or or 9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene Inherited human disorders show Recessive inheritance Two recessive alleles are needed to show disease Heterozygous parents are carriers of the disease-causing allele Probability of inheritance increases with inbreeding (mating between close relatives) Dominant inheritance One dominant allele is needed to show disease Dominant lethal alleles are usually eliminated from the population 0 Copyright © 2009 Education, Inc. 0 Parents Normal Dd Offspring Sperm Eggs dd Deaf d Dd Normal (carrier) DD Normal D D d Dd Normal (carrier) Normal Dd x 0 Genetic testing of parents Fetal testing: biochemical and karyotype analyses Amniocentesis Chorionic villus sampling Maternal blood test Fetal imaging Ultrasound Fetoscopy Newborn screening 9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy 0 Copyright © 2009 Education, Inc. Video: Ultrasound of Human Fetus 0 Needle inserted through abdomen to extract amniotic fluid Suction tube inserted through cervix to extract tissue from chorionic villi Ultrasound monitor Fetus Placenta Chorionic villi Uterus Cervix Amniocentesis Chorionic villus sampling (CVS) Ultrasound monitor Fetus Placenta Uterus Cervix Centrifugation Fetal cells Amniotic fluid Several weeks Biochemical tests Karyotyping Fetal cells Several hours 0 VARIATIONS ON MENDEL’S LAWS Copyright © 2009 Education, Inc. 9.11 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 0 Copyright © 2009 Education, Inc. 0 P generation 1 – 2 1 – 2 1 – 2 1 – 2 1 – 2 1 – 2 F1 generation F2 generation Red RR Gametes Gametes Eggs Sperm RR rR Rr rr R r R r R r Pink Rr R r White rr 0 HH Homozygous for ability to make LDL receptors hh Homozygous for inability to make LDL receptors Hh Heterozygous LDL receptor LDL Cell Normal Mild disease Severe disease Genotypes: Phenotypes: Another Example of Incomplete Dominance 9.12 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 0 Copyright © 2009 Education, Inc. 9.12 Many genes have more than two alleles in the population Co-dominance Neither allele is dominant over the other Expression of both alleles is observed as a distinct phenotype in the heterozygous individual Observed for type AB blood 0 0 Blood Group (Phenotype) Genotypes O A ii IAIA or IAi Red Blood Cells Carbohydrate A Antibodies Present in Blood Anti-A Anti-B Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Anti-B O A B AB B IBIB or IBi Carbohydrate B AB IAIB — Anti-A 9.13 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 0 Copyright © 2009 Education, Inc. 0 Clumping of cells and clogging of small blood vessels Pneumonia and other infections Accumulation of sickled cells in spleen Pain and fever Rheumatism Heart failure Damage to other organs Brain damage Spleen damage Kidney failure Anemia Paralysis Impaired mental function Physical weakness Breakdown of red blood cells Individual homozygous for sickle-cell allele Sickle cells Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped 9.14 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 0 0 P generation 1 – 8 F1 generation F2 generation Eggs Sperm 1 – 8 1 – 8 1 – 8 1 – 8 1 – 8 1 – 8 1 – 8 1 – 8 1 – 8 1 – 8 1 – 8 1 – 8 1 – 8 1 – 8 1 – 8 aabbcc (very light) AABBCC (very dark) AaBbCc AaBbCc 1 –– 64 15 –– 64 6 –– 64 1 –– 64 15 –– 64 6 –– 64 20 –– 64 0 Fraction of population Skin color 1 –– 64 15 –– 64 6 –– 64 20 –– 64 9.15 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 0 Copyright © 2009 Education, Inc. B=Brown G=Green/Hazel b=blue g=lighter eyes THE CHROMOSOMAL BASIS OF INHERITANCE Copyright © 2009 Education, Inc. 9.16 Chromosome behavior accounts for Mendel’s laws Mendel’s Laws correlate with chromosome separation in meiosis The law of segregation depends on separation of homologous chromosomes in anaphase I/II The law of independent assortment depends on alternative orientations of chromosomes in metaphase I 0 Copyright © 2009 Education, Inc. 0 F1 generation R Metaphase I of meiosis (alternative arrangements) r Y y R r Y y R r Y y All round yellow seeds (RrYy) 0 F1 generation R Metaphase I of meiosis (alternative arrangements) r Y y R r Y y R r Y y All round yellow seeds (RrYy) Anaphase I of meiosis Metaphase II of meiosis R y r Y r y R Y R r Y y R r Y y 0 F1 generation R Metaphase I of meiosis (alternative arrangements) r Y y R r Y y R r Y y All round yellow seeds (RrYy) Anaphase I of meiosis Metaphase II of meiosis R y r Y r y R Y R r Y y R r Y y 1 – 4 R y Ry R y r Y 1 – 4 rY r Y 1 – 4 ry r y 1 – 4 RY R Y R Y Gametes Fertilization among the F1 plants :3 9 :3 :1 F2 generation r y 9.18 Crossing over produces new combinations of alleles Linked Genes Located close together on the same chromosome Tend to be inherited together Can be separated by crossing over Recombinant chromosomes are formed Thomas Hunt Morgan demonstrated this in early experiments Geneticists measure genetic distance by recombination frequency 0 0 Gametes Tetrad Crossing over B a b a a b A B A B A b 9.17 Genes on the same chromosome tend to be inherited together Example studied by Bateson and Punnett Parental generation: plants with purple flowers, long pollen crossed to plants with red flowers, round pollen The F2 generation did not show a 9:3:3:1 ratio Most F2 individuals had purple flowers, long pollen or red flowers, round pollen 0 Experiment Parental phenotypes Recombination frequency = Black vestigial Black body, vestigial wings GgLl Offspring Female Male Gray long 965 944 206 185 ggll Gray vestigial Black long Gray body, long wings (wild type) Recombinant phenotypes 391 recombinants 2,300 total offspring Explanation = 0.17 or 17% G L g l g l g l GgLl (female) ggll (male) G L g l g L g l g l g l g l g l g l G L Sperm Eggs Offspring g L G l G l 0 Experiment Parental phenotypes Recombination frequency = Black vestigial Black body, vestigial wings GgLl Offspring Female Male Gray long 965 944 206 185 ggll Gray vestigial Black long Gray body, long wings (wild type) Recombinant phenotypes 391 recombinants 2,300 total offspring = 0.17 or 17% 0 Explanation G L g l g l g l GgLl (female) ggll (male) G L g l g L g l g l g l g l g l g l G L Sperm Eggs Offspring g L G l G l 9.19 Geneticists use crossover data to map genes Genetic maps Show the order of genes on chromosomes Arrange genes into linkage groups representing individual chromosomes 0 Copyright © 2009 Education, Inc. 0 Chromosome 9.5% Recombination frequencies 9% 17% g c l 0 Mutant phenotypes Short aristae Black body (g) Cinnabar eyes (c) Vestigial wings (l) Brown eyes Long aristae (appendages on head) Gray body (G) Red eyes (C) Normal wings (L) Red eyes Wild-type phenotypes SEX CHROMOSOMES AND SEX-LINKED GENES Copyright © 2009 Education, Inc. 9.20 Chromosomes determine sex in many species X-Y system in mammals, fruit flies XX = female; XY = male X-O system in grasshoppers and roaches XX = female; XO = male Z-W in system in birds, butterflies, and some fishes ZW = female, ZZ = male Chromosome number in ants and bees Diploid = female; haploid = male 0 Copyright © 2009 Education, Inc. 0 X Y 0 (male) Sperm (female) 44 + XY Parents’ diploid cells 44 + XX 22 + X 22 + Y 22 + X 44 + XY 44 + XX Egg Offspring (diploid) 0 22 + X 22 + XX 0 76 + ZZ 76 + ZW 0 16 32 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 can be passed from mother to son and mother to daughter X-linked genes can be passed from father to daughter Y-linked genes can be passed from father to son 9.21 Sex-linked genes exhibit a unique pattern of inheritance 0 Copyright © 2009 Education, Inc. 0 0 Female Male XR XR Xr Y XR Y XR Xr Y Xr XR Sperm Eggs R = red-eye allele r = white-eye allele 0 Female Male XR Xr XR Y XR Y XR XR Y XR XR Sperm Eggs Xr XR Xr Y Xr 0 Female Male XR Xr Xr Y XR Y XR Xr Y Xr XR Sperm Eggs Xr Xr Xr Y Xr 9.22 CONNECTION: Sex-linked disorders affect mostly males Males express X-linked disorders such as the following when recessive alleles are present in one copy Hemophilia Colorblindness Duchenne muscular dystrophy 0 Copyright © 2009 Education, Inc. 0 Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis 9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution Similarities in Y chromosome sequences Show a significant percentage of men related to the same male parent Demonstrate a connection between people living in distant locations 0 Copyright © 2009 Education, Inc. Homologous chromosomes Alleles, residing at the same locus Meiosis Gamete from other parent Fertilization Diploid zygote (containing paired alleles) Paired alleles, alternate forms of a gene Haploid gametes (allele pairs separate) Incomplete dominance Red RR Single gene Single characters (such as skin color) Multiple characters Pleiotropy Polygenic inheritance Multiple genes White rr Pink Rr Genes located on (b) (a) at specific locations called alternative versions called if both same, genotype called expressed allele called inheritance when phenotype In between called unexpressed allele called if different, genotype called chromosomes heterozygous (d) (c) (f) (e) loci alleles homozygous dominant recessive Incomplete dominance