Patterns of Inheritance Chapter 9 Patterns of Inheritance

Purebreds and Mutts-A Difference of Heredity Genetics is the science of heredity A common genetic background will produce offspring with similar physical and behavioral traits Purebred dogs show less variation than mutts True-breeding individuals are useful in genetic research Behavioral characteristics are also influenced by environment

9.1 The science of genetics has ancient roots MENDEL'S LAWS 9.1 The science of genetics has ancient roots Early attempts to explain heredity have been rejected by later science Hippocrates' theory of Pangenesis Particles from each part of the body travel to eggs or sperm and are passed on Early 19th-century biologists' blending hypothesis Traits from both parents mix in the offspring

9.2 Experimental genetics began in an abbey garden Gregor Mendel hypothesized that there are alternative forms of genes, the units that determine heritable traits Mendel crossed pea plants that differed in certain characteristics Could control matings Developed true-breeding varieties Traced traits from generation to generation

Terminology of Mendelian genetics Self-fertilization: fertilization of eggs by sperm-carrying pollen of the same flower Cross-fertilization (cross): fertilization of one plant by pollen from a different plant True-breeding: identical offspring from self-fertilizing parents Hybrid: offspring of two different varieties

P generation: true-breeding parents F1 generation: hybrid offspring of true-breeding parents F2 generation: offspring of self-fertilizing F1 parents

Petal Stamen Carpel Removed stamens White from purple flower Stamens Parents (P) Purple Transferred pollen from stamens of white flower to carpel of purple Stamens Pollinated carpel matured into pod Planted seeds from pod Offspring (F1) Petal Stamen Carpel

LE 9-2d 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

9.3 Mendel's law of segregation describes the inheritance of a single characteristic From his experimental data, Mendel developed several hypotheses There are alternative forms (alleles) of genes that account for variation in inherited characteristics For each characteristic, an organism inherits two alleles, one from each parent Homozygous: two identical alleles Heterozygous: two different alleles

If the two alleles of an inherited pair differ The dominant allele determines the organism's appearance The recessive allele has no noticeable effect on the organism's appearance The law of segregation: A sperm or egg carries only one allele for each inherited trait, because allele pairs separate from each other during gamete production

An organism's appearance does not always reveal its genetic composition Phenotype: Expressed (physical) traits Genotype: Genetic makeup

have purple flowers have white flowers LE 9-3a P generation (true-breeding parents)  Purple flowers White flowers F1 generation All plants have purple flowers Fertilization among F1 plants (F1  F1) F2 generation 3 4 of plants 1 4 of plants have purple flowers have white flowers

Genetic makeup (alleles) LE 9-3b Genetic makeup (alleles) P plants PP pp Gametes All P All p F1 plants (hybrids) All Pp Gametes 1 2 1 2 P p Sperm P p F2 plants Phenotypic ratio 3 purple : 1 white P PP Pp Eggs Genotypic ratio 1 PP : 2 Pp : 1 pp p Pp pp

9.4 Homologous chromosomes bear the two alleles for each characteristic Alternative forms of a gene reside at the same locus on homologous chromosomes Supports the law of segregation

LE 9-4 Gene loci Dominant allele P a B P a b Recessive allele Genotype: PP aa Bb Homozygous for the dominant allele Homozygous for the recessive allele Heterozygous

9.5 The law of independent assortment is revealed by tracking two characteristics at once Dihybrid cross Mate true-breeding parents differing in two characteristics The F1 generation exhibits only the dominant phenotype The F2 generation exhibits a phenotypic ratio of 9:3:3:1 Mendel's law of independent assortment: each pair of alleles segregates independently of other allele pairs during gamete formation

contradict hypothesis LE 9-5a Hypothesis: Dependent assortment Hypothesis: Independent assortment P generation RRYY rryy rryy RRYY rryy Gametes RY ry Gametes RY  ry F1 generation RrYy RrYy Sperm Sperm 1 4 RY 1 4 rY 1 4 Ry 1 4 ry 1 2 RY 1 2 ry 1 4 RY 1 2 RY RRYY RrYY RRYy RrYy F2 generation Eggs 1 4 rY 1 2 ry RrYY rrYY RrYy rrYy Eggs Yellow round 1 4 Ry 9 16 RRYy RrYy RRyy Rryy 3 16 Green round Actual results contradict hypothesis 1 4 ry Yellow wrinkled RrYy rrYy Rryy rryy 3 16 Actual results support hypothesis 1 16 Green wrinkled

LE 9-5b Blind Blind Phenotypes Genotypes Mating of heterozygotes Black coat, normal vision B_N_ Black coat, blind (PRA) B_nn Chocolate coat, normal vision bbN_ Chocolate coat, blind (PRA) bbnn Mating of heterozygotes (black, normal vision) BbNn  BbNn Phenotypic ratio of offspring 9 black coat, normal vision 3 black coat, blind (PRA) 3 chocolate coat, normal vision 1 chocolate coat, blind (PRA)

9.6 Geneticists use the testcross to determine unknown genotypes A testcross can reveal an unknown genotype Mate an individual of unknown genotype and a homozygous-recessive individual Each of the two possible genotypes (homozygous or heterozygous) gives a different phenotypic ratio in the F1 generation

Two possibilities for the black dog: LE 9-6 Testcross:  Genotypes B_ bb Two possibilities for the black dog: BB or Bb Gametes B B b b Bb b Bb bb Offspring All black 1 black : 1 chocolate

9.7 Mendel's laws reflect the rules of probability Events that follow probability rules are independent events One such event does not influence the outcome of a later such event The rule of multiplication: The probability of two events occurring together is the product of the separate probabilities of the independent events The rule of addition: The probability that an event can occur in two or more alternative ways is the sum of the separate probabilities of the different ways

LE 9-7 F1 genotypes Bb male Formation of sperm Bb female Formation of eggs 1 2 1 2 B b B B B b 1 2 B 1 4 1 4 F2 genotypes 1 2 b B b b b 1 4 1 4

9.8 Genetic traits in humans can be tracked through family pedigrees CONNECTION 9.8 Genetic traits in humans can be tracked through family pedigrees The inheritance of many human traits follows Mendel's laws The dominant phenotype results from either the heterozygous or homozygous genotype The recessive phenotype results from only the homozygous genotype Family pedigrees can be used to determine individual genotypes

LE 9-8a Dominant Traits Recessive Traits Freckles No freckles Widow’s peak Straight hairline Free earlobe Attached earlobe

Dd Joshua Lambert Dd Abigail Linnell D ? John Eddy D ? Hepzibah LE 9-8b Dd Joshua Lambert Dd Abigail Linnell D ? John Eddy D ? Hepzibah Daggett D ? Abigail Lambert dd Jonathan Lambert Dd Elizabeth Eddy Dd Dd dd Dd Dd Dd dd Female Male Deaf Hearing

9.9 Many inherited disorders in humans are controlled by a single gene CONNECTION 9.9 Many inherited disorders in humans are controlled by a single gene Thousands of human genetic disorders follow simple Mendelian patterns of inheritance Recessive disorders Most genetic disorders Can be carried unnoticed by heterozygotes Range in severity from mild (albinism) to severe (cystic fibrosis) More likely to occur with inbreeding

Some serious, but nonlethal, disorders (achondroplasia) Dominant disorders Some serious, but nonlethal, disorders (achondroplasia) Lethal conditions less common than in recessive disorders Cannot be carried by heterozygotes without affecting them Can be passed on if they do not cause death until later age (Huntington's disease)

Parents Normal Dd Normal Dd  Sperm D d Dd Normal (carrier) DD Normal LE 9-9a Parents Normal Dd Normal Dd  Sperm D d Dd Normal (carrier) DD Normal D Offspring Eggs Dd Normal (carrier) d dd Deaf

9.10 New technologies can provide insight into one's genetic legacy CONNECTION 9.10 New technologies can provide insight into one's genetic legacy New technologies can provide insight for reproductive decisions Identifying carriers Tests can distinguish parental carriers of many genetic disorders Fetal testing Amniocentesis and chorionic villus sampling (CVS) allow removal of fetal cells to test for genetic abnormalities

LE 9-10a Amniocentesis Chorionic villus sampling (CVS) Ultrasound monitor Needle inserted through abdomen to extract amniotic fluid Suction tube inserted through cervix to extract tissue from chorionic villi Ultrasound monitor Fetus Fetus Placenta Placenta Chorionic villi Uterus Cervix Cervix Uterus Amniotic fluid Centrifugation Fetal cells Fetal cells Biochemical tests Several weeks Several hours Karyotyping

Video: Ultrasound of Human Fetus 1 Fetal imaging Ultrasound imaging uses sound waves to produce a picture of the fetus Newborn screening Some genetic disorders can be detected at birth by routine tests Ethical considerations How will genetic testing information be used? Video: Ultrasound of Human Fetus 1

VARIATIONS ON MENDEL'S LAWS 9.11 The relationship of genotype to phenotype is rarely simple Mendel's principles are valid for all sexually reproducing species However, most characteristics are inherited in ways that follow more complex patterns

9.12 Incomplete dominance results in intermediate phenotypes Dominant allele has same phenotypic effect whether present in one or two copies Incomplete dominance Heterozygote exhibits characteristics intermediate between both homozygous conditions Not the same as blending

LE 9-12a P generation Red RR White rr  Gametes R r F1 generation Pink Sperm 1 2 1 2 R r 1 2 Red RR Pink rR R F2 generation Eggs 1 2 Pink Rr White rr r

LE 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 Normal Mild disease Severe disease

9.13 Many genes have more than two alleles in the population In a population, multiple alleles often exist for a single characteristic Example: human ABO blood group Involves three alleles of a single gene AB blood group is an example of codominance-both alleles are expressed in heterozygotes

LE 9-13 Blood Group (Phenotype) Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Genotypes O A B AB Anti-A Anti-B O ii IAIA or IAi A Anti-B IBIB or IBi B Anti-A AB IAIB

9.14 A single gene may affect many phenotypic characteristics Pleiotropy: a single gene may influence multiple characteristics Example: sickle cell disease Allele causes production of abnormal hemoglobin in homozygotes Many severe physical effects Heterozygotes generally healthy

Most common inherited disorder among people of African descent Allele persists in population because heterozygous condition protects against malaria

LE 9-14 Individual homozygous for sickle-cell allele Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickle-cells 5,555 Clumping of cells and clogging of small blood vessels Breakdown of red blood cells Accumulation of sickled cells in spleen Physical weakness Heart failure Pain and fever Brain damage Damage to other organs Spleen damage Anemia Impaired mental function Pneumonia and other infections Kidney failure Paralysis Rheumatism

9.15 A single characteristic may be influenced by many genes Polygenic inheritance is the additive effects of two or more genes on a single phenotypic characteristic Example: human skin color Controlled by at least three genes

Fraction of population LE 9-15 P generation  aabbcc (very light) AABBCC (very dark) F1 generation  AaBbCc AaBbCc 1 64 6 64 15 64 20 64 15 64 6 64 1 64 Sperm 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 20 64 F2 generation 1 8 1 8 15 64 1 8 1 8 Fraction of population Eggs 1 8 6 64 1 8 1 8 1 64 1 8 Skin color

9.16 The environment affects many characteristics Many characteristics result from a combination of genetic and environmental factors Nature vs. nurture is an old and hotly contested debate Only genetic influences are inherited

9.17 Genetic testing can detect disease-causing alleles CONNECTION 9.17 Genetic testing can detect disease-causing alleles Predictive genetic testing may inform people of their risk for developing genetic diseases Used when there is a family history but no symptoms Increased use of genetic testing raises ethical, moral, and medical issues

THE CHROMOSOMAL BASIS OF INHERITANCE 9.18 Chromosome behavior accounts for Mendel's laws Chromosome theory of inheritance Genes occupy specific loci on chromosomes Chromosomes undergo segregation and independent assortment during meiosis Thus, chromosome behavior during meiosis and fertilization accounts for inheritance patterns

(alternative arrangements) Fertilization among the F1 plants LE 9-18 All round yellow seeds (RrYy) F1 generation R y r Y R r r R Metaphase I of meiosis (alternative arrangements) Y y Y y R r r R Anaphase I of meiosis Y y Y y R r r R Metaphase II of meiosis Y y Y y Y y Y y Y Y y y Gametes R R r r r r R R 1 4 RY 1 4 ry 1 4 rY 1 4 Ry Fertilization among the F1 plants F2 generation 9 :3 :3 :1 (See Figure 9.5A)

9.19 Genes on the same chromosome tend to be inherited together Linked genes Lie close together on the same chromosome Tend to be inherited together Generally do not follow Mendel's law of independent assortment

Not accounted for: purple round and red long Experiment Purple flower PpLl  PpLl Long pollen Observed offspring Prediction (9:3:3:1) Phenotypes Purple long Purple round Red long Red round 284 21 55 215 71 24 Explanation: linked genes Parental diploid cell PpLl P L p l Meiosis Most gametes P L p l Fertilization Sperm P L p l P L P L P L Most offspring P L p l Eggs p l p l p l P L p l 3 purple long : 1 red round Not accounted for: purple round and red long

9.20 Crossing over produces new combinations of alleles During meiosis, homologous chromosomes undergo crossing over Produces new combinations of alleles in gametes Percentage of recombinant offspring is called the recombination frequency

LE 9-20a A B a b A B a b A b a B Tetrad Crossing over Gametes

Thomas Hunt Morgan performed some of the most important studies of crossing over in the early 1900s Used the fruit fly Drosophila melanogaster Established that crossing over was the mechanism that "breaks linkages" between genes

LE 9-20c Offspring Experiment Gray body, long wings (wild type) Black body, vestigial wings  GgLl ggll Female Male Offspring Gray long Black vestigial Gray vestigial Black long 965 944 206 185 Parental phenotypes Recombinant phenotypes Recombination frequency = 391 recombinants 2,300 total offspring = 0.17 or 17% Explanation G L g l GgLl (female) ggll (male) g l g l G L g l G l g L g l Eggs Sperm G L g l G l g L g l g l g l g l Offspring

9.21 Geneticists use crossover data to map genes Morgan and his students greatly advanced understanding of genetics Alfred Sturtevant used crossover data to map genes in Drosophila Used recombination frequencies to map the relative positions of genes on chromosomes

LE 9-21b Chromosome g c l 17% 9% 9.5% Recombination frequencies

Mutant phenotypes Wild-type phenotypes LE 9-21c 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 9.22 Chromosomes determine sex in many species Many animals have a pair of chromosomes that determine sex Humans: X-Y system Male is XY; the Y chromosome has genes for the development of testes Female is XX; absence of a Y chromosome allows ovaries to develop

(male) (female) 44 + XY 44 + XX Parents’ diploid cells 22 + X 22 + Y LE 9-22a (male) (female) 44 + XY 44 + XX Parents’ diploid cells 22 + X 22 + Y 22 + X Sperm Egg 44 + XX 44 + XY Offspring (diploid)

Other animals have other sex-determination systems X-O (grasshopper, roaches, some other insects) Z-W (certain fishes, butterflies, birds) Chromosome number (ants, bees) Different plants have various sex-determination systems

LE 9-22b 22 + X XX 76 + ZW ZZ 32 16

9.23 Sex-linked genes exhibit a unique pattern of inheritance Sex-linked genes are genes for characteristics unrelated to sex that are located on either sex chromosome In humans, refers to a gene on the X chromosome Because of linkage and location, the inheritance of these characteristics follows peculiar patterns Example: eye color inheritance in fruit flies follows three possible patterns, depending on the genotype of the parents

LE 9-23b Female Male XRXR  Xr Y Sperm Xr Y Eggs XR XRXr XRY R = red-eye allele r = white-eye allele

LE 9-23c Female Male XRXr  XRY Sperm XR Y XR XRXR XRY Eggs Xr XrXR

LE 9-23d Female Male XRXr  Xr Y Sperm Xr Y XR XRXr XRY Eggs Xr Xr Xr

CONNECTION 9.24 Sex-linked disorders affect mostly males In humans, recessive sex-linked traits are expressed much more frequently in men than in women Most known sex-linked traits are caused by genes (alleles) on the X chromosome Because the male has only one X chromosome, his recessive X-linked characteristic will always be exhibited Females with the allele are normally carriers and will exhibit the condition only if they are homozygous Examples: red-green color blindness, hemophilia, Duchenne muscular dystrophy

Queen Victoria Albert Alice Louis Alexandra Czar Nicholas  of Russia LE 9-24b Queen Victoria Albert Alice Louis Alexandra Czar Nicholas  of Russia Alexis