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Patterns of Inheritance
Chapter 9 Patterns of Inheritance
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Do Now In your own words, in 3 lines, describe what genetics is, and how traits are expressed.
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Purebreds and Mutts–A Difference of Heredity
Purebred dogs Are very similar on a genetic level due to selective breeding
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Mutts, or mixed breed dogs on the other hand
Show considerably more genetic variation
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9.1 The science of genetics has ancient roots
MENDEL’S LAWS 9.1 The science of genetics has ancient roots The historical roots of genetics, the science of heredity date back to ancient attempts at selective breeding Pangenesis-particles travel from each part of an organism’s body to the eggs or sperm and are then passed to the next generation. Changes that occur in various parts of the body during an organism’s life are passed on in this way.
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9.2 Experimental genetics began in an abbey garden
Modern genetics Began with Gregor Mendel’s quantitative experiments with pea plants Petal Stamen Carpel Figure 9.2 A Figure 9.2 B
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Mendel crossed pea plants that differed in certain characteristics
And traced traits from generation to generation 1 Removed stamens from purple flower White Stamens Carpel 2 Transferred pollen from stamens of white flower to carpel of purple flower Parents (P) Purple 3 Pollinated carpel matured into pod 4 Planted seeds from pod Offspring (F1) Figure 9.2 C
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Mendel hypothesized that there are alternative forms of genes
The units that determine heritable traits 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 Figure 9.2 D
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Self-fertilization-sperm-carrying pollen grains released from the stamens land on the egg-containing carpel of the same flower. Cross-fertilization-fertilization of one plant by pollen from a different plant. True-breeding-Varieties of plants in which self-fertilization produces offspring that are identical to the parents Hybrid-the offspring of two different varieties
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Cross-(hybridization)-the cross-fertilization itself
P Generation-the true-breeding parents F1 Generation-the hybrid offspring of the P generation F2 Generation-the offspring of the F1 plants self-fertilization, or fertilization with each other.
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9.3 Mendel’s law of segregation describes the inheritance of a single characteristic
Monohybrid cross-a breeding experiment in which the parental varieties differ in only one trait. From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic Allele-alternative versions of a gene. Figure 9.3 A
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For each characteristic, an organism inherits two alleles, one from each parent
Homozygous-an organism has two identical alleles for a gene Heterozygous-an organism has two different alleles for a gene
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If the two alleles of an inherited pair differ
Then one determines the organism’s appearance and is called the dominant allele The other allele Has no noticeable effect on the organism’s appearance and is called the recessive allele
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Mendel’s law of segregation
Predicts that allele pairs separate from each other during the production of gametes Applies to all sexually reproducing organisms. Figure 9.3 B
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Phenotype-an organism’s expressed, physical traits
Genotype-an organism’s genetic makeup P plants Genetic makeup (alleles) PP pp Gametes All P All p F1 plants (hybrids) All Pp 1 2 1 2 P p Gametes 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
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9.4 Homologous chromosomes bear the two alleles for each characteristic
Alleles (alternative forms) of a gene Reside at the same locus on homologous chromosomes Dominant allele Gene loci P a B P a b Recessive allele Genotype: PP aa Bb Heterozygous Homozygous for the dominant allele Homozygous for the recessive allele Figure 9.4
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9.5 The law of independent assortment is revealed by tracking two characteristics at once
By looking at two characteristics at once Mendel tried to determine how two characteristics were inherited
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Actual results support hypothesis
Mendel’s law of independent assortment States that alleles of a pair segregate independently of other allele pairs during gamete formation Hypothesis: Dependent assortment Hypothesis: Independent assortment P generation RRYY rryy RRYY rryy Gametes RY ry Gametes RY ry F1 generation RrYy RrYy Sperm Sperm 1 4 1 4 1 4 1 4 1 2 RY ry RY ry 1 2 RY ry 1 4 RY 1 2 F2 generation RY RRYY RrYY RRYy RrYy Eggs 1 4 1 2 ry ry Eggs RrYY rrYY RrYy rrYy 9 16 Yellow round 1 4 Ry RRYy RrYy RRyy Rryy 3 16 Green round 1 4 ry Actual results contradict hypothesis 3 16 Yellow wrinkled RrYy rrYy Rryy rryy Actual results support hypothesis 1 16 Green wrinkled Figure 9.5 A
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Heterozygous x Heterozygous 9:3:3:1
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The phenotypic ratio resulting from a dihybrid cross showing independent assortment is expected to be 9:3:3:1. Blind Blind Phenotypes Genotypes 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) Figure 9.5 B
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9.6 Geneticists use the testcross to determine unknown genotypes
The offspring of a testcross, a mating between an individual of unknown genotype and a homozygous recessive individual can reveal the unknown’s genotype 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 Figure 9.6
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9.7 Mendel’s laws reflect the rules of probability
Inheritance follows the rules of probability Number of times an event is expected to happen Probability= Number of times an event could happen
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The rule of multiplication
Calculates the probability of two independent events The rule of addition Calculates the probability of an event that can occur in alternate ways F1 genotypes Bb male Formation of sperm Bb female Formation of eggs 1 2 1 2 B b 1 2 B B B b B 1 4 1 4 F2 genotypes 1 2 b B b b b 1 4 1 4 Figure 9.7
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Do Now Using a six-sided die, what is the probability of rolling either a 5 or a 6? 1/6 + 1/6 = 1/3
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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 Dominant Traits Recessive Traits Freckles No freckles Widow’s peak Straight hairline Free earlobe Attached earlobe Figure 9.8 A
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Family pedigrees Can be used to determine individual genotypes
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 Figure 9.8 B
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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 Some autosomal disorders in humans Table 9.9
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Recessive Disorders Most human genetic disorders are recessive
A carrier of a genetic disorder who does not show symptoms is most likely to be heterozygous for the trait and able to transmit it to offspring. Parents Normal Dd Normal Dd X Sperm D d Dd Normal (carrier) D DD Normal Offspring Eggs Dd Normal (carrier) dd Deaf d Figure 9.9 A
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Dominant Disorders Some human genetic disorders are dominant
Figure 9.9 B
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CONNECTION 9.10 New technologies can provide insight into one’s genetic legacy New technologies Can provide insight for reproductive decisions
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Identifying Carriers For an increasing number of genetic disorders
Tests are available that can distinguish carriers of genetic disorders
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Fetal Testing Amniocentesis and chorionic villus sampling (CVS)
Allow doctors to remove fetal cells that can be tested for genetic abnormalities Amniocentesis Chorionic villus sampling (CVS) Ultrasound monitor Needle inserted through abdomen to extract amniotic fluid Ultrasound monitor Suction tube inserted through cervix to extract tissue from chorionic villi Fetus Fetus Placenta Placenta Chorionic villi Uterus Cervix Cervix Uterus Amniotic fluid Centrifugation Fetal cells Fetal cells Biochemical tests Several weeks Several hours Karyotyping Figure 9.10 A
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Fetal Imaging Ultrasound imaging
Uses sound waves to produce a picture of the fetus Figure 9.10 B
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Newborn Screening Some genetic disorders can be detected at birth
By simple tests that are now routinely performed in most hospitals in the United States
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Ethical Considerations
New technologies such as fetal imaging and testing Raise new ethical questions
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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 But genotype often does not dictate phenotype in the simple way his laws describe
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Do Now Find the phenotype of an individual the offspring of an individual from a RR mother and rr father using incomplete dominance where RR-red and rr-white.
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It exhibits incomplete dominance
9.12 Incomplete dominance results in intermediate phenotypes When an offspring’s phenotype is in between the phenotypes of its parents It exhibits incomplete dominance P generation Red RR White rr Gametes R r F1 generation Pink Rr Genotypes: 1 2 1 2 HH Homozygous for ability to make LDL receptors Hh Heterozygous hh Homozygous for inability to make LDL receptors Gametes R r Sperm Phenotypes: 1 2 1 2 R r LDL 1 2 Red RR Pink rR LDL receptor R F2 generation Eggs 1 2 Pink Rr White rr r Cell Normal Mild disease Severe disease Figure 9.12 A Figure 9.12 B
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9.13 Many genes have more than two alleles in the population
The expression of both alleles for a trait in a heterozygous individual illustrates codominance. RR rr Rr
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The ABO blood type in humans Involves three alleles of a single gene
The alleles for A and B blood types are codominant And both are expressed in the phenotype 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 — Figure 9.13
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Sickle-cell disease represents codominance and pleiotropy.
9.14 A single gene may affect many phenotypic characteristics In pleiotropy, a single gene may affect phenotype in many ways Sickle-cell disease represents codominance and pleiotropy. 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 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 Impaired mental function Pneumonia and other infections Paralysis Rheumatism Kidney failure Figure 9.14
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Essentially the converse of pleiotropy
9.15 A single characteristic may be influenced by many genes Polygenic inheritance creates a continuum of phenotypes where a single phenotypic characteristic is determined by the additive effects of two or more genes Essentially the converse of pleiotropy P generation aabbcc (very light) AABBCC (very dark) F1 generation AaBbCc AaBbCc Sperm 1 64 6 64 15 64 20 64 15 64 6 64 1 64 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 Eggs 1 8 Fraction of population 1 8 6 64 1 8 1 64 1 8 Figure 9.15 Skin color
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9.16 The environmental affects many characteristics
Many traits are affected, in varying degrees By both genetic and environmental factors Figure 9.16
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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
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THE CHROMOSOMAL BASIS OF INHERITANCE
9.18 Chromosome behavior accounts for Mendel’s laws Genes are located on chromosomes Whose behavior during meiosis and fertilization accounts for inheritance patterns Chromosome Theory of Inheritance The behavior of chromosomes during meiosis and fertilization accounts for patterns of inheritance.
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The chromosomal basis of Mendel’s laws
F1 generation All round yellow seeds (RrYy) R r y 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) Figure 9.18
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Not accounted for: purple round and red long
9.19 Genes on the same chromosome tend to be inherited together Linked Genes Tend to be inherited together because they reside close together on the same chromosome Do not follow the laws of independent assortment Experiment Purple flower PpLI PpLI Long pollen Observed Prediction Phenotypes offspring (9:3:3:1) Purple long Purple round Red long Red round 284 21 55 215 71 24 Explanation: linked genes P L Parental diploid cell PpLI P I Meiosis Most gametes P L P I Fertilization Sperm P L P I P L P L P L Most offspring P L P I Eggs P I P I P I P L P I 3 purple long : 1 red round Not accounted for: purple round and red long Figure 9.19
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9.20 Crossing over produces new combinations of alleles
The mechanism that "breaks" the linkage between linked genes is crossing-over. A B a b A B a b A b a B Tetrad Crossing over Gametes Figure 9.20 A
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Thomas Hunt Morgan Performed some of the early studies of crossing over using the fruit fly Drosophila melanogaster Figure 9.20 B
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Demonstrated the role of crossing over in inheritance
Morgan’s experiments Demonstrated the role of crossing over in inheritance Experiment Black body, vestigial wings Gray body, long wings (wild type) GgLI ggll Female Male Offspring Gray long Black vestigial Gray vestigial Black long 965 944 206 185 Parental phenotypes Recombinant phenotypes 391 recombinants Recombination frequency = = 0.17 or 17% 2,300 total offspring Explanation G L g l GgLI (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 Figure 9.20 C
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9.21 Geneticists use crossover data to map genes
Morgan and his students Used crossover data to map genes in Drosophila Figure 9.21 A
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Recombination frequencies
Can be used to map the relative positions of genes on chromosomes. Crossing-over is the mechanism that "breaks" the linkage between linked genes. Mutant phenotypes Short aristae Black body (g) Cinnabar eyes (c) Vestigial wings (l) Brown eyes Chromosome g c l 17% 9% 9.5% Recombination frequencies Long aristae (appendages on head) Gray body (G) Red eyes (C) Normal wings (L) Red eyes Wild-type phenotypes Figure 9.21 B Figure 9.21 C
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SEX CHROMOSOMES AND SEX-LINKED GENES
9.22 Chromosomes determine sex in many species In mammals, a male has one X chromosome and one Y chromosome And a female has two X chromosomes (male) (female) 44 + XY Parents’ diploid cells 44 + XX 22 + X 22 + Y 22 + X Sperm Egg 44 + XX Offspring (diploid) 44 + XY Figure 9.22 A
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The Y chromosome Has genes for the development of testes The absence of a Y chromosome Allows ovaries to develop
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Other systems of sex determination exist in other animals and plants
22 + XX 22 + X Figure 9.22 B 76 + ZW 76 + ZZ Figure 9.22 C 32 16 Figure 9.22 D
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9.23 Sex-linked genes exhibit a unique pattern of inheritance
All genes on the sex chromosomes Are said to be sex-linked In many organisms The X chromosome carries many genes unrelated to sex
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In Drosophila White eye color is a sex-linked trait
Figure 9.23 A
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The inheritance pattern of sex-linked genes
Is reflected in females and males Female Male Female Male Female Male XR XR Xr Y XR Xr XR Y XR Xr Xr Y Sperm Sperm Sperm Xr Y XR Y Xr Y Eggs XR XR Xr XR Y XR XR XR XR Y XR XR Xr XR Y Eggs Eggs R = red-eye allele r = white-eye allele Xr Xr XR Xr Y Xr Xr Xr Xr Y Figure 9.23 B Figure 9.23 C Figure 9.23 D
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9.24 Sex-linked disorders affect mostly males
CONNECTION 9.24 Sex-linked disorders affect mostly males Most sex-linked human disorders Are due to recessive alleles Are mostly seen in males Queen victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis Figure 9.24 A Figure 9.24 B
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A male receiving a single X-linked allele from his mother
Will have the disorder A female Has to receive the allele from both parents to be affected
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Recessive sex-linked human conditions
Red-green color blindness Hemophilia Muscular dystrophy
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