Observing Patterns in Inherited Traits
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Observing Patterns in Inherited Traits
Chapter 11
Hsueh-Fen Juan
Oct. 16, 2012
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Impacts, Issues:The Color of Skin
Like most human traits, skin color has a genetic basis; more than 100 gene products affect the synthesis and deposition of melanins
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Video: Genetics of skin color
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11.1 Mendel, Pea Plants, and Inheritance Patterns
Recurring inheritance patterns are observable outcomes of sexual reproduction
Before the discovery of genes, it was thought that inherited traits resulted from a blend of parental characters
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Mendel’s Experimental Approach
Mendel was a monk with training in plant breeding and mathematics
He studied the garden pea (Pisum sativum), which breeds true for a number of traits
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Fig. 11-3, p. 170
carpel anther
A Garden pea flower, cut in half. Sperm form in pollen grains, which originate in male floral parts (anthers). Eggs develop, fertilization takes place, and seeds mature in female floral parts (carpels).
B Pollen from a plant that breeds true for purple flowers is brushed onto a floral bud of a plant that breeds true for white flowers. The white flower had its anthers snipped off. Artificial pollination is one way to ensure that a plant will not self-fertilize.
C Later, seeds develop inside pods of the cross-fertilized plant. An embryo in each seed develops into a mature pea plant.
D Each new plant’s flower color is indirect but observable evidence that hereditary material has been transmitted from the parent plants.
Garden Pea Plant:
Self Fertilization and Cross-Fertilization
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Animation: Crossing garden pea plants
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Terms Used in Modern Genetics
Genes• Heritable units of information about traits• Parents transmit genes to offspring• Each gene has a specific locus on a
chromosome
Diploid cells (chromosome number 2n) have pairs of genes on homologous chromosomes
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Terms Used in Modern Genetics
A mutation is a permanent change in a gene• May cause a trait to change• Alleles (等位基因 ) are different molecular forms
of a gene
A hybrid has nonidentical alleles for a trait• Offspring of a cross between two individuals that
breed true for different forms of a trait are hybrids
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Terms Used in Modern Genetics
An individual with nonidentical alleles of a gene is heterozygous for that gene (異型合子 )
An individual with identical alleles of a gene is homozygous for that gene (同型合子 )
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Terms Used in Modern Genetics
An allele is dominant (顯性 ) if its effect masks the effect of a recessive (隱性 ) allele paired with it• Capital letters (A) signify dominant alleles;
lowercase letters (a) signify recessive alleles• Homozygous dominant (AA)• Homozygous recessive (aa)• Heterozygous (Aa)
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Terms Used in Modern Genetics
Gene expression (基因表現 )• The process by which information in a gene is
converted to a structural or functional part of a cell or body
• Expressed genes determine traits
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Terms Used in Modern Genetics
Genotype (基因型 )• The particular alleles an individual carries
Phenotype (表型 )• An individual’s observable traits
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Terms Used in Modern Genetics
P stands for parents, F for filial (offspring)
F1: First generation offspring of parents
F2: Second generation offspring of parents
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11.1 Key ConceptsWhere Modern Genetics Started
Gregor Mendel gathered the first experimental evidence of the genetic basis of inheritance
His meticulous work gave him clues that heritable traits are specified in units
The units, which are distributed into gametes in predictable patterns, were later identified as genes
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11.2 Mendel’s Law of Segregation
Garden pea plants inherit two “units” of information for a trait, one from each parent
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Testcrosses
Testcross (試交 )• A method of determining if an individual is
heterozygous or homozygous dominant• An individual with unknown genotype is crossed
with one that is homozygous recessive (AA x aa) or (Aa x aa)
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Monohybrid Experiments
Monohybrid experiments (單性雜交 )• Testcrosses that check for a dominance
relationship between two alleles at a single locus (某基因位上,等位基因僅兩種才能用 )
• May be crosses between true breeding (homozygous) individuals (AA x aa), or between identical heterozygotes (Aa x Aa)
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Mendel’s Monohybrid Experiments
Mendel used monohybrid experiments to find dominance relationships among pea plant traits• When he crossed plants that bred true for white
flowers with plants that bred true for purple flowers, all F1 plants had purple flowers
• When he crossed two F1 plants, ¾ of the F2 plants had purple flowers, ¼ had white flowers
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Segregation of Alleles at a Gene Locus
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Mendel’s Monohybrid Experiments
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Calculating Probabilities
Probability• A measure of the chance that a particular
outcome will occur
Punnett square (旁氏表 )• A grid used to calculate the probability of
genotypes and phenotypes in offspring
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Construction of a Punnett Square
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Phenotype Ratios in a Monohybrid Experiment
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Phenotype Ratios in a Monohybrid Experiment
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Mendel’s Law of Segregation (分離律 )
重點:單性雜交實驗驗證了分離律 Mendel observed a phenotype ratio of 3:1 in the
F2 offspring of his monohybrid crosses
• Consistent with the probability of the aa genotype in the offspring of a heterozygous cross (Aa x Aa)
This is the basis of Mendel’s law of segregation in modern terms (當時尚未知染色體的存在 )1.Diploid cells have pairs of genes on pairs of
homologous chromosomes
2.The two genes of each pair separate during meiosis, and end up in different gametes
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11.2 Key ConceptsInsights from Monohybrid Experiments
Some experiments yielded evidence of gene segregation: When one chromosome separates from its homologous partner during meiosis, the alleles on those chromosomes also separate and end up in different gametes
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11.3 Mendel’s Law of Independent Assortment
Mendel’s law of independent assortment• Many genes are sorted into gametes
independently of other genes
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Dihybrid Experiments
Dihybrid experiments (兩性雜交 )• Tests for dominance relationships between
alleles at two loci • Individuals that breed true for two different traits
are crossed (AABB x aabb)
• F2 phenotype ratio is 9:3:3:1 (four phenotypes)
• Individually, each dominant trait has an F2 ratio of 3:1 – inheritance of one trait does not affect inheritance of the other
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Independent Assortment at Meiosis
Fig. 11-8, p. 174
One of two possible alignments The only other possible alignment
a Chromosome alignments at metaphase I:
A A a a A A a a
B B b b b b B B
b The resulting alignments at metaphase II:
A A a a A A a a
B B b b b b B B
c Possible combinations of alleles in gametes:
B A A B b a a b b A A b B a a B
AB ab Ab aB
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Fig. 11-9a, p. 175
P generation
parent plant homozygous
for purple flowers
and long stems
parent plant homozygous
for white flowers
and short stems
A Meiosis in homozygous individuals results in one kind of gamete. AABB aabb
B A cross between plants homozygous for two different traits yields one possible combination of gametes:
AB x ab
Mendel’s Dihybrid Experiments
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Fig. 11-9b, p. 175
F2 generation
AaBb AaBb AaBbAll F1 offspring are AaBb, with purple flowers and tall stems.
C Meiosis in AaBb dihybrid plants results in four kinds of gametes:
AB Ab aB ab
These gametes can meet up in one of 16 possible wayswhen the dihybrids are crossed (AaBb X AaBb):
F1 generation
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Fig. 11-9c, p. 175
AB Ab aB ab
AB AABB AABb AaBB AaBb
Ab AABb AAbb AaBb Aabb
aB AaBB AaBb aaBB aaBb
ab AaBb Aabb aaBb aabb
D Out of 16 possible genetic outcomes of this dihybrid cross, 9 will result in plants that are purple-flowered and tall; 3, purple-flowered and short; 3, white-flowered and tall; and 1, white-flowered and short. The ratio of phenotypes of this dihybrid cross is 9:3:3:1.
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Mendel’s Law of Independent Assortment
Mendel’s dihybrid experiments showed that “units” specifying one trait segregated into gametes separately from “units” for other traits (亦即一對等位基因之分離,與另一對等位基因之分離毫不相干 )
重點:兩性雜交實驗驗證了自由配合律 Exception: Genes that have loci very close to
one another on a chromosome tend to stay together during meiosis (基因連鎖則不成立 )
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11.3 Key ConceptsInsights from Dihybrid Experiments
Some experiments yielded evidence of independent assortment: Genes are typically distributed into gametes independently of other genes
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11.4 Beyond Simple Dominance
Mendel focused on traits based on clearly dominant and recessive alleles; however, the expression patterns of genes for some traits are not as straightforward
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Codominance in ABO Blood Types
Codominance (等顯性 )• Two nonidentical alleles of a gene are both fully
expressed in heterozygotes, so neither is dominant or recessive
• May occur in multiple allele systems
Multiple allele systems (複對偶基因 )• Genes with three or more alleles in a population• Example: ABO blood types
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Codominance in ABO Blood Types
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Incomplete Dominance
Incomplete dominance• One allele is not fully dominant over its partner• The heterozygote’s phenotype is somewhere
between the two homozygotes, resulting in a 1:2:1 phenotype ratio in F2 offspring
Example: Snapdragon color• RR is red• Rr is pink• rr is white
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Incomplete Dominance in Snapdragons
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Fig. 11-11a, p. 176
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Fig. 11-11b, p. 176
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Epistasis Epistasis (上位基因 )• Two or more gene products influence a trait• Typically, one gene product suppresses the
effect of another, so the resulting phenotype is somewhat unexpected.
• 通常會有一個基因在眾基因的交互作用中佔優勢,稱此基因為上位基因 (例:藍眼瑪莉花色基因 )
Example: Coat color in dogs• Alleles B and b designate colors (black or brown)• Two recessive alleles ee suppress color• Allele E promotes the deposition of melanin in fur,
but two recessive alleles (ee) reduce it.
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Epistasis in Chicken Combs
Dramatic variations in their combs
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Animation: Comb shape in chickens
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Epistasis in Coat Colors
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Fig. 11-13a, p. 177
EB Eb eB eb
EB black black blackEEBB EEBb EeBB
blackEeBb
Eb black chocolate black chocolateEEBb EEbb EeBb Eebb
eB black black yellow yellowEeBB EeBb eeBB eeBb
EeBb Eebb eeBbeb black chocolate yellow yellow
eebb
Allele B (black) is dominant to b (brown). Allele E promotes the deposition of melanin in fur, but two recessive alleles (ee) reduce it.
EE/Ee/ee為上位基因,決定是否有黑色素,因此只要出現基因型 ee,不論是 BB, Bb 或 bb,皆因無黑色素而呈表型 yellow
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Pleiotropy
Pleiotropy (基因多效性 )• One gene product
influences two or more traits
• Example: Some tall, thin athletes have Marfan syndrome, a potentially fatal genetic disorder
• 單基因 (產物 )多表型
FBN1, which encodes a connective protein called fibrillin-1Long fibers of fibrillin impart elasticity to the tissues of the heart, skin, blood vessels, tendons and other body parts.
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11.5 Linkage Groups
The farther apart two genes are on a chromosome, the more often crossing over (互換 ) occurs between them
Linkage group (基因連鎖群 )• All genes on one chromosome are called a
linkage group• Linked genes are very close together; crossing
over rarely occurs between them
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Linkage and Crossing Over
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Animation: Crossover review
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The Distance Between Genes
The probability that a crossover event will separate alleles of two genes is proportional to the distance between those genes
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11.6 Genes and the Environment
Expression of some genes is affected by environmental factors such as temperature, altitude, or chemical exposure
The result may be variation in traits
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Effects of Temperature on Gene Expression
Enzyme tyrosinase (酪胺酸酶 ), works at low temperatures
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Animation: Coat color in the Himalayan rabbit
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Effects of Altitude on Gene Expression
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Effects of Predation on Gene Expression
Predators of daphnias emit chemicals that trigger a different phenotype
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11.7 Complex Variations in Traits
Individuals of most species vary in some of their shared traits
Many traits (such as eye color) show a continuous range of variation
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Continuous Variation
Continuous variation• Traits with a range of small differences• The more factors that influence a trait, the more
continuous the distribution of phenotype
Bell curve• When continuous phenotypes are divided into
measurable categories and plotted as a bar chart, they form a bell-shaped curve
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Continuous Variation and the Bell Curve
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Regarding the Unexpected Phenotype
Phenotype results from complex interactions among gene products and the environment• Enzymes and other gene products control steps
of most metabolic pathways• Mutations, interactions among genes, and
environmental conditions may affect one or more steps
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11.4-11.7 Key ConceptsVariations on Mendel’s Theme
Not all traits appear in Mendelian inheritance patterns• An allele may be partly dominant over a
nonidentical partner, or codominant with it• Multiple genes may influence a trait; some genes
influence many traits• The environments also influences gene
expression