chapter 13 observing patterns in inherited traits
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Chapter 13 Observing Patterns in Inherited Traits. 13.1 Menacing Mucus. Cystic fibrosis (CF) is the most common fatal genetic disorder in the United States The CFTR gene encodes CFTR protein which maintains a thin film of water on the surface of the epithelial sheets - PowerPoint PPT PresentationTRANSCRIPT
Albia Dugger • Miami Dade College
Chapter 13Observing Patterns in Inherited Traits
13.1 Menacing Mucus
• Cystic fibrosis (CF) is the most common fatal genetic disorder in the United States
• The CFTR gene encodes CFTR protein which maintains a thin film of water on the surface of the epithelial sheets
• A deletion of three base pairs (ΔF508, deletion) prevents proper membrane trafficking of CFTR so it can’t do its job
• Mucus obstructs the airways and bacteria infect the intestine and lungs – most CF patients live no longer than thirty years
Figure 13-1a p203
ATP
ΔF508
13.2 Mendel, Pea Plants, and Inheritance Patterns
• Recurring inheritance patterns are observable evidence of how heredity works
• Before the discovery of genes, it was thought that inherited traits resulted from a blend of parental characters
Mendel’s Experimental Approach
• Gregor 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
• Mendel discovered that traits of offspring of cross-fertilized pea plants often appear in predictable patterns
• Mendel’s work led him to conclude that hereditary information passes from one generation to the next in discrete units
Gregor Mendel
Garden Pea Plant: Self Fertilization and Cross-Fertilization
carpelA
anther
BC
D
E
ANIMATED FIGURE: Crossing garden pea plants
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Terms Used in Modern Genetics
• Genes are heritable units of information about traits
• Each gene has a specific locus on a chromosome
• Diploid cells (chromosome number 2n) have pairs of genes on homologous chromosomes
• Alleles are different molecular forms of a gene
Loci of Some Human Genes
Figure 13-3a p205
ribosomal RNA
skin pigmentation
15
(Tay–Sachs disease)
fibrillin 1 (Marfan syndrome)
Figure 13-3b p205
17
BRCA1 (breast, ovarian cancer)
NF1 (neurofibromatosis)
(Canavan disease)
serotonin transporter
Growth hormone
p53 tumor antigen
Figure 13-3c p205
19 LH, β chain
HCG, β chain
(Warfarin resistance)
green/blue eye color
brown hair color
insulin receptor
LDL receptor (coronary artery disease)
Figure 13-3d p205
20
GHRH (acromegaly)
prion protein(Creutzfeldt–Jakob disease)
oxytocin
Figure 13-3e p205
(green-deficient color blind)X(red-deficient color blind)
(hemophilia A)
(hemophilia B)
XIST X chromosomeinactivation control
IL2RG (SCID-X1)
dystrophin (muscular dystrophy)
(anhidrotic ectodermal dysplasia)
Terms Used in Modern Genetics
• The particular set of alleles that an individual carries is the individual’s genotype
• An individual with two identical alleles of a gene is homozygous for that gene
• An individual with nonidentical alleles of a gene is heterozygous for that gene
Terms Used in Modern Genetics
• A hybrid is the heterozygote offspring of a cross between two individuals that breed true for different forms of a trait
• An individual’s genotype determines its phenotype, which refers to an individual’s observable traits
• Any mutated gene is a new allele, whether or not it affects phenotype
Terms Used in Modern Genetics
• An allele is dominant if its effect masks the effect of a recessive allele paired with it• Capital letters (P) signify dominant alleles; lowercase
letters (p) signify recessive alleles• Homozygous dominant (PP)• Homozygous recessive (pp)• Heterozygous (Pp)
Genotypes Give Rise to Phenotypes
Pp(heterozygous atthe P gene locus)
genotype:
phenotype:
PP(homozygous fordominant allele P)
pp(homozygous forrecessive allele p)
Take-Home Message:
How do alleles contribute to traits?
• Gregor Mendel discovered the role of alleles in inheritance by breeding pea plants and tracking traits of their offspring
• Genotype refers to the particular set of alleles carried by an individual’s somatic cell; phenotype refers to the individual’s set of observable traits; genotype is the basis of phenotype
• A homozygous individual has two identical alleles at a particular locus; a heterozygous individual has nonidentical alleles at the locus
• Dominant alleles mask the effects of recessive ones in heterozygous individuals
13.3 Mendel’s Law of Segregation
• Pairs of genes on homologous chromosomes separate during meiosis, so they end up in different gametes
• Mendel showed that garden pea plants inherit two “units” of information for a trait, one from each parent
Gene Segregation
• Homologous chromosomes (and all the alleles they carry) segregate into separate gametes during meiosis
• Plants homozygous for the dominant allele (PP) can only make gametes that carry the allele P
• Plants homozygous for the recessive allele (pp) can only make gametes that carry the allele p
• Heterozygous plants produce both type of gametes
Calculating Probabilities
• Probability• A measure of the chance that a particular outcome will
occur
• Punnett square• A grid used to calculate the probability of genotypes and
phenotypes in offspring
Stepped Art
gametes (p)
meiosis II
gametes (P)
DNA replication
meiosis I
1 2
zygote (Pp)
3
female gametes
mal
e g
amet
es
4
Figure 13-5 p206
Figure 13-5b p206
mal
e g
amet
es
female gametes
Testcrosses
• A testcross is 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 (PP x pp) or (Pp x pp)
Monohybrid Crosses
• A monohybrid cross is a testcross that checks for a dominance relationship between two alleles at a single locus
• May be a cross between true breeding (homozygous) individuals (PP x pp), or between identical heterozygotes (Pp x Pp)
Generations in a Monohybrid Cross
• P stands for parents, F for filial (offspring)
• F1: First generation offspring of parents
• F2: Second generation offspring of parents
Mendel’s Monohybrid Crosses
• Mendel used monohybrid crosses 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
Table 13-1 p207
Mendel’s Dihybrid Cross
parent plant homozygous
for purple flowers and long stemsPPTT pptt
dihybridPpTt
four types of gametes
parent plant homozygous
for white flowers and short stems
1
2
3
4
PPTT PPTt PpTT PpTt
PPTt PPtt PpTt Pptt
PpTT PpTt ppTT ppTt
PpTt Pptt ppTt ppttPT Pt pT pt
PT pt
PT Pt pT pt
PP
Pt
pT
pt
Stepped Art
Offspring of Mendel’s Monohybrid Cross
Mendel’s Law of Segregation
• Mendel observed a phenotype ratio of 3:1 in the F2 offspring of his monohybrid crosses• Consistent with the probability of the pp genotype in the
offspring of a heterozygous cross (Pp x Pp)
• This is the basis of Mendel’s law of segregation• Diploid cells have pairs of genes on pairs of homologous
chromosomes • The two genes of each pair separate during meiosis, and
end up in different gametes
Take-Home Message:
What is Mendel’s law of segregation?
• Diploid cells carry pairs of genes, on pairs of homologous chromosomes
• The two genes of each pair are separated from each other during meiosis, so they end up in different gametes
• Mendel discovered patterns of inheritance in pea plants by tracking the results of many monohybrid crosses
ANIMATED FIGURE: Monohybrid cross
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13.4 Mendel’s Law of Independent Assortment
• Mendel’s law of independent assortment• During meiosis, members of a pair of genes on
homologous chromosomes get distributed into gametes independently of other gene pairs
Dihybrid Crosses
• Dihybrid crosses test for dominance relationships between alleles at two loci
• Individuals that breed true for two different traits are crossed (PPTT x pptt)
• 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
The Contribution of Crossovers
• Independent assortment also occurs when the genes are on the same chromosome, but far enough apart that crossing over occurs between them very frequently
• Genes that have loci very close to one another on a chromosome tend to stay together during meiosis and not assort independently
Linkage Groups
• All genes on one chromosome are called a linkage group
• The farther apart two genes are on a chromosome, the more often crossing over occurs between them
• Linked genes are very close together; crossing over rarely occurs between them
• The probability that a crossover will separate alleles of two genes is proportional to the distance between those genes
Take-Home Message: What is Mendel’s law of independent assortment?
• Each member of a pair of genes on homologous chromosomes tends to be distributed into gametes independently of how other genes are distributed during meiosis
ANIMATED FIGURE: Independent assortment
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ANIMATED FIGURE: Dihybrid cross
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13.5 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
Codominance
• Codominance• 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• Genes with three or more alleles in a population• Example: ABO blood types
Codominance in ABO Blood Types
Phenotypes (blood type):
Genotypes:
O
OO
BABA
AA
or
AO AB
BB
or
BO
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
Figure 13-10 p210homozygous (RR) homozygous (rr)heterozygous (Rr)
Figure 13-10b p210
INTERACTION: Incomplete dominance
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Epistasis
• Epistasis• Two or more gene products influence a trait• Typically, one gene product suppresses the effect of
another
• Example: Coat color in dogs• Alleles B and b designate colors (black or brown)• Two recessive alleles ee suppress color
Coat Colors in Labrador Retrievers
Figure 13-11b p211
Pleiotropy
• A pleiotropic gene influences multiple traits
• Example: Some tall, thin athletes have Marfan syndrome, a potentially fatal genetic disorder
Take-Home Message
Are all alleles dominant or recessive?
• An allele may be fully dominant, incompletely dominant, or codominant with its partner on a homologous chromosome
• In epistasis, two or more gene products influence a trait
• The product of a pleiotropic gene influences two or more traits
13.6 Nature and Nurture
• Variations in traits aren’t always the result of differences in alleles – many traits are influenced by environmental factors
Environment and Gene Expression
• The environment affects the expression of many genes, which in turn affects phenotype – including behavioral traits
• We can summarize this relationship as:
genotype + environment → phenotype
Environment and Epigenetics
• Environmentally driven changes in gene expression patterns can be permanent and heritable
• Such changes are implemented by gene controls such as chromatin modifications and RNA interference that act on DNA itself
• Example: Many environmental factors affect DNA methylation patterns, enhancing or suppressing gene expression
Effects of Temperature on Gene Expression
Figure 13-14a p212
c Maturecutting at lowelevation (30meters abovesea level)
a Maturecutting athigh elevation(3,060 metersabove sealevel)
b Maturecutting atmid-elevation(1,400 metersabove sealevel)
Figure 13-15a p213A Light micrograph of a living water flea.
Figure 13-15b p213
B Electron micrographs comparing Daphnia body form that develops in the presence of few predators (left) with the form that develops in the presence of many predators (right). Note the difference in the length of the tail spine and the pointiness of the head. Chemicals emitted by thewater flea’s insect predators provoke the change.
Mood Disorders in Humans
• Environment is a factor in schizophrenia, bipolar disorder, depression, and other mood disorders
• Example: Stress-induced depression causes methylation-based silencing of a particular nerve growth factor – some antidepressants work by reversing this methylation
• Future treatments for many disorders may involve deliberate modification of epigenetic marks in one’s DNA
Take-Home Message: Is genotype the only factor that gives rise to phenotype?
• The environment influences gene expression, and therefore can alter phenotype
• Cell-signaling pathways link environmental cues with epigenetic marks such as methylation and other chromatin modifications
ANIMATION: Coat color in the Himalayan rabbit
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13.7 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
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
Continuous Variation in Height (Females)
Continuous Variation in Height (Males)
The Bell Curve
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 result in unpredictable traits
• Example: Camptodactyly can affect any fingers on either or both hands
Camptodactyly
Take-Home Message:Do all traits occur in distinct forms?
• The more genes and other factors that influence a trait, the more continuous is its range of variation
• Unpredictable phenotypes can be caused by interactions among genes with a range of expression among individuals