observing patterns in inherited traits
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Observing Patterns in Inherited Traits. Chapter 10. Early Ideas about Heredity. People knew that sperm and eggs transmitted information about traits Blending theory Problem: Would expect variation to disappear Variation in traits persists. Gregor Mendel. - PowerPoint PPT PresentationTRANSCRIPT
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Observing Patterns in Inherited Traits
Chapter 10
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Early Ideas about Heredity
• People knew that sperm and eggs transmitted information about traits
• Blending theory
• Problem:– Would expect variation to disappear– Variation in traits persists
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Gregor Mendel
• Strong background in plant breeding and mathematics
• Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring
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The Garden Pea Plant
• Self-pollinating
• True breeding (different alleles not normally introduced)
• Can be experimentally cross-pollinated
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Genetic TermsA pair of homologous chromosomes
A gene locus
A pair of alleles
Three pairs of genes
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Genes
• Units of information about specific traits
• Passed from parents to offspring
• Each has a specific location (locus) on a chromosome
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Alleles
• Different molecular forms of a gene
• Arise by mutation
• Dominant allele masks a recessive
allele that is paired with it
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Allele Combinations
• Homozygous – having two identical alleles at a locus– AA or aa
• Heterozygous – having two different alleles at a locus– Aa
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Genotype & Phenotype
• Genotype refers to particular genes an individual carries
• Phenotype refers to an individual’s observable traits
• Cannot always determine genotype by observing phenotype
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Tracking Generations
• Parental generation P
mates to produce
• First-generation offspring F1
mate to produce
• Second-generation offspring F2
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Monohybrid Crosses
• Use F1 offspring of parents that breed true for different forms of a trait:(AA x aa = Aa)
• The experiment itself is a cross between two identical F1 heterozygotes, which are the “monohybrids” (Aa x Aa)
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Monohybrid Crosses
fertilization produces heterozygous offspring
meiosis II
meiosis I
(chromosomes duplicated
before meiosis)
Homozygous dominant parent
Homozygous recessive parent
(gametes) (gametes)
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Mendel’s Monohybrid Cross Results
F2 plants showed dominant-to-recessive ratio that averaged 3:1
Trait Studied Dominant Form
Recessive Form
F2 Dominant-to-Recessive Ratio
SEED SHAPE
SEED COLOR
POD SHAPE
POD COLOR
FLOWER COLOR
FLOWER POSITION
STEM LENGTH
2.96:1
3.01:1
2.95:1
2.82:1
3.15:1
3.14:1
2.84:1787 tall 277 dwarf
651 long stem 207 at tip
705 purple 224 white
152 yellow428 green
299 wrinkled882 inflated
6,022 yellow 2,001 green
5,474 round 1,850 wrinkled
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Probability
The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached
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Punnett Square of a Monohybrid Cross
Female gametes
Male gametes
Dominant phenotype can arise 3 ways,recessive only 1
aA
aaAa
AaAAA
a
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F1 Results of One Monohybrid Cross
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F2 Results of Monohybrid Cross
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Testcross
• Individual that shows dominant phenotype is crossed with individual with recessive phenotype
• Examining offspring allows you to determine the genotype of the dominant individual
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Mendel’s Theory of Segregation
• An individual inherits a unit of information (allele) about a trait from each parent
• During gamete formation, the alleles segregate from each other
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Dihybrid Cross
Experimental cross between individuals that are homozygous for different
versions of two traits
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A Dihybrid Cross - F1 Results
homozygous dominant parent plant (purple flowers, tall stem)
AB abX
1 2
3
homozygous recessive parent plant (white flowers, short stem)
F1 OUTCOME
AABB aabb
All F1 plants are AaBb heterozygotes (purple flowers, tall stems)
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F1 Results of Mendel’s Dihybrid Crosses
• All plants displayed the dominant form
of both traits
• We now know:
– All plants inherited one allele for each trait
from each parent
– All plants were heterozygous (AaBb)
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Phenotypic Ratios in F2
Four Phenotypes:– Tall, purple-flowered (9/16)
– Tall, white-flowered (3/16)
– Dwarf, purple-flowered (3/16)
– Dwarf, white-flowered (1/16)
AaBb X AaBb
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Explanation of Mendel’s Dihybrid Results
If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible
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Independent Assortment
• Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait
• Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis
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Independent AssortmentNucleus of a diploid (2n)reproductive cell with two pairs of homologouschromosomes
Possible alignmentsof the two homologouschromosomes duringmetaphase I of meiosis
The resulting alignments at metaphase II
Allelic combinationspossible in gametes
1/4 AB 1/4 ab 1/4 Ab 1/4 aB
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Tremendous Variation
Number of genotypes possible in
offspring as a result of independent
assortment and hybrid crossing is
3n
(n is the number of gene loci
at which the parents differ)
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Impact of Mendel’s Work
• Mendel presented his results in 1865
• Paper received little notice
• Mendel discontinued his experiments in 1871
• Paper rediscovered in 1900 and finally appreciated
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Dominance Relations
• Complete dominance
• Incomplete dominance– Heterozygote phenotype is somewhere
between that of two homozyotes
• Codominance– Non-identical alleles specify two
phenotypes that are both expressed in heterozygotes
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Genetics of ABO Blood Types: Three Alleles
• Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells
• Two alleles (IA and IB) are codominant when paired
• Third allele (i) is recessive to others
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ABO Blood Type:Allele Combinations
• Type A - IAIA or IAi
• Type B - IBIB or IBi
• Type AB - IAIB
• Type O - ii
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ABO Blood Type: Glycolipids on Red Cells
• Type A - Glycolipid A on cell surface
• Type B - Glycolipid B on cell surface
• Type AB - Both glyocolipids A & B
• Type O - Neither glyocolipid A nor B
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ABO and Transfusions
• Recipient’s immune system will attack
blood cells that have an unfamiliar
glycolipid on surface
• Type O is universal donor because it
has neither type A nor type B glycolipid
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Flower Color in Snapdragons: Incomplete Dominance
Red-flowered plant X White-flowered plant
Pink-flowered F1 plants
(homozygote) (homozygote)
(heterozygotes)
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Flower Color in Snapdragons: Incomplete Dominance
Pink-flowered plant X Pink-flowered plant
White-, pink-, and red-flowered plants in a 1:2:1 ratio
(heterozygote) (heterozygote)
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Flower Color in Snapdragons: Incomplete Dominance
• Red flowers - two alleles allow them to make a red pigment
• White flowers - two mutant alleles; can’t make red pigment
• Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment
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Comb Shape in Poultry
Alleles at two loci (R and P) interact
• Walnut comb - RRPP, RRPp, RrPP, RrPp
• Rose comb - RRpp, Rrpp
• Pea comb - rrPP, rrPp
• Single comb - rrpp
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Pleiotropy
• Alleles at a single locus may have effects on two or more traits
• Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia
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Genetics of Sickle-Cell Anemia
• Two alleles1) HbA
Encodes normal beta hemoglobin chain2) HbS
Mutant allele encodes defective chain
• HbS homozygotes produce only the defective hemoglobin; suffer from sickle-cell anemia
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Pleiotropic Effects of HbS/HbS
• At low oxygen levels, cells with only HbS hemoglobin “sickle” and stick together
• This impedes oxygen delivery and blood flow
• Over time, it causes damage throughout the body
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Genetics of Coat Color in Labrador Retrievers
• Two genes involved- One gene influences melanin production
• Two alleles - B (black) is dominant over b (brown)
- Other gene influences melanin deposition• Two alleles - E promotes pigment deposition
and is dominant over e
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Allele Combinations and Coat Color
• Black coat - Must have at least one
dominant allele at both loci
– BBEE, BbEe, BBEe, or BbEE
• Brown coat - bbEE, bbEe
• Yellow coat - Bbee, BbEE, bbee
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Albinism
• Phenotype results when pathway for melanin production is completely blocked
• Genotype - Homozygous recessive at the gene locus that codes for tyrosinase, an enzyme in the melanin-synthesizing pathway
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Campodactyly: Unexpected Phenotypes
• Effect of allele varies:
– Bent fingers on both hands
– Bent fingers on one hand
– No effect
• Many factors affect gene expression
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Continuous Variation
• A more or less continuous range of small differences in a given trait among individuals
• The greater the number of genes and
environmental factors that affect a trait,
the more continuous the variation in
versions of that trait
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Human Variation
• Some human traits occur as a few discrete types– Attached or detached earlobes – Many genetic disorders
• Other traits show continuous variation– Height– Weight– Eye color
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Describing Continuous Variation
Range of values for the traitNu
mb
er o
f in
div
idu
als
wit
hso
me
valu
e o
f th
e t
rait
(line of bell-shaped curve indicates continuous variation in population)
Range of values for the traitN
um
ber
of
ind
ivid
ual
s w
ith
som
e va
lue
of
the
tra
it
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Temperature Effects on Phenotype
• Himalayan rabbits are Homozygous for an allele that specifies a heat-sensitive version of an enzyme in melanin-producing pathway
• Melanin is produced in cooler areas of body
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Environmental Effects on Yarrow Plants