Observing Patterns in Inherited Traits

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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 Terms
A 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 individuals 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
Homozygous
Homozygous
dominant parent
recessive parent
(chromosomes
duplicated
before meiosis)
meiosis
I
meiosis
II
(gametes)
(gametes)
fertilization
produces
heterozygous
offspring
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Mendels Monohybrid Cross Results
F2 plants showed dominant-to-recessive ratio that
averaged 31
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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
Dominant phenotype can arise 3 ways, recessive
only 1
Male gametes
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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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Mendels 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
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F1 Results of Mendels 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
AaBb X
AaBb

• Four Phenotypes
• Tall, purple-flowered (9/16)
• Tall, white-flowered (3/16)
• Dwarf, purple-flowered (3/16)
• Dwarf, white-flowered (1/16)

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Explanation of Mendels 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 Assortment
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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 Mendels 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 TypeAllele 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

• Recipients 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 121 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 cant 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 alleles
• 1) HbA
• Encodes normal beta hemoglobin chain
• 2) 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
(line of bell-shaped curve indicates continuous
variation in population)
Number of individuals with some value of the trait
Number of individuals with some value of the trait
Range of values for the trait
Range of values for the trait
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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