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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
• Strong background in plant breeding and
mathematics
• Using pea plants, found indirect but
observable evidence of how parents
transmit genes to offspring
The Garden Pea Plant
• Self-pollinating
• True breeding (different alleles
not normally introduced)
• Can be experimentally crosspollinated
Genetic Terms
A pair of homologous
chromosomes
A gene locus
A pair of alleles
Three pairs of genes
Genes
• Units of information about specific traits
• Passed from parents to offspring
• Each has a specific location (locus) on a
chromosome - akin to a postal address!
Alleles
• Different molecular forms of a gene (green
eye vs. brown eyes)
• Arise by mutation
• Dominant allele masks a recessive allele
that is paired with it (brown eye allele and
green eye allele = brown eye are
dominant)
Allele Combinations
• Homozygous
– having two identical alleles at a locus
– AA or aa (Brown OR Green eyes)
• Heterozygous
– having two different alleles at a locus
– Aa (Brown allele AND Green allele)
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
Tracking Generations
• Parental generation
mates to produce
P
• First-generation offspring
mate to produce
F1
• Second-generation offspring
F2
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)
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Monohybrid Crosses
Homozygous
dominant parent
Homozygous
recessive parent
(chromosomes
duplicated
before meiosis)
meiosis
I
meiosis
II
(gametes)
(gametes)
fertilization
produces
heterozygous
offspring
Mendel’s Monohybrid Cross Results
F2 plants
showed
dominant-torecessive ratio
that averaged
3:1
Probability
The chance that each outcome of a
given event will occur is proportional to
the number of ways that event can be
reached
Roll a die
(how many sides = 6)
(probability of rolling a 3 = 1/6)
Punnett Square of a
Monohybrid Cross
Female gametes
A
Male
gametes
A
AA
a
Aa
a
Aa
aa
Dominant
phenotype
can arise 3
ways,
recessive
only 1
F1 Results of One
Monohybrid Cross
F2 Results of
Monohybrid Cross
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
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
Dihybrid Cross
Experimental cross between individuals
that are homozygous for different
versions of two traits
A Dihybrid Cross - F1 Results
AB
1
AABB
homozygous
dominant parent
plant (purple
flowers, tall stem)
X
ab
2
aabb
homozygous
recessive parent
plant (white flowers,
short stem)
3 F1 OUTCOME
All F1 plants are AaBb
heterozygotes (purple
flowers, tall stems)
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)
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)
Explanation of Mendel’s
Dihybrid Results
If the two traits
are coded for
by genes
on separate
chromosomes,
sixteen gamete
combinations
are possible
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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
Independent Assortment
Nucleus of a
diploid (2n)
reproductive cell
with two pairs of
homologous
chromosomes
Possible alignments
of the two homologous
chromosomes during
metaphase I of meiosis
The resulting
alignments
at metaphase II
Allelic
combinations
possible in
gametes
1/4 AB
1/4 ab
1/4 Ab
1/4 aB
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)
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
Dominance Relations
• Complete dominance
• Incomplete dominance
– Heterozygote phenotype is somewhere
between that of two homozyotes (evening
primrose - red, white, and pink)
• Codominance
– Non-identical alleles specify two
phenotypes that are both expressed in
heterozygotes (Blood groups of humans)
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
ABO Blood Type:
Allele Combinations
• Type A - IAIA or IAi
• Type B - IBIB or IBi
• Type AB - IAIB
• Type O - ii
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
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
Flower Color in Snapdragons:
Incomplete Dominance
Red-flowered plant X White-flowered plant
(homozygote)
(homozygote)
Pink-flowered F1 plants
(heterozygotes)
Flower Color in Snapdragons:
Incomplete Dominance
Pink-flowered plant X Pink-flowered plant
(heterozygote)
(heterozygote)
White-, pink-, and red-flowered plants
in a 1:2:1 ratio
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
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
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
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
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
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
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
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 melaninsynthesizing pathway
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
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
Number of individuals with
some value of the trait
(line of bell-shaped curve indicates
continuous variation in population)
Number of individuals with
some value of the trait
Describing Continuous
Variation
Range of values for the trait
Range of values for the trait
Temperature Effects
on Phenotype
• Himalayan rabbits are
Homozygous for an allele
that specifies a heatsensitive version of an
enzyme in melaninproducing pathway
• Melanin is produced in
cooler areas of body
Environmental Effects
on Yarrow Plants