Observable Patterns of Inheritance
Chapter 14
Earlobe Variation
• Whether a person is born with attached or detached earlobes depends on a single gene
• Gene has two molecular forms (alleles)
Earlobe Variation
• You inherited one allele for this gene from each parent
• Dominant allele specifies detached earlobes
• Recessive allele specifies attached lobes
Dominant & Recessive Alleles
• If you have attached earlobes, you inherited two copies of the recessive allele
• If you have detached earlobes, you may have either one or two copies of the dominant allele
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 cross-pollinated
Genes
• Units of information about specific traits
• Passed from parents to offspring
• Each has a specific location (locus) on a chromosome
Alleles
• Different molecular forms of a gene
• Arise by mutation
• Dominant allele masks a recessive
allele that is paired with it
Allele Combinations
• Homozygous Chromosomes– having two identical alleles at a locus– AA or aa
• Heterozygous Chromosomes– having two different alleles at a locus– Aa
Genetic TermsA pair of homologous chromosomes
A gene locus
A pair of alleles
Three pairs of genes
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 P
mates to produce
• First-generation offspring F1
mate to produce
• Second-generation offspring F2
F1 Results of One Monohybrid Cross
F2 Results of Monohybrid Cross
Mendel’s Monohybrid Cross Results
787 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
F2 plants showed dominant-to-recessive ratio that averaged 3:1
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
Probability
The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached
Punnett Square of a Monohybrid Cross
Female gametes
Male gametes
A a
A
a Aa
AA Aa
aa
Dominant phenotype canarise 3 ways,recessive only one
Test Cross
• Individual that shows dominant (heterozygous or Homozygous dominant) phenotype is crossed with individual with recessive phenotype
• Examining offspring allows you to determine the genotype of the dominant individual
Punnett Squares of Test Crosses
Homozygous recessive
a a
A
a aa
Aa Aa
aa
Homozygous recessive
a a
A
A Aa
Aa Aa
Aa
Two phenotypes All dominant phenotype
Dihybrid Cross
Experimental cross between individuals that are homozygous for different
versions of two traits
A Dihybrid Cross - F1 Results
AABB aabbx
AaBb
AB AB ab ab
TRUE-BREEDING PARENTS:
GAMETES:
F1 HYBRID OFFSPRING:
purple flowers, tall
white flowers,dwarf
All purple-flowered, tall
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
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
Explanation of Mendel’s Dihybrid Results
If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible
aB
AB
AB
abAb
Ab
aB
ab
1/4
1/4
1/4
1/4
1/4 1/4 1/4 1/4
AaBb aabbAabb aaBb
AABB AABb AaBB AaBb
AABb AAbb AaBb Aabb
AaBb aaBB aaBbAaBB
1/161/161/161/16
1/161/161/161/16
1/161/161/161/16
1/16 1/16 1/16 1/16
16 Allele Combinations in F2
aB
AB
AB
abAb
Ab
aB
ab
1/4
1/4
1/4
1/4
1/4 1/4 1/4 1/4
AaBb aabbAabb aaBb
AABB AABb AaBB AaBb
AABb AAbb AaBb Aabb
AaBb aaBB aaBbAaBB
1/161/161/161/16
1/161/161/161/16
1/161/161/161/16
1/16 1/16 1/16 1/16
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
Metaphase I
Metaphase II:
Gametes:
1/4 AB 1/4 ab 1/4 Ab 1/4 aB
A A A A
A A A A
AAAA
B B
B B
BB
B B
BBBB
a a a a
aa aa
aaaa
bb b b
bb b b
b b b b
OR
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
• Codominance– Non-identical alleles specify two
phenotypes that are both expressed in heterozygotes
Flower Color in Snapdragons: Incomplete Dominance
Red-flowered plant X White-flowered plant
Pink-flowered F1 plants
(homozygote) (homozygote)
(heterozygotes)
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)
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
Genetics of ABO Blood Types: Three Alleles: Codominance
• 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
Pleitropy
• 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 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
Pleiotrophic 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
Epistasis
• Interaction between the products of gene pairs
• Common among genes for hair color in mammals
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 melanin-synthesizing pathway
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
Campodactyly: Unexpected Phenotypes
• Effect of allele varies:
– Bent fingers on both hands
– Bent fingers on one hand
– No effect
• Many factors affect gene expression
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
Describing Continuous Variation
Range of values for the trait
Nu
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 trait
Nu
mb
er o
f in
div
idu
als
wit
hso
me
valu
e o
f th
e t
rait
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
Environmental Effects on Plant Phenotype
• Hydrangea macrophylla
• Action of gene responsible for floral color is influenced by soil acidity
• Flower color ranges from pink to blue
Web Sites
• BSCI 124 Lecture Notes -- Mendelian Genetics
• Biology Project - site map
• Genome Project
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