Transcript You Light Up My Life

Observable Patterns
of Inheritance
Chapter 20
Earlobe Variations
• 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
Gregor Mendel
• Strong background in plant breeding
and mathematics
• Using pea plants, found indirect but
observable evidence of how parents
transmit genes to offspring
Alleles
• Different molecular forms of a gene
• Arise by mutation
• Dominant allele masks a recessive
allele that is paired with it
Allele Combinations
• Homozygous
– having two identical alleles at a locus
– AA or aa
• Heterozygous
– having two different alleles at a locus
– Aa
Genetic Terms
A 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
mates to produce
P
• First-generation offspring
mate to produce
F1
• Second-generation offspring
F2
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
Segregation
Parents:
CC
cc
(meiosis)
Gametes:
C
(meiosis)
C
c
c
AA parent produces only A gametes;
aa parent produces only a gametes
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
C
Male
gametes
c
C
CC
Cc
c
Cc
cc
Dominant
phenotype can
arise three ways,
recessive only
one
Test Cross
• Individual that shows dominant
phenotype is crossed with individual
with recessive phenotype
• Examining offspring enables you to
determine the genotype of the dominant
individual
Punnett Squares of
Test Crosses
c
c
C
Cc
Cc
c
cc
cc
Two phenotypes
c
c
C
Cc
Cc
C
Cc
Cc
All dominant phenotype
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:
A
A a
a
B
B b
b
OR
A
A a
a
b
b B
B
Metaphase II:
Gametes:
A
A
a
a
A
A
a
a
B
B
b
b
b
b
B
B
B
A
B
A
1/4 AB
b
a
b
a
1/4 ab
b
A
b
A
1/4 Ab
B
a
B
a
1/4 aB
Dihybrid Cross
Experimental cross between individuals
that are homozygous for different
versions of two traits
Dihybrid Cross - F1 Results
Parents:
Gametes:
F1 offspring:
ccdd
(smooth chin,
no dimples)
cd
cd
CCDD
(chin fissure,
dimples)
CD
CcDd
CD
Allele Combinations in F2
CcDd
1/4 CD 1/4 Cd 1/4 cD 1/4 cd
1/4
CD
1/16
1/16
1/16
1/16
CCDD CCDd CcDD CcDd
1/4
Cd
1/16
1/16
1/16
CCDd CCdd CcDd
1/16
Ccdd
1/4
cD
1/16
1/16
CcDD CcDd
1/16
ccDD
1/16
ccDd
1/4
cd
1/16
CcDd
1/16
ccDd
1/16
ccdd
CcDd
1/16
Ccdd
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
Sickle-Cell Anemia
• 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
Campodactyly:
Unexpected Phenotypes
• Effect of allele varies:
– Bent fingers on both hands
– Bent fingers on one hand
– No effect
• Many factors affect gene expression
Polygenic Traits
• Result from the combined expression of
several genes
• Skin color, eye color
• Population may show continuous
variation
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