Transcript Chapter 9

Chapter 9
Student 2015
Part 3
9.8 CONNECTION: Genetic traits in humans can be
tracked through family pedigrees
 Mendel’s laws apply to inheritance of many human
traits.
 Wild-type traits, those prevailing in nature, are not
necessarily specified by dominant alleles.
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Figure 9.8A
Dominant Traits
Recessive Traits
Freckles
No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
9.8 CONNECTION: Genetic traits in humans can be
tracked through family pedigrees
 A pedigree
• shows the inheritance of a trait in a family through multiple
generations,
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Figure 9.8B
First generation
(grandparents)
Second generation
(parents, aunts,
FF
and uncles)
or
Ff
Third generation
(two sisters)
Female
Male
Attached
Free
Ff
ff
Ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
9.9 CONNECTION: Many inherited disorders in humans
are controlled by a single gene
 Inherited human disorders can show either
1. Autosomal recessive disorder
– heterozygous parents are carriers of the disease-causing allele,
and
» Ex. Cystic fibrosis
– the probability of inheritance increases with inbreeding
2. Autosomal dominant disorders
– one dominant allele is needed to show disease and
» Achondroplasia
» Huntington’s disease
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VARIATIONS ON MENDEL’S
LAWS
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9.11 Incomplete dominance results in intermediate
phenotypes
 Mendel’s pea crosses always looked like one of the
parental varieties, called complete dominance.
 incomplete dominance: the appearance of F1 hybrids
falls between the phenotypes of the two parental
varieties.
• neither allele is dominant over the other and
• expression of both alleles occurs.
• Example: cross a red flower with a white flower
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Figure 9.11A
P generation

Red
RR
White
rr
Gametes R
r
F1 generation
Pink hybrid
Rr
Gametes
1
2 R
1
2 r
Sperm
1
1
R
2
2 r
F2 generation
1 R
2
RR
rR
1 r
2
Rr
rr
Eggs
Codominant alleles
• Both alleles are
expressed, see both
characteristics.
• Erminette chickens
• What did this chickens
parents look like? What
were their genotypes?
9.12 Many genes have more than two alleles in the
population (multiple alleles)
 Human ABO blood group phenotypes involve three
alleles for a single gene.
 The four human blood groups, A, B, AB, and O, result
from combinations of these three alleles.
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9.13 A single gene may affect many phenotypic
characters
 Pleiotropy occurs when one gene influences many
characteristics.
 Sickle-cell disease is a human example of pleiotropy.
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Figure 9.13B
An individual homozygous for the sickle-cell allele
Produces sickle-cell (abnormal) hemoglobin
The abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickled cell
The multiple effects of sickled cells
Damage to organs
Other effects
Kidney failure
Heart failure
Spleen damage
Brain damage (impaired
mental function,
paralysis)
Pain and fever
Joint problems
Physical weakness
Anemia
Pneumonia and other
infections
9.14 A single character may be influenced by many
genes
 Polygenic inheritance: a single phenotypic character
results from the additive effects of two or more genes.
 Human skin color
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Figure 9.14
P generation

aabbcc
AABBCC
(very light) (very dark)
F1 generation

AaBbCc AaBbCc
Sperm
1
8
F2 generation
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
Fraction of population
Eggs
1
8
1
8
1
8
1
8
1
8
1
64
6
64
15
64
20
64
15
64
6
64
1
64
Skin color
9.15 The environment affects many characters
 Many characters result from a combination of heredity
and the environment. For example,
• skin color /sunlight,
• susceptibility to diseases
• Intelligence/ experieces
 Only genetic influences are inherited.
 ABO blood type vs blood count
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THE CHROMOSOMAL BASIS OF
INHERITANCE
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9.16 Chromosome behavior accounts for Mendel’s laws
 The chromosome theory of inheritance states that
• genes occupy specific loci (positions) on chromosomes and
• chromosomes undergo segregation and independent
assortment during meiosis.
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9.16 Chromosome behavior accounts for Mendel’s laws
 Mendel’s laws correlate with chromosome separation in
meiosis.
• The law of segregation depends on separation of
homologous chromosomes in anaphase I.
• The law of independent assortment depends on alternative
orientations of chromosomes in metaphase I.
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9.17 SCIENTIFIC DISCOVERY: Genes on the same
chromosome tend to be inherited together
 Bateson and Punnett studied plants that did not show a
9:3:3:1 ratio in the F2 generation. What they found was an
example of linked genes, which
• are located close together on the same chromosome and
• tend to be inherited together.
 Linked alleles can be separated by crossing over, forming
recombinant gametes.
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Figure 9.17
The Experiment
Purple flower

PpLl
Phenotypes
PpLl
Long pollen
Observed
offspring
284
21
21
55
Purple long
Purple round
Red long
Red round
Prediction
(9:3:3:1)
215
71
71
24
The Explanation: Linked Genes
PL
Parental
diploid cell
PpLl
pl
Meiosis
Most
gametes
pl
PL
Fertilization
Sperm
PL
Most
offspring
pl
PL
PL
PL
pl
pl
pl
PL
pl
PL
Eggs
pl
3 purple long : 1 red round
Not accounted for: purple round and red long