Fig. 12.2
Chapter 12: Patterns of Inheritance
Chapter 13: Chromosomes, Mapping,
and the Meiosis-Inheritance
Connection
Patterns of Inheritance
Chapter 12
Early Ideas of Heredity
• Before the 20th century, 2 concepts were
the basis for ideas about heredity:
- heredity occurs within species
- traits are transmitted directly from parent to
offspring
• Led to the belief that inheritance is a
matter of blending traits from the parents.
3
Early Ideas of Heredity
• Botanists in the 18th and 19th centuries
produced hybrid plants.
• When the hybrids were crossed with each
other, some of the offspring resembled the
original strains, rather than the hybrid
strains.
• This evidence contradicted the idea that
traits are directly passed from parent to
offspring.
4
Fig. 12.3-3
Early Ideas of Heredity
• Gregor Mendel
• Studied the pea plant:
1. other research showed that pea hybrids
could be produced
2. many pea varieties were available
3. peas are small plants and easy to grow
4. peas can self-fertilize or be cross-fertilized
6
Early Ideas of Heredity
• Gregor Mendel’s experimental method:
- Produce true-breeding strains for each trait
he was studying
- Cross-fertilize true-breeding strains having
alternate forms of a trait
– perform reciprocal crosses as well
- Allow the hybrid offspring to self-fertilize and
count the number of offspring showing each
form of the trait
7
8
Fig. 12.3-1
Fig. 12.3-2
Monohybrid Crosses
• Monohybrid cross: a cross to study only
2 variations of a single trait
• Mendel produced true-breeding pea strains
for 7 different traits
- each trait had 2 alternate forms (variations)
- flower color, pea color, pea shape...
• Mendel cross-fertilized the 2 true-breeding
strains for each trait
11
Monohybrid Crosses
• F1 generation (1st filial generation):
- offspring produced by crossing 2 truebreeding strains
• For every trait Mendel studied, all F1 plants
resembled only 1 parent
• No plants with characteristics intermediate
between the 2 parents were produced
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13
Monohybrid Crosses
• F2 generation (2nd filial generation): the
offspring resulting from the self-fertilization
of F1 plants
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16
Monohybrid Crosses
• F2 generation (2nd filial generation): the
offspring resulting from the self-fertilization
of F1 plants
- Dominant traits: the form of each trait most
commonly expressed in the F1 plants
- Recessive traits: the form of the trait not
seen in the F1 plants; hidden traits
17
Monohybrid Crosses
• F2 plants exhibited both forms of the trait in
a very specific pattern:
- ¾ plants with the dominant form
- ¼ plant with the recessive form
• The dominant to recessive ratio was 3 : 1.
• Mendel discovered the ratio is actually:
- 1 true-breeding dominant plant
- 2 not-true-breeding dominant plants
- 1 true-breeding recessive plant
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20
Monohybrid Crosses
• Gene: information for a trait passed from
parent to offspring
• Alleles: alternate forms of a gene
• Homozygous: having 2 of the same allele
• Heterozygous: having 2 different alleles
21
Monohybrid Crosses
• Genotype: total set of alleles of an
individual
- PP = homozygous dominant
- Pp = heterozygous
- pp = homozygous recessive
• Phenotype: outward appearance of an
individual
- physical manifestation of genotype
22
Monohybrid Crosses
• Principle of Segregation:
• Two alleles for a gene segregate during
gamete formation
• homologous chromosome separation during
meiosis
• Alleles are re-paired at random, one from
each parent, during fertilization.
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25
Monohybrid Crosses
• Some human traits are controlled by a
single gene.
- some of these exhibit dominant inheritance
- some of these exhibit recessive inheritance
• Carriers are heterozygous
• Pedigree analysis is used to track
inheritance patterns in families.
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28
Dihybrid Crosses
• Dihybrid cross: examination of 2 separate
traits in a single cross
- for example: RR YY
x rryy
• The F1 generation of a dihybrid cross
(RrYy) shows only the dominant phenotypes
for each trait.
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30
Dihybrid Crosses
• The F2 generation is produced by crossing
members of the F1 generation with each
other or allowing self-fertilization of the F1.
- for example
RrYy x RrYy
• The F2 generation shows all four possible
phenotypes in a set ratio:
9:3:3:1
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32
Dihybrid Crosses
• Principle of Independent Assortment:
• In a dihybrid cross, the alleles of each
gene assort independently.
33
Probability – Predicting Results
• Rule of addition: the probability of 2
mutually exclusive events occurring
simultaneously is the sum of their individual
probabilities.
• When crossing Pp x Pp, the probability of
producing Pp offspring is:
• probability of obtaining Pp (1/4), PLUS
probability of obtaining pP (1/4)
¼ + ¼ = ½
34
Probability – Predicting Results
• Rule of multiplication: the probability of 2
independent events occurring simultaneously
is the PRODUCT of their individual
probabilities.
• When crossing Rr Yy x RrYy, the
probability of obtaining rr yy offspring is:
- probability of obtaiing rr = ¼
- probability of obtaining yy = ¼
probability of rr yy = ¼ x ¼ = 1/16
35
Testcross
• Testcross: a cross used to determine the
genotype of an individual with dominant
phenotype
- cross the individual with unknown genotype
(e.g. P_) with a homozygous recessive (pp)
- the phenotypic ratios among offspring are
different, depending on the genotype of the
unknown parent
36
37
Extensions to Mendel
• Mendel’s model of inheritance assumes:
- each trait is controlled by a single gene
- each gene has only 2 alleles
- there is a clear dominant-recessive
relationship between the alleles
• Most genes do not meet these criteria.
• Other types of inheritance:
• Polygenic, Pleiotropy, Incomplete dominance,
codominance
38
Extensions to Mendel
• Polygenic inheritance occurs when
multiple genes are involved in controlling the
phenotype of a trait.
• The phenotype is an accumulation of
contributions by multiple genes.
• These traits show continuous variation
and are referred to as quantitative traits.
- For example – human height
39
40
Extensions to Mendel
• Pleiotropy refers to an allele which has
more than one effect on the phenotype.
• This can be seen in human diseases such
as cystic fibrosis or sickle cell anemia.
• In these diseases, multiple symptoms can
be traced back to one defective allele.
41
Extensions to Mendel
• Incomplete dominance: the heterozygote
is intermediate in phenotype between the 2
homozygotes.
• Codominance: the heterozygote shows
some aspect of the phenotypes of both
homozygotes.
42
43
Extensions to Mendel
The human ABO blood group system:
-multiple alleles: there are 3 alleles of the I
gene (IA, IB, and i)
-codominance: IA and IB are dominant to i
but codominant to each other
44
45
Extensions to Mendel
• The expression of some genes can be
influenced by the environment.
- for example: coat color in Himalayan rabbits
and Siamese cats
- an allele produces an enzyme that allows
pigment production only at temperatures below
30oC
46
Extensions to Mendel
47
Extensions to Mendel
• The products of some genes interact with
each other and influence the phenotype of
the individual.
• Epistasis: one gene can interfere with the
expression of another gene
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49
Chromosomes, Mapping, and the
Meiosis-Inheritance Connection
Chapter 13
Chromosome Theory
• Chromosomal theory of inheritance
- developed in 1902 by Walter Sutton
- proposed that genes are present on
chromosomes
- based on observations that homologous
chromosomes pair with each other during
meiosis
- supporting evidence was provided by work
with fruit flies
51
Chromosome Theory
• T.H. Morgan isolated a mutant white-eyed
Drosophila
• Red-eyed female X white-eyed male gave
a F1 generation of all red eye
• Morgan concluded that red eyes are
dominant
52
Fig. 13.1
Chromosome Theory
• Morgan crossed F1 females X F1 males
• F2 generation contained red and white-
eyed flies but all white-eyed flies were male
• Testcross of a F1 female with a white-eyed
male showed the viability of white-eyed
females
• Morgan concluded that the eye color gene
is linked to the X chromosome
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56
Sex Chromosomes
• Sex determination in Drosophila is based
on the number of X chromosomes
- 2 X chromosomes = female
- 1 X and 1 Y chromosome = male
• Sex determination in humans is based on
the presence of a Y chromosome
- 2 X chromosomes = female
- having a Y chromosome (XY) = male
57
Sex Chromosomes
• Sex-linked traits: traits controlled by
genes present on the X chromosome
• Sex-linked traits show inheritance patterns
different than those of genes on
autosomes.
- In many organisms, the Y chromosome is
greatly reduced or inactive.
- Genes on the X chromosome are present in
only 1 copy in males
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59
Sex Chromosomes
• Dosage compensation ensures an equal
expression of genes from the sex
chromosomes even though females have 2
X chromosomes and males have only 1.
- In each female cell, 1 X chromosome is
inactivated and is highly condensed into a Barr
body.
• Females heterozygous for genes on the X
chromosome are genetic mosaics.
- Phenotype depends on which X chromosome
inactivated
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61
Chromosome Theory Exceptions
• Mitochondria and chloroplasts contain
genes.
- Traits controlled by these genes do not follow
the chromosomal theory of inheritance
- Genes from mitochondria and chloroplasts are
often passed to the offspring by only one parent
62
Chromosome Theory Exceptions
• Maternal inheritance: uniparental (oneparent) inheritance from the mother
- The mitochondria in a zygote are from the egg
cell; no mitochondria come from the sperm
during fertilization
- In plants, the chloroplasts are often inherited
from the mother, although this is species
dependent
63
Genetic Mapping
• The science of determining the location of
a gene on a chromosome
- Early geneticists realized that they could
obtain information about the distance between
genes on a chromosome.
• This is genetic mapping
• Mapping is based on genetic
recombination (crossing over) between
genes.
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66
Genetic Mapping
• To determine the distance between genes:
- dihybrid organisms are testcrossed
- offspring resembling the dihybrid parent result
from homologues that were not involved in the
crossover
- offspring resulting from a crossover are called
recombinant progeny
67
Genetic Mapping
The distance between genes is proportional to
the frequency of recombination events.
recombination
frequency
recombinant progeny
=
total progeny
1% recombination = 1 map unit (m.u.)
1 map unit = 1 centimorgan (cM)
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Genetic Mapping
• Determining the order of genes can be
done with a three-point testcross
• The frequency of double crossovers is the
product of the probabilities of each individual
crossover
• Therefore, the classes of offspring with the
lowest numbers represent the double
crossovers and allow the gene order to be
determined
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Human Genetic Disorders
• Some human genetic disorders are caused
by altered proteins:
- the altered protein is encoded by a mutated
DNA sequence
- the altered protein does not function correctly,
causing a change to the phenotype
- the protein can be altered at only a single
amino acid (e.g. sickle cell anemia)
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Fig. 13.11
Human Genetic Disorders
• Some genetic disorders are caused by a
change in the number of chromosomes.
- Nondisjunction during meiosis can create
gametes having one too many or one too few
chromosomes
- Fertilization of these gametes creates
trisomic or monosomic individuals
- Down syndrome is trisomy of chromosome 21
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Human Genetic Disorders
• Nondisjunction of sex chromosomes can
result in:
- XXX triple-X females
- XXY males (Klinefelter syndrome)
- XO females (Turner syndrome)
- OY nonviable zygotes
- XYY males (Jacob syndrome)
81
Sex Determination in Humans
• The sex chromosomes, X and Y, are a
homologous pair:
– this pair is unique because X and Y carry
different sets of genes
– the Y chromosome has genes that determine
maleness
– the X chromosome has a variety of genes on
it
• XX = female; XY = male
• X_ = female; _Y = does not survive
82
Sex Determination in Humans
• Genotype: X__
• Sex: Female
• Phenotype: Turner’s Syndrome
– short stature (less than 5’)
– ‘webbed’ neck and other physical
characteristics
– infertility
83
Sex Determination in Humans
• Genotype: XXX
• Sex: Female
• Phenotype: ‘Super-females’, metafemales
– tall stature
– longer legs and torso
– may have learning disabilities or emotionally
underdeveloped
– commonly labeled as ‘trouble makers’ in
school
84
Sex Determination in Humans
• Genotype: XYY
• Sex: Male
• Phenotype: ‘Super-males’
– produce higher levels of testosterone
– may be taller than average
– no known significant abnormalities
85
Sex Determination in Humans
• Genotype: XXY or XXXY
• Sex: Male
• Phenotype: Klinefelter Syndrome
– produce very little testosterone
– taller and more overweight than average
– may have feminine characteristics
– sterile or nearly sterile
– most have normal cognitive abilities
– can be treated with testosterone early in life
86
Table 13.2-1
Table 13.2-2
89
Human Genetic Disorders
• Genomic imprinting occurs when the
phenotype exhibited by a particular allele
depends on which parent contributed the
allele to the offspring
• A specific partial deletion of chromosome 15
results in:
- Prader-Willi syndrome if the chromosome is
from the father
- Angelman syndrome if it’s from the mother
90
Human Genetic Disorders
• Genetic counseling can use pedigree
analysis to determine the probability of
genetic disorders in the offspring.
• Some genetic disorders can be diagnosed
during pregnancy:
- amniocentesis collects fetal cells from the
amniotic fluid for examination
- chorionic villi sampling collects cells from
the placenta for examination
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