Post on 29-Aug-2018
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Patterns of Inheritance/Mendelian
GeneticsChapter 9, 12
Student Learning Goals & Achievement Scale – Biology
Mendel’s Laws, Genetics, and Patterns of Inheritance
SC.912.L.16.1
• Goals: Use Mendel’s Laws of Segregation and Independent
Assortment to analyze patterns of inheritance.
• 4 - Explore Mendel’s Laws of Segregation and Independent
Assortment to analyze patterns of inheritance.
• 3 - Use Mendel’s Laws of Segregation and Independent
Assortment to analyze patterns of inheritance.
• 2 - Summarize Mendel’s Laws of Segregation and Independent
Assortment to analyze patterns of inheritance.
• 1 – Define Mendel’s Laws of Segregation and Independent
Assortment to analyze patterns of inheritance.
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Learning Objectives1. Describe how Mendel was able to control how
his pea plants were pollinated.
2. Describe the steps in Mendel’s experiments on
true-breeding garden peas.
3. Distinguish between dominant and recessive
traits.
4. State two laws of heredity that were developed
from Mendel’s work
5. Describe how Mendel’s results can be
explained by scientific knowledge of genes and
chromosomes3
Learning Objectives
6. Differentiate between the genotype and phenotype of an organism.
7. Explain how probability is used to predict the results of genetic crosses.
8. Use a Punnett square to predict the results of a monohybrid and dihybrid
genetic crosses.
9. Explain how a testcross is used to show the genotype of an individual
whose phenotype expresses the dominant trait.
10. Differentiate a monohybrid cross from a dihybrid cross.
11. Distinguish between sex chromosomes and autosomes.
12. Explain the role of sex chromosomes in sex determination.
13. Describe how an X or Y-linked gene affects the inheritance of traits.
14. Explain the effect of crossing-over on the inheritance of genes in linkage
groups.
15. Distinguish between chromosome mutations and gene mutations.
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Learning Objectives
16. Analyze pedigrees to determine how
genetic traits and genetic disorders are
inherited.
17.Explain the inheritance of the ABO blood
type
18.Explain how geneticists can detect and
treat genetic disorders.
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Schedule and Announcements
• Quiz Thursday December 3
• Exam 3- Tuesday December 8 over 9, 12
• Semester Exam Tuesday December 15 @
2:15 (cumulative)
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Early Ideas of Heredity
• Before the 20th century, 2 concepts were
the basis for ideas about heredity:
1.) heredity occurs within species
2.) traits are transmitted directly from parent to
offspring (The homunculus myth)
• This led to the belief that inheritance is a
matter of blending traits from the parents.
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Gregor Mendel
• Mendel song
(http://www.youtube.com/watch?v=
2xpTz7SUbnc)
• Born in 1822
• Education: University of Vienna
– Failed exit examinations
• Returned to monastery
• Mendel published his work in 1865.
• That work was lost until ca. 1900.
• With the “rediscovery” of Mendel’s
conceptual work the hunt was on for
the physical nature of the gene.
• What was it and how did it function?
• These questions were largely
answered from 1940’s through the
1960’s and lead to the biotech
revolution beginning of the 1970’s.
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The Garden Pea
• Pisum sativum
• Easy to grow
• Produces many varieties
• Male and female organs
in the same flower
– Self-fertilization
– Cross-fertilization
• What if Mendel choose to
work with sheep instead?
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Early Ideas of Heredity
• Mendel’s experimental method:
1. produce true-breeding strains for each trait he was studying
2. cross-fertilize true-breeding strains having alternate forms of a trait
-perform reciprocal crosses as well
3. allow the hybrid offspring to self-fertilize and count the number of offspring showing each form of the trait
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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)
– Mendel cross-fertilized the 2 true-breeding
strains for each trait
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Monohybrid Crosses
• F1 generation (1st filial generation):
offspring produced by crossing 2 true-
breeding 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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Monohybrid Crosses
• F1 generation: offspring resulting from a cross of true-breeding parents
• F2 generation: offspring resulting from the self-fertilization of F1 plants
• dominant: the form of each trait expressed in the F1 plants
• recessive: the form of the trait not seen in the F1 plants
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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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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
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Monohybrid Crosses
• genotype: total set of alleles of an
individual
– PP = homozygous dominant
– Pp = heterozygous
– pp = homozygous recessive
• phenotype: outward appearance of an
individual
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Monohybrid Crosses
• Principle of Segregation
Two alleles for a gene segregate during
gamete formation and are rejoined at
random, one from each parent, during
fertilization.
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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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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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Dihybrid Crosses
• Principle of Independent Assortment
In a dihybrid cross, the alleles of each gene
assort independently.
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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
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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
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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
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Extensions to Mendel
• Mendel’s model of inheritance assumes
that:
– 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.
Degrees of Dominance
• Complete dominance occurs when
phenotypes of the heterozygote and
dominant homozygote are identical
• In incomplete dominance, the phenotype of
F1 hybrids is somewhere between the
phenotypes of the two parental varieties
• In codominance, two dominant alleles affect
the phenotype in separate, distinguishable
ways
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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.
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• Tay-Sachs disease is fatal; a dysfunctional
enzyme causes an accumulation of lipids in
the brain
– At the organismal level, the allele is recessive
– At the biochemical level, the phenotype (i.e., the
enzyme activity level) is incompletely dominant
– At the molecular level, the alleles are codominant
Multiple Alleles
• Most genes exist in populations in more than
two allelic forms
• For example, the four phenotypes of the ABO
blood group in humans are determined by
three alleles of the gene: IA, IB, and i.
• The enzyme (I) adds specific carbohydrates to
the surface of blood cells
• The enzyme encoded by IA adds the A
carbohydrate, and the enzyme encoded by IB
adds the B carbohydrate; the enzyme encoded
by the i allele adds neither
Figure 11.11
Carbohydrate
(b) Blood group genotypes and phenotypes
Allele
Red blood cellappearance
Genotype
noneBA
IB
Phenotype(blood group)
iIA
IAIB iiIAIA or IAi IBIB or IBi
BA OAB
(a) The three alleles for the ABO blood groups and theircarbohydrates
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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 variationand are referred to as quantitative traits.
• For example – human height
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Epistasis
• In epistasis, a gene at one locus alters the
phenotypic expression of a gene at a second
locus
• For example, in Labrador retrievers and many
other mammals, coat color depends on two
genes
• One gene determines the pigment color (with
alleles B for black and b for brown)
• The other gene (with alleles C for color and c
for no color) determines whether the pigment
will be deposited in the hair
Figure 11.12
¼
¼
¼
¼
¼ ¼¼¼ BE Be
BE
be
BBEE
bbee
BbEE BbEe
bE be
bE
Be
BBEe
BbEE bbEE bbEeBbEe
BBEe BbEe BbeeBBee
BbEe bbEe Bbee
9 : 4: 3
Eggs
Sperm
BbEe BbEe
Polygenic Traits
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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.
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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
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Extensions to Mendel
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Figure 11.14
WW
or
Ww
ww
ww ww
ww ww
WwWw
Ww Ww
No widow’s peakWidow’s peak
wwWw
1st generation(grandparents)
3rd generation(two sisters)
2nd generation(parents,aunts, anduncles)
Affected male
Affected female
Male Female
Key
Mating
Attachedearlobe
Freeearlobe
Offspring, inbirth order(first-born on left)
FF
or
Ff
ff
ff ff
Ff ff
FfFF or
Ff
Ff Ff
ffFf
(a) Is a widow’s peak a dominant or recessive trait? (b) Is an attached earlobe a dominant
or recessive trait?
The Behavior of Recessive Alleles
• Recessively inherited disorders show up only
in individuals homozygous for the allele
• Carriers are heterozygous individuals who
carry the recessive allele but are
phenotypically normal
• Most people who have recessive disorders
are born to parents who are carriers of the
disorder
Figure 11.15
Parents
Sperm
NormalAa
NormalAa
EggsAA
Normal
AaNormal(carrier)
AaNormal(carrier)
aaAlbino
A
a
A a
Sickle-Cell Disease: A Genetic Disorder
with Evolutionary Implications
• Sickle-cell disease affects one out of 400
African-Americans
• The disease is caused by the substitution of a
single amino acid in the hemoglobin protein in
red blood cells
• In homozygous individuals, all hemoglobin is
abnormal (sickle-cell)
• Symptoms include physical weakness, pain,
organ damage, and even paralysis
• Heterozygotes (said to have sickle-cell trait)
are usually healthy but may suffer some
symptoms
• About one out of ten African-Americans has
sickle-cell trait, an unusually high frequency of
an allele with detrimental effects in
homozygotes
• Heterozygotes are less susceptible to the
malaria parasite, so there is an advantage to
being heterozygous
Figure 11.UN05
In the whole population,some genes have morethan two alleles
Pleiotropy
Relationship amongalleles of a single gene Description Example
Codominance
Multiple alleles
Incomplete dominance
of either allele
Complete dominance
of one allele
One gene is able to affectmultiple phenotypiccharacters
Both phenotypesexpressed inheterozygotes
Heterozygous phenotypeintermediate betweenthe two homozygousphenotypes
Heterozygous phenotypesame as that of homo-zygous dominant
ABO blood group alleles
Sickle-cell disease
IAIB
IA, IB, i
CRCR CRCW CWCW
PP Pp
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Figure 11.UN06
Polygenic inheritance A single phenotypiccharacter is affected bytwo or more genes
The phenotypicexpression of one geneaffects the expressionof another gene
Epistasis
Relationship amongtwo or more genes Description Example
AaBbCc AaBbCc
BbEe BbEe
BE
BE bE
bE
be
be
Be
Be
9 : 3 : 4
Chromosomes, Mapping, and the
Meiosis-Inheritance Connection
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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
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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 eyes
• Morgan concluded that red eyes are
dominant
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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
• Chromosomal basis
of sex linkage
– White-eyed male flies
X red-eyed females
– F1 flies all have red
eyes
– F2 flies, all of the
white-eyed flies are
males because the Y
chromosome lacks
the white gene
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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
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Sex Chromosomes• In many organisms, the Y chromosome is
greatly reduced or inactive.
• genes on the X chromosome are present in only 1 copy in males
• sex-linked traits: controlled by genes present on the X chromosome
• Human X-linked disorders
– Color blindness, Muscular dystrophy, Hemophilia, Fragile X syndrome
• Sex-linked traits show inheritance patterns different than those of genes on autosomes.
Royal Hemophilia Pedigree
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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.
Genetic basis behind a calico
cat
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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
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Chromosome Theory Exceptions
• Maternal inheritance: uniparental (one-
parent) 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
Human X Chromosome Gene
Map
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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)
Sickle-Cell Anemia
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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
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Down Syndrome
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Human Genetic Disorders• Nondisjunction of sex chromosomes can
result in:Syndrome Sex Disorder Chromosome
#
Spontaneous
abortions
Live
births
Turner F XO 45 1/18 1/ 2,500
Klinefelter M XXY OR
XXXY
47 or 48 1/300 1/800
Poly-X F XXX OR
XXXX
47 or 48 0 1/ 1,500
Jacobs M XYY 47 ? 1/1,000
Down M or F Trisomy 21 47 1/40 1/800
Abnormalities in the # of sex
chromosomes
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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
Amniocentesis
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Chorionic villi sampling
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