Download Genetics: Mendel and The Gene Idea

Mendel and the Gene Idea
Chapter 14
BCOR 012
January 20, 22, and 25, 2010
Mendel and the Gene Idea
Chapter 14
I. Mendel and his contribution to biology
A. A brief biography of Mendel
B. Why his work was revolutionary
C. Mendel’s organism
D. Brief review of meiosis
E. So what did Mendel do and why is it
important?
F. Mendel's First Law of Inheritance, the law
of segregation
G. Dihybrid Crosses and Independent Assortment
Gregor Mendel,
1822-1884
Garden pea
(Pisum sativum - Fabaceae)
Mendel and the Gene Idea
Chapter 14
I. Mendel and his contribution to biology
A. A brief biography of Mendel
B. Why his work was revolutionary
C. Mendel’s organism
D. Brief review of meiosis
E. So what did Mendel do and why is it
important?
F. Mendel's First Law of Inheritance, the law
of segregation
G. Dihybrid Crosses and Independent Assortment
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se pre a
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this or a
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ture
.
Each cell in the leaf of a pea plant has seven
pairs of chromosomes. How many
chromosomes in the egg cell?
A.
B.
C.
D.
E.
14
28
7
21
The question cannot be answered with the
information given
A short review of meiosis ...
Most organisms, including ourselves and garden peas, are
diploid. That means they have two sets of chromosomes per
cell.
One of these sets came from the mother, via the egg, and the
other came from the father, via the sperm.
Meiosis is also called
reductional division
because the number of
chromosomes in each
cell is reduced by half.
During meiosis, members
of the two sets separate
into separate cells.
Mendel’s Explanation of his Observations
(in present-day language):
1) Alternative versions of genes (that is to say, different
alleles) account for variation in inherited characters.
2) For each character, an organism inherits two alleles,
one from each parent.
3) If an organism is heterozygous at a particular gene
locus, the organism’s appearance (phenotype) will be
determined by the dominant allele.
4) The two alleles for each character segregate during
gamete production.
This fourth point is also called Mendel’s Law
of Segregation:
Alleles segregate into separate gametes
during meiosis.
Some useful Genetic Vocabulary:
• Homozygous: an individual having an
identical pair of alleles at a locus is
homozygous at that locus.
(example: PP )
• Heterozygous: an individual having
different alleles at a locus is heterozygous at
that locus
(example: Pp)
P - purple color
(dominant allele)
p - white color
(recessive allele)
Note that the probability
of a white-flowered plant
in the F2 generation is
0.25, or 25%
(This is a
Punnett square.)
When solving a problem involving a
cross in Mendelian genetics:
• First ask yourself: what kind of gamete (genotype)
can each parent produce?
• And in what proportion will each be produced?
• Then figure out all possible ways in which these
gametes can combine at syngamy
• Note that a Punnet square is one way to approach
the problem, but not necessarily the most efficient
way.
Here a Punnet square
has been used to
solve the problem
Another way to approach the problem:
• Half (0.5) of the heterozygous mother’s eggs carry the P allele and half
carry the p allele.
• Similarly, half of the heterozygous father’s sperm cells carry the P allele
and half carry the p allele.
• Thus the progeny will include 0.5 P x 0.5 P = 0.25 PP, (0.5 P x 0.5 p)+
(0.5 P x 0.5 p) = 0.5 Pp, and 0.5 p x 0.5 p, = 0.25 pp
More useful Genetic Vocabulary:
• Genotype: The specific genetic makeup of
an organism, as contrasted with the actual
characteristics of an organism (its
phenotype).
• Phenotype: The observable characteristics
of an organism, as opposed to the set of
genes it possesses (its genotype).
A testcross: a pea
plant showing the
dominant phenotype
- but whose genotype
is unknown - is bred
to a pea plant showing
the recessive phenotype.
The results permit the
unknown parent’s
genotype to be
determined.
Mendel and the Gene Idea
Chapter 14
I. Mendel and his contribution to biology
A. A brief biography of Mendel
B. Why his work was revolutionary
C. Mendel’s organism
D. Brief review of meiosis
E. So what did Mendel do and why is it
important?
F. Mendel's First Law of Inheritance, the law
of segregation
G. Dihybrid Crosses and Independent Assortment
Mendel’s Law of Independent Assortment:
Pairs of alleles segregate independently
during meiosis.
Mendel and the Gene Idea
Chapter 14
II. The Relationship Between Genotype and Phenotype
A. Incomplete Dominance
B. Codominance
C. Pleiotropy: Epistasis
D. Polygenic traits
E. Environmentally Induced Variation
III. Pedigree Analysis in Humans
A. Recessive alleles
B. Dominant alleles
IV. Tools for detection of genetic disorders
A. Pedigree analysis and risk assessment
B. Amniocentesis
C. Chorionic villus sampling
The Relationship Between
Genotype and Phenotype
The inheritance of
flower color in
snapdragons provides
a useful example of
incomplete dominance.
In incomplete dominance,
a heterozygous genotype
creates an intermediate
phenotype.
Human Blood Types
In the human population, there are three alleles
determining blood groups:
IA
IB
i
Of course, any given individual will possess only one
(if homozygous at the blood group locus) or two
(if heterozygous at that locus) of these. Your genotype
at the blood group locus determines your blood type.
Human Blood Types
Phenotype
Type A
Type B
Type AB
Type O
Genotype
IA IA or IA i
IB IB or IB i
IA IB
ii
Human blood group alleles IA and
IB demonstrate codominance.
In codominance, neither phenotype is
dominant. Instead, the individual expresses
both phenotypes.
Epistasis is the condition in which the genotype
at one locus affects the expression of the genotype
at another locus.
Locus C determines
whether pigment is
deposited or not
Locus B determines
coat color, but its
expression depends on the
genotype at the C locus.
Coat color in mice: an
example of epistatic
interaction between
two loci.
< water level
The submersed and emersed leaves of water
marigold (Megalodonta beckii, Asteraceae)
demonstrate an environmental contribution
to the phenotype.
Mendel and the Gene Idea
Chapter 14
II. The Relationship Between Genotype and Phenotype
A. Incomplete Dominance
B. Codominance
C. Pleiotropy: Epistasis
D. Polygenic traits
E. Environmentally Induced Variation
III. Pedigree Analysis in Humans
A. Recessive alleles
B. Dominant alleles
IV. Tools for detection of genetic disorders
A. Pedigree analysis and risk assessment
B. Amniocentesis
C. Chorionic villus sampling
Human Pedigree Analysis: the Conventions
Pedigree Analysis of a Benign Condition in Humans: the
Attached Earlobe
I
II
III
Q. Is the trait inherited as a dominant or a recessive condition?
A. Recessive. You can tell because the affected daughter in
generation III was born to unaffected parents.
For those traits exhibiting recessive gene
action:
• affected individuals may be born to
unaffected parents
• the trait is not manifested in every
generation
The pedigree of a family with cystic fibrosis, a recessively inherited
disorder.
The pedigree of a family with Huntington’s disease, a
dominantly inherited disorder
For those traits exhibiting dominant gene action:
• affected individuals have at least one affected parent
• the phenotype generally appears every generation
• two unaffected parents only have unaffected offspring
His son, the 60s legend Arlo
Guthrie, did not inherit the disease.
Woody Guthrie, American folk hero and composer of “This Land is
Your Land,” died of Huntington’s disease in 1967.
Mendel and the Gene Idea
Chapter 14
II. The Relationship Between Genotype and Phenotype
A. Incomplete Dominance
B. Codominance
C. Pleiotropy: Epistasis
D. Polygenic traits
E. Environmentally Induced Variation
III. Pedigree Analysis in Humans
A. Recessive alleles
B. Dominant alleles
IV. Tools for detection of genetic disorders
A. Pedigree analysis and risk assessment
B. Amniocentesis
C. Chorionic villus sampling
Know the Difference:
•
•
•
•
•
•
meiosis vs. mitosis
gene vs. allele
genotype vs. phenotype
homozygous vs. heterozygous
dominant vs. recessive
monohybrid vs. dihybrid
Solving Problems in Mendelian Genetics
A corn plant of genotype XxYyZz is crossed with a plant of
genotype XXYyzz. Assume the genes assort independently.
Of their offspring, what proportion will:
a. be heterozygous at all three loci?
b. be homozygous at all three loci?
c. be homozygous for the dominant
allele at all three loci?
d. have the genotype XXYyZz?
1/2 X 1/2 X 1/2 = 1/8
1/2 X 1/2 X 1/2 = 1/8
1/2 X 1/4 X 0 = 0
1/2 X 1/2X 1/2 = 1/8