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Lecture for Tuesday September 23, 2003
What’s due?
Today’s lecture:
Review material from 9/18
CH2 problem set
Assignments:
Human Pedigrees
CH4 problems:
1-5, 8, 10, 11, 14,
16, 17, 21, 22
What’s due Thursday 9/25?
CH3 problem set
Begin CH4
Reading assignment:
Omit sections 4.8 and 4.9
Today’s Lab:
*Exam I is one week from today!
Maize segregating ears:
Dihybrid cross and chisquare analysis
Review:
Trihybrid cross- A genetic cross between two individuals involving
three characters (also referred to as a three-factor cross)
The Forked-Line Method (branch diagram):
Recall:
*The F1 that result from a monohybrid cross (AA x aa) all have the
genotype Aa and the phenotype represented by A
*The F2 that result from a cross between 2 individuals from the F1, have a
phenotypic ratio of 3:1
*Assume independent assortment of the 3 gene pairs
KEY: We are
examining the
resulting
phenotypes!
Review Chi-Square Analysis:
Mendel’s monohybrid and dihybrid ratios are predictions based on
the following assumptions:
1. Each allele is dominant or recessive
2. Random segregation of alleles
3. Independent assortment
4. Fertilization is random
NOTE:
*The outcomes of 2-4 are “chance events” and are subject to random fluctuation
*As sample size increases, the average deviation from expected results decreases
Establishing a null hypothesis (H0):
States that there is no difference between the observed and expected data
An Example (for a monohybrid cross):
The observed phenotypic ratio is 3:1
Review Chi-Square Analysis:
The null hypothesis is analyzed statistically:
*It may be rejected or
*It may fail to be rejected
Chi-Square (X2) Analysis:
*Examines deviation between observed and expected numbers
*Accounts for sample size
X2
(o-e)2
= S
e
Interpretation:
*determine df (n-1)
*typically use p value of 0.05 or greater (i.e. 0.01, 0.001)
*reject or fail to reject null hypothesis
Review Chi-Square Analysis:
p value (probability): consider as a percentage (i.e. 0.05 = 5%)
*A level of error that is acceptable to the researcher in analysis of data
*5% of the time your result (or outcome) is due to chance
*95% of the time your results are not due to chance
*If your calculated X2
is GREATER than that
shown at p = 0.05,
then you reject your
null hypothesis
Example from Table 3.1:
Calculated X2 = 0.53
*Therefore, we CAN
NOT reject our null
hypothesis!
Human Pedigrees
Pedigree- a family tree that shows the phenotype of a particular trait
for each family member
*Shaded symbol=expressed phenotype
= Female
*Individuals KNOWN to be heterozygous
= Male
are half shaded
=Unknown
*Horizontal lines connect parents, vertical
lines lead to offspring
*Proband (p)= individual in whom a genetically determined trait of
interest is first determined
Chapter 4: Modification of Mendelian Ratios
Allele- (short for allelomorph) alternative forms of the same gene
*Wild-type allele- allele that occurs most frequently
in a population (arbitrarily designated as “normal”);
usually dominant
*Mutant allele- allele that contains modified genetic
information and often specifies an altered gene product
Conventional symbols for alleles:
Tall = D
Dwarf = d
recessive allele- initial letter of the name of the
recessive trait, lowercased and italicized
dominant allele- same letter in uppercase
Genetic nomenclature is extremely diverse!
Example:
BRCA1 or BRCA2- (humans) Breast Cancer susceptibility
SUPERMAN- (Arabidopsis) regulates genes involved in floral development
Incomplete or Partial Dominance
Cross between parents with
contrasting traits:
Red flowers or white flowers
Offspring with an
intermediate phenotype:
pink flowers
Incomplete dominance- expression of a heterozygous phenotype which is
distinct from, and often intermediate to, that of either parent
Incomplete or Partial Dominance con’t
CRCR x CWCW
CRCW
CRCW x CRCW
¼ CRCR
½ CRCW
¼ CRCW
Codominance:
Codominance- Condition in which the phenotypic effects of a gene’s alleles
are fully and simultaneously expressed in the heterozygote
Example:
MN Blood group- red blood cells contain a transmembrane
glycoprotein (glycophorin); two different forms of this
protein exist, M and N
Genotype Phenotype
LMLM
M
LMLN
MN
LNLN
N
We can predict genotypic
and phenotypic ratios
LMLM X LMLN
¼ LMLM
½ LMLN
¼ LMLN
Multiple Alleles:
Multiple Alleles- three or more alleles of the same gene
Examples:
*Table 4.1: over 100 alleles at a given locus in Drosophila
*ABO Blood group in humans
*Characterized by the presence of glycoprotein antigens
on the surface of red blood cells
*Distinct from the M and N antigens
*Also exhibits codomiance
Genotype
IAIA
IAIO
IBIB
IBIO
IAIB
IO IO
Antigen
A
A
B
B
A,B
Neither
Phenotype
A
A
B
B
AB
O
Lethal Alleles:
Lethal Allele- recessive allele in which a homozygous genotype
results in death
Example: Coat color in mice
*A = agouti = wild-type allele
*AY = yellow = mutant allele
Dominant Lethal:
Huntington’s
disease (H);
heterozygous
individuals (Hh)
have late onset
Combining modified modes of inheritance:
Gene interaction:
Individual characteristics (discrete phenotypes) are often under the
control of more than one gene
Epistasis- from the greek “stoppage”, interaction between genes
such that one gene interferes with or prevents the expresion of
another gene
Example:
In Drosophila, the recessive gene eyeless (when homozygous)
prevents the expression of eye color genes present in genome
Novel phenotypes due to gene interaction
Example:
disc-shaped fruit (AABB) X long fruit (aabb)
F1 are all AaBb and disc-shaped
F2 Ratio
9/16
3/16
3/16
1/16
Genotype
A-BA-bb
aaBaabb
Phenotype
disc
sphere
sphere
long
Final
phenotypic
ratio
9/16 disc
6/16 sphere
1/16 long