Review:
*Unit factors in pairs- genetic characteristics are controlled
by unit factors that exist in pairs in individual organisms
*Principle of Dominance and Recessiveness- one factor is
dominant over the recessive one
*Law of Segregation- During gamete formation the paired unit
factors segregate randomly so that each gamete receives one
or the other
*Law of Independent Assortment – During gamete formation,
segregating pairs of unit factors assort independently of each
other
Dihybrid cross
Example:
P1
X
yellow, round
green, wrinkled
GGWW
ggww
F1
All
yellow, round
GgWw
Dihybrid cross con’t:
F1
F1
X
All
yellow, round
All
yellow, round
GgWw
GgWw
F2
The Trihybrid Cross:
Trihybrid cross
Example:
Theoretical gene pairs represented by the symbols A, B, and C
P1
Gametes:
AABBCC
ABC
F1
Gametes:
aabbcc
X
abc
AaBbCc
ABC
ABc
AbC
Abc
aBC
aBc
abC
abc
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
The Forked-Line Method (branch diagram):
The Forked-Line Method (branch diagram):
*NOTE: We are assuming that independent assortment of these 3
gene pairs is a random process!
Mendel Rediscovered:
Why did Mendel’s work go unnoticed for so long?
Charles Darwin and Alfred Russel Wallace:
continuous variation – offspring were a blend of parents’ phenotypes
*1879 Walter Flemming
*early 20th century
*Hugo de Vries
*Karl Correns
*Erich Tcshermak
*1902 Walter Sutton and Theodor Boveri
Chromosomal Theory of Inheritance
How do we account for genetic variation?
*Independent assortment
*Crossing over
*Random fertilization
Independent Assortment:
Cross over:
Human Pedigrees
Pedigree
= Female
= Male
=Unknown
*Proband (p)
Chapter 4: Modification of Mendelian Ratios
Allele
*Wild-type allele
*Mutant allele
Conventional symbols for alleles:
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!
Incomplete or Partial Dominance
Cross between parents with
contrasting traits:
Red flowers or white flowers
Offspring with an
intermediate phenotype:
pink flowers
Codominance:
Example:
MN Blood group- red blood cells contain a transmembrane
glycoprotein (glycophorin); two different forms of this
protein exist, M and N
Multiple Alleles:
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