Lecture 3: Jan. 25
Transmission genetics:
independent assortment
Human pedigrees
The 7 traits in garden pea studied by Mendel
Molecular basis of the wrinkled seed coat mutation
The wrinkled seed coat
mutant is due to the
insertion of a foreign
sequence in the wild
type smooth seed
coat gene. As a
consequence, the
mutant gene is longer
and runs slower in a
size-separation gel.
Results of a monogenic genetic cross
(involving 2 alleles of the same gene)
Genetic and molecular explanation of dominance
in the F1 and 3:1 segregation in the F2 generation
A diagram like
this is called a
Punnett square
The smooth F2 plants segregate in F3
while the wrinkled ones breed true
Genetic and molecular explanation of a testcross
Results of a dihybrid cross (involving
alleles of 2 genes)
The coat color
gene and seed
shape genes
assort (segregate)
independently
Independent segregation of alleles of 2 genes
leads to 4 kinds of gametes in equal proportion
Random fertilization of the 4 kinds of gametes
generates the 9:3:3:1 phenotypic ratio
Punnett square
for a dihybrid
cross
Genotype
ratio
Phenotype
ratio
Genotype and phenotype ratios in the F2
of a dihybrid cross
The W gene is segregating 1 WW : 2 Ww : 1ww
A backcross of the F1 to the double recessive
parent (a testcross) yields a 1:1:1:1 ratio
Results of a trihybrid cross - 1
(involving 3 genes, W, G and P)
Results of a trihybrid cross
Punnett square for a
trihybrid cross
would have 64
boxes. Here it is
broken up into 3
dihybrid diagrams,
each with 16 squares
Symbols used in a human pedigree diagram
A human pedigree showing the inheritance of a
dominant disease gene.
The diseased individuals are present in every generation
(indicates a dominant disease) and males and females are
both about equally affected (indicates autosomal inheritance)
Inheritance of an autosomal recessive
disease gene
The heterozygous individuals are phenotypically wild type. In this
pedigree, there are only 3 affected individuals (III.2, III.4, IV.5).
Mating of two heterozygotes is required to produce an affected child.
A human pedigree showing the inheritance of
a polymorphic DNA marker
There is no masking of one allele by another allele
for DNA markers (codominance of alleles is seen)
Mendel’s laws apply to all organisms because
the mechanisms of meiosis and fertilization are the same
Independent
assortment of
alleles for 2
different genes
yields 4 kinds of
gametes in 1:1:1:1
ratio in all
organisms (peas
and humans, for
example).
Incomplete dominance between 2 alleles
of the same gene yields a 1:2:1 phenotypic
ratio in F2 (not 3:1)