Single Gene Inheritance
Chapter 2
Gene are located on chromosomes
- the two members of an homologous pair carry alleles for
the same genes and, therefore, affect the same traits.
The stuff that makes up chromosomes
Chromatin – 25% protein + 75% DNA
Chromatin condenses
into chromosomes
at cell division
Packaging of DNA
Helix
Nucleosome
Fibers
Loops
Loops
Coils
Chromosome
A chromosome,
A closer view
Definitions
Phenotype - the visible expression of information
contained in the genetic make-up of an individual
Genotype - the genetic make-up, latent or expressed.
The sum of all genes present in an individual.
P1 - the parental lines
F1 - the first filial generation, the offspring resulting
from a cross. F2/ F3 the second and third
generations.
Genetic polymorphisms
The stuff of genetic variation
1 locus (monogenic),
2 different alleles
3 combos possible
Mendel’s Principles of Inheritance
1) Genetic characteristics are controlled by
unit factors (elemente) that exist in pairs in
individual organisms.
These unit factors represent units of inheritance
today called genes.
Alternative forms of a single gene are called alleles.
Individuals can have identical alleles (homozygous)
or different alleles (heterozygous) for a single
gene.
An example: seed color
o
+
X
o
P1 (parental)
F1
Self-pollination of the F1
X
o
+
3
o
1
Dominance/Recessiveness
When two unlike alleles responsible for a single
character are present in a single individual, one is
dominant (expressed) to the other which is said to
be recessive (silent).
o
+ GG
X
o
P1
gg
F1
Gg
Segregation
During formation of the gametes, the paired alleles
separate or segregate randomly so that each
gamete receives either with equal likelihood.
Possible sperm alleles
Possible
egg alleles
G
g
G
g
GG
Gg
Gg
gg
Punnett square
Sample Problem
• Flower position in pea plants is another
single gene trait. Axial flower position is
dominant to terminal. If we cross a true
breeding plant with axial flowers to one
with terminal flowers, what phenotypes can
we expect in the F1 progeny? in the F2 of a
selfing of the F1?
Sample Problem
• Having dimples is a dominant trait in
humans. I have no dimples but my father
and mother do. All my siblings have
dimples as well. Am I adopted??
More on Single Gene Inheritance
Query
P1
F1
red X white
pink
Self F1
F2
pink X pink
red pink white
1
2
1
Incomplete (partial) dominance
Self F1
pink X pink
(R1R2) (R1R2)
R1
Possible sperm cells
R1
R1R1
R2
R1R2
pink
Possible
egg cells
R2
R1R2
pink
Intermediate expression of the phenotype.
R2R2
But what about the yellow ?
This could be the effect of multiple
alleles involved in color
production.
Multiple alleles
An example: the ABO blood types in humans
Genotype
Phenotype
IAIA
IAIO
A
I BI B
I BI O
B
IAIB
AB
IOIO
O
Sample problem
• Red-green color blindness is a sex linked,
recessive allele of a single gene trait. My
brother-in-law is red-green color blind. My
sister is not, but my father might have been.
My sister and her husband have 4 sons. My
mother (their grandmother) was worried
that they would be color blind. Should she
have been concerned?
Genes can be Pleiotropic
-single gene, multiple effects
ex. dwarf
Chromosomal Basis of
Inheritance
Meiosis
(the prelude to
sexual
reproduction)
For sexual reproduction to occur, chromosomes must be
duplicated and divided between the gametes.
Meiosis I
Meiosis II
Meiosis
There are 2 steps of meiosis - 2 cell divisions,
but only
1 replication of chromosomes.
Each gamete contains only one member of each
homologous pair.
Summary
Mendel’s “rules of the game”
1) Genes occur in pairs - Genetic characteristics are
controlled by genes that exist in pairs.
2) Dominance/Recessiveness- When two unlike alleles
responsible for a single character are present in a single
individual, one is dominant (expressed) to the other which is
said to be recessive (silent).
3) Segregation- during formation of the gametes, the paired
unit factors separate or segregate randomly.
Discovering genes via Mutant
analysis
• Generating mutants
– Chemical mutagenesis (EMS)
– base transition, point mutation
– Radiation
– deletions
– Transposons/ T-DNA tags
– insertion/deletions (indels)
And
observing segregation ratios
• Planned crosses and Punnett squares, Pedigree
analysis
– Mutant or polymorphism analysis
• Autosomal dominant/recessive
• Sex-linked genes