Illustration of Mitosis
Illustration of Meiosis
For an organism with 46 chromosomes, there are 223 = 8,388,608 possible gametes (not including variability due to crossing over)
Genetics
Starr et al
Chapter 13
Origins of Genetics
•  What is a ‘trait’?
–  Height?
–  Hair color?
–  Intelligence?
–  Complex traits mix, merge, average
•  Various transmission theories ….
–  From organism to gametes to organism
•  How ???
–  Darwin’s ‘gemmules’
•  Minute particles in all cells, gathered by germ cells ---> new organism
•  Allows inheritance of acquired characteristics
–  Lamarck’s hypothesis
»  Disproven
»  Khrushchev and Lysenko: Mendel-Morganism a capitalist plot
–  Genes on chromosomes distributed to various tissues that express them
•  (This has been disproven!!
–  all cells are totipotent in theory
–  all cells contain a full complement of DNA)
•  And then: Gregor Mendel
Definitions
•  Homozygote –  2 copies of the same allele at a locus
•  Heterozygote –  1 copy of each of 2 different alleles
•  Dominant alleles mask the presence of another allele
•  Recessive alleles are masked by dominant alleles
•  Biochemistry of dominance
–  Protein-product of one allele often suffices for normal function
•  Dominant allele is the normal, functioning version of gene
•  Recessive allele is a nonfunctional mutant
•  Genotype
–  An individual’s combination of alleles
•  Phenotype
–  The appearance of the individual
Here are 2 inbred mice – completely homozygous at all loci.
Assume that black is dominant to white.
x
If you mate these mice, what color are the offspring?
Homozygous
recessive parent
Homozygous
dominant parent
Here’s the cross:
A
Parents are homozygous
F1 progeny are identical
genotypically and
phenotypically
A
a
A
(chromosomes
duplicated before
meiosis)
A
A
a
A
a
a
a
a
meiosis I
They are all heterozygous.
A
A
A
a
A
a
a
a
meiosis II
A
A
A
a
A
a
a
a
gametes
F1
A
a
fertilization produces
heterozygous offspring
Now you have heterozygous mice – different color allele on
each chromosome.
x
If you mate these mice, what color are the offspring?
Here’s the shortcut: Punnett Squares
Self-crossing of F1 to produce F2 : Principle of segregation
Gametes are either A (dominant) or a (recessive)
Random fusion of gametes produces 4 possible zygotes
The phenotypes are in the ratio of 3:1
The genotypes are in the ratio of 1 : 2 : 1 female gametes
male gametes
A
a
A
a
A
a
A
aa
a
A
A
Aa
aa
a
Aa
a
A
a
Aa
A
AA
Aa
aa
a
Aa
aa
Dihybrid Crosses and Independent Assortment
What happens if we cross a purple-flowered and tall plant (dominant alleles) with a
white-flowered and short plant (recessive alleles)?
AABB
purpleflowered
tall parent
(homozygous
dominant)
AB
X
ab
F1 OUTCOME: All F1 plants purple-flowered, tall
(AaBb heterozygotes)
AaBb
aabb
whiteflowered
dwarf parent
(homozygous
recessive)
Now we have two different loci and two alleles of each in the cross. If
these loci are on different chromosomes their alleles on the homologs will
assort into the gametes independently of their parental origin.
OR
C c
c
D
Dd
d
anaphase I
C
C
c
c
D
D
d
d
D
D
C
C
C
1/4 CD
d
c
gametes
metaphase
II
d
c
1/4 cd
C
Cc
c
d
d D
D
C
C
c
c
d
d
D
D
d
C
d
C
D
c
1/4 Cd
D
c
1/4 cD
gametes
AABB
purpleflowered
tall parent
(homozygous
dominant)
AB
aabb
whiteflowered
dwarf parent
(homozygous
recessive)
ab
X
F1 OUTCOME: All F1 plants purple-flowered, tall
(AaBb heterozygotes)
AaBb
AaBb
ADDING UP THE F2 COMBINATIONS POSSIBLE:
meiosis, gamete formation
9/16 or 9 purple-flowered, tall
3/16 or 3 purple-flowered, dwarf
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/16
1/16
1/16
1/16
AABB AABb AaBB AaBb
1/16
1/16
1/16
1/16
AABb AAbb AaBb Aabb
1/16
1/16
1/16
1/16
AaBB AaBb aaBB aaBb
1/16
1/16
1/16
1/16
AaBb Aabb aaBb aabb
Possible outcomes of cross-fertilization
3/16 or 3 white-flowered, tall
1/16 or 1 white-flowered, dwarf
Determining Outcomes of Matings
1.  Identify all possible gametes
2.  Then set up a Punnett square to figure out all possible genotypes
in F2 3.  Convert to phenotypes
Example: Tall Purple (TTPP) x short white (ttpp)
Gametes TP and tp
F1 = TtPp, tall and purple
F1 gametes are TP, Tp, tP, tp
F2 genotypes are TTPP, TTPp, TTpp, TtPP, TtPp, Ttpp, ttPP, ttPp, ttpp F2 phenotypes are
Tall Purple : Tall white : short purple : short white
::
9
: 3
:
3
:
1
The Heritage of Mendel
•  Mendel –  Published results in 1866
•  Proceedings of the Natural History Society of
Brünn
•  Paper was essentially ignored
–  Became abbot in 1868
–  Died in 1884
•  1900: rediscovery by Correns and deVries
•  1900-1930
–  Proving that Mendelian genetics is universal
–  Identifying basis for seeming exceptions
•  Linkage
•  Sex linked genes
•  Complex inheritance
–  Developing model systems
•  Fruit fly
•  Mouse
•  Plants were more difficult
–  Developing practical applications
Linkage
•  Chromosomes segregate randomly into gametes
•  Genes on separate chromosomes segregate independently
•  Genes on the same chromosome are linked
–  Segregate together
•  Unless crossing over occurs between them
–  Probability of crossover increases with increasing distance
•  This means that linkage decreases with increasing distance
•  Mendel’s peas
–  6 traits are on 6 different chromosomes
–  7th trait very weakly linked –  Luck? •  Mendel’s laws apply to all diploid eukaryotic organisms
•  Did Mendel miss anything else major?
–  YES!
•  Sex-linked inheritance (Friday)
•  Epistatic interactions
Incomplete Dominance
Let’s go back to those inbred mice ….
x
If you mate these mice, what color are the offspring?
Well, it depends … An albino mouse can’t make any pigment. One does not really know
what pigment genes it carries – it can’t make any of them!
This is called epistatic inheritance – a gene at a different locus
‘trumps’ expression of the genes I’m interested in.
If the albino mouse carries genes for agouti (“brown”), the
offspring will all be brown!
x
If the albino mouse carries the alleles for black, the offspring
will all be black.
Epistatic Interaction:
coat color in
Labrador retrievers
Interaction between 2 genes for comb shape in chickens
P:
RRpp
(rose comb)
X
rrPP
(pea comb)
F1:
RrPp
(all walnut comb)
X
F2:
9/16 walnut
(RRPP, RRPp, RrPP, or RrPp)
3/16 rose
(RRpp or Rrpp)
3/16 pea
(rrPP or rrPp)
1/16 single
(rrpp)
ROSE COMB
PEA COMB
SINGLE COMB
comb
WALNUT COMB
R_P_
R _
P _
rrpp
Human Eye Color:
It’s not that simple!
Eye color is more
complex than the
traits we have
talked about so far.
Different colors
result from
different amounts
of melanin
deposited in the iris.
Even many ‘blueeyed’ people have
some genes for
brown eye color
So --
it is possible for 2
(sort-of) blue-eyed
parents to have a
(sort-of) browneyed child.
Fig. 11.16, p. 186
To Be Continued as… Human Genetics
•
•
•
•
Sex linkage
Chromosomal abnormalities
Screening
etc…