Genetics&
The Work of Mendel
HOMOLOGOUS CHROMOSOME
Pairs of matching chromosomes, with one being inherited from each parent.
ALLELES- alternative forms of the same gene. When 1 allele masks (hides) the effect of another, that allele is
called DOMINANT and the hidden allele is called RECESSIVE.
Dominant alleles are represented by a CAPITAL letterRecessive alleles are represented by a LOWERCASE letter
A b C d e
A B c d E
Chromosome from MOM
Chromosome from DAD
ALLELES
What are Dominant Genes?
Hereford: white face is dominant
Horned is recessive to polled.
PHENOTYPE (what the organism LOOKS like)Examples of phenotypes: blue eyes, brown fur, striped fruit,
yellow flowers
GENOTYPE the genes present in the DNA of an organism 1 gene from MOM + 1 gene from DAD= 2 genes (2 letters) for
offspring
3 possible GENOTYPES:"TT" – Homozygous Dominant "Tt" - Heterozygous "tt" – Homozygous Recessive
Genotype Symbol
Genotype Vocabulary
Phenotype
TThomozygous DOMINANT
or purebred tall
tall
Ttheterozygous
or hybrid
tall
tthomozygous RECESSIVE
or purebred
short
short
Gregor Mendel (1822-1884)
Austrian monk
Called the “Father of Genetics" for his study of the inheritance of 7 traits in pea plants.
Mendel collected data for 7 pea traits
Pollen transferred from white flower to stigma of purple flower
all purple flowers result
Mendel’s work
?
self-pollinate
• Bred pea plants– cross-pollinate
true breeding parents– raised seed & then
observed traits– allowed offspring
to self-pollinate & observed next generation
2nd
generation
3:175%purple-flower peas
25%white-flower peas
Looking closer at Mendel’s work
Parents
100%1st
generation(hybrids)
100%purple-flower peas
Xtrue-breedingpurple-flower peas
true-breeding white-flower peas
self-pollinate
Making crosses
• Can represent alleles as letters– flower color alleles P or p– true-breeding purple-flower peas PP– true-breeding white-flower peas pp
PP x pp
PpF1
P X
purple white
all purple
P P
p Pp Pp
p Pp Pp
all the F1 (1st filial generation) offspring
are purple colored hybrids
100% purple offspring
P p
P
p
Pp x Pp
Heterozygous - hybrid
Heterozygous - hybrid
Punnett squares
Pp x Pp
P pmale / sperm
P
p
fem
ale
/ eg
gs
PP
75%
25%
3:1
25%
50%
25%
1:2:1
%genotype
%phenotype
PP Pp
Pp pp pp
Pp
Pp
1st
generation(hybrids)
Aaaaah,phenotype & genotype
can have different ratios
Monohybrid Cross
• Example:Cross between two heterozygotes for brown eyes (Bb)
BB = brown eyesBb = brown eyesbb = blue eyes
B
b
B b
Bb x Bb
malegametes
female gametes
Monohybrid Cross
BB
Bb
Bb
bb
B
b
B b
Bb x Bb
1/4 = BB - brown eyed1/2 = Bb - brown eyed1/4 = bb - blue eyed
1:2:1 genotype 3:1 phenotype
Dihybrid cross
• Other of Mendel’s experiments followed the inheritance of 2 different characters – seed color and
seed shape– dihybrid crosses
Dihybrid cross
true-breedingyellow, round peas
true-breedinggreen, wrinkled peasx
YYRR yyrrP
100%F1generation(hybrids)
yellow, round peas
Y = yellowR = round
y = greenr = wrinkled
self-pollinate
9:3:3:19/16
yellowround peas
3/16greenround peas
3/16yellow
wrinkledpeas
1/16green
wrinkledpeas
F2generation
YyRr
Dihybrid Cross
• Example:cross between round and yellow heterozygous pea seeds.
R = roundr = wrinkledY = yellowy = green
RY Ry rY ry RY Ry rY ry x RY Ry rY ryRY Ry rY ry possible gametes produced
RrYyRrYy x RrYyRrYy
Dihybrid cross
YyRr YyRr
YR Yr yR yr
YR
Yr
yR
yr
YYRR
x
YYRr YyRR YyRr
YYRr YYrr YyRr Yyrr
YyRR YyRr yyRR yyRr
YyRr Yyrr yyRr yyrr
9/16yellowround
3/16greenround
3/16yellow
wrinkled
1/16green
wrinkled
YyRr
Yr yRYR yr
YyRr
YR yr
or
Incomplete Dominance
• Incomplete dominance is when both alleles
are expressed and results in an intermediate
phenotype. The hybrid is a blend of both
alleles.
• When expressing incomplete dominant
alleles, both alleles are represented by
different capitalized letters.
Incomplete dominance
• Hybrids have “in-between” appearance– RR = red flowers– rr = white flowers– Rr = pink flowers
• make 50% less color
RR Rr rr
RRWW
RW
Incomplete dominance
true-breedingred flowers
true-breeding white flowers
XP
100%
100% pink flowers1st
generation(hybrids)
self-pollinate
25%white
2nd
generation
25%red 1:2:1
50%pink
R R
W
W
Homozygous
HomozygousIncompleteDominanceP (parental)generation
RW RW
RW RW
R R
W
W
Incomplete Dominance F1 (First Filial)
generation
100% pink offspring
R W
R
W
Heterozygous
Heterozygous
Incomplete Dominance
F1 x F1
Incomplete dominance
R Wmale / sperm
R
Wfem
ale
/ eg
gs RR
RW WW
RW
25%
1:2:1
25%
50%
25%
1:2:1
%genotype
%phenotype
RR
RW
RW
WW
25%
50%
RW x RW
Co-dominance
• Some alleles are equally strong and neither are masked
by the other. Alleles which are equally expressed are said
to be "co-dominant".
• The hybrid is a mixture of both alleles.