Transcript File
Complete dominance
◦ One allele is
expressed over
another in the
heterozygote
◦ Mendel’s pea traits
1.
Example: pure tall x pure short
T = tall t = short
TT x tt
Genotype ratio
T T
0TT:4Tt:0tt
t Tt Tt
Phenotype ratio
t Tt Tt
4 tall: 0 short
Incomplete dominance
◦ The heterozygous condition has
an intermediate phenotype
2.
Example: pure red x pure white
R = red R’ = white
RR x R’R’
Genotype
ratio
R
R
0RR:4RR’:0R’R’
R’ RR’ RR’
Phenotype ratio
R’ RR’ RR’
0red:4pink:0white
Codominance
◦ Both alleles are equally
expressed in the heterozygote
3.
Example: pure black x pure white
CB = black CW = white
C B C B x C WC W
Genotype ratio
CB
CB
CW CBCW CBCW
0CBCB :4CBCW :0CWCW
Phenotype ratio
CW CBCW CBCW 0black:4checkered:0white
4.
Multiple Alleles
◦ When one trait has more than 2
alleles
◦ Produces multiple phenotypes
◦ Example: human blood type has 3
alleles
IA, IB, i
Genotype
Phenotype
IAIA or IAi
A type blood
IBIB or IBi
B type blood
IAIB
AB type blood
ii
O type blood
The alleles indicate the type of
marker proteins found on the red
blood cells
Thomas Hunt Morgan
accidentally
discovered sex
chromosomes when
studying fruit flies
Humans
have 23 pairs of
chromosomes
22 pairs are autosomes and the 23rd
pair is sex chromosomes
Human
males have
sex chromosomes
XY
Human
females
have sex
chromosomes XX
Human Sex-linked Disorders
(caused by recessive alleles)
1.
Red-green colorblindness
2.
Hemophilia A
3.
Duchenne’s
muscular dystrophy
males
pass their X-linked allele to
ALL their daughters (never to
sons)
Heterozygous (carrier) females
have a 50% chance of passing a
recessive allele to all her children
males receiving the recessive X
from Mom will ALWAYS express
the trait
Red-green colorblindness is a
disorder caused by a sex-linked
recessive gene. Normal vision is
dominant.
XB = normal vision
Xb = colorblindness
(Y chromosomes don’t carry the
gene)
Find
the results of a cross
between a colorblind man and a
homozygous normal woman.
XBXB x XbY
XB
XB
Xb
XBXb XBXb
Y
X B Y XB Y
Genotypes
M: all XBY
F: all XBXb (carriers)
Phenotypes
M: all normal
F: all normal
Find
the results of a cross
between a normal woman, whose
father was colorblind, and a
normal man.
XBXb x XBY
XB
XB
Y
Xb
XBXB XBXb
XBY
XbY
Genotypes
M: ½ XBY, ½ XbY
F: ½ XBXB, ½ XBXb
Phenotypes
M: ½ normal, ½ colorblind
F: all normal
Wilma
(mom)
Fred
(dad)
Pebbles
(daughter)
Polygenic Inheritance
◦ When one trait is controlled by
multiple genes
◦ Produces a bell curve of
phenotypes
◦ Ex. Skin color, hair color
Each
dominant
allele
contributes
one unit of
color to the
phenotype