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