12.2 Notes
Special Inheritance


Autosomes--22 pairs of chromosomes in a
human
Sex chromosomes--23rd (last) pair of
chromosomes

Ex. XX-female XY-male

The male always determines sex of offspring
22 Autosomes
Sex chromosomes

Sex-linked traits--traits located on sex
chromosomes
R
 Ex. In drosophila (fruit flies), X = red
eyes, Xr = white eyes (located on X
chromosome)

Cross white eye male (XrY) with red eye female (XRXR)
Xr
Y
XR
XRXr
XRY
XR
XRXr
XRY
Phenotypic ratio--50% male 50% female
100% red eyes
White-eyed male (XrY)
Red-eyed
female
(XRXR)
Offspring: All red eyed

Cross red eye male (XRY) with red eye female (XRXr)
XR
Y
XR
XRXR
XRY
Xr
XRXr
XrY
Phenotypic ratio--Female: 100% red-eye
Male: 50% red-eye 50% white-eye
Whiteeyed male
(XRY)
Red-eyed
female
(XRXr)

Simple Mendelian Inheritance-Inheritance controlled by either
dominant or recessive alleles

Not all traits are either dominant or
recessive, some are in between

Incomplete dominance—the phenotype
of the heterozygous individual is an
intermediate (blend) of the 2 homozygous
individuals (neither allele is dominant)



RR= red flower
WW= white flower
RW(heterozygous) = pink flower
R
R
W
RW
RW
W
RW
RW
Cross red with white--The offspring are all pink because that
color is in between 2 homozygous phenotypes
Red (RR)
White (WW)
W
RW
RW
RW
RW
W
Offspring: All pink flowers

Co-dominance--The phenotype of both
homozygous individuals to be present in
the heterozygous individual



BB = black chicken
WW= white chicken
BW (heterozygous) = black and white chicken
B
B
W
BW
BW
W
BW
BW
Cross black with white chicken--The offspring are all black and
white because that is a combination of both homozygous phenotypes

Blood type



An example of co-dominance in humans
Example of multiple alleles, there are
more than 2 alleles for this trait
It is determined by the presence or
absence of proteins (chains of amino
acids) on the surface of blood cell

Mixing incompatible blood types can cause
the cells to clump together, which can cause
a person to die

Human Blood Types
Phenotype
Genotype
Blood cell surface
molecules
Type A
IAIA or IAi
A molecule
Type B
IBIB or IBi
B molecule
Type AB
IAIB
A and B molecules
Type O
ii
No molecules



Alleles IA and IB –are co-dominant to each
other
Allele i –is recessive to both IA and IB
Type O blood—universal donor


Has no proteins on the blood cells so any blood
type can receive it
Type AB blood—universal acceptor

Has both A and B proteins on blood cells so this
blood type can receive type A, B, AB, or O blood

Cross parent with A ( IAi) blood with
a parent with B blood (IBi)
IA
i
IB
IAIB
IBi
i
IAi
ii
Genotypic ratio
1 IA IB : 1 IA i : 1 IB i : 1 ii
Phenotypic ratio (blood type)—
1 type AB : 1 type A : 1 type B : 1 type O

Multiple alleles— traits controlled by
more than 2 alleles

Ex. Mouse hair color is controlled by
different alleles, black, brown, gray, albino,
etc…

Polygenic inheritance--Trait that is
controlled by 2 or more different genes

Ex. Blood pressure is controlled by genes
for weight, cholesterol, kidney function,
etc…