th
13
Friday March
Day 1
What are the male and
female sex chromosomes?
Human Genetics
Codominance

Two alleles are expressed (multiple
alleles) in heterozygous individuals.
 Example:




1.
2.
3.
4.
blood type
type
type
type
type
A =
B =
AB=
O =
IAIA or IAi
IBIB or IBi
IAIB
ii
Codominance Problem
 Example:
homozygous male Type
B (IBIB) x heterozygous female
Type A (IAi)
IA
i
IB
IAIB
IBi
IB
IAIB
IBi
1/2 = IAIB
1/2 = IBi
Another Codominance Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IAi
IBi
i
IAi
IBi
1/2 = IAi
1/2 = IBi
Codominance
 Question:
If a boy has a blood type O
and his sister has blood type
AB, what are the genotypes and
phenotypes of their parents?
 boy
- type O (ii)
 girl - type AB (IAIB)
Codominance
 Answer:
IA
IB
i
i
IAIB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
Sex-linked Traits
 Traits
(genes) located on
the sex chromosomes
 Sex chromosomes are X
and Y
 XX genotype for females
 XY genotype for males
 Many sex-linked traits
carried on X chromosome
Sex-linked traits
Morgan performed experiments that proved
some genes are on the X chromosome
2. Used fruit flies – Drosophila
a. Easily and inexpensively raised in lab glassware
b. Females mate only once and then lay hundreds
of eggs
c. Generation time is short
d. They have same sex chromosomes as humans
1.
MORGAN’S EXPERIMENT
1.
Crossed red eye female with white eye
male – got all red eyed offspring
a. Red must be dominant
2.
Crossed the red eye offspring – expected
3 red eye to 1 white eye
a. He got that, but only males were white eyed
3.
Eye color must be on the X sex
chromosome
SEX-LINKED PROBLEMS
XR = red eyes
Xr = white eyes
XRXR = red-eyed female
XRXr = red-eyed female (carrier)
XrXr = white-eyed female
XRY = red-eyed male
XrY = white-eyed male
SEX-LINKED PROBLEMS
Example: Cross a red-eyed heterozygous
female with a white-eyed male
XRXr x XrY
XR Xr
25% red-eyed female
Xr XRXr XrXr
25% white-eyed female
25% red-eyed male
25% white-eyed male
Y
XRY XrY
CARRIERS
1.
2.
3.
Females that are heterozygous for the
trait
Carriers do not show the recessive
abnormality; they pass it on to male
offspring
Males cannot be carriers – their either
dominant or recessive
SEX-LINKED TRAITS IN HUMANS
1.
Color blindness
a. Caused by a
recessive Xlinked allele
b. Females are
carriers
2.
Hemophilia
a. Results in excessive
bleeding
b. Chemical missing
for normal clotting
c. Recessive allele
3.
Sex-limited traits
a. Only expressed in the
presence of sex
hormones and
observed in one sex
or the other
b. Both sexes carry the
alleles, but one sex
will show the trait
c. Color in animals
d. Beard growth in
males
4.
Sex-influenced traits
a. Are expressed in
both sexes, but they
are expressed
differently
b. Allele for
baldness – in males
it is dominant, in
females it is
recessive
Polygenic Inheritance

- When 2 or more genes
affect a single trait
- Examples are height and
skin color
Environmental influence
Environment can affect trait just
as much as genotype
- In a Siamese cat, the enzyme
for black fur is only affective in
cold temperatures – that’s why
black is only on extremities
- Nutrition affects growth in
humans