Download 5.1 Human Inheritance File

Describe human traits with only 2 alleles
Describe traits with multiple alleles
Explain blood-type alleles and heredity
Describe sex determination
Explain sex-linked heredity
Define carrier and explain what it means to be a
carrier
Describe examples of traits with 2 alleles
•Some human traits are controlled
by a single gene with two alleles
•Dimples
•Cleft in the chin
•Attached/unattached ear lobes
•Widow’s peak
Check:
What is one human trait controlled by
one gene with only two alleles?
• Some human traits are controlled by single
genes with multiple alleles
• Example: Blood type (A, B, AB, O)
• Most human traits are controlled by multiple
genes that act together
Explain blood-type alleles and heredity
There are four types of blood – A, B, AB, and O
Three alleles control human blood type:
Two of the alleles are codominant: IA and IB
One allele is recessive: i
Explain blood-type alleles and heredity
Possible genotypes:
IA IB = AB blood type
IA i or IAIA= A blood type
IB i or IBIB = B blood type
i i = O blood type
CHECK:
What are the three alleles that control human blood
type?
What are the four blood types?
Which blood type alleles are codominant?
If the genotype is ii what is the blood type?
POINT > Explain blood-type alleles and heredity
Suppose a heterozygote with type A blood has
offspring with a heterozygote with type B blood.
What type blood will the children have?
A
I i
x
B
I i
Type B heterozygous parent
B
I
Type A
heterozygous
parent
A
I
i
A
B
I I
A
I
B
I
ii
i
25% type AB 25% type A
25% type B
i
25% type O
i
CHECK:
Suppose a homozygous woman with type A blood
has offspring with a man with type AB blood. What
are the genotype percentages of the offspring?
A
A
I I
x
A
B
I I
POINT > Describe sex determination
Human body cells 23 pairs of chromosomes
(= 46 total)
22 pairs of “regular” chromosomes are called
autosomes
1 pair of sex chromosomes = X and Y
Girls have two X chromosomes (XX)
Boys have one X and one Y chromosome (XY)
POINT > Describe sex determination
During meiosis, the sex chromosomes split apart
(like the other 22 pairs)
In women, all egg cells have an X chromosome
In men, half of the sperm cells get an X and the
other half get a Y
The male sperm cell (X or Y) determines the sex
of the offspring
The
formation of
egg and
sperm cells:
1 sperm cell

4 sperm cells
1 egg cell

1 really
good egg
cell
CHECK:
How many autosomes do humans have?
What are the sex chromosomes in a woman?
What are the sex chromosomes in a man?
Explain sex-linked heredity
The genes for some human traits are carried on
the sex chromosomes
They are called sex-linked genes, and the traits
are sex-linked traits
Like other traits, they can have dominant and
recessive alleles
POINT > Explain sex-linked heredity
FEMALES (XX)
A dominant allele on one X will cover up a
recessive allele on the other X
XRXr
If “r” is a recessive allele that causes a disease,
this girl is not affected
A girl would have to be XrXr to have the disease
POINT > Explain sex-linked heredity
MALES (XY)
If there is a recessive allele on the X it will be
expressed
XrY
This boy will have the disease
Many recessive sex-linked traits carried on the X
chromosome will show up more in males –
ex. colorblindness, hemophilia
CHECK:
If “g” is a sex-linked recessive allele that causes a
disease, which people would have the disease?
G
a)X Y
g
XY
b)
c) XGXg
d) XGXG
e) XgXg
POINT > Define carrier and explain what
it means to be a carrier
A carrier is a person who has one recessive allele
for a trait and one dominant allele (heterozygous)
We usually use the term “carrier” to refer to a trait
that is a recessive disease
A carrier does not have the trait, but can pass on the
recessive allele to offspring
In the case of sex-linked traits, only females can be
carriers
POINT > Define carrier and explain what it
means to be a carrier
What if a female carrier for colorblindness has
offspring with a colorblind man? What will the
children have?
C = allele for good color vision
c = allele for colorblind
C
c
X X
x
c
X Y
Female carrier
C
X
Colorblind
man
c
X
c
X
C
c
X X
c
X
c
X
Y
C
X
c
X
Y
Y
25% colorblind girls
25% carrier girls
25% good vision boys
25% colorblind boys
CHECK:
What is a carrier?
Does a carrier have the disease?
Homework
Read 5.1 pages144-150
S.A. #1-3
GRAS pp. 69-71