Human Genetics
It’s all in the…
Inheritance
• Parents and offspring often
share observable traits.
• Grandparents and
grandchildren may share traits
not seen in parents.
• Why do traits disappear in one
generation and reappear in
another?
A pedigree is a chart that shows how a
trait and the genes that control it are
inherited within a family.
Genetic predictions
Ellen’s brother Michael has
sickle cell anemia, an
autosomal recessive
disease.
What is the chance that
Ellen’s child has a sickle cell
anemia allele (a)?
Ellen and Michael’s
parents must be carriers.
A
a
A
AA Aa
a
Aa
aa
Ellen is not affected and
cannot carry aa genotype
Chance Ellen is a carrier = 2/3
Chance child inherits sickle
cell allele = 1/2
Ellen Michael
?
Overall chance child carries
sickle cell allele from Ellen =
2/3 x 1/2 = 1/3
Autosomal recessive inheritance
• Heterozygous individuals
carry the recessive allele
but exhibit the dominant
phenotype.
• Males and females are
equally affected and may
transmit the trait.
• May skip generations.
Sex chromosomes determine
gender
•Human males are the have two
different sex chromosomes,
(XY).
•Human females only have one
(XX).
What determines gender?
Number of X chromosomes,
or presence of Y
chromosome?
X0
Female with Turner syndrome
XXY
Male with Klinefelter syndrome
X-linked traits
Males
• One X chromosome
Females
• Two X chromosomes
• Inherited from mother
• Inherited from both parents
• Two possible genotypes
• XNY
• XnY
• Three possible genotypes
• XNXN
• XNXn
• XnXn
• Heterozygous individuals are
carriers of recessive traits- they
can pass it down to their
offspring- especially their male
children!
• Have trait or do not have traitthey can’t be a carrier!
X-linked recessive traits
• Always expressed in
males if the male inherits
the recessive gene from
the mother.
• Affected males: Inherited
from mother
• Females need to be
homozygous recessive to
show the trait
• Affected females : Fathers
affected or mother had the
trait or carried it!
X-linked dominant inheritance
Expressed in females
with one copy.
Males are often more
severely affected.
Typically associated
with miscarriage or
lethality in males.
BLOOD TYPE
• Human blood type genes are an example of a
codominant trait- meaning a trait with more than
one set of alleles.
Phenotype : A
Genotype : AO, AA
B
BO, BB
AB
AB
O
OO