Download Principles of Heredity

Human Genetics
Inheritance Patterns for
Autosomal and Sex-Linked Genes
Variations in Genetic Patterns
Loci, Genes and Alleles
• Locus = area on chromosome
where gene is located
(plural = loci)
• Gene = sequence of DNA that
codes for a protein, gives rise to
physical trait
• Alleles = alternative forms of a
gene
– Dominant allele masks other alleles
– Recessive allele is masked
Locus 1
Locus 2
Locus 3
recessive allele
Separation of
Alleles by Meiosis
dominant allele
T
t
centromere
Each sex cell will
carry only one allele
for each gene
T
T
t
Chromosomes
duplicate
t
Meiosis I
T
T
t
Meiosis II
Meiosis II
T
t
T
t
t
Autosomal Inheritance
for One Gene
Donald and Darla are members of a population
at high risk for carrying the recessive allele, t, for
Tay-Sachs disease. Individuals with at least one
copy of the dominant allele, T, do not have the
disease. A genetic counselor orders tests to
determine the alleles that Donald and Darla have
for this gene. The results show that they each
have one copy of allele T and one copy of allele t.
What are the chances that Donald and Darla
could have a child with Tay-Sachs disease,
inherited with the alleles tt?
Autosomal Inheritance of One Gene
phenotype =
physical
characteristic
TT or Tt = no disease
tt = Tay-Sachs disease
Donald, no disease
Darla, no disease
Tt
T
t
homozygous
dominant
Tt
T
t
TT
Tt
Tt
tt
heterozygous
(carrier)
genotype = set
of alleles
Chances with each
pregnancy:
75% no disease
25% Tay-Sachs disease
homozygous
recessive
Autosomal Inheritance
for Two Genes
WW or Ww = widow’s peak
FF or Ff = Freckles
ww = no widow’s peak
ff = no freckles
Suppose George and Gina are both heterozygous for
these traits, what are the chances that they will have
a child with freckles but no widow’s peak?
George
FfWw
FW Fw fW fw
x
FfWw
Gina
FW Fw fW fw
Each sex cell will carry one allele for each of the two genes,
in all possible combinations.
FW
Fw
fW
fw
FFWW
FFWw
Ff WW
Ff Ww
Fw
FFWw
FFww
Ff Ww
Ff ww
fW
Ff WW
Ff Ww
ff WW
ff Ww
fw
Ff Ww
Ff ww
ff Ww
ff ww
FW
Child with freckles but no widow’s peak
F_ww = 3/16
Child with freckles and a widow’s peak
F_W_ = 9/16
Child with no freckles but with a widow’s peak
ff W_ = 3/16
Child with no freckles, no widow’s peak
ff ww = 1/16
Variations in Genetic Patterns:
Incomplete Dominance
Neither allele masks the other and both are
observed as a blending in the heterozygote
Example: Hair Curling
HH = curly hair
HH’= wavy hair
H’H’ = straight hair
Variations in Genetic Patterns: Codominance
Neither allele masks the other so that effects
of both alleles are observed in heterozygotes
without blending
Example: ABO Blood Type
Alleles A and B are codominant.
Alleles A and B are completely dominant over o.
Type A
Type B
Type O
Blood Type
Genotypes
Type A
AA or Ao
Type B
BB or Bo
Type AB
AB
Type O
oo
Effects of both
alleles observed
in phenotype
Type AB
Variations in Genetic Patterns: Multiple Alleles
Three or more alleles exist for one trait
[Note: A person can only carry any two
of these alleles at once.]
Examples: ABO Blood Type
and also Rh Factor
Phenotype
Genotype*
Rh Positive
Rh Negative
RR or Rr
rr
Protein on Red
Blood Cells
Rhesus Protein
None
*Although there are multiple R alleles, R1, R2, R3, etc. all are
completely dominant over all of the r alleles, r1, r2, r3, etc.
ABO Blood Type and Rh Factor are controlled by
separate genes. They are inherited independently.
Example of Multiple Alleles and Codominance
Mom
oorr
Dad
oorr
Type A, Rh positive x Type B, Rh negative
(mother is Type O, Rh-)
AoRr
AR
(father is Type O, Rh-)
x
Borr
Br ABRr
Ar
ABrr
oR
BoRr
or
Borr
or
Aorr
ooRr
oorr
AoRr
Child with Type AB, Rh negative blood
ABrr = 1/8
Variations in Genetic Patterns: Polygenic Inheritance
Many genes affect one trait
Example: Skin color
Number of Skin Color*
Dominant (Phenotype)
Alleles
0
White
Genotypes
% Pigmentation*
aabb
Aabb or aaBb
0-11%
1
Light Black
2
Medium Black AAbb or AaBb or
aaBB
26-40%
3
Dark Black
41-55%
4
Darkest Black AABB
AABb or AaBB
12-25%
56-78%
*Based on a study conducted in Jamaica.
Example of Polygenic Inheritance
Grandma
aabb
Medium Black Woman X Darkest Black Man
(her mother is white)
AABB
AaBb
AB
AB
Ab
AABB
AABb
Darkest
Black
Dark
Black
aB
ab
AaBB
AaBb
Dark
Black
Medium
Black
¼ Darkest Black; ½ Dark Black; ¼ Medium Black
Variations in Genetic Patterns:
Sex Influenced Genes
Autosomal genes that have different
dominance patterns in males and females
Example: Pattern Baldness
Caused by a dominant allele in males,
but a recessive allele in females
bb
BB or Bb
BB or Bb
bb
Inheritance for Sex-linked Genes
Males carry only one copy of genes on the X chromosome
Females can be homozygous or heterozygous these genes
Hemophilic Male
Non-hemophilic
Female
Gramps
XhY
(father is hemophilic)
XhY
H = no hemophilia
h = hemophilia
Xh
x
XHXh
XH
XHXh
Xh
XhXh
XHY
XhY
Y
Chance for each pregnancy:
¼ non-hemophilic females; ¼ hemophilic females
¼ non-hemophilic males; ¼ hemophilic males