Transcript Complex Patterns of Inheritance

Complex Patterns of
Inheritance
Mendel’s Laws
Law of Segregation:
• Organisms inherit two copies of each
gene, one from each parent
• Organisms donate only one copy of each
gene in their gametes. Thus, the two
copies of each gene segregate, or
separate, during gamete formation.
Law of Independent Assortment: the alleles
of different genes separate independently of
one another during gamete formation
I. Sex-linked traits
• A sex-linked trait is a trait whose allele is
located on either the X or Y chromosome.
A) Hairy Ears: Gene on the Y
Chromosome
B) Most sex-linked traits on X chromosome
are recessive
• Examples include: hemophilia, red-green
colorblindness, and a form of muscular
dystrophy
A person with red-green color
blindness sees a number 2
Hemophilia
• In this pedigree, only males are
affected, and sons do not share the
phenotypes of their fathers.
– Thus, hemophilia is linked to a sex
chromosome–the X.
• Expression of hemophilia often skips
generations.
– Thus, it is recessive.
Extensive
bruising of
the left
forearm and
hand in a
patient with
hemophilia.
Genotypes for females
XHXH = normal blood clotting, non-carrier
XHXh = normal blood clotting, carrier of gene
XhXh = female with hemophilia
Queen Victoria of the
United Kingdom was a
carrier of hemophilia
Genotypes for Males
• XHY = normal blood clotting
• XhY = male with hemophilia
• Tsarevich Alexei of Russia
had hemophilia
Cross a carrier mother with a
normal father.
No daughters with hemophilia, ½ of
sons with hemophilia
XH
Xh
XH
XHXH
XHXh
Y
XHY
XhY
Cross a hemophiliac father with a
normal (non-carrier) mother.
All daughers are carriers, no sons
have hemophilia.
XH
XH
Xh
XHXh
XHXh
Y
XHY
XHY
II. Incomplete Dominance
• An individual displays a trait that is
intermediate between the two parents
• Example: a red snapdragon
crossed with a white
snapdragon produces
pink offspring
Red = RR, White = WW, Pink = RW
Make a Punnett square for a red
plant crossed with a white plant
All offspring are pink, with
RW genotype
W
W
R
R
RW
RW
RW
RW
What happens if you cross two
pink snapdragons?
Offspring are 1 red, 2 pink, and
1 white
R
W
R
W
RR
RW
RW
WW
1 red (RR), 2 pink (RW),
1 white (WW)
Incomplete
dominance
in horse
coat color
III. Codominance
• Two dominant alleles are expressed at the
same time
• Roan coat color in horses or cows is an
example
• A roan horse has both red and white hairs
• Roan 
has red
and white
hairs
Parents
 
Codominance
in chicken
feather color.
Black and
White feathers

IV. Multiple Alleles
• Genes with three or more alleles for a
particular trait.
• Examples are the ABO blood types, fur
color in many animals.
Coat color in rabbits
Phenotype
Allele
ch
Pattern of
Inheritance
Dominant to all
other alleles
Dominant to
Himalayan and
white
Dominant to
white
c
recessive
Dark gray coat
C
Chinchilla
(silver-gray)
cch
Himalayan
(white with dark
ears)
White
1. List all possible genotypes for a:
a) dark gray-coated rabbit
CC, Ccch, Cch, Cc
b) chinchilla rabbit
cchcch, cchch, cchc
c) Himalayan rabbit
c hc h, c hc
d) white rabbit
cc
2. Predict the phenotype for a rabbit with a chcch
genotype
chinchilla
Predict the phenotype for a rabbit with a Cch
genotype
dark gray
3. Would it be possible to obtain white rabbits if
one parent is white and the other is chinchilla?
Yes, if the chinchilla parent has a white allele
4. Would it be possible to obtain chinchilla
rabbits if one parent is Himalayan and
the other is white?
No, because chinchilla is dominant to
both Himalayan and white, so the allele
couldn’t be “hiding”.
5. A chinchilla rabbit is mated with a
Himalayan. Some offspring are white.
What are the parents’ genotypes?
Himalayan is chc and chinchilla parent is
cchc
V. Polygenic traits
• Traits that are influenced by 2 or more
genes
• Examples include human height, weight,
hair color, eye color, and skin color
Coat color in Labrador retrievers
Coat color in Labrador retrievers
VI. Epistasis
• One gene that interferes with the
expression of other genes.
• Example is with albinism.
• One gene interferes with all other genes
for pigment production.
VII. Traits influenced by the
environment.
• Color of hydrangea flowers (blue in acidic
soil, pink in basic soil)
• Color of arctic fox
• Human height, skin color, behavior