Download Inheritance of Traits

Inheritance of Traits
Exceptions to Mendelian Inheritance
Chapter 7, Sections 1-2
In your textbook
Incomplete Dominance
• The heterozygote – individual with one dominant
and one recessive allele for the trait - displays a
phenotype (physical appearance) that is a
blending of the dominant and recessive alleles
• ex. In certain flowers, R is the allele for red color
and r is the allele for white. The Rr flowers will be
pink (an intermediate between red and white)
Incomplete Dominance, cont’d
Incomplete Dominance
• An example of incomplete dominance in humans
is sickle-cell anemia.
• A person with sickle-cell anemia has two sickle-cell
alleles; causes heart and circulatory problems.
• A person with sickle-cell trait has one sickle-cell allele
and one normal allele; usually only has complications
when oxygen levels are low (maybe stress or high
altitude).
• An unaffected person has two normal alleles.
• This diagram shows the possible combinations of traits
in offspring when both parents are heterozygous (sickle
cell trait); these parents have a 25% chance of having a
child with full-blown sickle cell anemia.
Codominance
• The heterozygote displays a phenotype that
shows both the dominant and recessive alleles at
the same time
• ex. In certain horses and cattle, the alleles for
coat color are CR (red) and CW (white). Neither
allele is dominant over the other.
• CRCR has a red coat
• CWCW has a white coat
• CRCW has a roan coat (individual hairs may be either
red or white, mixed together throughout the coat).
Codominance
•If a pure red horse is
crossed with a pure white
horse, all of the offspring
will be roan.
•If two roan horses are
bred with each other,
offspring may be red
(25% chance), roan
(50%), or white (25%).
Polygenic Inheritance
• Some traits are influenced / controlled by multiple
genes.
• The combination of all of the alleles present in the
individual determine the phenotype.
• Therefore, these traits are sometimes described
as additive.
• Many of these traits are distributed along a bell
curve in a population.
• ex. height, skin color, eye color
Polygenic Inheritance, cont’d
While the majority of offspring
have an intermediate level of
pigment, a smaller percentage
would have extremely light or
extremely dark pigmentation.
Multiple Alleles
• Some traits have more than two possible alleles
that contribute to the genotype and phenotype.
• ex. Human blood types:
• IA – “A” allele
• IB – “B” allele
• i – “O” allele
Multiple Alleles, cont’d
• When there are more
than two alleles, multiple
phenotypes are possible.
• Note: In human blood
types, A and B are
codominant with each
other; both are
completely dominant
over O.
Phenotype
Possible
genotypes
A
IAIA or IAi
B
IBIB or IBi
AB
IAIB
O
ii
Multiple Alleles, cont’d
Crossing heterozygous
type A and B parents (at
left) and an AB parent
with an O parent (below).
Sex-Linked Traits
• The genes for certain traits are associated
with / found on the sex chromosomes. Most
sex-linked traits in humans are carried on
the larger X chromosome.
• Therefore, females (XX) will have two
alleles for these traits while males (XY) will
only have one allele.
Sex-Linked Traits, cont’d
The Punnett square above
illustrates which sex
chromosomes are passed on by
the male and female parents.
Sex-Linked Traits, cont’d
• Recessive sex-linked traits are more common in
males because there is no possibility of a
dominant allele on the Y chromosome “masking”
any recessive allele that may be present on the X
chromosome.
• Females can be homozygous dominant,
heterozygous, or homozygous recessive.
• Males will always express whichever allele they have.
• ex. hemophilia, colorblindness
Sex-Linked Traits, cont’d
The Punnett Square at left
shows possible combinations of
traits passed on by a father with
normal vision and a mother who
is a carrier for colorblindness.
50% of the daughters will have
normal vision
50% of the daughters will be
carriers for colorblindness
** Note that when considering genotype and
phenotype ratios for sex-linked traits, both the
trait and the gender must be considered.
50% of the sons will have
normal vision
50% of the sons will be
colorblind