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Variations on Mendel’s principles
Incomplete dominance
Co-dominance
Multiple alleles
Sex-linked alleles
Incomplete Dominance
and
Codominance
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Incomplete Dominance
 F1 hybrids have an appearance somewhat in
between (intermediate ) the phenotypes of the two
parental varieties.
 Example: snapdragons (flower)
 red (RR) x white (WW)
W
 RR = red flower

WW = white flower
W
R
R
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Incomplete Dominance
W
W
R RW
RW
R RW
RW
produces the
F1 generation
All RW = pink
(heterozygous pink)
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Incomplete Dominance
5
Many genes have more than two alleles in the
population
Ex. three alleles for ABO blood type in humans
IA, IB, i
Co-dominance
 Two alleles are expressed equally in multiple
alleles in heterozygous individuals.
 Example: blood type




1.
2.
3.
4.
type A
type B
type AB
type O
=
=
=
=
IAIA or IAi
IBIB or IBi
IAIB (equally expressed)
ii (recessive alleles)
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Co-dominance Problem
 Example: homozygous male Type B (IBIB)
X
heterozygous female Type A (IAi)
IA
i
IB
IAIB
IBi
1/2 = IAIB
(A & B = equally
expressed)
IB
IAIB
IBi
1/2 = IBi (B
dominant over i)
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Another Codominance Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IAi
IBi
i
IAi
IBi
1/2 = IAi
1/2 = IBi
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Co-dominance
Question:
If a boy has a blood type O and
his sister has blood type AB,
what are the genotypes and
phenotypes of their parents?
boy - type O (ii) X girl - type AB (IAIB)
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Co-dominance
Answer:
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Sex-linked Traits
Traits (genes) located on the sex
chromosomes
Sex chromosomes are X and Y
XX genotype for females
XY genotype for males
Many sex-linked traits carried on
X chromosome
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Sex-linked Traits
Example: Eye colour in fruit
flies
Sex Chromosomes
fruit fly
eye colour
XX chromosome - female
Xy chromosome - male
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Sex-linked Trait Problem
 Example: Eye colour in fruit flies

(red-eyed male) x (white-eyed female)
XRY
x
XrXr
 Remember: the Y chromosome in males
does not carry these traits.
Xr
Xr
 RR = red eyed
 Rr = red eyed
R
X
 rr = white eyed
 XY = male
Y
 XX = female
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Sex-linked Trait Solution:
Xr
XR
XR
Xr
Y
Xr Y
Xr
XR
Xr
Xr Y
50% red eyed
female
50% white eyed
male
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Female Carriers
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Polygenic traits - A single trait may be
influenced by many genes
Fraction of population
 Quantitative traits
skin colour, height, eye colour
Skin pigmentation
Sex-linked disorders affect mostly males
 Most sex-linked human
disorders are due to
recessive alleles
 Ex: hemophilia,
red-green color blindness
These traits appear mostly in males. Why?
 If a male receives a single X-linked recessive allele from his
mother, he will have the disorder; while a female has to
receive the allele from both parents to be affected
Pedigree Chart: Inheritance Pattern for
an X-linked Recessive Disease
 A high incidence of hemophilia has plagued the royal
families of Europe
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Fetal testing can spot many inherited
disorders early in pregnancy
 Karyotyping and biochemical tests of fetal cells can help
people make reproductive decisions
 Fetal cells can be obtained through amniocentesis
Amniotic
fluid
Amniotic
fluid
withdrawn
Centrifugation
Fluid
Fetal
cells
Fetus
(14-20
weeks)
Biochemical
tests
Placenta
Figure 9.10A
Uterus
Cervix
Several
weeks later
Cell culture
Karyotyping
 Chorionic villus sampling is another procedure that
obtains fetal cells for karyotyping
Fetus
(10-12
weeks)
Several hours
later
Placenta
Suction
Chorionic villi
Fetal cells
(from chorionic villi)
Karyotyping
Some
biochemical
tests
Figure 9.10B
 Examination of the fetus with ultrasound is another
helpful technique
PGD - Preimplantation Genetic Diagnosis
genetic analysis of embryos from in vitro fertilization (IVF)
before inserting into womb
Figure 9.10C, D