1. Patterns of Genetic Inheritance

2. Points to Ponder
What is the genotype and the phenotype of an individual?
What are the genotypes for a homozygous recessive and dominant individuals and a heterozygote individual?
Be able to draw a punnett square for any cross (1-trait cross, 2-trait cross and a sex-linked cross).
What are Tay-Sachs disease, Huntington disease, sickle-cell disease, and PKU?
How are each of the above inherited?
What is polygenic inheritance?
What is a multifactorial trait?
What is sex-linked inheritance?
Name 3 X-linked recessive disorders.
What is codominance?
What is incomplete dominance?
What do you think about genetic profiling?

3. These traits are genetically inherited
20.1 Genotype and phenotype
These traits are genetically inherited
Answer these questions about your inheritance:
Do you have a widow’s peak?
Are your earlobes attached or unattached?
Do you have short or long fingers?
Do you have freckles?
Can you roll your tongue?
Do you have Hitchhiker’s thumb?

4. 20.1 Genotype and phenotype
Genotype – specific genes for a particular trait written with symbols
Alleles: alternate forms of a specific gene at the same position (locus) on a gene (e.g. allele for unattached earlobes and attached lobes); alleles occur in pairs
Dominant gene: will be expressed and will mask a recessive gene (Tt or TT)
Recessive allele: allele that is only expressed when a gene has two of this type of allele
Homozygous dominant genotype: 2 dominant alleles (TT or AA)
Homozygous recessive genotype: 2 recessive alleles (tt or aa)
Heterozygous genotype: one dominant allele and one recessive allele (Tt or Aa)

5. 20.1 Genotype and phenotype
Phenotype – the physical or outward expression of the genotype
Genotype Phenotype
EE unattached earlobe
Ee unattached earlobe
ee attached earlobe
What are your genotype and phenotype?

6. Understanding genotype & phenotype
20.1 Genotype and phenotype
Understanding genotype & phenotype
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
egg E e E E e e
sperm
fertilization EE ee Ee
growth and
development EE ee Ee
unattached earlobe
attached earlobe
unattached earlobe
Allele Key
E= unattached earlobes
e= attached earlobes

7. What about your inheritance?
20.2 One-and Two-trait inheritance
What about your inheritance?
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a. Widow's peak: WW or Ww
b. Straight hairline: ww
c. Unattached earlobes: EE or Ee
d. Attached earlobes: ee
e. Short fingers: SS or Ss
f. Long fingers: ss
g. Freckles: FF or Ff
h. No freckles: ff
a: © Corbis RF; b: © Dynamic Graphics/PictureQuest RF; c-f: © The McGraw-Hill Companies, Inc./Bob Coyle photographer;
g: © Corbis RF; h: © Creatas/PunchStock RF

8. Crosses 20.2 One-and Two-trait inheritance
One-trait cross – considers the inheritance of one characteristic
e.g. WW x Ww
Two-trait cross – considers the inheritance of two characteristics
e.g. WWTT x WwTT
Gametes only carry one allele, so if an individual has the genotype Ww what are the possible gametes that this individual can pass on?
Answer: either a W or a w but not both
Another example:
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
gametes
meiosis
Offspring
Parents f ff
no freckles

9. 20.2 One-and Two-trait inheritance
Punnett squares
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Punnett squares are the use of a grid that diagram crosses between individuals by using the possible parental gametes
These allow one to figure the probability that an offspring will have a particular genotype and phenotype
Parents Ff Ff
Key F
Freckles
eggs f
No freckles
Freckles F f
No freckles F FF Ff
Phenotypic Ratio
3:1
sperm
Freckles f Ff ff
No freckles
Offspring

10. Practicing punnett squares
20.2 One-and Two-trait inheritance
Practicing punnett squares
eggs
What would a punnett square involving a man (M) with a genotype Ff and a women (F) with a genotype Ff look like?
F – freckles
f – no freckles
M/F F f FF Ff ff
sperm

11. Practicing ratios F f FF Ff ff M/F
20.2 One-and Two-trait inheritance
Practicing ratios
Genotypic ratio: the number of offspring with the same genotype
Phenotypic ratio: the number of offspring with the same outward appearance
What is the genotypic ratio?
1: 2: 1 (1 FF: 2 Ff: 1 ff)
What is the phenotypic ratio?
3: 1 (3 with freckles and 1 with no freckles)
eggs
M/F F f FF Ff ff
sperm

12. 20.2 One-and Two-trait inheritance
Monohybrid crosses
Monohybrid cross – an experimental cross in which parents
are identically heterozygous at one gene pair (e.g. Aa x Aa)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Parents
Parents WW ww Ww ww
Key W
Widow’ s peak
Key
eggs
eggs w
Straight hairline W
Widow’s peak
Widow’ s peak w w w
Straight hairline w w
Straight hairline
Widow’s peak
Straight hairline W Ww Ww W Ww Ww
Phenotypic Ratio
sperm
sperm
1:1
Phenotypic Ratio 1
Widow’ s peak W Ww Ww w ww ww
All
Widow’ s peak
Straight hairline
Offspring
Offspring a. b.

13. Possible gametes for two traits
20.2 One-and Two-trait inheritance
Possible gametes for two traits
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
one pair
Allele Key
Cell has
two pairs of
homologues. W =
Widow’s peak w =
Straight hairline
one pair S =
Short fingers s =
Long fingers
either or
MEIOSIS I S S s s S S s s W W W W w w w w
MEIOSIS II S S s s S S s s W W w w w w W W S S s s S S s s W W w w w w W W WS ws wS Ws

14. Dihybrid cross (a type of two-trait cross)
20.2 One-and Two-trait inheritance
Dihybrid cross (a type of two-trait cross)
Dihybrid cross – an experimental cross usually involving parents that are homozygous for different alleles of two genes and results in a 9:3:3:1 genotypic ratio for the offspring
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
P generation
WWSS
wwss
P gametes WS ws F 1
generation
WwSs

15. Practicing a punnett square for a 2-trait cross
20.2 One-and Two-trait inheritance
Practicing a punnett square for a 2-trait cross
What would the punnett square look like for a dihybrid cross between a male that is WWSS and a female that is wwss?
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. F 1
gametes F WS Ws wS ws WS
WWSS
WWSs
WwSS
WwSs F 2
generation 2 Ws
sperm
WwSs
Wwss
WwSs
Wwss wS
WwSS
WwSs
wwSS
wwSs ws
WwSs
Wwss
wwSs
wwss
Offspring
Allele Key
W = Widow’s peak
w = Straight hairline
S = Short fingers
s = Long fingers
Phenotypic Ratio
9:3:3:1 9
Widow’ s peak, short fingers
Widow’ s peak, long fingers 3
Straight hairline, short fingers
Straight hairline, long fingers

16. Autosomal recessive disorder
20.2 One-and Two-trait inheritance
Autosomal recessive disorder
Individuals must be homozygous recessive to have the disorder
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. I aa A? II A? Aa Aa A? *
Relatives
III Aa Aa A? A? IV aa aa A?
Key aa =
affected Aa =
carrier (unaffected) A A =
unaffected
Autosomal recessive disorders A? =
unaffected •
Affected children can have
unaffected parents.
(one allele unknown) •
Heterozygotes (Aa) have an unaffected phenotype. • T
wo affected parents will always have affected children. •
Affected individuals with homozygous unaffected mates will
have unaffected children. •
Close relatives who reproduce are more likely to have
affected children. •
Both males and females are affected with equal frequency.

17. Autosomal dominant disorder
20.2 One-and Two-trait inheritance
Autosomal dominant disorder
Individuals that are homozygous dominant and heterozygous will have the disorder
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. I Aa Aa * II aa Aa A? aa aa aa
III Aa Aa aa aa aa aa
Key AA =
affected Aa =
affected
Autosomal dominant disorders A? =
affected
• Affected children will usually have
an affected parent.
(one allele unknown)
aa = unaffected
• Heterozygotes (Aa) are affected.
• Two affected parents can produce an unaffected child.
• Two unaffected parents will not have affected children.
• Both males and females are affected with equal frequency .

18. Genetic disorders of interest
20.2 One-and Two-trait inheritance
Genetic disorders of interest
Tay-Sachs disease: lack of the enzyme that breaks down lipids in lysosomes resulting in excess and eventually death of a baby
Cystic fibrosis: Cl- do not pass normally through a cell membrane resulting in thick mucus in lungs and other places often causing infections
Phenylketonuria (PKU): lack of an enzyme needed to make a certain amino acid and affects nervous system development
Sickle-Cell disease: red-blood cells are sickle shaped rather than biconcave that clog blood vessels
Huntington disease: huntington protein has too many glutamine amino acids leading to the progressive degeneration of brain cells

19. Genetic disorders 20.2 One-and Two-trait inheritance Many neurons in
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cl - Cl -
H2O - Cl
H2O Cl - Cl -
H2O
Many neurons in
normal brain.
nebulizer
defective
channel
percussion
vest
thick mucus
Loss of neurons in
Huntington brain.
© Pat Pendarvis
(woman): © Steve Uzzell; 20.12(normal and Huntington brain): Courtesy Dr. Hemachandra Reddy, The Neurological Science Institute, Oregon Health & Science University

20. Polygenic inheritance
20.3 Beyond simple inheritance
Polygenic inheritance
Polygenic traits - two or more sets of alleles govern one trait
Each dominant allele codes for a product so these effects are additive
Results in a continuous variation of phenotypes
Environmental effects cause intervening phenotypes
e.g. skin color ranges from very dark to very light
e.g. height vary among
Multifactorial trait – a polygenic trait that is particularly influenced by the environment
e.g. skin color is influenced by sun exposure
e.g. height can be affected by nutrition

21. Polygenic inheritance
20.3 Beyond simple inheritance
Polygenic inheritance
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
most
are
this
height
Number of Men
few
few 62 64 66 68 70 72 74
short
tall
Height in Inches
Courtesy University of Connecticut/Peter Morenus, photographer;

22. Demonstrating environmental influences on phenotype
20.3 Beyond simple inheritance
Demonstrating environmental influences on phenotype
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Himalayan rabbit’s coat color influenced by temperature
There is an allele responsible for melanin production that appears to be active only at lower temperatures
The extremities have a lower temperature and thus the ears, nose paws and tail are dark in color
© H. Reinhard/Arco Images/ Peter Arnold

23. 20.3 Beyond simple inheritance
Incomplete dominance
Occurs when the heterozygote is intermediate between the 2 homozygotes
Example:
(curly hair) CC x SS (straight hair)
CS (wavy hair)

24. 20.3 Beyond simple inheritance
Codominance
Occurs when the alleles are equally expressed in a heterozygote
Example:
(Type A blood) AA x BB (Type B blood)
AB (Type AB blood that has characteristics
of both blood types)

25. Multiple allele inheritance
20.3 Beyond simple inheritance
Multiple allele inheritance
The gene exists in several allelic forms
A person only has 2 of the possible alleles
A good example is the ABO blood system
A and B are codominant alleles
The O alleles is recessive to both A and B therefore to have this blood type you must have 2 recessive alleles

26. Multiple allele inheritance
20.3 Beyond simple inheritance
Multiple allele inheritance
Based on what you know what type of blood would each of the following individuals have in a cross between Ao and Bo?
possible genotypes: phenotypes:
AB Type AB blood
Bo Type B blood
Ao Type A blood
oo Type O blood

27. Blood type inheritance
20.3 Beyond simple inheritance
Blood type inheritance
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Parents I B
Key i I A i
Blood type A
Blood type B
Blood type AB
eggs
Blood type O I A i
Phenotypic Ratio I B I A I B I B i
1:1:1:1
sperm 1 1 1 i I A i ii 1
Offspring

28. Sex-linked inheritance
Traits are controlled by genes on the sex chromosomes
X-linked inheritance: the allele is carried on the X chromosome
Y-linked inheritance: the allele is carried on the Y chromosome
Most sex-linked traits are X-linked

29. X-linked inheritance: Color blindness
20.4 Sex-linked inheritance
X-linked inheritance: Color blindness
Cross:
XBXb x XBY
Possible offspring:
XBXB normal vision female
XBXb normal vision female
XBY normal vision male
XbY normal vision male
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Parents X B
Key Y X B X b X B
Normal vision X b
Color-blind
eggs
Normal vision
Color- blind X B X b
Phenotypic Ratio X B X B X B X B X b
Females All
sperm
1:1
Males 1 Y X B Y X b Y 1
Offspring

30. 20.4 Sex-linked inheritance
X-linked disorders
More often found in males than females because recessive alleles are always expressed
Most X-linked disorders are recessive:
Color blindness: most often characterized by red-green color blindness
Muscular dystrophy: characterized by wasting of muscles and death by age 20
Fragile X syndrome: most common cause of inherited mental impairment
Hemophilia: characterized by the absence of particular clotting factors that causes blood to clot very slowly or not at all

31. X-linked disorders 20.4 Sex-linked inheritance X B Y b Key
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. X B Y b
Key
=Unaffected female
=Carrier female
=Color-blind female
=Unaffected male
=Color-blind male
grandfather
daughter
grandson
X-linked Recessive
Disorders
• More males than females are affected.
• An affected son can have parents who have the normal
phenotype.
• For a female to have the characteristic, her father must
also have it. Her mother must have it or be a carrier.
• The characteristic often skips a generation from the
grandfather to the grandson.
• If a woman has the characteristic, all of her sons will
have it.

32. X-linked disorders: Hemophilia
20.4 Sex-linked inheritance
X-linked disorders: Hemophilia
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Key
Unaffected male
Hemophiliac
Unaffected female
Carrier
Queen Victoria
Prince Albert
4 children of 9
are shown V
ictoria
Frederick III
(Germany)
Alice
Louis IV
(Hesse)
Princess
Helena of
Waldeck
Leopold
(died at 31)
Beatrice
Prince Henry of
Battenberg
12 children of 26
are shown
Henry
Irene
Frederick
(died at 3)
Alexandra
Nicholas II
(Russia)
Alice
Alexander
(Earl of
Athlone)
Alfonso XII
(Spain)
Victoria
Leopold
(died at 32)
6 children of 34
are shown
Waldemar
(died at 56)
Henry
(died at 4)
Alexei
(murdered)
Rupert
(died at 21)
Alfonso
(died at 31)
Gonzalo
(died at 20)
(queen): © Stapleton Collection/Corbis; (prince): © Huton Archive/Getty Images