Key Knowledge:
patterns of inheritance in sexually reproducing
organisms:
one gene locus, monohybrid cross including
dominance, recessiveness, co-dominance; multiple
alleles, two gene loci: dihybrid cross,
pedigree analysis: autosomal, sex-linked
inheritance, test cross.
Chapter 9
Basic Concepts of Inheritance
 Diploid organisms
 One gene
 two alleles
 both alleles equally viable
 Exception
 More than two alleles at one gene (e.g. ABO blood
system)
 Lethal genotypes (e.g. Manx cats can only be
heterozygous [Mm] because the homozygous
dominant genotype [MM] is lethal).
The Punnett Square Analysis of
Inheritance
The Eight Steps
The Eight Steps
1. Set up Genetic Hypothesis
2. Assign Alleles
3. Show P phenotypes
4. Show P genotypes
5. Show P ova/sperm punnet square
6. Show F1 genotypes and frequencies
7. Show F1 phenotypes and percentages
8. Answer the question
The Eight Steps
1. Set up Genetic Hypothesis
Number of genes
Number of alleles per gene
Which allele is dominant and which allele is
recessive
e.g. 1 gene, 2 alleles, red allele is dominant to
white allele
The Eight Steps
2. Assign Allele symbols
R = Dominant red colour allele
r = recessive white colour allele
The Eight Steps
3. Show P (parental) phenotypes
Red coloured flowers × Red coloured flowers
The Eight Steps
4. Show P genotypes
Rr × Rr
The Eight Steps
5. Show P ova/sperm punnett square
R
R RR
r Rr
r
Rr
rr
The Eight Steps
6. Show F1 (Filial) genotypes and frequencies
¼RR : ½Rr : ¼rr
The Eight Steps
7. Show F1 phenotypes and percentages
75% red coloured flowers and
25% white coloured flowers
The Eight Steps
8. Answer the question
There will be 75% red coloured flowers and 25% white
coloured flowers
The Eight Steps
1. Set up Genetic Hypothesis
2. Assign Alleles
3. Show P phenotypes
4. Show P genotypes
5. Show P ova/sperm punnet square
6. Show F1 genotypes and frequencies
7. Show F1 phenotypes and percentages
8. Answer the question
Monohybrid crosses
Hypothesis:
one gene and two
alleles, round is
dominant to
wrinkled
Dihybrid crosses
Hypothesis: two genes
each with two alleles, green
is dominant to yellow and
round is dominant to
wrinkled
The Punnett square on
the right shows the
resulting genotypes when
two heterozygous parents
with RrYy genotype are
crossed together.
Test crosses
 A test cross may be used for either a monohybrid
cross or a dihybrid cross.
 Test crosses are carried out in order to determine
the unknown genotype of an individual.
 In order to do a test cross, the individual is mated
with a homozygous recessive individual (aa or
aabb).
 The frequency of phenotypes of the progeny are
then analysed in order to determine the genotype
of the individual being tested.
Multiple Alleles
 The ABO blood group system is the most




important blood type system in human blood
transfusion.
ABO blood types are also present in three other
great apes (chimpanzees, bonobos and gorillas)
Blood groups are inherited from both parents. The
ABO blood type is controlled by a single gene (the
ABO gene) on the long arm of chromosome 9
(9q34).
The gene has three different alleles: i, IA and IB. i
codes for O blood type, IA for A blood type and IB
codes for B blood type.
An individual can only carry two of the three alleles.
Multiple Alleles
 The gene has three alleles: i, IA and IB. i codes for O
blood type, IA for A blood type and IB codes for B
blood type.
 The i allele is recessive to both the IA and IB alleles
 The IA allele and IB alleles are codominant
Genotype Phenotype
I AI A
A
IAi
IBIB
IB i
I AI B
ii
A
B
B
AB
O
Linked Genes
 Genetic linkage is a term which describes the
tendency of certain loci (genes) to be inherited
together.
 Loci on the same chromatid are physically close to
one another and tend to stay together during
meiosis and are thus genetically linked (i.e. they do
not assort independently of each other).
Reading pedigrees
 Males represented by squares
 Females represented by circles.
 Filled in squares or circles indicate that the individual
has the condition
 Patterns of inheritance often indicate the mode of
inheritance
Pedigree Symbols
Female (Unaffected)
Female (Affected)
Male (Unaffected)
Male (Affected)
Female (Carrier)
Female (Unaffected Deceased)
Male (Carrier)
Male (Unaffected Deceased)
Nomenclature of a Human Pedigree
Generation
(Roman Numerals)
Relationship Line
I
If possible, male partner
should be placed left of
female partner on relationship
line.
Line of Descent
Sibship Line
Individual’s Line
II
1
2
3
Individual within Generation
Siblings should be listed left to right in birth order (oldest to youngest).
23-23
Modes of Inheritance
 There are four modes of inheritance
 Autosomal dominant
 Autosomal recessive
 X-linked dominant
 X-linked recessive
 Each of these four modes of inheritance can be
deciphered from a pedigree.
Autosomal Recessive Inheritance
 Usually there is no previous family history
 The most likely place to find a second affected
child is a sibling of the first
Autosomal Recessive
 Inbreeding increases the chance of observing
an autosomal recessive condition
 E.g. Cystic fibrosis, sickle cell anaemia, Tay
Sachs disease.
23-27
Cystic Fibrosis
 Cystic fibrosis is the most common lethal genetic
disorder among Caucasians.
 A chloride ion (Cl-) transport protein is defective in
affected individuals.
 Normally when a chloride ion passes through a
membrane, water follows.
 In cystic fibrosis patients, a reduction in water results
in a thick mucus which accumulates in bronchial
passageways and pancreatic ducts.
Phenylketonuria (PKU)
 Individuals with phenylketonuria lack an enzyme
needed for the normal metabolism of phenylalanine,
 Coded by a gene on chromosome 12.
 Newborns are regularly tested for elevated
phenylalanine in the urine.
 If the infant is not put on a phenylalanine-restrictive
diet in infancy until age seven when the brain is fully
developed, brain damage and severe mental
retardation result.
Autosomal Dominant Inheritance
 All affected individuals should have an affected parent
 Both sexes should be equally affected
 Roughly 50% of the offspring of an affected individual
should also be affected
 e.g. Huntington’s disease, achondroplastic dysplasia
(dwarfism), Neurofibromatosis.
23-32
Huntington’s Disease
 Individuals with Huntington’s disease experience
progressive degeneration of the nervous system and
no treatment is presently known.
 Most patients appear normal until middle age.
 The gene coding for the protein huntingtin contains
many more repeats of glutamines than normal.
Huntington’s disease
X-linked Recessive
 Gene located on the X chromosome
 More males than females affected (XaY) (males have

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


only one X from mother)
Females can only be affected if the father is affected
and mother is a carrier (heterozygous) or affected
(homozygous)
An affected female (XaXa) will pass the trait to all her
sons and daughters will be carriers if father is not
affected
Males cannot be carriers (only have one X so either
affected or not)
Can skip generations
e.g. colour blindness, haemophilia, Duchene
muscular dystrophy
X-linked Recessive Pedigrees
• Trait is rare in pedigree
• Trait skips generations
• Affected fathers
CANNOT pass to their
sons
• Males are more often
affected than females
X-linked Dominant
 Dominant gene on X chromosome
 Affected males pass to all daughters and none of
their sons: Genotype= XBY
 If the mother has an X-linked dominant trait and
is homozygous (XBXB), all children will be
affected
 If Mother heterozygous (XBXb) 50% chance of
each child being affected
 e.g. Fragile X syndrome, Vitamin D resistant
rickets, brown teeth enamel.
X-linked Dominant Pedigrees
• Trait is common in pedigree
• Affected fathers pass to ALL of their daughters
• Males and females are equally likely to be affected
X-linked Dominant Diseases
• X-linked dominant diseases are extremely unusual.
• Often, they are lethal (before birth) in males and only
seen in females in heterozygous form. Homozygous
dominant genotype is usually lethal
Problems...
What is the pattern of inheritance?
What are IV-2’s odds of being a carrier?
Sample pedigree - cystic fibrosis
What can we say about
I-1 and I-2?
What can we say about
II-4 and II-5?
What are the odds that
III-5 is a carrier?
What is the inheritance pattern?
What is the genotype of III-1, III-2, and II-3?
What are the odds that IV-5 would have an affected
son?
III-1 has 12 kids with an unaffected wife
8 sons - 1 affected
4 daughters - 2 affected
Does he have reason to be concerned about paternity?