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Powerpoint Lecture Outline
Human Genetics
Concepts and Applications
Seventh Edition
Ricki Lewis
Prepared by
Mary King Kananen
Penn State Altoona
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Chapter 4
Mendelian Inheritance
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Inheritance
• A child inherits half
of its genes from
each parent
• How are traits
assorted with each
generation?
Figure 4.1
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Gregor Mendel
• Research in plant breeding
• Without knowledge of DNA, cells, or
chromosomes
• Described the units of inheritance and
how they pass from generation to
generation
• Not recognized during his lifetime
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Gregor Mendel
•Experimented from 1857-1863
•Developed the laws of inheritance
•Used
Controlled plant breeding
Careful recordkeeping
Large numbers
Statistics
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Mendel Studied Transmission of
Seven Traits in the Pea Plants
Figure 4.2
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True Breeding Plants
Offspring have the same trait as parent
Examples:
• Round seeded parents
– produce all round seeded offspring
• Yellow seeded parents
– produce all yellow seeded offspring
• Short parents
– produce all short offspring
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Monohybrid Cross
• True breeding plants with two forms of a
trait are crossed
• Progeny show only one form of the trait
• The observed trait is dominant
• The masked trait is recessive
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Monohybrid Cross
Parental generation (P1)
Tall X Short
F1
All Tall
F2
¼ Short ¾ Tall
Figure 4.3
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Alleles
• Mendel’s units (or “elementen”) are alleles
• Versions of the same gene or DNA sequence.
• Differ in DNA sequence at one or more sites.
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Mendel's Law of Segregation
• Each plant possesses two units (alleles)
for each trait
• Alleles separate in the formation of
gametes
• Gametes contain ONE allele for each
trait
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Figure 4.4
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Mendel’s Data
Table 4.1
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Terms
Genotype
• The alleles present in an individual
– Homozygous carry the same alleles
– Heterozygous carry different alleles
TT or tt
Tt
Phenotype
• Indicates the trait observed
Tall or Short
Wild Type
• Most common phenotype
Mutant phenotype
• A product of a change in the DNA
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Punnett Square
• Represent
particular
genes in
gametes
and how
they may
combine
Figure 4.5
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Test Cross
Identifies
parents with
an unknown
genotype
Figure 4.6
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Autosomal Inheritance
• Human autosomal traits are
located on the non sex
chromosomes (1-22)
• They may be inherited as
– Autosomal dominant or
– Autosomal recessive
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Autosomal Dominant
• Homozygous dominant
and heterozygotes
exhibit the affected
phenotype
• Males and females are
equally affected and
may transmit the trait
Figure 4.7
• Affected phenotype
does not skip
generation
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Autosomal Recessive
• Only homozygous
recessive individuals
exhibit the affected
phenotype
• Males and females
are equally affected
and may transmit the
trait
• May skip generations
Figure 4.8
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Box Figure 4.1
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Examples of
Mendelian Disorders in Humans
• See Table 4.2 in text, p. 80
• Thousands of examples are described in
the database
Online Mendelian Inheritance in Man
www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM
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Probability
• The likelihood that an event will occur
• The probability that a coin will land heads up is ½
• The probability that a heterozygous individual (Bb)
will produce a gamete with the B allele is ½
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Solving Genetics Problems
• List genotypes and phenotypes for the
trait
• Determine the genotypes of the parents
• Possible gametes
• Possible genotypes of offspring
• Repeat for successive generations
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Genetics Problems
Examples
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Meaning of Dominance and Recessive
• Whether an allele is dominant or
recessive is important in determining
risk and critical in medical genetics
• Reflect the characteristics or abundance
of a protein
• Recessive traits have “loss of function”
• Dominant traits have “gain of function”
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Maintaining Detrimental or Lethal
Alleles in a Population
• Recessive traits are maintained in a
population in the heterozygotes
• How can a dominant lethal allele such
as Huntington disease be maintained in
a population?
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Mendel’s Second Law
of Independent Assortment
• Two genes on different chromosomes
segregate their alleles independently
• The inheritance of one does not influence
the chance of inheriting the other
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Law of Independent Assortment
Figure 4.9
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Independent Assortment
of Two Traits
Figure 4.10
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Independent Events
The probability of simultaneous independent events
= the product of the probability of each event
Example:
• If both parents are heterozygous (Bb)) what is the
probability that they will produce a BB child?
• Probability of a sperm with B allele = ½
• Probability of a ova with B allele = ½
• Probability of a BB child is ½ X ½ = ¼
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Independent Events
• If both parents are
heterozygous for
two genes, what is
the probability of
having a
homozygous
recessive child?
Figure 4.11
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Figure 4.12
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Dependent Events
The probability of dependent events
= the sum of probability of each event
Example
•Parents are heterozygous for a trait, R.
•What is the chance that their child carries at least
one dominant R allele?
•Probability of child carrying RR = ¼
•Probability of child carrying Rr = ½
•Probability of child carrying R_ = ¼ + ½ = ¾
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Pedigrees
symbolic representations of family relationships
and inheritance of a trait
Figure 4.13
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A Pedigree with Consanguinity
Figure 4.14a
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Autosomal Recessive Inheritance
Albinism
Figure 4.15
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Autosomal Dominant Inheritance
Brachydactyly
Figure 4.16
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Inconclusive
Figure 4.17
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Genetic Predictions
Ellen’s brother Michael has
sickle cell anemia, an autosomal recessive
disease.
What is the probability that Ellen’s child has a
sickle cell anemia allele?
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Figure 4.18a
Ellen and Michael’s parents
must be heterozygous
S
s
S
SS
Ss
s
Ss
ss
Ellen is not affected and
cannot be aa
•Probability Ellen is a carrier = 2/3
•Probability child inherits sickle cell allele = ½
•Probability child carries sickle cell allele from Ellen
= 2/3 x 1/2 = 1/3
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Applied Questions Pedigrees
• Applied section pedigrees or other
examples could be added here for
instructors to use in classroom activities
• It may make the presentation
excessively long. Perhaps, just make it
a separate file.
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Achondroplasia
Text Figure 4.1
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Chands Syndrome
Text Figure 4.20
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Congenital Insensitivity to Pain
Text Figure 4.3
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Tay-Sachs Disease
Text Figure 4.4
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Sclerosteosis
Figure 4.5
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Freckles
Text Figure 4.6
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