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Chapter 4
Genetics: From Genotype to
Phenotype
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From Genotype to Phenotype
Structural Genes
Regulatory Genes
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From Genotype to Phenotype (cont’d)
Genotype
Phenotype
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From Genotype to Phenotype (cont’d)
• The ABO Blood Type System
– Recessive, dominant and co-dominant alleles
• Obesity: A Complex Interaction
– Genes, environment, and phenotype
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Mendelian Genetics
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Gregor Mendel (1822-1884)
Experiments in particulate inheritance
Discovered dichotomous variation
Developed a series of postulates about
inheritance
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Gregor Mendel
 A monk living in what is
now the Czech
Republic
 Crossed different
strains of purebred
plants and studied
their progeny.
 His work illustrates the
basic rules of
inheritance.
Mendel’s Postulates
• Hereditary characteristics are controlled by
particulate unit factors that exist in pairs in
individual organisms
• When an individual has two different unit
factors responsible for a characteristic, only
one is expressed and is said to be dominant to
the other, which is said to be recessive
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Mendel’s Postulates (cont’d)
• Mendel’s Law of Segregation: during the
formation of gametes, the paired unit
factors separate or segregate randomly so
that each sex cell receives one or the other
with equal likelihood
• Mendel’s Law of Independent Assortment:
during gamete formation, segregating pairs
of unit factors assort independently of each
other
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Hybrids
 Offspring of parents that differ from each
other with regard to certain traits or certain
aspects of genetic makeup; heterozygotes.
Traits Mendel Studied in Peas
Principle of Segregation
 Discrete units, or genes, occur in pairs because
chromosomes occur in pairs.
 During gamete production, members of each gene
pair separate so each gamete contains one member
of a pair.
 During fertilization, the full number of chromosomes
is restored and members of a gene or allele pairs
are reunited.
Results When One Trait (Height)
Is Considered at a Time
Dominance
 Dominant traits are governed by an allele that can
be expressed in the presence of another, different
allele.
 Dominant alleles prevent the expression of
recessive alleles in heterozygotes.
Recessiveness
 Recessive traits are not expressed in
heterozygotes.
 For a recessive allele to be expressed, there
must be two copies of the allele.
Alleles
 Alternate forms of a gene.
 Alleles occur at the same locus on a pair of
chromosomes and influence the same trait.
 However, because they are slightly different, their
action may result in different expressions of that
trait.
 The term is sometimes used synonymously with gene.
Locus
 The position on a chromosome where a
given gene occurs.
 The term is sometimes used
interchangeably with gene, but this usage
is technically incorrect.
Phenotypes
 The observable or detectable physical
characteristics of an organism; the detectable
expressions of genotypes, frequently
influenced by environment.
Principle of
Independent Assortment
 The distribution of one pair of alleles into
gametes does not influence the distribution of
another pair.
 The genes controlling different traits are
inherited independently of one another.
Random Assortment
 The chance distribution of chromosomes to
daughter cells during meiosis; along with
recombination, a source of genetic variation
(but not new alleles) from meiosis.
Mendelian Traits
 Characteristics that are influenced by alleles
at only one genetic locus.
 Examples include many blood types, such as
ABO.
 Many genetic disorders, including sickle- cell
anemia and Tay-Sachs disease, are also
Mendelian traits.
Mendelian Inheritance in Humans
 Over 19,000 human traits are known to be inherited
according to Mendelian principles (Online Mendelian
Inheritance in Man www.ncbi.nlm.nih.gov/omim/)
 The human ABO blood system is an example of a
simple Mendelian inheritance.
 The A and B alleles are dominant to the O allele.
 Neither the A or B allele are dominant to one
another; they are codominant and both traits are
expressed.
Codominance
 The expression of two alleles in
heterozygotes.
 In this situation, neither allele is dominant or
recessive; thus, both influence the phenotype.
What Dominance Doesn’t Mean
 Complete determinant of phenotype
 Better or stronger
Polygenic Inheritance
 Polygenic traits, or continuous traits, are
governed by alleles at two or more loci, and
each locus has some influence on the
phenotype.
 Hair, eye and skin color are polygenic traits
Linkage and Assortment
• Linkage: genes found on the same
chromosome are said to be linked. The closer
together two genes are, the greater the
linkage
• Crossing Over: makes possible the
independent assortment of linked genes
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X-Linked Disorders
Genetic conditions that result from mutations to genes
on the X chromosome. They are almost always
expressed in males, who have only one copy
of the X chromosome.
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Mendelian Genetics in Humans
• Over the past century, hundreds of human
disorders and diseases have been
cataloged, which can be explained in terms
of Mendelian genetic transmission
– Earlobe form
– Phenylthiocarbamide (PTC) tasting or nontasting
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Mendelian Genetics in Humans (cont’d)
• The Online Mendelian Inheritance in Man
(OMIM) Web site:
(http://www.ncbi.nlm.nih.gov/entrez/quer
y.fcgi?db=OMIM ) provides an
extraordinary database on genetic
conditions in humans, from the most
innocuous to the most lethal
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Genetics Beyond Mendel
• Qualitative variation
• Phenotypic variation that can be characterized as
belonging to discrete, observable categories
• Quantitative variation
• Phenotypic variation that is characterized by the
distribution of continuous variation within a
population
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Genetics Beyond Mendel (cont’d)
• Polygenic Traits, the Phenotype, and the
Environment
– Heritability
• The proportion of total phenotypic variability observed for a
given trait that can be ascribed to genetic factors
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Genetics Beyond Mendel (cont’d)
Variability caused by genetics
Variability caused by genetics + Variability caused by the environment
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Genetics Beyond Mendel (cont’d)
• Heritability and IQ Test Score Performance
– IQ Test Score Performance
• Exhibit continuous variation in human populations, with a
normal distribution
• Genetics is an important factor in producing the variation
observed within populations
• Both genetics and environment effect IQ score variation
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Phenylketonuria: Illustrating Mendelian
and Post-Mendelian Concepts
• Autosomal recessive condition that leads to the
accumulation of large quantities of the amino acid
phenylalanine, which causes mental retardation and
other phenotypic abnormalities
– Screening in newborns
– Nutritional prognosis
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Genes and Environments
• Environment
– From a gene’s perspective the “environment” is
made up of other genes
• The genetic environment is just as critical to the
production of phenotypes as any other kind of
environment
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