Essentials of Biology
Sylvia S. Mader
Chapter 10
Lecture Outline
Prepared by: Dr. Stephen Ebbs
Southern Illinois University Carbondale
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
10.1 Mendel’s Laws
• Gregor Mendel was a 19th century Austrian
monk who derived a series of laws that
described the genetic patterns of heredity from
one generation to the next.
• These laws account for the variability between
members of a family.
• Mendel derived these laws from his experiments
with pea plants.
10.1 Mendel’s Laws (cont.)
Mendel’s Experimental Procedure
• The study of peas was advantageous
because they can both self-pollinate and
cross-pollinate.
– When peas self-pollinate, the progeny are
genetically identical to the parent.
– When peas cross-pollinate, variation occurs.
• Mendel tracked traits such as seed color,
seed shape, and flower color.
Mendel’s Experimental Procedure
(cont.)
• Mendel applied mathematical laws of
probability to interpret the results.
• From the results Mendel derived the
particulate theory of inheritance.
• The particulate theory of inheritance is
based on the inheritance of particles from
each parent, which we now know are
genes.
Mendel’s Experimental Procedure
(cont.)
Mendel’s Experimental Procedure
(cont.)
Mendel’s Experimental Procedure
(cont.)
One-Trait Inheritance
• Mendel performed crosses with truebreeding pea lines to observe the results.
– The original parents were called the P
generation.
– The first generation of offspring were called
the first filial (F1) generation.
– The second generation of offspring were
called the second filial (F2) generation.
One-Trait Inheritance (cont.)
• The results of genetic crosses can be predicted
using a Punnett square.
• In a Punnett square, the possible male and
female gametes of each parent are arranged on
the horizontal and vertical axes.
• The squares represent every possible
combination of gametes that could combine to
form a zygote.
One-Trait Inheritance (cont.)
• The Punnett square can be used to
understand the results of one of Mendel’s
crosses.
– When true-breeding tall plants were crossed
with true-breeding short plants, the F1
generation was all tall.
– When the F1 generation self-pollinated, ¾ of
the progeny were tall and ¼ were short (3:1).
– Thus the F1 character for shortness was
passed on by the tall F1 generation.
One-Trait Inheritance (cont.)
• Mendel’s mathematical approach offered an
explanation for the 3:1 pattern.
– The F1 parents each contained one copy of the
hereditary factor for height, one dominant and one
recessive (they are heterozygous for the factor).
– The factors separated when the gametes were
formed during meiosis, each gamete would get either
the tall or short gene.
– When random fusion of the gametes occurred during
fertilization, the combinations were brought together
in a 3:1 ratio, as indicated by the Punnett square.
One-Trait Inheritance (cont.)
One-Trait Testcross
• A testcross can be performed to determine
if the F1 progeny carries a recessive
factor.
• A testcross crosses the F1 generation with
true-breeding tall plants to observe the
distribution of height in the progeny.
• A 1:1 ratio of tall:short in the progeny
confirms that a recessive factor is present.
One-Trait Testcross (cont.)
One-Trait Testcross (cont.)
• It is also possible that the F1 generation
carries two dominant factors (homozygous
for the factor).
• If true, then the testcross would produce
progeny that were all tall.
One-Trait Testcross (cont.)
One-Trait Testcross (cont.)
• From the results of these one-trait crosses,
Mendel formed the law of segregation.
– Each individual carries two factors for each trait.
– The factors segregate (separate) when gametes form
during meiosis.
– Each gamete contains only one factor from each pair.
– Fertilization gives each new individual two factors for
each trait.
The Modern Genetics View
• Mendel’s results can be stated in more
modern terms.
– Traits are controlled by two alleles.
– The dominant allele has the ability to mask
the recessive allele.
– The dominant allele is typically designated
with a capital letter and the recessive a
lowercase letter.
– Alleles occur on homologous chromosomes at
a specific location called the gene locus.
The Modern Genetics View
(cont.)
Genotype Versus Phenotype
• It is possible that two organisms with
different allelic combinations can have the
same outward appearance.
• For example, if the dominant allele for
finger length is “S” for short and the
recessive allele is “s” for long, the
individuals that are SS and Ss both have
short fingers.
Genotype Versus Phenotype (cont.)
• Separate terms are needed to describe each
condition.
• The genotype of an organism describes the two
alleles that are present and the condition that
this combination creates.
• For example, if an organism has two S alleles,
the genotype is SS, or homozygous dominant for
finger length.
Genotype Versus Phenotype (cont.)
• The phenotype of an organism refers to
the physical appearance.
• For example, organisms that were either
SS or Ss would have short fingers.
Genotype Versus Phenotype (cont.)
Two-Trait Inheritance
• Mendel also performed crosses between
plants that differed in two traits.
• One example was a cross between tall
plants with green pods and short plants
with yellow pods.
– Tall green pod plants are homozygous
dominant for both traits (TTGG).
– Short yellow pod plants are homozygous
recessive for both traits (ttgg).
Two-Trait Inheritance (cont.)
• The progeny from the cross (the F1 generation)
were allowed to self-pollinate to generate the F2
generation.
• From this cross, two possible patterns would be
expected in the F2 generation.
– If the dominant factors (T and G) segregate during
meiosis together, progeny will all be tall with green
pods.
– If the factors segregate separately, then four possible
phenotype could result.
Two-Trait Inheritance (cont.)
• The results of the cross produced four
phenotypes
– Tall with green pods
– Tall with yellow pods
– Short with green pods
– Short with yellow pods
• These results demonstrated that the two
factors segregated independently.
Two-Trait Inheritance (cont.)
• Based upon these results, Mendel
formulated the law of independent
assortment.
– Each pair of factors segregates (assorts)
independently of other factors.
– All possible combinations of factors occur in
the gametes.
Two-Trait Inheritance (cont.)
Two-Trait Inheritance (cont.)
Two-Trait Testcross
• Because the fruit fly Drosophila melanogaster
has a variety of heritable mutations, this insect
has been used extensively in genetic research.
• Two mutations displayed by this fly are wing
length and body color.
– Wild type (normal) flies have long wings and gray
bodies.
– Mutant flies can have short wings, ebony bodies, or
both.
– Both of these mutations are recessive traits.
Two-Trait Testcross (cont.)
• Fruit flies that have long wings and gray bodies
could be heterozygous or homozygous for each
trait.
• When the genotype is in doubt, it can be written
as L__G__ to indicate that there is one dominant
allele but the other is unknown.
• A two-trait testcross can be used to determine
genotype of an L__G__ organism.
Two-Trait Testcross (cont.)
• In a two-trait testcross, a dominant
L__G__ fly is crossed with a recessive fly
of known genotype (llgg).
• A heterozygous dominant fly, or dihybrid,
would produce four possible gametes.
– LG
– Lg
– lG
– lg
Two-Trait Testcross (cont.)
• The homozygous recessive fly can only
form gametes containing lg.
• Since the homozygous parent only
contributes lg, the other parent determines
the phenotype of the offspring.
• Since the heterozygous parent would
provide four gamete types, the offspring
should be present in a 1:1:1:1 ratio.
Two-Trait Testcross (cont.)
Two-Trait Testcross (cont.)
• But if the parent is homozygous dominant
(LLGG), then the gametes would only
contain LG.
• In this case, the testcross would produce
offspring that had only the dominant
phenotypes.
Mendel’s Laws and Probability
• Mendel realized that the results of his genetic
crosses followed rules of probability.
• The rule of multiplication says that the chance of
two events occurring together is the product of
their chances of occurring separately.
• For example, the chance of getting two tails
when you flip two coins is:
½X½=¼
Mendel’s Laws and Probability
(cont.)
• Since alleles in a two-trait cross segregate
independently, we can determine the probability
of each allele pair.
Ll X ll: Probability of ll = ½
Gg X gg: Probability of gg = ½
• The probability of obtaining the llgg genotype:
½X½=¼
Mendel’s Laws and Meiosis
• Mendel’s laws are intimately related to the
events of meiosis.
• The laws of segregation and independent
assortment relate to the separation of
homologous chromosomes during Meiosis
I.
Mendel’s Laws and Meiosis
(cont.)
Mendel’s Laws and Meiosis
(cont.)
10.2 Beyond Mendel’s Laws
• Mendel’s study of inheritance dealt with
simple, independently-segregating traits.
• There are other patterns of inheritance
other than the dominance/recessive
relationship Mendel observed.
• The environment can also influence the
phenotype of an organism.
Incomplete Dominance
• In incomplete dominance, the progeny show a
phenotype intermediate to the parents.
• For example in four-o’clock flowers, a cross
between parents with red or white flowers yields
progeny with pink flowers.
• In humans, wavy hair is an example of
incomplete dominance for curly and straight hair.
Incomplete Dominance (cont.)
Multiple-Allele Traits
• For some factors, such as human blood
type, there are more than two alleles.
– IA = A antigen on red blood cells
– IB = B antigen on red blood cells
– i = Neither A or B antigen on red blood cells
• Each person has only two of these
possible three alleles.
Multiple-Allele Traits (cont.)
• The combination of these alleles produce
a person’s blood type.
– IA and IB alleles are dominant over i.
– IAi and IAIA genotypes produce type A blood.
– IBi and IBIB genotypes produce type B blood.
– The ii genotype produces type O blood.
– IA and IB alleles are codominant, meaning that
neither is dominant over the other.
– The IAIB genotype produces type AB blood.
Multiple-Allele Traits (cont.)
Polygenic Inheritance
• In polygenic inheritance, more than one pair of
alleles determines the phenotype.
• Each dominant allele is additive to the overall
phenotype.
• Human skin color is a polygenic trait.
– Skin color is determined by three genes.
– The more dominant alleles a person has, the darker
their skin color.
Polygenic Inheritance (cont.)
Polygenic Inheritance (cont.)
• Multifactorial traits are polygenes that are also
controlled by environmental influences.
• There are several examples of multifactorial
traits.
–
–
–
–
Skin color
Palate and lip disorders
Allergies
Some cancers
Environment and the Phenotype
• In some cases, the environment can affect
phenotype more than genetics.
• For example, temperature can affect the
color of primroses and Himalayan rabbits.
• The control of phenotype by genetics
and/or environment leads to the nature
versus nurture concept.
Environment and the Phenotype
(cont.)
Pleiotropy
• Pleiotropy occurs when one gene has
more than one effect.
• Pleiotropy is often seen in human disease,
leading to syndromes, or groups of
symptoms related to a genetic mutation.
– Marfan syndrome
– Sickle cell disease
Pleiotropy (cont.)
10.3 Sex-Linked Inheritance
• The sex chromosomes, X and Y, not only
determine gender but carry genes.
• The Y allele carries 26 genes mostly
related to gender.
• The X allele carries genes for gender as
well as genes unrelated to gender, the Xlinked genes.
10.3 Sex-Linked Inheritance
(cont.)
X-Linked Alleles
• Drosophila genetics can be used to
understand sex linked genes.
• Consider a cross between a red-eyed
female and a white-eyed male.
– The F1 generation all had red eyes.
– The F2 generation showed the typical 3:1
ratio, but all white-eyed flies were males.
– The eye color gene is carried on the X
chromosome as an X-linked, mutant allele.
X-Linked Alleles (cont.)
An X-Linked Problem
• X-linked genes are indicated as the X
chromosome with an allele.
– XR = red eyes in Drosophila
– Xr = white eyes in Drosophila
• There are several possible genotypes.
–
–
–
–
–
XR XR = red-eyed female
XR Xr = red-eyed female
Xr Xr = white-eyed female
XRY = red-eyed male
XrY = white-eyed male
An X-Linked Problem (cont.)
• Heterozygous XRXr females are carriers of
the white-eyed trait
– The white-eyed trait is present but not shown.
– Carriers are capable of passing that trait on to
offspring.
• In this case, males can’t be X-linked
carriers because the X-linked gene has to
be expressed.
10.4 Inheritance of Linked
Genes
• Some genes are linked, meaning that they
are carried on the same chromosome.
• Linked genes are inherited together
because they cannot segregate during
meiosis.
• The linked alleles on the same
chromosome form a linkage group.
10.4 Inheritance of Linked
Genes (cont.)
• Since Drosophila has only four
chromosomes, there must be many genes
on each.
• For example, chromosome II has genes
for eye color, wing type, body color, leg
length, and antennae type.
10.4 Inheritance of Linked
Genes (cont.)
Constructing a Chromosome
Map
• The relative position of genes on a chromosome
can be illustrated with a chromosome map.
• The crossing-over that can occur during meiosis
can be used to construct a chromosome map.
• Recall that if crossing-over does occur, the
gametes produced are recombinant gametes.
Constructing a Chromosome
Map (cont.)
Constructing a Chromosome
Map (cont.)
• Consider a cross between a heterozygous
gray-bodied red-eyed fly and black-bodied
purple-eyed fly.
• Because these traits are linked, a 1:1 ratio
of the progeny is expected.
Constructing a Chromosome
Map (cont.)
Constructing a Chromosome
Map (cont.)
• However, when the cross is performed a
small percentage of the offspring have
either gray bodies and purple eyes or
black bodies and red eyes.
• This mixing of traits from the two parents
occurs when there is crossing-over
between the genes of a linkage group
during meiosis I.
Constructing a Chromosome
Map (cont.)
Linkage Data
• The frequency of a crossing-over event between
two genes in a linkage group is proportional to
the distance between them.
• By convention, a cross-over frequency of 1%
indicates a distance between the two genes of 1
map unit.
• If the frequency of recombinant phenotypes in
the offspring is 6%, the genes are 6 map units
apart.
Linkage Data (cont.)
• The frequency of crossing-over can be used to
map multiple genes on a chromosome.
• For example, assume a series of crosses were
performed to determine the distance between
three pairs of alleles on a single chromosome.
– Black body
– Purple eyes
– Vestigial wings
Linkage Data (cont.)
• From the frequency of recombinant
progeny, the distances between alleles are
determined and a map created.
– The distance between black-body and purpleeye alleles is 6 map units.
– The distance between purple-eye and
vestigial wing alleles is 12.5 map units.
– The distance between black-body and
vestigial wing alleles is 18.5 map units.
Linkage Data (cont.)