Genetics: Mendel and Beyond
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Genetics: Mendel and Beyond
By the end of this chapter you should be able to:
• Describe Mendel’s Experiments and the Laws of
Inheritance
• Predict inheritance patterns of monohybrid and
dihybrid crosses using a punnett square
• Explain examples of gene interactions
• Distinguish between genes and chromosomes
• Explain sex determination and
• Predict sex-linked inheritance
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The Foundations of Genetics
• Applied genetics (plant and animal breeding) has
been used for five thousand years ago or more.
• The foundation for the science of genetics is
credited to Gregor Mendel who used varieties of
peas to conduct experiments on inheritance.
• Mendel’s research was ignored until the turn of
the twentieth century.
• Meiosis provides an explanation for Mendel’s
theory.
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The Foundations of Genetics
• Plants have some desirable characteristics for
genetic studies:
 They can be grown in large quantities.
 They produce large numbers of offspring.
 They have relatively short generation times.
 Many have both male and female reproductive
organs, making self-fertilization possible.
 It is easy to control which individuals mate.
Figure 10.1 A Controlled Cross between Two Plants
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Mendel’s Experiments and the Laws of Inheritance
• Mendel looked for characters that had welldefined alternative traits and that were truebreeding.
• Mendel developed true-breeding strains to be
used as the parental generation, designated P.
• The offspring from the cross of the P parents are
called the first filial generation, designated F1.
• When F1 individuals are crossed to each other or
self-fertilized, their progeny are designated F2.
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Mendel’s Experiments and the Laws of Inheritance
• Mendel’s experiment 1:
 A monohybrid cross involves one character
(seed shape) and different traits (spherical or
wrinkled).
 SS x ss → Ss
 The F1 seeds were all spherical; the wrinkled
trait failed to appear at all.
 Because the spherical trait completely masks
the wrinkled trait when true-breeding plants
are crossed, the spherical trait is considered
dominant and the wrinkled trait recessive.
We represent the dominant trait with a capital
letter and the recessive trait with a small case
letter.
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Mendel’s Experiments and the Laws of Inheritance
• Mendel’s experiment 1 continued:
 The F1 generation was allowed to self-pollinate
to produce F2 seeds.
 Ss x Ss → 3Ss, 1 ss
 3 are heterozyous smooth and 1 is
homozygous wrinked.
 In the F2 generation, the ratio of spherical
seeds to wrinkled seeds was 3:1.
Figure 10. 3 Mendel’s Experiment 1 (Part 1)
Figure 10. 3 Mendel’s Experiment 1 (Part 2)
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Mendel’s Experiments and the Laws of Inheritance
• From these results, Mendel reached several conclusions:
 The units responsible for inheritance are discrete
particles that exist in pairs and separate during
gamete formation; this is called the particulate
theory.
 Each pea has two units of inheritance for each
character.
 During production of gametes, only one of the pair for
a given character passes to the gamete.
 When fertilization occurs, the zygote gets one unit
from each parent, restoring the pair.
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Mendel’s Experiments and the Laws of Inheritance
• Mendel’s units of inheritance are called genes (a
portion of the chromosomal DNA that resides at a
specific locus and codes for a particular function);
different forms of a gene are called alleles.
• True-breeding individuals have two copies of the
same allele (i.e., they are homozygous).
• Some smooth-seeded plants are Ss or
heterozygous.
• The physical appearance of an organism is its
phenotype (what it looks like); the actual
composition of the organism’s alleles for a gene is
its genotype. Homozygous dominant;
heterozygous dominant; homozygous recessive
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• An organism’s trait does not always reveal its
genetic composition. Why?
• Heterozygous genotypes yield phenotypes
showing the dominant trait.
• The same phenotype can result from different
genotypes.
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Mendel’s Experiments and the Laws of Inheritance
• Mendel’s first law is called the law of
segregation: The two alleles of a trait segregate
(separate) during meiosis.
• Each gamete receives one member of a pair of
alleles.
• Determination of possible allelic combinations can
be accomplished by a Punnett square.
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Punnett Square
• Another way to demonstrate this is through the use
of a punnett square
S
S
s
Ss
Ss
s
Ss
Ss
S
s
S
SS
Ss
s
Ss
ss
• Since one character with two contrasting traits are
being crossed, this is called a monohybrid cross
Figure 10.4 Mendel’s Explanation of Experiment 1
Figure 10.5 Meiosis Accounts for the Segregation of Alleles (Part 1)
Figure 10.5 Meiosis Accounts for the Segregation of Alleles (Part 2)
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Mendel’s Experiments and Laws of Inheritance
• How do we determine if a purple flowering plant is
SS or Ss?
• We could cross the purple flowering plant of
unknown genotype with a true breeding purple
flowering plant or a white flowering plant
• SS x SS (unknown) or SS x Ss (unknown)
all spherical
all spherical
• ss x SS (unknown) or ss x Ss (unknown)
all spherical
2 spherical: 2 wrinkled
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Mendel’s Experiments and the Laws of Inheritance
• Crossing an unknown with a homozygous
recessive is called a test cross.
• An individual with a dominant trait is crossed with
a true-breeding recessive (homozygous
recessive).
• The appearance of the recessive phenotype in
half the offspring indicates that the parent is
heterozygous.
Figure 10.6 Homozygous or Heterozygous?
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Mendel’s Experiments and the Laws of Inheritance
• Mendel’s second law, the law of independent
assortment, states that alleles of different genes
assort into gametes independently.
• This can be shown by using a dihybrid crosses.
• For example, in pea plants purple flowers are
dominant over white flowers and green pods are
dominant over yellow pods.
• Cross a purebreeding purple flowering plant with
green pods with a white flowering plant with
yellow pods.
• Random fertilization of gametes results in all
heterozygous offspring.
•
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Dihybrid Crosses
• PPGG x ppgg = PpGg
• If we allow these plants to self-pollinate, PpGg x
PpGg, what are the possible offspring?
• Each parent could produce four different gametes
– PG, Pg, pG, or pg
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Dihybrid Crosses
PG
PG
Pg
pG
pg
Pg
pG
pg
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Dihybrid Crosses
PG
Pg
pG
pg
PG PPGG PPGg PpGG PpGg
Pg PPGg PPgg PpGg Ppgg
pG PpGG PpGg ppGG ppGg
pg
PpGg Ppgg ppGg ppgg
• Putting these into a punnett square results
in a 9:3:3:1 ratio
Figure 10.7 Independent Assortment
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Mendel’s Experiments and the Laws of Inheritance
• Humans cannot be studied using planned
crosses.
• Therefore, human geneticists rely on pedigrees.
• Human pedigrees do not show clear proportions.
• Outcomes for small samples fail to follow the
expected outcomes closely.
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Mendel’s Experiments and the Laws of Inheritance
• If neither parent has a given phenotype, but it
shows up in their offspring, the trait is recessive
and the parents are heterozygous.
• Half of the children from such a cross will be
carriers (heterozygous for the trait).
• The chance of any one child’s getting the trait is
1/4.
Figure 10.11 Recessive Inheritance
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Mendel’s Experiments and the Laws of Inheritance
• A pedigree analysis of the dominant allele for
Huntington’s disease shows that:
 Every affected person has an affected parent.
 About half of the offspring of an affected
person are also affected (assuming only one
parent is affected).
 The phenotype occurs equally in both sexes.
Figure 10.10 Pedigree Analysis and Dominant Inheritance
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Polydactyl
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figure 10-11.jpg
Figure 10.11
Figure 10.11
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Albinism
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Huntington’s Disease
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Marfan’s
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Marfan’s
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Alleles and Their Interactions
• Differences in alleles of genes consist of slight
differences in the DNA sequence at the same
locus, resulting in slightly different protein
products.
• This is a mutation. Alleles can mutate randomly.
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Alleles and Their Interactions
• A population can have more than two alleles for a
given gene.
• In rabbits, coat color is determined by one gene
with four different alleles. Five different colors
result from the combinations of these alleles.
• Even if more than two alleles exist in a population,
any individual can have no more than two of
them: one from the mother and one from the
father.
• Also send in ABO blood typing.
Figure 10.12 Inheritance of Coat Color in Rabbits
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Alleles and Their Interactions
• A white snapdragon crossed with a red
snapdragon, gives an intermediate phenotype –
pink
• Incomplete Dominance
• In the case of snapdragons, one allele codes for
an enzyme that leads to the formation of red
pigment. The other allele does not code for
pigment production.
Figure 10.13 Incomplete Dominance Follows Mendel’s Laws
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• The F2 offspring, however, demonstrate
Mendelian genetics. For self-fertilizing F1
pink flowers the F2 progeny have a
phenotypic ratio of 1 red:2 pink:1 white.
• Other examples include roan cattle and
blue Andalusian chickens
• Tay Sachs Disease is also an example of
incomplete dominance.
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Incomplete Dominance
• Persons with Tay Sachs lack a crucial enzyme to
metabolize a type of lipid. The lipids accumulate
in the brain interfering with normal function.
Causes regression of nervous system, - blind,
deaf, bedridden, inability to move limbs
• Cherry red spot in retina is one indication
• Heterozygotes have an intermediate level of the
lipid metabolizing enzyme
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Tay Sachs
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Tay Sachs
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Alleles and Their Interactions
• In codominance, the two different alleles are both
expressed in the heterozygotes.
• In the human ABO blood group system the alleles for
blood type are IA, IB, and IO. We inherit two of these
three alleles.
 Two IA, or IA and IO, results in type A.
 Two IB, or IB and IO, results in type B.
 Two IO results in type O.
 IA and IB results in type AB. The alleles are called
codominant.
Figure 10.14 ABO Blood Reactions Are Important in Transfusions
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ABO Genetic Problems
• A couple have their blood typed before marriage.
They both are AB. What types of blood might
their children have? Explain
• A woman sues a man for child support. She has
type A blood, her child type O, and the man type
B. Could the man be the father? Why or why not?
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ABO Genetic Problems
• A wealthy, elderly couple die together in a car
accident. Soon a young man shows up to claim
their fortune, contending that he is their only son
who ran away from home when he was a young
man. Other relative dispute his claim. Hospital
records show that the deceased couple were
blood types AB and O. The claimant is type O.
Do you think the claimant is an impostor? Explain.
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Pleiotropic alleles
• Most genes have multiple phenotypic effects.
• An example is the coloration pattern and crossed
eyes of Siamese cats, which are both caused by
the same allele.
• These unrelated characters are caused by the
same protein produced by the same allele.
• Another example is sickle cell anemia.
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Sickle Cell Anemia
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Gene Interactions
• In epistasis a gene at one locus alters the
phenotypic expression of a gene at a second
locus.
• An example is coat color in mice:
 The B allele produces a banded pattern, called
agouti. The b results in unbanded hairs.
 The genotypes BB or Bb are agouti or banded.
The genotype bb is black.
 Another locus determines if any coloration
occurs. The genotypes AA and Aa have color
and aa are albino.
 Cross two AaBb mice. What is the
phenotype?
Figure 10.15 Genes May Interact Epistatically
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Gene Interactions
• When two homozygous strains of plants or animals
are crossed, the offspring are often phenotypically
stronger, larger, and more vigorous than either
parent.
• This phenomenon is called hybrid vigor.
Hybridization is now a common agricultural
practice used to increase production in plants.
Figure 10.16 Hybrid Vigor in Corn
The Environment Affects Gene Action
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• Genotype and environment interact to determine
the phenotype
• Environmental variables such as light,
temperature, and nutrition can affect the
translation of genotype into a phenotype
• Examples – Siamese cats, hydrangea flowers
• Twin Studies
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Genes and Chromosomes
• Homologous chromosomes can exchange
corresponding segments during prophase I of
meiosis (crossing over).
• Genes that are close together tend to stay
together.
• The farther apart on the same chromosome
genes are, the more likely they are to separate
during recombination.
Figure 10.19 Crossing Over Results in Genetic Recombination
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Sex Determination in Humans
• Sex chromosomes carry genes that determine
whether male or female gametes are produced.
• In humans, the Y chromosome has a sexdetermining region - SRY
• The SRY gene codes for a functional protein. If
this protein is present, testes develop; if not,
ovaries develop.
• Some XY individuals lacking a small portion of the
Y chromosome are phenotypically female.
• Some XX individuals with a small piece of the Y
chromosome are male.
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Sex linked traits
• The Y chromosome carries few genes (20). The
X chromosome carries many genes. This
difference generates a special type of inheritance
called sex-linked inheritance
• Two well-know traits carried on the X
chromosome are colorblindness and hemophilia.
• Sex linked traits tend to be expressed with greater
frequency in males.
• Can you explain why?
• This is due to the fact that many of these diseases
are only on the X chromosome and not the Y
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Hemophilia
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Colorblindness
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Figure 10.24 Red-Green Color Blindness Is a Sex-Linked Trait in Humans
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Sex Determination and Sex-Linked Inheritance
• Pedigree analysis of X-linked recessive phenotypes:
 The phenotype appears much more often in
males than in females.
 A male with the mutation can pass it only to his
daughters.
 Daughters who receive one mutant X are
heterozygous carriers.
 The mutant phenotype can skip a generation if
the mutation is passed from a male to his
daughter and then to her son.
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Sex Determination and Sex-Linked Inheritance
• Disorders can arise from abnormal sex
chromosome constitutions.
• Turner syndrome is characterized by the XO
condition and results in females who physically
are slightly abnormal but mentally normal and
usually sterile.
• The XXY condition, Klinefelter syndrome, results
in males who are taller than average and always
sterile.
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Non-Nuclear Inheritance
• Mitochondria, chloroplasts, and other plastids
possess a small amount of DNA.
• Some of these genes are important for organelle
assembly and function.
• Mitochondria and plastids are passed on by the
mother only, as the egg contains abundant
cytoplasm and organelles.
• A cell is highly polyploid for organelle genes.
• Organelle genes tend to mutate at a faster rate.