Gregor Mendel
• Strong background in
plant breeding and
mathematics
• Using pea plants, found
indirect but observable
evidence of how parents
transmit genes to
offspring
In an Abbey Garden
– Mendel studied garden peas because they
• were easy to grow,
• came in many readily distinguishable varieties,
• are easily manipulated, and
• can self-fertilize.
• Hybrids are the offspring of two different purebred
varieties.
– The parental plants are the P generation.
– Their hybrid offspring are the F1 generation.
– A cross of the F1 plants forms the F2 generation.
Monohybrid Cross
Experimental intercross between
two F1 heterozygotes
AA X aa
Aa (F1 monohybrids)
Aa X Aa
?
How Did Gregor Mendel Lay the
Foundation for Modern Genetics?
• The relationships among genes, alleles,
and chromosomes
Monohybrid Crosses
– Mendel developed four hypotheses from the
monohybrid cross, listed here using modern
terminology (including “gene” instead of “heritable
factor”).
1. The alternative versions of genes are called alleles.
Genes
• Units of information about specific traits
• Passed from parents to offspring
• Each has a specific location (locus) on a
chromosome
Alleles
• Different molecular forms of a gene
• Arise by mutation
• Dominant allele masks a recessive allele
that is paired with it
Monohybrid Crosses
2. For each inherited character, an organism inherits two
alleles, one from each parent.
– An organism is homozygous for that gene if both alleles are
identical.
– An organism is heterozygous for that gene if the alleles are
different.
Allele Combinations
• Homozygous
– having two identical alleles at a locus
– AA or aa
• Heterozygous
– having two different alleles at a locus
– Aa
Monohybrid Crosses
3. If two alleles of an inherited pair differ,
– then one determines the organism’s appearance and is called the
dominant allele and
– the other has no noticeable effect on the organism’s appearance
and is called the recessive allele.
4. Gametes carry only one allele for each inherited character.
– The two alleles for a character segregate (separate) from each other
during the production of gametes.
– This statement is called the law of segregation.
Monohybrid Crosses
– Geneticists distinguish between an organism’s
physical appearance and its genetic makeup.
• An organism’s physical appearance is its phenotype.
• An organism’s genetic makeup is its genotype.
Mendel’s Theory
of Segregation
• An individual inherits a unit of information
(allele) about a trait from each parent
• During gamete formation, the alleles
segregate from each other
Dihybrid Cross
Experimental cross between
individuals that are heterozygous for
different versions of two traits
Mendel’s Law of Independent
Assortment
– A dihybrid cross is the mating of parental varieties
differing in two characters.
– What would result from a dihybrid cross? Two
hypotheses are possible:
1. dependent assortment or
2. independent assortment.
Mendel’s Law of Independent Assortment
– Mendel’s dihybrid cross supported the hypothesis
that each pair of alleles segregates independently of
the other pairs during gamete formation.
– Thus, the inheritance of one character has no effect
on the inheritance of another.
– This is called Mendel’s law of independent
assortment.
– Independent assortment is also seen in two
hereditary characters in Labrador retrievers.
Using a Testcross to Determine an
Unknown Genotype
– A testcross is a mating between
• an individual of dominant phenotype (but unknown
genotype) and
• a homozygous recessive individual.
The Rules of Probability
– Mendel’s strong background in mathematics helped
him understand patterns of inheritance.
– The rule of multiplication states that the probability
of a compound event is the product of the separate
probabilities of the independent events.
Tremendous Variation
Number of genotypes possible in offspring as a
result of independent assortment and hybrid
crossing is
3n
(n is the number of gene loci
at which the parents differ)
Summary of Mendel's Results:
1. The F1 offspring showed only one of the two parental traits,
and always the same trait.
2. Results were always the same regardless of which parent donated
the pollen.
3. The trait not shown in the F1 reappeared in the F2
in about 25% of the offspring.
4. Traits remained unchanged when passed to offspring:
they did not blend in any offspring but behaved as separate units.
5. Reciprocal crosses showed each parent made
an equal contribution to the offspring.
Mendel's Conclusions:
1. Evidence indicated factors could be hidden or
unexpressed, these are the recessive traits.
2. The term phenotype refers to the outward appearance
of a trait, while the term genotype is used for the
genetic makeup of an organism.
3. Male and female contributed equally to the offsprings'
genetic makeup: therefore the number of traits was
probably two (the simplest solution).
4. Upper case letters are traditionally used to denote
dominant traits, lower case letters for recessives.
Gene - a unit of inheritance that usually is directly responsible for one trait or
character.
Allele - an alternate form of a gene. Usually there are two alleles for every gene,
sometimes as many a three or four.
Homozygous - when the two alleles are the same.
Heterozygous - when the two alleles are different, in such cases the dominant allele
is expressed.
Dominant - a term applied to the trait (allele) that is expressed regardless
of the second allele.
Recessive - a term applied to a trait that is only expressed when the second allele is
the same (e.g. short plants are homozygous for the recessive allele).
Phenotype - the physical expression of the allelic composition for the trait under
study.
Genotype - the allelic composition of an organism.
Punnett squares - probability diagram illustrating the possible offspring of a mating.
Dominance Relations
Complete dominance
Incomplete dominance
Codominance
VARIATIONS ON MENDEL’S LAWS
– Some patterns of genetic inheritance are not
explained by Mendel’s laws.
– In incomplete dominance, F1 hybrids have an
appearance between the phenotypes of the two
parents.
Codominance: ABO Blood Types
• The gene that controls ABO type codes for the
enzyme that dictates structure of a glycolipid
on blood cells
• Two alleles (IA and IB) are codominant when
paired
• Third allele (i) is recessive to others
Pleiotropy
• Alleles at a single locus may have effects on
two or more traits
– Sickle-Cell anemia
-results in abnormal hemoglobin proteins, and
-causes disk-shaped red blood cells to deform
into a sickle shape with jagged edges.
Continuous Variation
• A more or less continuous range of small
differences in a given trait among individuals
• The greater the number of genes and
environmental factors that affect a trait, the
more continuous the variation in versions of
that trait
Environmental Effects on Phenotype
• Genotype and environment interact to
produce phenotype
– Himalayan rabbit ice pack experiment
– Transplantation of plant cuttings to different
elevations
– Human depression
THE CHROMOSOMAL BASIS OF
INHERITANCE
– The chromosome theory of inheritance states that
• genes are located at specific positions (loci) on
chromosomes and
• the behavior of chromosomes during meiosis and
fertilization accounts for inheritance patterns.
Genetic Recombination: Crossing Over
• Crossing over can
– separate linked alleles,
– produce gametes with recombinant gametes, and
– produce offspring with recombinant phenotypes.
• The percentage of recombinant offspring among the
total is called the recombination frequency.
Crossover Frequency
Proportional to the distance that separates
genes
A
B
C
D
Crossing over will disrupt linkage between A
and B more often than C and D
Linked Genes
– Linked genes
• are located close together on a chromosome and
• tend to be inherited together.
– Thomas Hunt Morgan
• used the fruit fly Drosophila melanogaster and
• determined that some genes were linked based on the
inheritance patterns of their traits.
– Dominant traits are not necessarily
• normal or
• more common.
– Wild-type traits are
• those seen most often in nature and
• not necessarily specified by dominant alleles.
Family Pedigrees
– A family pedigree
• shows the history of a trait in a family and
• allows geneticists to analyze human traits.
The Y Chromosome
• Fewer than two dozen genes identified
• One is the master gene for male sex
determination
– SRY gene (sex-determining region of Y)
• SRY present, testes form
• SRY absent, ovaries form
The X Chromosome
• Carries more than 2,300 genes
• Most genes deal with nonsexual traits
• Genes on X chromosome can be expressed
in both males and females
Pedigree
Symbols
male
female
marriage/mating
offspring in order of birth,
from left to right
Individual showing trait being
studied
sex not specified
I, II, III, IV...
generation
Genetic Abnormality
• A rare, uncommon version of a trait
• Polydactyly
– Unusual number of toes or fingers
– Does not cause any health problems
– View of trait as disfiguring is subjective
Genetic Disorder
• Inherited conditions that cause mild to severe
medical problems
• Why don’t they disappear?
– Mutation introduces new rare alleles
– In heterozygotes, harmful allele is masked, so it
can still be passed on to offspring
Human Disorders Controlled by a
Single Gene
– Many human traits
• show simple inheritance patterns and
• are controlled by single genes on autosomes.
Autosomal
Dominant Inheritance
Trait typically appears in
every generation if both
parents carry the gene.
(Huntington disease,
Achondroplasia)
Achondroplasia
• Autosomal dominant inheritance
• In homozygous form usually leads
to stillbirth
• Heterozygotes display a type of dwarfism
• Have short arms and legs relative to other
body parts
Recessive Disorders
– Most human genetic disorders are recessive.
– Individuals who have the recessive allele but appear
normal are carriers of the disorder.
Recessive Disorders
– Prolonged geographic isolation of certain
populations can lead to inbreeding, the mating of
close relatives.
– Inbreeding increases the chance of offspring that are
homozygous for a harmful recessive trait.
Autosomal Recessive Inheritance
Patterns
• If parents are both heterozygous,
child will have a 25% chance of
being affected
SEX CHROMOSOMES AND SEX-LINKED
GENES
– Sex chromosomes influence the inheritance of
certain traits. For example, humans that have a pair
of sex chromosomes designated
• X and Y are male or
• X and X are female.
X-Linked Recessive Inheritance
• Males show disorder more than
females
• Son cannot inherit disorder from
his father
Sex-Linked Genes
– Any gene located on a sex chromosome is called a
sex-linked gene.
• Most sex-linked genes are found on the X chromosome.
• Red-green colorblindness is
– a common human sex-linked disorder and
– caused by a malfunction of light-sensitive cells in the eyes.
Examples of X-Linked Traits
• Color blindness
– Inability to distinguish among some
or all colors
• Hemophilia
– Blood-clotting disorder
– 1/7,000 males has allele for hemophilia A
– Was common in European royal families
Aneuploidy
• Individuals have one extra or less
chromosome
• (2n + 1 or 2n - 1)
• Major cause of human
reproductive failure
• Most human miscarriages are
aneuploids
Polyploidy
• Individuals have three or more of each type
of chromosome (3n, 4n)
• Common in flowering plants
• Lethal for humans
– 99% die before birth
– Newborns die soon after birth
How Accidents during Meiosis Can Alter Chromosome
Number
– If nondisjunction occurs, and a normal sperm
fertilizes an egg with an extra chromosome, the
result is a zygote with a total of 2n + 1
chromosomes.
– If the organism survives, it will have
• an abnormal karyotype and
• probably a syndrome of disorders caused by the abnormal
number of genes.
Down Syndrome
• Trisomy of chromosome 21
• Mental impairment and a variety of
additional defects
• Can be detected before birth
• Risk of Down syndrome increases dramatically when
mothers are over age 35
Turner Syndrome
• Inheritance of only one X (XO)
• 98% spontaneously aborted
• Survivors are short, infertile females
– No functional ovaries
– Secondary sexual traits reduced
– May be treated with hormones, surgery
Klinefelter Syndrome
• XXY condition
• Results mainly from nondisjunction in mother
(67%)
• Phenotype is tall males
– Sterile or nearly so
– Feminized traits (sparse facial hair, somewhat
enlarged breasts)
– Treated with testosterone injections
XYY Condition
• Taller than average males
• Most otherwise phenotypically normal
• Some mentally impaired
• Once thought to be predisposed to criminal
behavior, but studies now discredit
Phenotypic Treatments
• Symptoms of many genetic disorders can be
minimized or suppressed by
– Dietary controls
– Adjustments to environmental conditions
– Surgery or hormonal treatments
Testing for Genetic Disorders
• Carrier screening
• Prenatal diagnosis
– Amniocentesis
– Chorionic villi sampling
– Fetoscopy
• Preimplantation diagnosis