PATTERNS OF INHERITANCE
GENETICS DEVELOPED FROM CURIOSITY ABOUT
INHERITANCE
GENETICS DEVELOPED FROM CURIOSITY
ABOUT INHERITANCE
• The Blending Hypothesis of Inheritance
– Traits are variations of particular characteristics.
• One plant might have red flowers, while another might
have purple flowers.
– The hypothesis in the early 1800s was the
blending hypothesis, which explained how
offspring inherited traits from both parents.
• Crossing yellow and red flowered plants would produce
orange flowered plants.
GENETICS DEVELOPED FROM CURIOSITY
ABOUT INHERITANCE
• All offspring would then be orange.
– This hypothesis was proven wrong after red
flowered plants produced yellow flowers and
some traits would disappear from one generation
to the next.
• Mendel’s Plant Breeding Experiments
– Gregor Mendel, a priest, was one of the first to
use the scientific method to the subject of
inheritance, giving rise to genetics, the study of
heredity.
GREGOR MENDEL
GENETICS DEVELOPED FROM CURIOSITY
ABOUT INHERITANCE
– Mendel developed the particulate hypothesis of
inheritance after working with pea plants for
seven years.
– According to the hypothesis, parents pass on to
their offspring genes responsible for the inherited
traits.
– These heritable factors retain their identity from
generation to generation.
– Mendel first identified true breeding plants that
produced identical offspring in each generation
after self fertilization.
GENETICS DEVELOPED FROM CURIOSITY
ABOUT INHERITANCE
• Purple always produced purple and white always
produced white.
– A bag was tied around each flower to prevent
cross pollination.
– He then crossed the true breeding purple plant
with the true breeding white plant. In a process
called cross-fertilization.
– Sperm of one plant fertilizes the eggs of a
different plant.
– He then wondered what the offspring traits would
be (flower colors).
MENDEL’S FERTILIZATION TECHNIQUE
DIFFERENT PEA PLANT TRAITS
REVIEW: CONCEPT CHECK 10.1, page 207
1. Explain how Mendel’s particulate hypothesis
is different from the blending hypothesis of
inheritance.
2. What is the difference between selffertilization and cross-fertilization?
3. Describe a pattern of inheritance that the
blending hypothesis fails to explain.
MENDEL DISCOVERED THAT INHERITANCE FOLLOWS
RULES OF CHANCE
• Mendel’s Principle of Segregation
– The offspring of two true breeding specimens are
hybrids.
– The parents are the P generation and the first
offspring the F₁ generation for filial.
– The F₂ generation is the product of the F₁ selffertilizing.
– In one experiment, Mendel crossed purple
flowered plants with white flowered plants, or a
monohybrid cross where the parents differ in only
one character.
P, F₁, & F₂ GENERATIONS
DOMINANT vs. RECESSIVE TRAITS
P, F₁, & F₂ GENERATIONS
P, F₁, & F₂ GENERATIONS
MENDEL DISCOVERED THAT INHERITANCE FOLLOWS
RULES OF CHANCE
• The F₁ hybrids were all purple.
• The F₂, after self-fertilization, were ¾ purple and ¼
white.
– Mendel concluded that there were two factors for
flower color, purple and white, each now called
genes.
– As seen in previous pictures, there were other
characteristics in the pea plants that Mendel
studied.
– The same pattern appeared in each F₁ generation.
MENDEL DISCOVERED THAT INHERITANCE FOLLOWS
RULES OF CHANCE
– As a result, Mendel developed four hypothesis
(use gene instead of factor):
1. There are alternative forms of genes (the gene for
flower color was purple in one form and white in
another) or alleles.
2. For each inherited character, the organism has two
alleles for the gene that controls that character, one
from each parent.
a.
b.
Both alleles the same, the individual is homozygous.
Both alleles different, the individual is heterozygous.
MENDEL DISCOVERED THAT INHERITANCE FOLLOWS
RULES OF CHANCE
3. The trait is dominant if only one of the two different
alleles affect or present that trait in the offspring, with
the allele being recessive, or non-appearing.
4. The two alleles for a character segregate (separate)
during the formation of gametes (sex cells) with each
gamete carrying only one allele for each character.
a. This is Mendel’s principle of segregation.
b. As the gametes join during fertilization, allele pairs
reform in the offspring.
MENDEL DISCOVERED THAT INHERITANCE FOLLOWS
RULES OF CHANCE
• Probability and Punnett Squares
– Crossing true-breeding (homozygous) purple with
white flowers will result in a purple (dominant)
flower,
P Purple(PP) X White(pp)
F₁ Purple(Pp)
where P is dominant and p is recessive, and
is heterozygous.
PENNY PROBABILITIES
X axis: 2 sides of penny 2 (½H + ½T)
Y axis: 2 sides of penny 1 (½H + ½T)
¼ HH
¼ HT
¼ HT
¼ TT
PUNNETT SQUARES
MENDEL DISCOVERED THAT INHERITANCE FOLLOWS
RULES OF CHANCE
– When the F₁ generation reproduces, each gamete
will receive only one allele for flower color, P or p,
with equal likelihood.
– The gametes will combine randomly and form
zygotes or pairs of alleles.
– The likelihood or probability of each pair forming
is key to the inheritance pattern in F₂.
– Punnett squares aid in the calculation of
probabilities by using grid patterns by showing all
the possible outcomes of a genetic cross.
MENDEL DISCOVERED THAT INHERITANCE FOLLOWS
RULES OF CHANCE
• Genotype and Phenotype
– Looking at the F₂ generation, the probabilities
using the Punnett square shows that ¼ of the F2
plants are homozygous for purple allele (PP).
– ½ (¼ +¼) of the F₂ are heterozygous (Pp), and be
purple, the dominant color.
– ¼ of the F₂ will be homozygous for the recessive
trait (pp) and be white.
MENDEL DISCOVERED THAT INHERITANCE FOLLOWS
RULES OF CHANCE
– Therefore, according to Mendel’s hypothesis, the
probabilities are ¾:¼ or 3:1, flower color (purple
to white).
– The phenotype is the observable trait and the
genotype is the genetic makeup, or combination
of alleles (PP, Pp, pp).
– The phenotypic ratio is 3 purple to 1 white and
the genotypic ratio is 1(PP):2(Pp):1(pp).
MENDEL DISCOVERED THAT INHERITANCE FOLLOWS
RULES OF CHANCE
• The Testcross
– A question arises when trying to determine the
genotype of an organism.
– For example, if a flower is purple, is it PP or Pp?
– To determine this, a testcross is done, which is
breeding an individual of an unknown genotype,
but dominant phenotype, with a homozygous
recessive individual.
TESTCROSS
TESTCROSS
MENDEL DISCOVERED THAT INHERITANCE FOLLOWS
RULES OF CHANCE
– The appearance of the cross will reveal the
genotype of the mystery plant.
– This is because the homozygous recessive plant
(pp) can contribute only a recessive allele to the
offspring, therefore the phenotype will specify the
allele contributed by the mystery plant.
• Mystery homozygous (PP) X homozygous recessive (pp)
yields heterozygous purple (Pp).
• Mystery heterozygous (Pp) X homozygous recessive
(pp) yields purple (Pp) and white (pp).
MENDEL DISCOVERED THAT INHERITANCE FOLLOWS
RULES OF CHANCE
• Mendel’s Principle of Independent
Assortment
– Mendel studied seven characteristics of the peas,
including shape and color.
• Round shape dominant to wrinkled
• Yellow color dominant to green
– How about a dihybrid cross, that is, crossing
organisms differing in two characters?
DIHYBRID CROSS
DIHYBRID CROSS
DIHYBRID CROSS
MENDEL DISCOVERED THAT INHERITANCE FOLLOWS
RULES OF CHANCE
– Mendel crossed true breeding round yellow seeds
(RRYY) with true breeding wrinkled green seeds
(rryy).
– The first parent produced only RY gametes, the
other ry.
– The union produced RrYy, hybrid heterozygotes,
with all having the dominant phenotype, round
and yellow.
– The hybrids grew into the F₁ plants, and were
allowed to self-fertilize.
MENDEL DISCOVERED THAT INHERITANCE FOLLOWS
RULES OF CHANCE
– Four phenotypes were produced in the F₂, RY, rY,
Ry, and ry, a ratio of 9 : 3 : 3 : 1.
• In fact, looking at the dihybrid Punnett square, this
shows the same outcome as two monohybrid crosses
occurring at the same time.
• Seed shape alone shows 12 plants with round seeds to
every 4 with wrinkled seeds, which is the 3 : 1 seen in
the monohybrid F₂ generation.
– Mendel tested every pea characteristic in the
dihybrid form and found that the 9:3:3:1 ratio
persisted.
MENDEL DISCOVERED THAT INHERITANCE FOLLOWS
RULES OF CHANCE
• As a result Mendel proposed the principle of
independent assortment.
• This states that gamete formation in F₂
crosses, a particular allele for one character
can be paired with either allele of another
character (R with a Y or y, r with a Y or y).
REVIEW: CONCEPT CHECK 10.2, page 213
1. What are the two possible gametes produced by a plant that
has the genotype Aa? Give the probability of each type of
gamete.
2. Use a Punnett square to predict the genotypes produced if the
plant in Question 1is self-fertilized. Calculate the probability
of each outcome.
3. List all the possible genotypes of a pea plant with purple
flowers and round seeds.
4. List the four possible allele combinations in the gametes of a
plant with genotype PpWw.
THERE ARE MANY VARIATIONS OF
INHERITANCE PATTERNS
• Intermediate Inheritance
– In the F₁ generations of Mendel, all the specimens
looked like the dominant homozygous parent
because there was one dominant allele to
produce the dominant phenotype.
– To see the recessive phenotype required two
recessive alleles.
– Some traits in organisms have neither allele as
dominant.
THERE ARE MANY VARIATIONS OF
INHERITANCE PATTERNS
– In these cases, the phenotype of the heterozygote
is intermediate between the phenotypes of the
two homozygotes, or is the intermediate
inheritance.
• A prime example is the Andalusian chicken.
• Black and white parents produce an F₁ hybrid called
“blues” that have grayish-blue feathers.
• Neither the black nor white allele is dominant, capital
and lower case letters are not used.
– C & B with a superscript B or W are used (CB or BW), with the
heterozygous chicken being CB CW , and blue.
INTERMEDIATE INHERITANCE
THERE ARE MANY VARIATIONS OF
INHERITANCE PATTERNS
– This is not the blending hypothesis because the
parent phenotypes can reappear in the F₂
generation.
– In the picture previously, the predicted
phenotypes in the F₂ are 1 black: 2 blue: 1 white,
and this ratio 1:2:1 is also the genotype.
• Multiple Alleles
– We have looked at no more than two contrasting
alleles for each inherited character, but in
populations several alleles exist for genes.
THERE ARE MANY VARIATIONS OF
INHERITANCE PATTERNS
– This expands the number of possibilities of
genotypes and phenotypes.
• Multiple alleles account for blood types.
– These typically are A, B, AB, or O with the letter referring to
the two carbohydrates (A and B) found on the surface of red
blood cells (rbc).
– So, a person’s rbc can be coated with one carbohydrate (type
A), or the other (type B), both (type AB), or none (type O).
– In the illustration to follow, there are various combinations
for the three alleles.
• IA (for carbohydrate A)
• IB (for carbohydrate B)
• i (for neither A or B)
BLOOD TYPE ALLELES
THERE ARE MANY VARIATIONS OF
INHERITANCE PATTERNS
• Each person inherits one of the alleles from each
parent, with six possible ways to pair the alleles, or six
possible genotypes.
• The alleles IA and IB exhibit codominance, or a
heterozygote expresses both traits.
• This is different from intermediate inheritance because
the phenotype is not intermediate but shows separate
traits of both alleles.
• Polygenic Inheritance
– Mendel’s pea plants exhibited seven characters
that occurred in two phenotypes.
THERE ARE MANY VARIATIONS OF
INHERITANCE PATTERNS
– Sometimes, intermediate inheritance,
codominance of alleles, or multiple types of alleles
can lead to more than two phenotypes in a
population.
– Multiple genes affecting characters leads to much
variation in phenotypes.
– Polygenic inheritance is when two or more genes
affect a single character.
– Height and skin color are just two examples.
• There could be 3 tall alleles (A, B, C) and 3 short alleles
(X, Y, Z).
POLYGENIC INHERITANCE
THERE ARE MANY VARIATIONS OF
INHERITANCE PATTERNS
– AABBCC would be the tallest, AXBBCC maybe next, and
XXYYZZ the shortest.
– The number of genes that affect a character
increases the potential combination of alleles.
• The Importance of Environment
– Environment has an affect on phenotype.
– Trees and plants are examples where leaves,
shape, and greenness are affected by wind and
sunlight.
THERE ARE MANY VARIATIONS OF
INHERITANCE PATTERNS
– Temperature can affect the fur color of a Siamese
cat, where black is found on the ears, face, feet,
and tail because these are cooler where black is
predominant.
– Nutrition can affect height, exercise body status,
sunlight darkens skin.
– The environment can even affect identical twins.
– Blood type is not affected by environment but
oxygen carrying capacity can be.
PHENOTYPES AND THE ENVIRONMENT
REVIEW: CONCEPT CHECK 10.3,
page 217
1. For a trait with intermediate inheritance, what is
the phenotypic ratio for F₂ offspring of a
monohybrid cross? How is that different from a
simple dominant-recessive cross?
2. Two parents have O blood. What blood type would
you expect for their child?
3. What is the likely mechanism for inheritance for a
character with large range of phenotypes? Explain.
4. Give three examples of human physical characters
affected by environment.
MEIOSIS EXPLAINS MENDEL’S PRINCIPLES
• Chromosome Theory of Inheritance
– From the 1800s to the beginning of the 20th
century, scientists discovered mitosis and meiosis
and saw the similarities between the behavior of
chromosomes and Mendel’s heritable factors.
– From this came the chromosome theory of
inheritance, which states that genes are located
on chromosomes, and the behavior of
chromosomes during meiosis and fertilization
accounts for inheritance patterns.
MEIOSIS EXPLAINS MENDEL’S PRINCIPLES
– Remember, during meiosis the chromosomes
undergo segregation and independent
assortment, Mendel’s two primary principles.
– Every diploid individual, pea plants or humans,
have two sets of homologous chromosomes.
• One from the male
• One from the female
– The alleles of a gene are in the same location on
the homologous chromosomes, or gene locus.
GENE LOCUS
CHROMOSOME THEORY OF INHERITANCE
MEIOSIS EXPLAINS MENDEL’S PRINCIPLES
– The homologous chromosomes can either have
the same allele or a different one at the locus,
making the organism either homologous or
heterozygous for each gene.
• Genetic Linkage and Crossing Over
– Genes located on separate chromosomes sort
independently of each other during meiosis.
– What happens to genes located on the same
chromosome?
MEIOSIS EXPLAINS MENDEL’S PRINCIPLES
– These alleles would normally not be sorted into
gametes independently but crossing over can
separate them.
– Genetic linkage is the tendency of the alleles on
one chromosome to be inherited together.
– Closeness to each other (genes) increases linkage.
– The further apart, crossover is more likely.
– Scientists can map distance between genes by
looking at the proportion of gametes with
recombined alleles.
MEIOSIS, LINKAGE, RECOMBINATION
REVIEW: CONCEPT CHECK 10.4,
page 219
1. Draw a picture of the possible arrangements
of chromosomes in the gametes produced by
a pea plant with genotype RRYy. Label the
gene loci. (Remember that these genes are
located on separate chromosomes).
2. Explain how the distance between two gene
loci on the same chromosome affects genetic
linkage.
SEX-LINKED TRAITS HAVE UNIQUE
INHERITANCE PATTERNS
• Sex-Linked Genes
– The sex chromosomes are the X and Y
chromosomes in many species.
– Eggs contain the X chromosome while the sperm
contains both the X and Y.
– Any gene located on the sex chromosome is a sexlinked gene.
• In humans, most of these are found on the X
chromosome.
SEX-LINKED GENES
SEX-LINKED TRAITS HAVE UNIQUE
INHERITANCE PATTERNS
– Thomas Hunt Morgan, while studying fruit flies,
discovered sex-linked genes.
• Normally they have red eyes, while white is rare.
• He crossed a white eyed male with a red eyed female,
and the F₁ offspring had red eyes (red eye dominant
allele).
• The F₂ crossed with each other showed the 3 : 1
phenotypic ratio of red to white.
• What surprised him was that none of the F₂ offspring
that had white eyes was female, indicating that eye
color must be related to sex.
SEX-LINKED TRAITS HAVE UNIQUE
INHERITANCE PATTERNS
– Morgan then thought that the gene involved must
be located on the X chromosome, as there was no
corresponding eye color locus on the Y
chromosome.
– The female(XX) carry two copies of the gene for
this character, and the male only one(XY).
– White is recessive, so females have to have the
while allele on both X chromosomes, while the
male needs only one on its X chromosome.
SEX-LINKED GENES
SEX-LINKED GENES
SEX- LINKED GENES
XRXR x XrY
↓
XRXr + XRY + XRXr + XRY [F₁ Generation]
↓
XRXR + XRY + XRXr + XrY [F₂ Generation]
SEX-LINKED TRAITS HAVE UNIQUE
INHERITANCE PATTERNS
• Sex-Linked Disorders
– Red-green color blindness and hemophilia are sexlinked, and, like the fruit fly, are seen more often
in males.
– Females must inherit two of the alleles, one from
each parent, to exhibit the trait.
SEX-LINKED DISEASES
REVIEW: CONCEPT CHECK 10.5,
page 221
1. Describe why one would expect to find more
white eyed fruit male fruit flies than whit
eyed females in natural populations.
2. A man with hemophilia and a homozygous
non hemophiliac woman have a son. Is it
possible that the son will inherit hemophilia?
Why not?
THE NUCLEUS CONTAINS AN
INFORMATION RICH GENOME
• DNA Packing in a Single Cell
– Each chromosome consists of one DNA molecule,
which is MUCH longer than the nucleus that
houses it.
• All the DNA in the 46 chromosomes placed end to end
would be 2 M long.
– A genome is the complete set of genetic material
in an organism, defined by its order of bases.
– Because of the unique property that DNA has for
folding, all of the genome can fit into the nucleus
HISTONES
NUCLEOSOMES
THE NUCLEUS CONTAINS AN
INFORMATION RICH GENOME
of a single cell.
– The DNA is wrapped around a protein called a
histone.
• Then it is wrapped in a tight helical fiber and coiled.
• The Human Genome Project
– Remember that the DNA double helix is a long
chain of nucleotides containing base pairs (A, T, C,
G)
– The order is different for each species and can
even be different within the species.
THE NUCLEUS CONTAINS AN
INFORMATION RICH GENOME
– By 1990, new DNA technology allowed scientists
to sequence the human genome that was
completed in a rough draft in 2000.
– Knowing just the sequence is only part of the
picture as knowing the functions of the
polypeptide chains in important.
– As we will see with Darwin, this project and
sequencing has allowed scientists to compare the
human genome to other species and embryonic
development.
HUMAN GENOME PROJECT
THE NUCLEUS CONTAINS AN
INFORMATION RICH GENOME
– Researchers all over the world can enter the
database for any research that they are doing.
– New medical treatments and medicines come
from knowing the genome.
– Reworking genetic sequences in genetic diseases
is now possible.
REVIEW: CONCEPT CHECK 12.1,
page 249
1. Draw a simple diagram showing the different
levels of DNA-packing within a nucleus.
2. How might a complete map of the human
genome be useful?
ACCIDENTS AFFECTING CHROMOSOMES
CAN CAUSE DISORDERS
• Down Syndrome
– Normal human karyotypes have 46 chromosomes,
or 23 pair.
– What if there are three chromosomes at the 21st
pair, instead of two?
– This is called a trisomy 21, which results from an
error during either stage of meiosis but more
commonly meiosis I.
– Most times , embryos with abnormal numbers of
TRISOMY 21 OR DOWN SYNDROME
ACCIDENTS AFFECTING CHROMOSOMES
CAN CAUSE DISORDERS
–
–
–
–
chromosomes results in spontaneous abortions
commonly known as a miscarriage.
Trisomy 21 embryos , for the most part, do
survive.
Its incidence is 1:700.
The syndrome associated with this defect is Down
syndrome, named after John Langdon Down who
described it in 1866.
The symptoms include characteristic facial
features, short height, heart defects, an impaired
FEATURES OF DOWN’S SYNDROME
ACCIDENTS AFFECTING CHROMOSOMES
CAN CAUSE DISORDERS
immune system, and differing degrees of mental
disability.
– Their life spans might be shorter than normal, but
they live happy and sometimes very productive
lives.
• Nonseparation of Chromosomes
– Trisomy 21 and other errors of chromosomes
number are usually caused by homologous
chromosomes or sister chromatids failing to
NONDISJUNCTION
ACCIDENTS AFFECTING CHROMOSOMES
CAN CAUSE DISORDERS
– separate during meiosis, called nondisjunction.
– If it occurs in meiosis I, it usually occurs during
anaphase I, as well as anaphase II during meiosis
II.
– The result is gametes with abnormal numbers of
chromosomes.
– For example, the sperm could fertilize an egg
having an extra chromosome 21, resulting in a
zygote with 3 copies of this chromosome rather
than 2.
ACCIDENTS AFFECTING CHROMOSOMES
CAN CAUSE DISORDERS
– The total number of chromosomes after
fertilization would be 47, not 46.
– Then, as you will remember, all the cells replicate
with 47 chromosome, resulting in an embryo with
47 chromosomes.
– The cause of nondisjunction is not fully known,
but occurs more frequently the later in life
pregnancy occurs.
– This would indicate that the older the mother is,
there is a difference in egg cell production.
ACCIDENTS AFFECTING CHROMOSOMES
CAN CAUSE DISORDERS
– Remember meiosis begins at puberty but has its
origin in the pre-egg cells in a girl’s ovaries before
birth.
– There is one egg produced and released each
month until menopause, around 50 y.o.
– The possibility then exists for a cell to be stopped
in the middle of meiosis for many years.
– The longer this period is, the greater the chance
that errors such as nondisjunction will occur, at
the completion of meiosis.
MATERNAL AGE AND DOWN
SYNDROME
ACCIDENTS AFFECTING CHROMOSOMES
CAN CAUSE DISORDERS
– One hypothesis for the cause, then, is the damage
to this cell during this lag time.
• Damaged Chromosomes
– Changes in chromosome structure can also lead to
disorders.
• Chromosomes can break, leading to new arrangements
that affect the genes.
– Duplication occurs when a part of the
chromosome is repeated.
ACCIDENTS AFFECTING CHROMOSOMES
CAN CAUSE DISORDERS
• When not fatal, they result in developmental
abnormalities.
– Deletion occurs when fragments of chromosomes
are lost.
• If a portion of the chromosomes is lost, then the genes
that code for those proteins are also lost.
• As proteins are essential to the body, losing some
might have deleterious effects on the body.
– Inversion is when a fragment is reversed from the
original.
DELETION, INVERSION,
TRANSLOCATION
ACCIDENTS AFFECTING CHROMOSOMES
CAN CAUSE DISORDERS
• Because most of the genes are still present, these are
less likely to cause harmful effects.
• Translocation happens when a fragment of one
chromosome attaches to a nonhomologous
chromosome.
• This can even occur in two different chromosomes
exchanging parts.
• Jumping Genes
– These are single genes that can move around that
Barbara McClintock, an American geneticist,
discovered this in the 1940s.
BARBARA McCLINTOCK AND
“JUMPING GENES”
ACCIDENTS AFFECTING CHROMOSOMES
CAN CAUSE DISORDERS
– She studied the genetic variation in corn and
found that some genetic material had the ability
to move from on location to another in a
chromosome, or to an entirely different
chromosome.
– This is not the same as translocation where a
whole piece of the chromosome moves but where
a gene moves.
– If they landed in the middle of other genes, and
they did, they could disrupt them.
TRANSPOSONS
ACCIDENTS AFFECTING CHROMOSOMES
CAN CAUSE DISORDERS
• Examples include pigment genes in corn, or spotted
kernels.
– These ‘jumping genes” are now called
transposons.
– Data now shows that all organisms have
transposons.
– Dr. McClintock won the Nobel prize for this in
1983.
– The transposon includes a gene that codes for an
enzyme which catalyzes movement of the gene by
ACCIDENTS AFFECTING CHROMOSOMES
CAN CAUSE DISORDERS
attaching to the ends of the transposon and
another site on the DNA.
– The enzyme then cuts the DNA and catalyzes
insertion of the transposon at the new site, and,
perhaps, disrupting the sequence of the new
gene.
REVIEW: CONCEPT CHECK 12.2,
page 253
1. What is the relationship between trisomy 21 and
Down syndrome? Describe how nondisjunction can
result in trisomy 21.
2. List and define four types of damage to
chromosome structure that can cause disorders.
3. What is a “jumping gene,” or transposon?
4. How is a mother’s age related to the probability of
nonseparation of chromosomes in her gametes?
MENDEL’S PRINCIPLES APPLY TO
HUMANS
• Working With Human Pedigrees
– Human subjects cannot control matting
capabilities such as we can with plants and
animals.
– What is done is to analyze the genetic patterns in
previous, current, and past family members.
– A trait such as attached or unattached earlobes
can be genetically transmitted to future
generations.
HUMAN PEDIGREES
MENDEL’S PRINCIPLES APPLY TO
HUMANS
– As you can see from the previous slide, Mendel’s
principles can be applied to this trait, knowing
what is dominant and recessive.
– A pedigree is like a family tree but it records and
traces the occurrence of phenotypic characters in
a family.
– Genotypes can be determined by examining the
patterns of the phenotype.
HUMAN PEDIGREES
MENDEL’S PRINCIPLES APPLY TO
HUMANS
• Disorders Inherited as Recessive Traits
– Pedigree analysis becomes more important when
the inherited alleles can cause serious problems.
– There are over one thousand genetic disorders
that have Mendelian inheritance patterns.
– A single gene, either dominant or recessive, cause
these disorders.
• Most human genetic disorders are recessive.
– The severity ranges from mild to lethal.
MENDEL’S PRINCIPLES APPLY TO
HUMANS
– Albinism (lack of skin and hair pigmentation) is
non-lethal while Tay-Sachs disease (nervous
system disorders is.
– Carriers are heterozygous for the trait, that is they
have one copy of the allele for the recessive
disorder.
– Using Mendel’s genetics and Punnett squares, one
can predict the probabilities of children of two
carriers that will have the disorder.
ALBINISM
TAY-SACHS DISEASE
INHERITED DEAFNESS
CYSTIC FIBROSIS
MENDEL’S PRINCIPLES APPLY TO
HUMANS
• Disorders Inherited as Dominant Traits
– This group is smaller in number than recessive
inheritance.
– Extra fingers and toes and achondroplasia
(dwarfism) are but two.
• In achondroplasia, the torso may be normal but the
arms and legs are short.
• The incidence is 1:25,000
• All people with this disorder are heterozygous (a single
copy of the allele) dominant.
ACHONDROPLASIA
MENDEL’S PRINCIPLES APPLY TO
HUMANS
• Homozygous for this condition is fatal.
– Dominant alleles in the population, that are fatal,
are rare because the affected individual dies
before producing any offspring.
– Recessive alleles that are fatal are more common
because they can pass from generation to
generation undetected as the heterozygote
before two carriers happen to produce a
homozygous offspring.
MENDEL’S PRINCIPLES APPLY TO
HUMANS
– There are lethal dominant disorders that don’t
cause death until adulthood.
– In these cases, the lethal gene could have already
been transmitted before the malady is known.
– Huntington’s disease is one of these disorders.
• In this case, a degeneration of the nervous system
occurs and does not begin until middle age, and is
eventually fatal.
• This disease has been tracked to chromosome 4 with a
test now available to detect this.
• Treatments by genetic engineering are already in the
works.
HUNTINGTON’S DISEASE
MENDEL’S PRINCIPLES APPLY TO
HUMANS
• Sex-Linked Disorders
– Previously, you learned most sex-linked alleles are
on the X chromosome.
• The male receives sex-linked alleles from his mother.
• The homologous Y chromosome is always inherited
from the father.
– The male, therefore, needs only one allele while
the female needs two to exhibit a trait.
– As previously noted, red-green colorblindness is
MENDEL’S PRINCIPLES APPLY TO
HUMANS
sex-linked and involves a break down of light
sensitive cells in the eyes.
– While rare, it is not impossible for females to
exhibit the trait.
– This would happen when the carrier female
married a male with the trait.
– 50% of the children, male or female, would have
the trait.
WHAT COLORBLIND PEOPLE SEE
MENDEL’S PRINCIPLES APPLY TO
HUMANS
• Predicting and Treating Genetic Disorders
– Genetic counselors collect and analyze data about
inheritance patterns and predict, explain, the
results and significance.
– Standard tests are performed on every baby born
in the US.
– One of these is Phenylketonuria, a recessive trait
in which the individual does not possess the
amino acid phenylalanine.
PHENYLKETONURIA
MENDEL’S PRINCIPLES APPLY TO
HUMANS
– Babies that test positive for this can be placed on
low phenylalanine diets that prevents the mental
disability that usually results from this disorder.
REVIEW: CONCEPT CHECK 12.3,
page 259
1. What information is collected to create a
pedigree for a particular trait?
2. Give examples for a recessive disorder, a
dominant disorder, and a sex-linked
recessive disorder, an describe how each is
inherited.
3. What does a genetic counselor do?
GENETIC CHANGES CONTIBUTE TO
CANCER
• Cancer Genes
– For the cell cycle to function properly, there are
two classes of genes that direct the production of
proteins for growth and division.
– One class of genes produce proteins called growth
factors which initiate cell division.
– The other class, tumor-suppressive genes,
produces proteins that stop cell division in certain
situations.
GENETIC CHANGES CONTIBUTE TO
CANCER
– This happens when cells contain damaged DNA or
when cells exceed a specific amount of space.
– Cancer can develop when several mutations to
these genes occurs.
– First, a mutation results in producing too much
growth factor or increasing growth factor activity.
– This gene may become a oncogene, or cancer
causing gene.
– These cells divide more frequently.
GENETIC CHANGES CONTIBUTE TO
CANCER
– Second, mutations to tumor-suppressor genes can
cause cancer.
– Remember, these stop cell division, and if there is
no control to stop them, the cells will continue to
divide out of control.
• “Inherited” Cancer
– Cancer is always a genetic disease in that it always
results from changes to DNA.
– The mutations that cause cancer start in the organ
where the cancer originated.
MUTATIONS AND CANCER
MUTATIONS AND CANCER
MUTATIONS AND CANCER
GENETIC CHANGES CONTIBUTE TO
CANCER
– These mutations do not affect the cells that give
rise to eggs or sperm, so they are not passed from
parent to child.
– There, however, can be a mutation to one or
more of these same genes in a cell that leads to a
gamete.
– These, then can be passed on to the offspring and
the risk of getting cancer.
• One of these is the mutated version of the BRAC1 gene,
a tumor-suppressing gene.
GENETIC CHANGES CONTIBUTE TO
CANCER
• Women who have this are at a greater risk for breast
cancer.
• It doesn’t mean that if one has this gene they will
develop cancer, just that they have a head start on
accumulating more mutations that can lead to cancer.
REVIEW: CONCEPT CHECK 12.4,
page 261
1. Compare and contrast the two classes of
genes involved in regulating the cell cycle.
2. Describe how a woman inheriting a mutated
BRCA1 gene is at a higher risk for breast
cancer.
3. What is an oncogene? What effect does an
oncogene have on a cell?