Transcript 2015 Pearson Education, Inc.

Chapter
5
Chromosomes and
Inheritance
Lecture Presentation
by Wendy Kuntz
© 2015 Pearson Education, Inc.
Chapter 5 Chromosomes and Inheritance:
Unit Hyperlinks
• 5.1 Cell division
• 5.2 What is structure of chromosome and
what is DNA?
• 5.3 Cell cycle?
• 5.4 Mitosis
• 5.5 Cytokinesis
• 5.6 Gametes
• 5.7 Meiosis
• 5.8 Mitosis vs. meiosis
• 5.9 Genetic variation
© 2015 Pearson Education, Inc.
Chapter 5 Chromosomes and Inheritance:
Unit Hyperlinks
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5.10 Meiosis mistakes
5.11 Mendelian genetics
5.12 Punnett square
5.13 Independent assortment
5.14 Pedigrees
5.15 Complex inheritance
5.16 Linked genes
5.17 Sex-linked genes
5.18 Clones
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5.1 Opening Questions: Cell birth and death
Did you know that between 50 and 70
billion of your cells die each day?
• Is your body making any new cells right
now? What kind?
• Are certain types of cells replaced
faster? What might be examples?
• Are certain types of cells never replaced
or slowly replaced? What might be
examples?
• Different cells have different life spans
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5.1 All living organisms consist of cells.
• A fundamental concept
in biology is the cell
theory, which states:
1. All life is cellular.
(all organisms are either a
single cell or made of
multiple cells)
2. All cells arise from
preexisting cells.
Some living organisms have just
one cell, but others have
trillions.
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5.1 Cell division is the formation of new
cells from preexisting cells
Cell division
provides for
1. Growth
2. Repair
3. Reproduction
•
Organisms use
cell division to
reproduce
sexually or
asexually
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Organisms can use cell
division to reproduce
sexually or asexually.
5.1 Sexual reproduction takes two parents
• Two parents produce
genetically unique
offspring.
• Gametes (egg and
sperm cells) are
formed via cell
division from adult
cells in the gonads
(testes and ovaries).
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5.1 Life cycle in sexual reproduction:
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5.1 Asexual reproduction only needs one
parent
• One parent produces
genetically identical
offspring.
• There is no sperm or egg
involved.
• Examples
• Protists- amoeba
• Plants - strawberries
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Asexual reproduction
Examples
Amoeba ( a protist) divides in two amoeba
Star fish – broken limb can grow into new star
fish
An African violet leaf can generate a new plant
(clone)
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5.1 Sexual vs. asexual reproduction
Complete the comparison table:
Sexual
Number of parents needed
Gametes? (yes/no)
Fertilization? (yes/no)
Number of chromosome sets
Offspring genetically unique?
(yes/no)
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Asexual
5.1 Sexual vs. asexual reproduction
Sexual
Asexual
Number of parents needed
2
1
Gametes? (yes/no)
YES
NO
Fertilization? (yes/no)
YES
NO
Number of chromosome sets
2
1
Offspring genetically unique?
(yes/no)
YES
NO
© 2015 Pearson Education, Inc.
5.2 Opening Questions: What is DNA?
• What type of information is stored in DNA?
• How different is your DNA from the person
sitting next to you?
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5.2 Opening Questions: What is DNA?
• What type of information is stored in DNA?
• How different is your DNA from the person
sitting next to you?
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5.2 DNA and genes:
• All life on Earth uses DNA as
the genetic material.
• The nucleus of every eukaryotic
cell contains long strands of
DNA complexed with proteins
called chromosomes.
• Each chromosome contains
genetic information in genes.
• A chromosome contains many
genes
– Average number of genes
per human chromosome is
around 1000
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5.2 DNA and genes:
• A gene is a length of DNA
that codes for the proteins
that make up our bodies.
• Genes are the unit of
inheritance
Genes are the unit
of inheritance.
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5.2 A closer look at the chromosome
• Inside the nucleus,
the chromosomal
DNA is wound
around proteins;
together they form
chromatin.
Most of the time
chromosomes are
unraveled as loose
chromatin.
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550,000x
11,000x
450x
5.2 Chromosome number: every human
body cell has 46 chromosomes
How many
chromosomes
did you inherit
from your
mother?
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5.2 Chromosomes at cell division have
unique properties
At cell division
chromosomes
1. Become tightly
packed
2. Duplicate-are double stranded
The two strands of the
chromosomes are called sister
chromatids.
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5.2 Chromosomes duplicate prior to division
• Sister chromatids
are joined at the
centromere.
• Replication of the
DNA occurs prior
to cell division
• Replication occurs
during the S phase
of the cell cycle
Formation of sister chromatids
means the cell is preparing to
divide.
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5.3 Opening Questions: Cell cycle
• Healthy cells only start dividing if there
is a need for replication.
Provide at least three examples of times
when a cell (animal, plant, protist) would
need to go through cell division.
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5.3 Opening Questions: Cell cycle
• Unhealthy cells may undergo
unregulated cell division.
• Cancer begins when a cell divides
although it should not.
What are some things you know about
cancer?
Why might cancer be so difficult to treat?
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5.3 Cells have regular cycles of growth and
division
• The cell cycle is an ordered sequence of
events in the “lifetime” of a cell.
• There are two broad phases:
1. Interphase
90% of cell’s lifetime
Normal cell functions
G1,S,G2
2. Mitotic phase
Active cell division (P,M,A,T)
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5.3 The cell cycle:
Healthy cells
only enter the
mitotic phase
if duplication
is needed.
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5.3 Most of a cell’s lifetime is spent in
interphase
During interphase, the cell
• Performs its normal functions
• Grows (G1 and G2 phase)
• Prepares for division by duplicating its
chromosomes ( S phase)
What are some normal
functions of cells?
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5.3 Active cell division is the mitotic phase
During the mitotic phase, the cell
• Undergoes active division (mitosis)
• Splits into two offspring cells
(cytokinesis)
The result of the mitotic
phase is two genetically
identical offspring cells.
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5.4 Opening Questions: Mitosis puzzle
The cells below are all undergoing the
process of cell division.
•
•
•
•
A
B
A. prophase
B. metaphase
C. anaphase
D. telophase
C
D
5.4 Mitosis is active cell division
Mitosis occurs in major stages.
• These stages help us think about how the
chromosomes are organized during
mitosis.
• However, cell division proceeds
seamlessly through all the stages.
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5.4 Interphase:
Early interphase (G1)
Cell is carrying out
its normal activities.
Chromosomes
duplicate (S)
Cell is preparing to
divide; generates
sister chromatids.
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5.4 Stages of mitosis
ProphaseChromosomes condense
Nuclear membrane dissolves.
Cell lays down mitotic spindle.
Metaphase
Chromosomes align
Sister chromatids line
up and attach to mitotic
spindle.
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5.4 Stages of mitosis
Anaphase
Chromosomes split
Sister chromatids are
pulled apart as mitotic
spindle retracts.
Telophase
Nucleus reforms
Two duplicated nuclei are
formed.
Cytokinesis occurs at the
end of telophase
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5.5 Cytokinesis is the final step in cell
division
• Cytokinesis is the
division of the
cytoplasm and is
the final step in the
cell cycle.
• The process of
cytokinesis is
different for plant
and animal cells
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5.5 Cytokinesis in animal cells:
• The parent animal cell is pinched into two,
leaving two independent offspring cells.
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5.5 Cytokinesis in plant cells:
• Plant cells divide their cytoplasm
by forming a cell plate
along the center
line of the cell.
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5.5 Review Questions:
Many chemotherapy drugs are used to treat
cancer by killing cells undergoing rapid
mitosis.
• What side effects have you heard of related
to chemotherapy treatment for cancer?
• With your understanding of mitosis, can
you explain some of the side effects of
chemotherapy?
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5.6 Opening Questions: How do you get one
from two?
• Most of the cells in your
body are diploid; they
have two copies of each
chromosome.
If your cells are diploid, how
could you reproduce without
doubling the chromosome
number in your offspring?
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5.6 Gametes are the answer!
• To prevent doubling
chromosome number
in offspring, sexually
reproducing organisms
need to make cells with a
single set of chromosomes.
• Gametes, or sex cells,
are haploid: they contain
only one copy of each
chromosome.
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5.6 Male and female gametes:
• Male gametes are called sperm.
– Human haploid number N=23
• Female gametes are called eggs.
– Same for the egg N=23
How many
chromosomes
are there in a
human sperm
cell?
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5.6 The human life cycle:
ADULTS
Every somatic cell in
your body is diploid,
with one set of
chromosomes derived
from your mother and
one from your father.
DEVELOPMENT
Through repeated
rounds of cell division, the
original zygote cell is duplicated,
eventually forming an embryo,
then a baby, and finally an adult.
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GAMETE FORMATION
In the adult gonads (testes in
males and ovaries in females),
a special kind of cell division
produces gametes. The male
gamete is the sperm and the
female gamete is the egg. As
the only haploid cells in your
body, gametes can be used
to form the next generation.
During fertilization, the
gametes (male sperm
and female egg) fuse.
Each contributes a
haploid number of
chromosomes to produce
a diploid zygote.
ZYGOTE
The zygote, or fertilized egg,
is the original cell that was
formed by the fusion of sperm
and egg. The zygote contains
one haploid set of chromosomes from the father and
one haploid set of chromosomes from the mother that
together make a unique
diploid set of chromosomes.
5.6 One pair of chromosomes makes a
person male or female
• The 46 chromosomes in human body are
organized as 23 homologous pairs.
• Of these, 22 pairs are autosomes.
• One pair is the sex chromosomes.
– Females are XX.
– Males are XY.
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5.6 Karyotypes are photographic inventories
of chromosomes taken at metaphase
• Chromosomes are
organized in
homologous pairs
from large to small
Can you tell if this a
male or a female?
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Metaphase
chromosomes
Normal male and female karyotypes
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5.7 Opening Questions: Why do we need to
produce sperm and eggs (gametes)?
• Explain why all sexually reproducing
organisms need both haploid and diploid
cells.
Remember:
Haploid cells have only one copy of each chromosome.
Diploid cells have two copies of each chromosome.
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5.7 Meiosis is the production of gametes
• Gametes (sperm and egg) are formed by
a special type of cell division, meiosis.
• Cells produced from meiosis are haploid.
• Like mitosis, meiosis occurs in stages.
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5.7 Meiosis occurs in stages
• Meiosis (like mitosis) starts with
chromosome duplication before division.
• In meiosis, there are then two rounds of
cell division.
• The result of meiosis is four haploid
offspring cells, all with one-half the number
of chromosomes.
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5.7 Meiosis interphase
In meiosis interphase, chromosomes
duplicate. After interphase, cells that are
producing gametes undergo two rounds of
division called meiosis I and meiosis II.
Remember that
chromosomes
come in
homologous
(matched)
pairs.
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Meiosis I
MEIOSIS I: HOMOLOGOUS CHROMOSOMES SEPARATE
INTERPHASE
Centrosomes
(with centriole pairs)
PROPHASE I
Sites of
crossing over
Spindle
Nuclear Chromatin
envelope
Chromosomes
duplicate.
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Sister
chromatids
Pair of
homologous
chromosomes
Homologous
chromosomes
pair up and
exchange
segments.
METAPHASE I
Microtubules
attached to
chromosome
ANAPHASE I
Sister chromatids
remain attached
Centromere
Pairs of
homologous
chromosomes
line up.
Pairs of
homologous
chromosomes
split up.
Meiosis II
MEIOSIS II: SISTER CHROMATIDS SEPARATE
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
AND
CYTOKINESIS
Sister
chromatids
separate
Haploid
daughter
cells forming
Cleavage
furrow
Two haploid
cells form;
chromosomes
are still doubled.
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During another round of cell division, the sister
chromatids finally separate; four haploid
daughter cells result, containing single
chromosomes.
5.8 Opening Questions: Meiosis vs. mitosis
Try to complete the comparison table below:
Meiosis
Where does it occur?
When is it needed?
How many/type offspring cells?
(haploid/diploid)
How many rounds of cell
divisions?
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Mitosis
5.8 Mitosis and meiosis compared
Meiosis
Mitosis
Where does it occur?
Ovaries/
testes
All body
cells
When is it needed?
Puberty
Lifetime
How many/type offspring cells?
(haploid/diploid)
How many rounds of cell
divisions?
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4 haploid 2 diploid
2
1
5.9 Opening Questions: How unique are
you?
• What is the probability that another human
on earth shares the exact same DNA as
you?
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5.9 Opening Questions: How unique are
you?
• What is the probability that another human
on earth shares the exact same DNA as
you?
Unless you have an identical twin,
you are genetically different from
any human that has every lived!
How is that possible?!
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5.9 Sexual reproduction leads to variation
Three major processes mean variation is
the norm for sexual reproduction:
1. Independent assortment
2. Random fertilization
3. Crossing over
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5.9 Independent assortment of
chromosomes leads to variation
• Chromosomes line
up by homologous
pairs during meiosis I.
• Maternal and paternal
chromosomes are
shuffled randomly.
Independent assortment:
223 = 8 million possible
arrangements of
chromosomes!
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Chromosomes
are shuffled
5.9 Random fertilization by sperm and egg
leads to variation
• The probability that
any one sperm will
fertilize any
particular egg is
extremely small.
Random fertilization:
8 million x 8 million =
64 trillion possible
arrangements of
chromosomes.
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Chromosomes
are shuffled
5.9 Crossing over during meiosis leads to
variation
• Chromosomes can
“swap” genetic
material, creating new,
unique combinations.
• Crossing over occurs
when homologous
chromosomes line up
during meiosis I,
during prophase I
Crossing over creates new hybrid chromosomes,
which increases gene variation.
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5.10 Opening Questions: What if meiosis
goes wrong?
• Jason and Laura are pregnant with their
third child. Since they are both over 35,
they opt to have an amniocentesis test.
The doctor comes back to them with a
karyotype that shows 47 chromosomes.
Imagine you are Jason and Laura’s
genetic counselor. How would you
explain the results?
© 2015 Pearson Education, Inc.
5.10 Meiosis can have mishaps!
• Non-disjunction is when
chromosomes fail to
separate properly.
• Resulting gametes will
have too few or too many
chromosomes.
Zygotes with abnormal chromosome number will
usually not develop or will have abnormalities.
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5.10 Examples of nondisjunction
• Trisomy 21 is a
condition in which
a person receives
three copies of
chromosome 21.
• The resulting
condition is called
Down syndrome.
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5.10 Examples of nondisjunction
• Sex chromosome
nondisjunction can
also occur.
• Each combination
of extra or missing
sex chromosomes
produces its own
syndromes.
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5.10 Other Examples of nondisjunction
•
•
•
•
Kleinfelters syndrome XXY (47 )
Jacob’s syndrome XYY (47)
Turner’s syndrome X0 (45)
Triple-X –XXX (47)
5.11 Opening Questions: Did you inherit
your good looks?
• Why do children resemble their parents?
• Why do families resemble each other?
• Is there anything you can’t inherit from
your parents?
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5.11 Our understanding of genetics starts
with Mendel
• Heredity is the transmission of
traits from one generation to the
next.
• Genetics is the study of
heredity.
• Gregor Mendel was the first to
deduce the basic principles of
inheritance.
• He introduced the concept of
dominance and recessive traits
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5.11 Character and traits are inherited
• Human eye color is a character,
or an inherited feature that varies
among individuals.
• Each possible variation of a
character is a trait.
• Blue eyes are recessive trait so in
order to have blue eyes you
inherit a recessive allele from
each parent (bb)
• Brown eyes are dominant
• So if you are either BB or Bb you
would have brown eyes
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5.11 Alleles are the individual units of
inheritance
• Traits derive from genes.
• Alternate forms of a particular gene are called alleles.
• Recessive alleles are indicated by a small letter (a)
while dominant alleles are indicated by a capital letter (A)
Matched set of
chromosomes,
one derived
from the father
(blue) and one
derived from the
mother (red)
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5.11 Genotype vs. phenotype
• An organism’s
phenotype is its
physical traits.
• Phenotypes are
indicated by word
descriptors ( like purple
flower or white flower)
• An organism’s genotype
is its underlying genetic
make-up, the alleles it is
carrying. Genotypes are
indicated by letters
• PP, Pp, pp
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5.11 Dominant vs. recessive alleles
• An individual who is
heterozygous has two different
alleles.
• A heterozygous purple flower
plant would be Pp
• The dominant allele will usually
determine an organism’s
appearance.
• Homozygous recessive pp plants
would have white flowers
In pea plants, purple (P)
is the dominant trait for
flower color. White (p) is
the recessive trait.
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5.12 Opening Questions: Can we predict the
inheritance patterns of genes?
• For some people the chemical PTC
(phenylthiocarbamide) tastes very bitter;
yet for others, it is tasteless. Scientists
report that the ability to taste PTC shows a
general pattern of dominant inheritance
(T and t).
What is the genotype of a non-taster?
Could a non-taster’s parents be tasters?
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5.12 A Punnett square can be used to
predict the results of a genetic cross
• In a genetic cross, two parents
(P generation) are crossed to
produce offspring (F1 generation).
×
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A Punnett square
can be used to
predict the offspring
that will result from a
genetic cross.
5.12 Punnett squares are predictions
• Punnett squares are named after a British
geneticist: Reginald Punnett.
• A Punnett square allows you to predict the
genotype and phenotype of the offspring.
• The simplest Punnett
square follows one
trait is a
monohybrid cross.
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Monohybrid crosses
If you cross a homozygous
dominant black Labrador
retriever (BB) with a
homozygous recessive
chocolate Labrador retriever
dog
(bb) the first generation F1
will all have black coats and
be heterozygous (Bb)
When you cross two
heterozygous (Bb) dogs you
will get a 3:1 ratio of black
dogs to chocolate dogs
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4:0
Monohybrid
crosses
Three types of
crosses
1. BB x bb (4:0)
2. Bb x Bb (3:1)
3. Bb x bb (2:2)
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• If you cross a
heterozygous black
Labrador retriever (Bb)
with a homozygous
recessive chocolate dog
(bb)
• You get 2:2 ratio
• 2 dogs which are
heterozygous (Bb) black
and 2 that are
homozygous (bb)
5.12 Punnett square: Monohybrid cross
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5.12 Alleles separate during meiosis
• The law of segregation states that the
two alleles for a character separate during
gamete formation.
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5.12 We can use a test cross to determine
an individual’s genotype
Is the genotype
of this black Lab
BB or Bb? To find
out, mate it with
a chocolate Lab.
×
Chocolate Lab
with genotype bb
For the test cross above, predict offspring
ratios for each possible genotype.
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5.12 Test cross results if black dog is BB:
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5.12 Test cross results if black dog is Bb:
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5.13 Opening Questions: Can we look at
more than one trait?
• In Labrador retrievers, breeders need to
keep track of traits for coat color (black is
dominant to chocolate) and hearing
(normal is dominant to deafness).
If you want all chocolate
coats, how would you
avoid breeding deaf
puppies?
© 2015 Pearson Education, Inc.
5.13 Alleles separate independently during
gamete formation
• The law of independent assortment states that
the inheritance of one character has no effect on
the inheritance of another. As long as the genes
are on separate chromosomes
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Which is dominant?
• There is a recessive gene for deafness
(d) on one chromosome. Normal hearing
(D) is dominant. So a dog must inherit two
recessive alleles to be deaf.
• the gene for coat color is on another
chromosome. Black (B) is dominant over
chocolate (b)
• So black dogs would be either BB or Bb
• For a dog to have a chocolate colored coat
he must inherit two recessive alleles (bb)
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5.13 Traits for coat color and hearing are an
example of independent assortment
All four kinds
of gametes are
equally likely
to be produced.
Genotype: BbDd
Phenotype: black
coat, hearing
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5.13 Independent assortment can be
observed during a dihybrid cross
• A dihybrid cross is one in which two
separate characters are studied.
MALE
Phenotype:
black coat
hearing
Genotype:
BbDd
×
FEMALE
Phenotype:
black coat
hearing
Genotype:
BbDd
What are the possible allele
combinations for the gametes?
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5.13 Dihybrid cross: BbDd xBbDd
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Dihybrid crosses would give 9:3:3:1 ratio
•
•
•
•
9:3:3:1
9 dogs would be black with normal hearing
3 dogs would be black and deaf
3 dogs would be chocolate and normal
hearing
• 1 dog would b chocolate and deaf
© 2015 Pearson Education, Inc.
5.14 Opening Questions: Can
understanding inheritance help us
understand disease?
• Katie and Dave are a healthy young
couple, but both have a sibling with cystic
fibrosis (a recessive disorder). Genetic
tests reveal they are both heterozygous.
They want to know their risk of having a
child with cystic fibrosis.
If you were their genetic counselor, how
would you explain the risks?
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5.14 Some human genetic characters are
controlled by one gene
• For example, the freckled phenotype is
dominant to the non-freckled phenotype.
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Human genetic diseases caused by a single
gene
• autosomal recessive- must have two recessive genes (aa), one
from each side of the family in order to have the disease; aa would
have the disorder. Carriers would be heterozygous (Aa) and normal
would be AA
• Examples: cystic fibrosis, sickle cell disease (anemia), TaySachs, Phenylketonuria (PKU)
• autosomal dominant- must have at least one dominant gene to
have the disease. AA and Aa would have disease, aa would be
normal.
• Examples: Achondroplasia, Huntington’s disease, familial
hypercholesterolemia
• X linked (recessive)- the mutant gene is present on the X
chromosome. Most individuals who have the disease are males. XcY
color blind male; XCXc female carrier. Females with condition are
homozygous recessive XcXc
• Examples include color blindness, hemophilia, and Duchene’s
Muscular dystrophy
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5.14 Many human genetic disorders are
recessive
• A carrier is a
heterozygous
individual.
• Carriers do not
have the disease,
but they can pass it
on to offspring.
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Genetic diseases in humans
• Autosomal recessive
disorders occur when a
child inherits a defective
gene from parents who are
carries (heterozygous)
• For example if both
parents are Cc then there
is ¼ chance of a child
being born with cystic
fibrosis for each
conception
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5.14 Pedigrees can be used to track genetic
traits in a family
Grandma and
grandpa had two
daughters, one of
whom married and
had four kids.
This daughter
does not have
the trait, but we
cannot tell if
she is Aa or AA.
This daughter has
the trait and
therefore must be
aa, having inherited
the recessive allele
from each parent.
This child has the trait, which is
how we know her father was a carrier
(she could not have inherited two
recessive alleles from her mother.)
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Because they produced a daughter
with the trait but don’t have it themselves,
both grandparents must be carriers.
Because this man
has a daughter with
the trait but does
not have the trait
himself, he must be
a carrier.
These three children must have received one a recessive
allele from their affected mother, but they don’t have
the trait. So they must be carriers themselves.
Familial Hypercholesterolemia
• Autosomal dominant
• Familial
Hypercholesterolemia is
an autosomal dominant
disease that affects both
men and women
equally. This means that
only one mutated gene
is necessary for the
effects of the disorder.
• Does not skip
generations
© 2015 Pearson Education, Inc.
5.15 Opening Questions: Can we always
count on Mendel’s laws?
• Farmers have long observed that crossing
cattle with red and white coats results in
offspring with a roan color (intermediate
color).
If coat color is a single gene with two
alleles (R and r), does the roan color make
sense according to Mendel’s laws?
If not, how would you explain it?
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Incomplete dominance
•
•
•
•
•
Phenotypes (3)
RR
Rr
red
roan
RR is Red
rr
Rr
rr is white
white
roan
Rr is roan
So if you cross two roan colored cows you
get a 1:2:1 ration
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5.15 Genetic inheritance has complexities
• Not all genes follow a
classic Mendelian
inheritance pattern.
• We often encounter
patterns that are
more complex.
• polygenic
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Polygenic
inheritance
• One trait
(phenotype)is
affected by
multiple genes
• Example of
polygenic
inheritance
includes
human skin
color and
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5.15 Sometimes both alleles are expressed
• For some genes there is a
pattern of incomplete
dominance.
• Individuals that are
heterozygous will have a
phenotype intermediate in
appearance.
Remember: in classic Mendelian genetics,
heterozygous individuals have the appearance
(phenotype) of the dominant gene.
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5.15 Flower color in snapdragons is a trait
with incomplete dominance
• Heterozygous
individuals show an
intermediate trait.
Now use a Punnett
square to predict the
outcome of Rr × Rr
for snapdragons.
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between two pink
snapdragons
exemplifies
incomplete
dominance
Only RR individuals
have a red phenotype.
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•
•
•
•
Rr x Rr
1 red (RR)
2 pink (Rr)
1 white (rr)
5.15 For most traits there are multiple alleles
• Classic Mendelian genetics only uses two
allele copies (such as R and r).
• Most genes actually have multiple alleles.
For any gene, how
many allele copies can
one person carry?
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Pearson
Education,
Inc.
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Education,
Inc.
5.15 Blood types in humans are the result of
multiple alleles- codominant
• Human blood types are
determined by a gene with three
alleles:
i, IA, IB.
• These three alleles can be
combined in six ways.
• A and B are co-dominant
• Type O is recessive Ii Ii
Alleles for blood type are also codominant, which
means both are expressed.
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5.15 Parents with different blood types
exemplify multiple alleles and codominance
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5.15 Genes may have multiple effects
• Polygenic alleles
• In some cases, one gene influences many
characters, a situation called pleiotropy.
• The sickle-cell mutation can cause many
physical changes.
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5.15 Many phenotypic characters are the
result of many genes
• Polygenic inheritance is the effect of
many genes on a single character.
• In humans, height and skin color are each
affected by several genes.
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Polygenic
inheritance
• One trait
(phenotype)is affected
by multiple genes
• Example of polygenic
inheritance includes
human skin color and
human height
• Polygenic inheritance
shows a bell curve
shape of phenotypes
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5.15 Many genes have both a genetic and
environmental component
• Some traits are entirely genetic, some are
a mix of environment and genetics, and
some traits are just environmental.
Only genetic influences are inherited!
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5.16 Opening Questions: What if traits are
on the same chromosome?
• So far, we have assumed that traits are on
different chromosomes.
• But what if they are on the same
chromosome? Will they still segregate
independently?
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5.16 Not all genes obey Mendel’s law of
independent assortment
• Linked genes are
located close
together on the same
chromosome and
tend to be inherited
together.
Linked genes display
different offspring
ratios compared to
unlinked genes.
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5.16 Crossing over is less likely to occur
for closely located genes
• Crossing over produces new hybrid
recombinant chromosomes.
• Genes located very near each other have
little chance of a crossover.
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5.17 Opening Questions: What if traits are
on the sex chromosomes?
• So far, we have assumed that traits are on
autosomes.
• But what if they are on the sex
chromosome (X or Y)?
• How might patterns of inheritance differ?
• What if there is a genetic disease just on
the X chromosome? What about
phenotype?
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5.17 Sex-linked genes are those carried on
the sex chromosomes
• Human cells contain 44 autosomes and
2 sex chromosomes:
XX = Female XY = Male
• Because females have
two X chromosomes
while males have only
one, sex-linked gene
display unusual
inheritance patterns.
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5.17 Hemophilia is a recessive mutation
carried on the X chromosome
• A single copy of the normal (dominant)
gene will prevent the disease.
• Men have a single X chromosome, so a
male carrier of the hemophilia gene will
have the disorder.
• In the last Russian
royal family, the son
had hemophilia,
which he inherited
from his mother.
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Hemophilia
• Genetic lack of factor VIII
causes bleeding disorder
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5.18 Opening Questions: Can science
harness cell division?
• In 2004, a
company called
Genetic Savings &
Clone produced
the first
commercially
cloned pet, a cat
named Little Nicky
(for a fee of
Would you clone your pet?
$50,000).
What are some other possible
scientific uses of cloning?
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5.18 Nuclear transfer can be used to
produce clones
• Biologists can artificially manipulate cell
division to produce clones.
– Clones are genetically identical individuals
born of a single parent.
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5.18 Cloning can be done through the
process of nuclear transplantation
•
•
•
•
Nucleus donor Jersey cow (super milk cow)
Egg donor- brown cow
Surrogate black angus cow
Baby cow- (Clone) of jersey super milk cow
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5.18 Cloned embryos can be used to
produce a new individual
• In reproductive cloning the embryo must
be transplanted into a surrogate.
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5.18 Cloned embryos can be used to
produce stem cells
• In therapeutic cloning, stem cells are
harvested from the cloned embryo.
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Stem Cell properties and types
• Stem cells have three general properties:
– they are capable of dividing and renewing themselves for
long periods;
– they are unspecialized;
– and they can give rise to specialized cell types
• Stem cells are pluripotent
• Types of natural stem cells
– Embryonic stem cells are from the blastula stage of an
embryo
– Umbilical stem cells
– Adult stem cells ( for example blood stem cells
(hemocytoblast )which gives rise to all kinds of blood
cells)
• Induced pluripotent stem cells (iPSCs)
– are adult cells that have been genetically reprogrammed
to an embryonic stem cell–like state
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