CHAPTER 17
17.1 Chromosomes, genes, and proteins
DNA controls cell func ons by controlling the produc on of proteins, whether
func onal (e.g., an bodies, enzymes, and hormones [all except sex hormones]) or
structural (e.g., hemoglobin, kera n, muscle fibers, membrane carriers, and
protein receptors for neurotransmi ers).
*Chromosomes are a length of DNA which contain gene c informa on in the form
of genes.
→ Genes are the lengths of DNA that code for proteins.
→ Alleles are alternate forms of genes.
!! The sequence of bases in a gene determines and codes for the sequence of
amino acids used to make a specific protein. Different sequences of amino acids
give different shapes of protein molecules and therefore give different func ons.
Humans have 2 sets of 23 chromosomes in each cell, giving a total of 46
chromosomes per diploid cell (a cell that has a nucleus containing 2 sets of
chromosomes, in which there is a pair of each type of chromosome). The only
haploid cells (cells that have a nucleus containing a single set of chromosomes) in
living organisms are the sex cells/ gametes.
The genotype of the homologous pair for a women’s zygote (fer lized egg) is XX.
The homologous pair for a man’s zygote is XY.
X chromosomes are larger than Y chromosomes, but whenever a Y chromosome is
present, it means the gender of the offspring is male.
gametes
X
X
X
XX
XX
Y
XY
XY
This means that there a 50% chance of the offspring being male or female, and
ul mately the gender of an offspring being impossible to predict.
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X and Y chromosomes do not only determine gender, but they also determine
other traits called sex-linked traits (a feature In which the gene responsible is
located on a sex chromosome and this makes the characteris c/trait more
common in one sex than the other.).
X and Y chromosomes have different genes called sex-linked genes and is what
causes sex-linked traits.
For example, the gene for red-green colorblindness is only present in the X
chromosome.
The five possible genotypes for the red-green colorblindness trait are:
Women : XBXB (woman with normal vision) , XBXb (carrier woman with normal
vision) , XbXb (colorblind woman)
Men: XBY (man with normal vision), XbY (colorblind man).
If a carrier woman mates with a man with normal vision:
XB
gametes
Xb
XB
XBXB XBXb
Y
X BY
XbY
There is a 50% chance of ge ng a female with normal vision, but 25% chance
(50% of that) of the female being a carrier.
On the other hand, there is a 25% chance of ge ng a colorblind male, and a 25%
chance of ge ng a male with normal vision.
Therefore:
-It is impossible to get a female offspring with a sex-linked trait without both
parents carrying that allele, but the female offspring can be carriers of the trait
(only if the gene is on the X chromosome).
-Male offspring cannot be carriers of any sex-linked traits since their 2 sex
chromosomes are different, meaning they either have the trait or they don’t. Also,
if a sex-linked gene is on the Y chromosome of the father, then all the male
offspring will have that trait, and it’s impossible for female offspring to have it.
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DNA, mRNA, and genes are responsible for the produc on of proteins.
Protein synthesis happens as follows:
*notes: the DNA does NOT leave the nucleus and mRNA is a copy of a gene made
in the nucleus.
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17.2 Mitosis
Mitosis is a nuclear division giving rise to gene cally iden cal cells.
There are many stages of mitosis, but it can’t happen without the exact replica on
of chromosomes prior. Mitosis is the division itself, so the replica on stage isn't a
part of the mitosis process.
Mitosis only has one division where the copies of chromosomes separate,
producing 2 daughter cells and maintaining the same chromosome number in
each.
Mitosis has many roles including:
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Growth
Repair of damaged ssues
Replacement of cells
Asexual reproduc on
A prime example of mitosis is stem cells.
Stem cells are unspecialized cells that divide by mitosis to produce daughter cells
that can become specialized for specific func ons.
Every cell in the body contains tens of thousands of the same genes because all of
them are gene cally iden cal to the first cell formed –the zygote–, but the reason
each cell has a different func on and is specialized is because some genes are
expressed while others aren’t.
A certain number of genes for each type of cell are expressed because the cell
only needs to make the specific proteins it needs.
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In an adult human, stem cells are only found in bone marrow and in the brain.
Stem cells in the bone marrow divide to produce specialized blood cells - red
blood cells, white blood cells (phagocytes and lymphocytes), and platelets.
Stem cells in the brain divide to produce new neurons – motor, sensory, and relay.
A be er understanding of stem cells in the future can lead to the discovery of
treatments for unsolved medical condi ons.
17.3 Meiosis
Meiosis is a reduc on division in which the chromosome number is halved from
diploid to haploid resul ng in gene cally different cells.
Meiosis is involved in the produc on of gametes. It has 2 divisions. Similar to
mitosis, meiosis also doesn't include the replica on of chromosomes stage since
it's the process of dividing post the replica on.
In the first division, the homologous chromosomes (chromosomes of the same
characteris cs) pair together and then divide.
The two ways they can divide is shown on page 325 and is what causes gene c
varia on.
In the second division, the cells formed separate their chromosomes once again
into 2 sister chroma ds.
Homologous chromosomes have genes for the same characteris c in the same
posi on. Basically, they're duplicated. The two chroma ds are joined by a
structure called a centromere.
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17.4 Monohybrid inheritance
Inheritance is the transmission of gene c informa on from genera on to
genera on.
This happens by genes and alleles.
There are two types of genes in inheritance:
1- dominant (an allele that is expressed when present, and is usually wri en as a
capital le er, e.g., T)
2- recessive (an allele that is expressed only when the dominant allele isn’t present,
and is usually wri en as a small le er, e.g., t)
Organisms have genotypes and phenotypes for its chromosomes:
1- Genotypes are the gene c make-up of an organism in terms of the alleles
present (e.g., Tt, TT, )
→ If the two alleles for the gene are iden cal, it is homozygous (e.g., TT, )
*two iden cal homozygous individuals breeding together is called pure breeding
→ If the two alleles for the gene are different, it is heterozygous (e.g., Tt)
2- Phenotypes are the observable features of an organism (e.g., brown eyes)
→ genotype + environment = phenotype
Types of inheritance:
1- Complete dominance
2- Codominance
Complete dominance is a situa on in which only one allele in the chromosome
contributes to the phenotype.
This means that if the dominant allele is present, it will take over the phenotype. If
only two recessive alleles are present, that will take over the phenotype.
On the other hand, codominance is a situa on which both alleles in the
chromosome contribute to the phenotype.
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Inheritance can be shown by gene c diagrams:
(a) Punne square
e.g.
(b) Pedigree diagram
In most cases, the filled shapes represent the offspring inheri ng the recessive
trait/phenotype.
I → first-genera on
II → second-genera on, first-genera on offspring
And so on.
In a test cross: (e.g., R dominant and r recessive)
1. The organism with the dominant trait is always crossed with an organism with
the recessive trait. (so, either Rr X rr or RR x rr and the results which show which.)
2. If any offspring show the recessive trait, the unknown genotype is
heterozygous. (Rr X rr)
3. If all the offspring have the dominant trait, the unknown genotype is
homozygous dominant. (RR X rr)
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Gene c diagrams can also be used to predict the results of monohybrid crosses.
Given that the tall plant allele is T and the dwarf plant allele is t.
Scenario 1:
A tall plant mates with another tall plant and have 4 offspring. 3 of them are tall
and 1 is short.
This means that 2 heterozygous plants mated.
gametes
T
T
TT
t
Tt
t
Tt
The ra o is 3:1 being tall:short
Scenario 2:
2 homozygous plants mate, one with the dominant trait and the other with the
recessive trait.
gametes
t
T
Tt
T
Tt
t
Tt
Tt
The ra o is 1:0 being tall:short
*This result will also appear when a heterozygous organism mates with a
dominant homozygous organism. (Tt X TT)
Scenario 3:
A heterozygous plant mates with a homozygous plant carrying the recessive trait.
gametes
t
T
Tt
t
Tt
The ra o is 1:1 being tall:short
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t
There are 4 blood groups, and each can be defined by a set of alleles. These
genotypes are the only possible types of blood type to have (normally).
phenotype
genotype
blood group A
IAIA, IAIO
blood group B
IBIB, IBIO
blood group AB
blood group O
IAIB
IOIO
*Alleles IA and IB are codominant, but dominant to allele Io.
A, B, and O are an gens.
An example of the inheritance in the ABO blood groups:
A woman with blood type B has children with a man with blood type B. One of
their children has blood type O. Create a complete gene c diagram to show this.
*note: gene c diagrams must include the parents’ phenotypes, genotypes, and
gametes.
If they had an offspring with blood group O, then both of them must have the O allele since its
recessive.
phenotype
genotype
gametes
blood group B
IBIO
IB and IO
blood group A
IAIO
IA and IO
gametes
IB
IA
IAIB
IO
IBIO
IO
IAIO
IOIO
¼ (25%) chance to get offspring with IAIB, blood group AB
¼ (25%) chance to get offspring with IBIO, blood group B
¼ (25%) chance to get offspring with IAIO, blood group A
¼ (25%) chance to get offspring with IOIO, blood group O
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