Download The Third PowerPoint (DNA and Sex Determination)

DNA and Sex Determination in
Humans
By the end of this lesson, you will never look at this family the same.
Chromosome: A threadlike structure of acids and protein found in the nucleus of most
living cells; carries genetic information
Genes: Characteristics of an organism that are inherited from their biological parents;
made up of DNA. Different “stretches” of DNA are genes that give us our
characteristics.
DNA: Stands for deoxyribonucleic acid. The chemical that genes are made up of. This
is the chain of ‘links’ that determines how the different cells in your body will
function.
To summarize: DNA is in genes, and genes are on chromosomes.
Genes Vs. Chromosomes Vs. DNA
Evidence for DNA
• Freidrich Miescher discovered the
substance of DNA in 1868, while
researching the nucleus of fish sperm. He
did not know it’s purpose.
• Scientists predicted that DNA held the
information of inheritance, but they
weren’t sure how.
• Using X-ray diffraction, Rosalind Franklin
discovered the structure of DNA as a
double helix in 1951. She was not noted
for this discovery until her death in 1958.
Rosalind Franklin
The Human Cell
Nearly every cell in the human
body contains 46 chromosomes
inside of its nucleus: 23 from
your mother, and 23 from your
father.
The exception to this rule is the
sex cells or gametes (i.e. the
sperm and the egg), which
have 23 chromosomes. Why
might this be?
If they each had 46 chromosomes, then when they
combine there would be a baby with 92
chromosomes, which would be disastrous. (It would
NOT be a super baby).
All of the chromosomes can be seen
on a Karyotype
Information about
•
•
•
A karyotype is the
arrangement of an
organism’s chromosomes
from one body cell
Karyotypes are made
using the amniotic fluid
(fluid in the amniotic sac
surrounding the fetus)
from a pregnant female.
Karyotypes show
– the number of
chromosomes
– the sex of the
individual
– large errors in
chromosome
structure
who you are is
contained in these
Typical male
karyotype
XY chromosomes
Typical female karyotype
Two X chromosomes
Sex Chromosomes
• Sex chromosomes determine
the gender of an individual
• Like the rest of your
chromosomes, you get one of
these from each of your
parents
• They control the primary and
secondary sexual
characteristics of humans
• Consist of an X chromosome
and Y chromosome
How do parents pass on genes to
their children?
• Traits are passed on from both
parents.
• You have some traits from your
mom and some from your dad.
• Because traits are coded for by
DNA, you have some of your mom’s
DNA and some of your dad’s DNA.
But your DNA is unique to you; no
one else has exactly the same DNA
as you (unless you are a twin).
Is it biologically beneficial to be
different from our parents?
Yes, in theory…
A child must be similar enough to its parents to survive in a
similar environment, but have the chance to be different
enough to survive in a changing environment.
It is a benefit to be genetically different from our parents, in
case we must survive in an environment that is different
from the one they live in. With different genes, we may
be better prepared.
People in drastically different environments have slightly
different genes. Can you think of any examples?
How are our traits a
combination of our parents
traits?
• Traits are not mixed when they are passed on.
– For example, if your mom has blue eyes and your dad has yellow eyes,
you will not have green eyes. You will either have blue or yellow eyes.
• Some traits are determined by dominance.
– If you have two genes in you that code for the same trait, one trait will
win over the other. The trait that “wins” is the dominant gene. The trait
that does not get expressed is the recessive gene.
• Some traits, like height, are determined by genes from both parents. There is
not one dominant gene for these traits.
Dominant Traits:
-attached or unattached earlobes
-widow’s peak hairline
Combination Traits:
-height
-hair color
Let’s take a breather and see
what the common traits in
this classroom are.
Dominant Traits
Each person has two genes for every trait.
• Dominant physical characteristics will show up unless both
genes code for the recessive trait.
Dominant trait: Unattached earlobes, “U”
Recessive trait: Attached earlobes, “A”
If your genes code:
UU
UA
AA
You will have:
Unattached
Unattached
Attached
Attached
“A”
Unattached
“U”
Evidence for Dominance
• In the late 1800’s, Gregor Mendel, a monk, was
studying pea plants.
• He noticed that when he bred two plants with
different traits, their offspring did not show a mix of
characteristics, but instead showed either
characteristic.
+
=
=
or
Predicting Physical Characteristics
Genes of Parent 1:
A
Genes of Parent 2:
• When a scientist knows the
characteristics of the parents,
he or she can predict the
characteristics of the offspring
using a Punnett-Square
• Genes of both parents are
labeled on the outside of the
big square.
• Each combination of the
parent’s genes in the small
squares is a possible
combination for the offspring.
B
A
AA
AB
B
AB
BB
Offspring could be:
AA, AB, AB, or BB
Probability (P)
• By knowing which genes the parents have, you can figure
out the probability those parent’s have of having a child
with a certain trait.
If the parents are both
dominant (DD):
D
D
D
DD
DD
D
DD
DD
There is a 100% chance that
the child will have the
dominant trait.
P = 4/4
If the parents are both
dominant but also carry
the recessive gene (DR):
D
R
D
DD
DR
R
DR
RR
D = dominant
R = recessive
P = Probability
It’s a fraction!
There is a 75% chance that their
child will have the dominant trait.
P = 3/4
Try It
Dominant Trait: Straight Thumb (S)
Recessive Trait: Hitchhiker’s Thumb (H)
If Parent 1 has Hitchhiker’s Thumb (HH)
And Parent 2 does not (SH)…
Parent 1:
S
H
H
SH
HH
S
What probability is there that a child will
have hitchhiker’s thumb (the recessive
Trait)?
3 out of 4 = 3/4
H
HH
HH
H
*Dominant traits are not always the
most common.
Designer “Genes”
 Alleles - two forms of a gene (dominant &
recessive)
 Dominant - stronger of two genes
expressed in the hybrid; represented by a
capital letter (R)
 Recessive - gene that shows up less often
in a cross; represented by a lowercase
letter (r)
copyright cmassengale
18
More Terminology
 Genotype - gene combination for a
trait (e.g. RR, Rr, rr)
 Phenotype - the physical feature
resulting from a genotype
(e.g. red, white)
copyright cmassengale
19
Genotypes
 Homozygous genotype - gene
combination involving 2 dominant or 2
recessive genes (e.g. RR or rr); also
called pure
 Heterozygous genotype - gene
combination of one dominant & one
recessive allele (e.g. Rr); also called
hybrid
copyright cmassengale
20
FACT OR FAKE:
Our bodies are made up of cells.
Our cells are constantly being replaced by new
cells, so in a way we’re a different person now
than we were 7-10 years ago.
(Mostly)
Yes, it’s true that individual cells
have a finite life span, and when
they die off they are replaced
with new cells. There are
between 50 and 75 trillion cells
in the body.... Each type of cell
has its own life span, and when
a human dies it may take hours
or day before all the cells in the
body die. (Forensic investigators
take advantage of this vaguely
morbid fact when determining
the cause and time of death of
homicide victims.)
However, red blood cells live for about four months, while white blood
cells live on average more than a year. Skin cells live about two or three
weeks. Colon cells have it rough: They die off after about four days. Sperm
cells have a life span of only about three days, while brain cells typically
last an entire lifetime (neurons in the cerebral cortex, for example, are not
replaced when they die).
Once DNA replicates,
how do cells replicate and divide?
•
•
The body grows by one cell dividing into two, and those two diving into four.
Skin cells live for many days, blood cells which carry oxygen live for months,
nerve cells last a lifetime.
One cell dividing
into two
One Cell
Let’s Listen to a Robot Explain Mitosis
and Meiosis:
“Normal” Cells Divide by the Process
of Mitosis
Cell with a single copy of DNA
DNA replicates to form chromosomes
(two copies of DNA)
Chromosomes line up in the middle
of the cell
Chromosomes are split. Half of each
chromosome travels to either end of the cell.
The cell divides to form two new cells
with their own DNA
“Sex Cells” divide by the Process of
Meiosis
Original cell with 46 chromosomes
Some DNA can detach from one chromosome
and switch with DNA from the chromosome
that holds similar genes. This is called
crossing over.
After two cell divisions, four cells with
23 chromosomes each are formed.
How do sex cells form new individuals?
• Sex cells in females are eggs.
• Sex cells in males are sperm.
• One sex cell from the mother
(an egg) merges with one sex
cell from the father (a sperm)
to form a gamete. This cell
continues to divide, until it
forms a complete organism.
• The individual grows through
mitosis (i.e. a single cell keeps
dividing as a human being
forms).
A Growing Human
One cell divides
into an identical
cell in mitosis
One cell divides into four new ones
in meiosis
Mitosis VS. Meiosis
• Mitosis
– Occurs in the creation of
normal cells
– One division
– Forms 2 cells
– Newly formed cells have
46 chromosomes
– Results in growth
• Meiosis
– Occurs only in the
process of producting
of sex cells (called
gametes)
– Two divisions
– Forms 4 cells
– Newly formed cells
have 23 chromosomes
– Allows for procreation
Atypical Meiosis
• Meiosis occurs correctly thousands of times throughout life –
sometimes it can go wrong!
Atypical Meiosis
• Nondisjunction:
– When homologous chromosomes do not separate during
meiosis which produces gametes with the wrong number
of chromosomes
So what happens???!!!
• During fertilization, if one of the gametes has the
wrong number of chromosomes the zygote will have
either too much or too little genetic material
Down Syndrome
• Common disorder that
results from a zygote with
47 chromosomes
• Down Syndrome is one of
the few genetic disorders
where an individual can
survive with an extra
chromosome.
• They have three copies of
chromosome 21
Turner Syndrome
• Occurs when a female is born with only 1 X
chromosome
– Females with turner syndrome cannot reproduce
Klinefelter Syndrome
• Males that are born
with two X
chromosomes and one
Y chromosome
– Males with Klinefelter
Syndrome produce less
sex hormone and cannot
father a child
Alzheimer Disease
• Individuals with Alzheimer disease lose the ability to create and keep
memories.
• This genetic disease is thought to be due to a combination of
environmental and genetic effects.
• Scientists do not know which genes affect Alzheimer Disease.
• They hypothesize that a certain protein that affects the structure of
nerves is over produced. This destroys nerves in the brain that are
responsible for storing memories.
Affected individual
MRI of the Brain
Cancer
• Like Alzheimer disease, cancer is caused by both genetic mutations and
environmental effects.
– Environmental effects, such as excess sun exposure, are thought to
be a partial cause of genetic mutations.
• There are many types of cancer, but they all involve un-controlled cell
growth.
Lance Armstrong,
7 time winner of the
Tour de France,
Testicular cancer
Bob Marley,
Nancy Regan,
Former First Lady, reggae musician,
Died of skin cancer
Breast cancer
in 1981
Sexual Reproduction “Life Hacks”
Cool, Outside of the Box Idea
OR
A Threat to Humanity?
Artificial Insemination (Animal Version)
 A veterinarian collects sperm from male animal & inserts into
a female animal of the same sex.
 This helps animals to conceive more quickly & more often.
 Used extensively in agriculture to help breed animals with
desirable traits.
 Ex. Dairy Cows, also used in Zoos to maintain or increase the
population of endangered species.
With humans,
this can work in a
similar fashion
through the use
of sperm banks.
In Vitro Fertilization
 Process that builds on artificial insemination
 Both sperm & eggs are collected from
animals/humans with desired traits
 Fertilization occurs in a lab, Petri dish
 Once eggs are fertilized the embryos can be
inserted into the female.
 These females are surrogate mothers, because they
are not genetically related to the embryo.
Eugenics
“Improving human genetic qualities”
• Eugenics is a social philosophy which
advocates the improvement of human
hereditary traits through various forms of
intervention.
Governments and Eugenics in the 20th
Century
•
•
•
•
•
•
Genetic screening
Birth control
Marriage restrictions
Segregation (racial and mentally ill)
Forced abortions or pregnancies
Genocide
Abortion
• Sometimes used
if it’s discovered
early that the child
will be born with
severe health
issues.
• Procedure is
more difficult
and dangerous
the longer the
fetus has
developed in the
womb.
Cloning an adult animal
Cloning a single adult animal, especially a mammal, is very complicated.
The most famous animal clone is
Dolly the sheep, who was born on
5th July 1996.
Dolly was not the first animal clone,
but the first mammal to be cloned
from an adult cell.
Did Dolly have a successful, happy,
healthy life? That’s for this class to
research...
What is nuclear transfer?
Dolly was created using a technique called nuclear transfer.
In this technique, the nucleus (i.e. DNA) from a
body cell of the adult (a somatic cell) is removed.
This nucleus is then inserted into an egg cell that
has had its own nucleus removed.
An electric shock makes the egg cell divide and develop like a normal fertilized egg. The
important difference is that it only contains the DNA from one, rather than two, animals.
Why couldn’t the DNA be taken from a sperm or egg cell?
Because they only contain half the genes of the animal.
But wait… what about
that A&W Burger
Family from earlier?