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Genetics
Review: Cells reproduce asexually by cell division. During mitosis, the chromosomes double
so that when the cell splits, each new cell will have the correct number of chromosomes.
Each new cell is identical to each other and to the original.
Other types of asexual reproduction include: budding, grafting, cloning, and runners. All
organisms that reproduce asexually have offspring genetically identical to the parent.
During sexual reproduction, a male sex cell (sperm) combines with a female sex cell (egg).
If each of these sex cells contained a full set of chromosomes, the resulting offspring
would have TWICE the directions it should have to survive (which would actually confuse it
and cause it to die). Therefore, sex cells can contain only HALF of the chromosomes, so
that when they combine, the resulting offspring has a full set. Each offspring contains
some random combination of genes from the chromosomes of each parent so each
offspring is unique from the others and from the original parents.
Traits are characteristics that are inherited (passed on) through genes.
Heredity is the passing of traits from parents to offspring. The study of this is called
genetics.
A gene is a short section of a chromosome that contains the directions for a trait. There
are thousands of genes on a single chromosome.
The different forms that a gene can have for a trait are called alleles. You get one allele
for each gene from each parent.
EX/ “Height” is an inherited trait in pea plants. There are two alleles for the
“height” gene:
1) short
2) tall
Since a seed has an allele from each parent, it can have either two short alleles, two
tall alleles, or one of each. Different seeds will have different combinations of alleles,
depending on meiosis.
Human body cells each contain 23 pairs of chromosomes….23 from the mother’s egg and 23
from the father’s sperm (46 total). Sex cells form by meiosis:
Fertilization occurs when the sperm and egg meet. The fertilized egg will develop into a
human with 46 chromosomes.
Organisms receive 2 alleles for each trait – one from the mother and one from the father.
Sometimes, one allele may be “stronger” than the other. The dominant (stronger) allele
hides or masks the recessive (weaker) allele if both are present.
EX/ Height in pea plants:
The “tall” allele is dominant; the “short” allele is recessive.
If a pea plant has two tall alleles, it will be tall.
If a pea plant has two short alleles, it will be short.
If a pea plant has one of each alleles, it will be tall, because this gene covers
up the other one because it is dominant. (Two important notes: it will NOT be medium
height because the dominant allele is the one that shows up; and this is only in pea plants,
not people)
genotype: (“type-o-gene”) the actual alleles an organism has for a trait
phenotype: what an organism looks like or shows for a trait
Pea plant genetics
Seed shape
seed color
pod shape
Dominant
round (R)
yellow (Y)
smooth (S) green (G)
tall (T)
Recessive
wrinkled (r)
green (y)
pinched (s) yellow (g)
short (t)
EX/
genotype
RR
rr
Rr




pod color
stem height
phenotype
round
wrinkled
round
If the alleles are the same for a trait, the organism is said to be purebred, or homozygous,
for that trait (ex/ TT or gg). If the alleles are different, the organism is said to be
hybrid, or heterozygous, for that trait (ex/ Tt or Gg).
A Punnett square is used to show the probability of the occurrence of a trait in offspring.
Example: Freckles are dominant in people. What is the probability that a man who is purebred dominant for
freckles with have a child without freckles if he marries a woman with no freckles?
The man’s genotype is FF. Since his wife has no freckles her genotype must be ff (because if she had the
dominant gene she would show freckles).
F
f
Ff
F
Ff
It is impossible for these parents to have a child
without freckles because the father can only
give the dominant gene to his children.
Outcome: 100% hybrid offspring
f
Ff
Ff
100% freckles
The 23rd pair of human chromosomes are also called the “sex chromosomes” because they
are different in males and females.
Females have two “X” shaped chromosomes (XX)
Males have an “X” and a “Y” shaped chromosome (XY)
These are codominant (both capital letters)
50% male
50% female
X
X
X
XX
XX
Y
XY
XY
The female can only give an X, but the male can give an X or a Y, depending on which sperm
fertilizes the egg.
Sex-Linked Traits
Sex-linked traits are those traits that are located on the sex chromosomes. Since the
male Y chromosome is missing a segment (one quarter of the X), any traits located on that
portion of the X chromosome will have a single gene instead of a pair for that trait in
males. This means that although a female would need both recessive alleles to show the
recessive trait, males can show the recessive trait if their only allele is recessive. This is
why sex-linked traits occur more in males than females. Examples of sex-linked traits
include baldness and color-blindness.
Female chromosomes:
Male chromosomes:
Pedigree charts are used to show
how traits are passed down to
individual family members.
Blending of Genes
Sometimes alleles can be codominant or show incomplete dominance so that both alleles
show up when present.
EX/ Hair curliness shows incomplete dominance.
Curly allele = C
Straight allele = S
So a person with both CS alleles has wavy hair
Examples:
If neither black fur (B) or white fur (W) is recessive in guinea pigs:
If they are codominant a guinea pig with BW would be black and white.
If they show incomplete dominance a guinea pig with BW would be gray.
Blood types show blending of genes: A and B are codominant to each other but O is
recessive.
Mutations
Whenever chromosomes duplicate then split,
there is a possibility for something to change.
This change can happen to a single gene or an
entire chromosome. Any change to a gene or a
chromosome is called a mutation.
Examples of chromosomal mutations are shown at right.
When the chromosomes line up after mitosis, sometimes
they don't separate correctly and a cell can end up with
too many or too few chromosomes. Some mutations
don't enable the cell to function at all and it doesn't
survive. Others allow the cell to survive with the change
and pass the change along to future cells during mitosis and
cell division.
If the mutation occurs in a sex cell, however, the mutation
can be passed on to the offspring and affect the entire
organism.
Many diseases are determined by genetic mutations. Examples include:
 Cystic fibrosis – the body produces abnormally thick mucus making it hard to breathe. This is a recessive
allele with a mutation in 3 bases of the DNA molecule.
 Sickle-Cell disease – People have sickle-shaped red blood cells that are not able to carry as much oxygen.
The alleles are co-dominant. (Being heterozygous can be beneficial though because of malaria)
 Hemophilia – a person's blood doesn't clot. Caused by a recessive allele on the X chromosome.
 Down Syndrome – A person has some degree of mental retardation and heart problems. People with Down
Syndrome have cells with an extra copy of Chromosome 21 because chromosomes fail to separate properly
during meiosis.
 Duschenne Muscular Dystrophy – The gene for making the protein dystrophin is affected. Dystrophin is
an important protein in muscle tissue so without it muscles get weaker. Usually caused by a recessive allele
on the X chromosome.
A mutation occurs when part of a gene or chromosome doesn’t copy properly. This
produces a different form of the trait called a variation. If the mutation causes a
variation that helps the organism to survive, it’s called an adaptation.
Sexual reproduction and mutations provide genetic variation in offspring, which helps a
species to survive.
If a mutation occurs in a body cell (ex/ skin cell), the mutation will affect only that cell
and its offspring. If a mutation occurs in a sex cell, the mutation can be passed on to the
offspring and affect all body cells.
Mutations can be helpful, harmful, or neither, depending on how it affects the organism’s
chance of survival (which depends on the mutation and the environment)
EX/ white buffalo in the wild vs white buffalo on a farm
If an organism is better adapted to its environment, it is more likely to survive and pass
along that adaptation to its offspring. We call this natural selection because nature
“selects” the best adapted organisms to live (also called “survival of the fittest”). After
many generations of offspring, a new species of organism can exist. This is called
evolution.
Darwin’s theory of evolution:
1. overproduction (organisms produce more offspring than can survive)
2. variation (organisms within a species are not all alike)
3. competition (limited resources force organisms within a species to compete for survival)
4. advantages of some variations = natural selection
If a species is unable to adapt to its environment, it becomes extinct.
Selective breeding is the process of choosing which organisms to breed with other
organisms to get specific traits in offspring. (For example, you breed a golden retriever
with a friendly disposition with another one that has a beautiful coat and hope that the
offspring will be both beautiful and friendly)
Cloning is a type of asexual reproduction because the offspring has the exact genetic
material as the parent. During cloning, the chromosomes of one organism are placed into
the egg of another and develop into a new organism.
Genetic Engineering
In genetic engineering, a gene is removed from a cell of one organism and placed into the
zygote of another organism. The zygote will grow into an organism possessing the gene for
that trait and exhibit that phenotype.
EX/ Scientists have taken the gene out of some fish that causes them to glow and put
that gene into puppy zygotes. The zygotes have grown into puppies that glow.
This is extremely helpful in medicines. Bacteria produce insulin for diabetics. This is also
used extensively in crops to produce crops with desirable traits. For example, scientists
have taken the antifreeze gene out of flounder (a fish) and put it in tomatoes so tomatoes
don't freeze.