Transcript Genetics & Inheritance - Parma City School District

Genetics & Inheritance
The Chromosome Theory of
Inheritance
Chromosome Theory of
Inheritance:
• Genes that code for various traits are
found on chromosomes which are made of
DNA and found in the nucleus of each cell
Thomas Hunt Morgan (1910): Studies
Drosophilia melanogaster ;common fruit
fly
Discovered the gene for eye color and wing
size were both located on the same
chromosome (X) = Linkage
Linkage = genes located on the same
chromosome
The closer the genes are on the
chromosome, the greater the likelihood of
crossing over
The Law of Independent Assortment does not
apply to linked genes!
Gene Linkage
Barbara McClintock & Harriet Creighton
(1950): Studied Zea mays ; corn

Looked at chromosome #9
They noticed that 2 copies of it were different
sizes
Conclusion = Crossing Over had occurred, an
abnormal event caused a piece of another
chromosome to attach itself to one of the
copies of chromosome #9
Crossing Over disrupts normal Linkage Groups!
Human Chromosomes
Chromosomes: Humans have 46 individual chromosomes in
every cell of the body except the sex cells
22 pairs = Autosomes (regular, information carrying
chromosomes)
1 pair = Sex Chromosomes, determines gender (XX =
female, XY = male)
Which Parent determines the sex
of the child?
MODES OF INHERITANCE
Different ways of inheriting genetic traits
1. Complete Dominance: dominant allele
completely masks out the recessive trait (AA, Aa)
 Autosomal Dominant = trait carried on an
autosome by a dominant gene (A)
 Autosomal recessive = trait carried by a
recessive allele (aa)
Example: flower color in pea plants
P = Purple, p = white




MODES OF INHERITANCE
2. Incomplete Dominance: One allele of a gene pair is not
fully dominant over the other; being heterozygous
produces an intermediate form, no masking occurs (AA,
Aa, aa)
Example: Flower color in snapdragons
RR = red, rr = white, Rr = pink
MODES OF INHERITANCE





3. Codominance: both alleles express themselves
when heterozygous (Blood types A, AB, B, O).
There is no true recessive trait.
AB Blood type is an example of Codominance =
Both alleles in the heterozygous form (IAIB ) end
up expressing themselves equally. Both traits
show up in the phenotype.
Example: coat color in horses
RR = red , WW = White , RW = roan
(RR = red , R’R’ = White , R R’ = roan)
 RR
x WW
RW x RW
MODES OF INHERITANCE
Polygenic Traits: trait is controlled by more
than one pair of genes (Eye color, Skin color,
Human Height). This usually results in
continuous variation. Polygenic inheritance is
considered the opposite of Pleiotropy.
4.
5. Pleiotropy = when a single gene has more
than one phenotypic expression. Example:
Sickle Cell Anemia = misshapen red blood
cells ultimately causes other problems such
as anemia, pneumonia, heart & kidney failure,
bone abnormalities, and impaired mental
functioning.
MODES OF INHERITANCE
 6.
Epistasis = one gene affects the
phenotypic expression of a second
gene. (Skin pigmentation) One gene
codes for color, the other codes for the
amount of pigment.
MODES OF INHERITANCE
 7.
Multiple Alleles: a gene having
more than two alleles (Blood types)





PHENOTYPES
A
B
*
AB
O
% of Population
39%
12%
4%
45%
GENOTYPES
IAIA , IAIi
IBIB , IBIi
IAIB
IiIi
Blood Types
Discovered in 1900 by Dr. Karl
Landsteiner
Based on the presence or absence of
specific agglutinogens (clotting factors)
on the surface of red blood cells (RBC’s
= Erythrocytes)
For example: a person with blood type A
posseses A-antigens and Anti-B
antibodies
The blood will clot if a foreign
Antigen is present
Donor vs. Recipient
• Universal Donor = Blood type O ; contains
no A or B antigens  No Clotting reaction
 Universal
Recipient = Blood type AB ;
contains both A & B antigens  Will
recognize antigens from any blood type
Rh Factor
 Rh
Factor = (Rhesus monkey) You
either have it (+) or you don’t (-)
 Sensitization can occur by:
– .)Rh+ blood transfused into Rh- person
– .)Rh- mother carries a fetus who is Rh+
 IAIA
x
IBIB
IAIix IBIi
MODES OF INHERITANCE
 8.
Sex-Linked Inheritance: trait
carried on the sex chromosomes;
usually the X (XX = female; XY = male)
 X-Linked Recessive = Xa (Colorblindness,
Hemophilia)
 X-Linked Dominant = XA; Y-Linked (Rare)
 Carrier
= person not affected by the trait
but can pass it on to offspring = XA Xa
Only females can be carriers for sexlinked traits because if a male has the
gene, he will also exhibit the trait!
MODES OF INHERITANCE
X – inactivation: one of the two X
chromosomes in a female does not
uncoil during embryonic development.
The chromosome that remains coiled is
called a Barr Body and contains genes
that will not get expressed. This could
cause a sex-linked trait to affect a
female that would normally be only a
carrier.
 9.
MODES OF INHERITANCE
 10.
Sex-Influenced Traits: expressed
in both sexes, but they are expressed
differently (Pattern Baldness)

B= Normal; b= Bald

female (bb) = bald; male (bb or Bb)
=bald
 BB
x Bb
Bb x Bb
MODES OF INHERITANCE
 11.
Sex-Limited Traits: autosomal
traits expressed in only one sex (Lion’s
mane)
MODES OF INHERITANCE
• 12. Dihybrid Crosses: follow 2
traits at a time (AaBb)
 Example:
A = purple flowers, a = white
B = Tall, b = short
 If you cross two parents, where the
father is AABB and the mother is aabb:
the possible gametes are AB x ab. This
can be determined using the F.O.I.L
method.
Dihybrid Crosses
F
= first
 O = outside
 I = inside
 L = last
= 100% AaBb
 AABB x aabb Results
-->
 RrYy x RrYy
GENETIC DISORDERS
 Disorders
or diseases related to a
persons genes or chromosomes;
inherited in the same ways as other
traits.
 2 Mechanisms exist:
Genetic Disorders
 1.
Inherited on Genes: inherited as a
trait (Autosomal, sex-linked, sex
influenced, etc...)
 colorblindness
 hemophilia “bleeder’s disease”
 muscular dystrophy
 albinism
 Progeria
Genetic Disorders
 2.
Chromosome Abnormalities: not
caused by a gene

A.) Extra or Missing Chromosomes.
Aneuploidy = abnormal chromosome
number
– Non-Disjunction = failure of chromosome pairs
to separate properly during meiosis, end up
with daughter cells having either too many or
not enough chromosomes in them.

Ex. Down Syndrome “Trisomy 21”
Genetic Disorders
B.) Mutated Chromosome = damaged DNA,
genes located in that section are damaged
 Deletion: missing gene or piece of
chromosome
 Duplication: extra piece, genes duplicated
 Translocation: gene switches chromosomes
 Inversion: fragment of gene gets turned
around

SCREENING FOR
DISORDERS
 1.
Karyotyping = genetic map of all
the chromosomes that an organism
possesses
 2. Amniocentesis = test done before
birth, take sample of amniotic fluid
(C.V.S.)
 3. Genetic Counseling = determine
family medical history
THE END!!