Download Heredity The passing of traits from parent to offspring

Heredity
The passing of traits from parent
to offspring
Genetics
The study of heredity
Gregor Mendel is known as the
“Father of Genetics”
In 1866 – Gregor Mendel, an Austrian monk,
published his findings about the inheritance in
garden pea plants.
Mendel noticed that certain types of garden
pea plants produced specific forms of a trait,
generation after generation.
He noticed some plants always produced green
seeds and others always produced yellow seeds.
He called these “true-breeding”.
Mendel performed cross pollination by
transferring the male gametes from the flower
of a yellow-seed plant to the female organ of a
green-seed plant.
Mendel called the green-seed plant and yellow
seed plant the Parent Generation (P generation).
Parent
Generation:
X
Yellow Peas
(male)
Green Peas
(female)
When Mendel grew the seeds from the cross
between the green-seed and yellow-seed plants
all of the offspring had yellow seeds.
The offspring of the P generation were called
the first filial generation (F1 generation)
Parent
Generation:
X
Yellow Peas
(male)
F1
Generation:
All Yellow Peas
Green Peas
(female)
Mendel wanted to know whether the trait was
no longer present, or whether it was hidden or
masked.
He planted the F1 generation of yellow-seeds
and allowed them to grow and self-fertilize, then
examined the seeds from that cross.
The offspring from the F1 generation were
called the second filial generation (F2
generation).
The F2 generation had 6022 yellow seeds and
2001 green seeds, which is almost a perfect 3:1
ratio.
P Generation:
X
Yellow Peas
(male)
Green Peas
(female)
F1 Generation:
All Yellow Peas
SELF FERTILIZATION
F2 Generation:
6022 Yellow Peas
2001 Green Peas
3:1 Ratio
Mendel concluded that there must be 2 forms of
the seed trait in the pea plants – yellow seed
and green seed – and that each was controlled
by a factor, which we now call an allele.
Allele
An alternative form of a single gene
passed from generation to
generation
ex. The gene for yellow seeds and
the gene for green seeds are just
different forms of a single gene.
Mendel said the 3:1 ratio he saw in his pea plant
experiments could be explained if the alleles
were paired in each of the plants.
Mendel called the form of the trait he saw in the
F1 generation dominant (yellow) and the form
of the trait that was masked in the F1 generation
recessive (green).
Dominant
Trait that appears in the F1
generation, and masks or hides
the expression of the recessive
gene
Indicated by a capital letter.
Recessive
Trait that is masked in the F1
generation by the dominant trait.
Indicated by a lowercase letter.
In Mendel’s experiment the yellow-seed form of
the trait is dominant, so it is represented by a
capital Y.
The green-seed form was recessive, so it is
represented by a lowercase y.
If an organism has two of the same alleles for a
trait it is homozygous.
Ex. Homozygous yellow seeds are YY.
Homozygous green seeds are yy.
Homozygous
An organism with two of the SAME
alleles for a specific trait.
YY or yy
An organism can have two different alleles for a
trait, and when this happens they are
heterozygous.
Ex:
Yy
The dominant trait observed in heterozygous
organisms, which means it will be yellow in
this case.
Heterozygous
An organism with two DIFFERENT
alleles for a specific trait.
Yy
Were the yellow plants in the F1
generation of Mendel’s experiments
homozygous or heterozygous?
The yellow plants could have been
homozygous OR heterozygous.
The outward appearance of an
organism does not always indicate
which pair of alleles is present.
Genotype
The genetic make-up of the alleles.
Ex. YY or yy or Yy in Mendel’s
plants
Phenotype
The physical appearance or expression
of an allele.
Ex. The phenotype of plants with the
genotype yy will be green plants.
Mendel used homozygous yellow seed
and homozygous green seed plants in
his P generation.
Each gamete from the yellow plant
contains one Y.
Each gamete from the green plant
contains one y.
Mendel’s Law of Segregation says
that two alleles for each trait
separate during Meiosis, and
during fertilization two alleles for
that trait unite.
Hybrid
An organism that is heterozygous
for a specific trait.
Ex. The F1 generation in Mendel’s
experiments were hybrids. (Yy)
When Mendel allowed the Yy
plants (F1 generation) to selffertilize he performed a
monohybrid cross.
Monohybrid Cross
A cross that involves hybrids for a
single trait.
Results:
Genotypic Ratio – 1:2:1
Phenotypic Ratio – 3:1
Punnett Square
A visual summary of the possible
alleles for any given trait.
Shows all the possible combinations of
genetic traits that results from the
crossing of the parent organims.
Once Mendel knew how a single trait
was inherited using a monohybrid
cross he could look at two or more
traits in the same plant.
In Garden Pea Plants round seeds
(R) are dominant to wrinkled seeds
(r), and yellow seeds (Y) are
dominant to green seeds (y).
If Mendel crossed homozygous yellow,
round seed pea plants with homozygous
green, wrinkled pea plants the cross would
result in all YyRr, yellow, round pea plants
(F1 generation).
The F1 generation are called DIHYBRIDS
because they are heterozygous for both
traits.
You can cross dihybrids using a dihybrid
cross.
Dihybrid Cross
A cross that involves two dihybrids.
Results in a phenotypic ratio of
9:3:3:1.
From Mendel’s dihybrid cross he
came up with the
Law of Independent Assortment.
Law of Independent
Assortment
Alleles randomly distribute during
gamete formation. Genes on
separate chromosomes sort
independently during meiosis.
The law of independent
assortment says that any four of
these combinations are equally
likely.
Genetic Recombination
The new combination of genes
produced by crossing over and
independent assortment.
To calculate the possible combinations
of genes due to independent
assortment you use the formula 2n.
n = the number of chromosomes
The possible # of combinations after
fertilization for humans would be:
223 X 223 = over 70 trillion
Genes that are close to each other
on the same chromosome are said
to be “linked”.
They usually travel together during
gamete formation.
Gene Linkage
Genes located close to one another on
the same chromosome that usually
travel together and do not segregate
independently.
Linked genes are an exception to
Mendel’s Law of Independent
Assortment.
Most species have diploid cells (2n),
but some have polyploidy cells.
Polyploidy
The occurrence of 1 or more extra
sets of all chromosomes in an
organism
Ex. Sugar Cane (8n)
Oats (6n)
Strawberries (8n)
If an organism has a heterozygous
genotype the phenotype will be
the DOMINANT trait.
BUT…. There’s an exception to
that….
If you cross a red flowered snap
dragon (RR) with a white flowered
snap dragon (rr) the heterozygous
offspring will have pink flowers
(Rr).
What’s up with that?....
INCOMPLETE DOMINANCE
The heterozygous type is an
intermediate phenotype between
the two homozygous phenotypes.
In most heterozygous organisms
the dominant phenotype is
expressed, but SOMETIMES both
alleles are expressed, like in sicklecell disease.
This is called CODOMINANCE.
So what’s Sickle Cell Disease?
It’s a disease common among people
of African descent (~9% of African
American’s have one form of the trait)
It affects red blood cells and their
ability to transport oxygen
Sickled shaped cells don’t
effectively transport oxygen
because they block circulation in
small blood vessels.
People are HETEROZYGOUS for the
trait have normal AND sickle cells
(codominance).
What disease do we know exists in
this area?
Scientists found that people who are
heterozygous for sickle cell have a higher
resistance to malaria.
So… the death rate due to malaria is lower
where the sickle-cell trait is higher.
Since less malaria exists in those area more
people live to pass on the sickle-cell trait to
their offspring. Consequently, sickle cell
continues to increase in Africa.
What would their baby look like?
What is this an example of?
So far we have talked about
inheritance involving two forms of
alleles for a trait (Like the peas
being Y or y).
Some things are determined by
multiple alleles, like your BLOOD
TYPE
The ABO Blood groups have 3
forms of alleles:
IA – Blood Type A I and I are
dominant to i
B
I – Blood Type B
i – Blood Type O
A
B
So in addition to blood being
determined by multiple alleles, it is
also what?
Blood types also have an Rh factor
that you get from your parents.
You can either be Rh+ or Rh-.
+
Rh
is dominant.
Some traits are controlled by genes
located on the X chromsome.
These are called SEX-LINKED traits.
Since males only have one X
chromosome they are more likely to
be affected by the recessive X-linked
traits than females.
Females would be less likely to express
the recessive X-linked trait because the
other X chromosome will mask the
trait.
Red-green color blindness is an
example of a recessive X-linked trait.
About 8% of males in the United States
have red-green color blindness.
XB - Normal
Xb - Red Green Color Blind
Y – Y chromsome
Imagine having a mom who is a carrier for redgreen color blindness and a father who is not
color blind.
Hemophilia is another recessive
sex-linked trait.
It is a disease that is characterized
by delayed clotting of the blood,
and it is more common in males
than in females.
Queen Victoria of England had a
son who died of hemophilia, and
two daughters who were carriers
for the disease.
Queen Victoria’s Pedigree
Some traits arise from the interaction
of multiple pairs of genes. They are
called polygenic traits.
Some examples of polygenic traits are:
skin color, height, eye color, and
fingerprint pattern
Cystic Fibrosis
Recessive Genetic Disorder
Excessive mucus production and
digestive and respiratory failure
affects 1 in 3500 Americans
Albinism
Recessive genetic disorder
Caused by altered genes, and results in
the absence of the skin pigment
melanin in hair and eyes.
Affects 1 in 17,000 people in the US
Tay-Sachs Disease
Recesive Genetic Disorder
Caused by the absence of a necessary enzyme that
breaks down fatty substances (on chromosome 15)
Causes a build up of fatty deposits in the brain and
mental disabilities; Causes a dark spot in the back of
the eye (how doctors are able to identify the disease)
Affects 1 in 2500 people in the US (usually affects
people of Jewish descent)
Galactosemia
Recessive Genetic Disorder
The body can’t break down/digest
galactose
Occurs in 1 in 50,000-70,000 people in
the US
Huntington’s Disease
Dominant Genetic Disorder
Affects the nervous system and occurs in one
out of 10,000 people in the US
Causes a decline of mental and neurological
functions
There is no cure or treatment
Achondroplasia
Dominant Genetic Condition
Results in short arms and legs and a large head
Caused by the gene that affects bone growth
being abnormal.
Occurs in 1 in every 25,000 people in the US.
There is no cure or treatment.
Selective Breeding
Selective Breeding
The process by which desired traits of
certain plants and animals are selected
and passed on to their future
generations.
Desired traits can be passed on to
future generations through
HYBRIDIZATION and INBREEDING.
Hybridization
The production of hybrids
Used commonly by farmers, animal
breeders, and gardners
Ex. Tomato Breeders
Disadvantage: Time consuming and
expensive
Inbreeding
Two closely related organisms are bred to have
the desired traits and to eliminate the undesired
ones in future generations.
Ex. Clydesdale Horses, Angus cattle, German
Shepherd Dogs
Disadvantage – harmful recessive traits also can
be passed to future generations.
Test Cross
When a breeder is producing a hybrid they
must determine the genotype of the
hybrid.
Breeders use a test cross, which involves
breeding an organism that has the
unknown genotype with one that is
homozygous recessive for the desired trait.
Genetic Engineering
Technology that involves manipulating
the DNA of one organism in order to
insert DNA of another organism.
Ex. Insert GFP (from jellyfish) in
mosquitoes to study certain diseases.
DNA tools for Genetic Engineering
DNA tools can be used to isolate specific
genes from the rest of the genome.
(Genome = the total DNA present in the
nucleus of each cell)
3 Main DNA tools for genetic engineering:
1. Restriction Enzymes
2. EcoRI
3. Gel Electrophoresis
Restriction Enzymes
Proteins that bind to specific DNA
sequences and cleave (cut) the DNA
within that sequence.
Scientists use restriction enzymes to
isolate specific genes or regions of the
genome.
Eco R1
A common restriction enzyme used by scientists
EcoR1 cuts DNA containing the sequence
GAATTC
When EcoR1 cuts the DNA it creates “sticky
ends” because they contain single-stranded DNA
that is complementary and can be joined with
other DNA fragments that have sticky ends.
Gel Electrophoresis
An electric current is used to
separate the DNA according to the
size of the fragments.
Smaller fragments move farther
and faster than the larger ones.
Load your samples into the gel
Connect your gel to electricity and
wait.
Look at your results (usually under UV
light)