Download III. Exploring Mendelian Genetics

III. Exploring Mendelian Genetics
Review :
Dominant trait – stronger trait that can
hide a weaker trait. Represented as a capital
letter, ex : B.
Recessive trait – weaker trait that can be
hidden by a stronger trait. Represented by a
lower case letter, ex : b. Must have 2 in order to
be shown.
Does segregation of alleles occur independently?
A. Independent Assortment
*Mendel performed
Dihybrid (2-factor)
crosses to determine if
alleles segregate
independently.
Dihybrid cross – cross in
which 2 traits are
considered at once, ex :
B = round seed/b = wrinkled
A = yellow color/a = green
AaBb x AaBb
It involves 16 possible
offspring.
Mendel’s Dihybrids
Mendel crossed true-breeding
(purebred) plants that
produced only round yellow
peas (RRYY) with plants that
produced wrinkled green peas
(rryy). F1 result = All hybrids
with genotype of RrYy.
*This cross only showed
which traits were dominant &
which were recessive. In
order to examine allele
segregation, a dihybrid cross
must be done with the F1
hybrid generation.
Results of Mono vs. Dihybrid crosses
-In a heterozygous cross of single traits, the
genotypic ratio would be 1 : 2 : 1 & the
phenotypic ratio would be 3 : 1.
-Mendel’s dihybrid crosses of F1 heterozygotes
resulted in values that were very close to the
phenotypic ratio of 9 : 3 : 3 : 1 predicted in the
Punnett square.
-These findings proved that the alleles for seed
color & shape segregate independently, a
principle known as independent assortment.
Dihybrid cross showing 9 : 3 : 3 : 1 ratio
B. Summary of Mendel’s Principles
*Mendel’s principles are the basis of modern
genetics as we know it today.
Mendel’s principles can be summarized into 3 basic laws :
1. Law of Segregation – states that each pair of genes
segregates (separates), during meiosis/gamete
formation.
2. Law of Independent Assortment – states that
alleles for different genes segregate into gametes
randomly & independently of each other.
3. Law of Dominance – states that the dominant
allele in a pair is expressed & the recessive allele
can be hidden.
C. Beyond Dominant & Recessive Alleles
*Exceptions to Mendel’s principles exist, ex :
many important traits are controlled by more
than one gene.
-Some alleles are neither dominant nor
recessive, & many traits are controlled by
multiple alleles or multiple genes.
Incomplete Dominance – when one allele is not
completely dominant over another, ex : snap
dragons can produce red flowers (RR) or white
flowers (WW). The heterozygous form (RW)
produces pink flowers.
*Color seems intermediate to parents.
Incomplete Dominance & Crosses
-Both alleles are partially expressed.
-Phenotypes of incompletely
dominant organisms appear to be a
blend of the parents, ex :
Snapdragon flowers.
-When pure red snapdragon flowers
are crossed with pure white ones,
heterozygous offspring look pink.
Problem : Cross a red snapdragon
with a white snapdragon &
determine the phenotypic &
genotypic ratios of the offspring.
What if we crossed 2 pink?
Codominance
Codominance – when
both alleles of a gene
contribute to the
phenotype of the
organism, ex : chickens
& erminette coloring,
horses with roan
coloring & human
proteins for cholesterol
level.
Codominance & Crosses
-Both are dominant, so both are
expressed at the same time.
-Phenotypes of codominant
organisms will show both dominant
alleles, ex : roan coloring in horses.
-When pure white horses are bred
with pure red horses, heterozygous
offspring look strawberry blonde.
-When examined closely one can see
both red and white hairs in a roan
horse’s coat.
Problem : Cross a red horse with a
white horse, then cross two of their
offspring & find genotypes &
phenotypes.
Multiple Alleles & Polygenic Traits
-Some human traits are controlled by a single
gene that has more than two alleles, or multiple
alleles.
Multiple alleles – when genes have more than
two alleles that code for a single trait, ex :
coloring in rabbits & human blood types.
Polygenic Traits – traits controlled by two or
more genes. Means, “Having many genes”.
Examples include brownish-red eyes in fruit flies
& skin color in humans.
Multiple Alleles & Blood types
*Human blood type is a great example of multiple alleles.
-There are 4 major blood types : A, B, AB, and O.
-The allele for blood type A and the allele for blood type B
are codominant & the allele for blood type O is recessive.
-A person who receives the alleles for A & O will be type A.
-A person who receives alleles for both A & B will be type
AB.
-A person who receives alleles for B & O will be type B.
-The only way that a person can have type O blood is if
they receive two recessive alleles for type O.
Universal Donors & Recipients
*Since type AB people
can receive all other
types of blood, they are
called the Universal
recipients.
*Since type O people
can donate blood to all
other blood types, they
are called the Universal
donors.
Rh Factor in Blood Types
*Besides the blood group ABO in humans there is also a blood
group known as Rh factor.
*Knowing a person’s blood group is critical because using the
wrong type of blood for a transfusion can be fatal.
-The Rh blood group is controlled by 1 gene with 2 alleles –
positive & negative.
-Rh stands for “rhesus monkey”, the animal in which the factor
was 1st found.
-The positive (Rh+)allele is dominant, so people who are
Rh+/Rh+ or Rh+/Rh- are said to be Rh-positive.
-Only individuals with 2 recessive Rh- alleles are Rh-negative.
-When medical workers refer to blood groups, they usually
mention both at the same time, ex : If a person has AB negative blood, they have the AB genotype for blood type &
two Rh- alleles.
Rh Factor Crosses
Problem : Knowing that people who are :
Rh+ = have the Rh antigen (dominant)
Rh- = do NOT have the Rh antigen
(recessive)
Cross a parent who is heterozygous for
the Rh factor with another heterozygous
parent. Use the 6 steps.
D. Genetics & the Environment
-Characteristics in organisms
are determined by interaction
between genes and the
environment, ex : Plant may
inherit tall/good fruit genes,
but if in poor soil conditions
with little water, it may be
stunted or become diseased.