Biology 3201 Unit 3 Genetics
Transcript of Biology 3201 Unit 3 Genetics
5/22/2015
1
Biology 3201 Unit 3 Genetics #1Mendelian Genetics - Introduction (Ch 16)
Mendelian genetics
✤ Genetics: the scientific study of heredity
✤ Heredity: the passing of traits from one generation to the
next
✤ Trait: any observable characteristic that an organism may
have
Ex. Eye colour, hair colour, height, tongue rolling
5/22/2015
2
Gregor Mendel
✤ An Austrian monk who studied genetics by
growing and cross breeding pea plants
✤ Why pea plants?
They reproduced quickly
Their traits were easily observable
They were readily available
They could self pollinate meaning he
could control which plants pollinated
Mendel's 1st experiment (part one)
✤ Took the seeds of a purebred tall plant and the seeds
of a purebred short plant and cross-pollinated them
and collected the seeds produced by this cross and
planted them
✤ Results: all of the offspring were tall (no short plants)
✤ So did the trait for short disappear??
5/22/2015
3
Mendel's 1st experiment (part two)
✤ Next, Mendel self pollinated the plants from the F1 generation.
These seeds were planted and grown.
✤ Results: Three quarters were tall and one quarter was short.
✤ To describe his findings Mendel said there was a dominant
factor at play causing the plants to be tall in the first cross and
that a recessive factor was being expressed in the second cross.
✤ Principle of Dominance: when an organism is crossed for a pair
of contrasting traits ONLY the dominant trait can be seen in the
hybrid. The recessive factor was hidden.
Mendelian Genetics Terminology
✤ Unit factors: these were the two factors that Mendel said affected the
expression of traits in organisms. Each organism has two factors for each
trait. We now call them GENES.
✤ Unit Theory of Inheritance: this was Mendel's theory where he believed
unit factors (now called genes) control the expression of traits in offspring
✤ Dominant factor: this was a factor that was ALWAYS expressed in an
organism. Represented by an uppercase (capital) letter in a Punnett
square
✤ Recessive factor: this was a factor that SOMETIMES was expressed in an
organism. Represented by a lowercase letter in a Punnett square
5/22/2015
4
Mendelian terminology cont...
✤ Allele: this is the form a gene can be. It can be
dominant or recessive but not both.
✤ F1 generation: also called First Filial Generation. This
is the first set of offspring that are produced by parents.
✤ F2 generation: also called the Second Filial
Generation. This is the second set of offspring created
from the offspring of the parents. These offspring are
the same thing as grandchildren.
Mendelian terminology cont...
✤ Pure breed/strain: a term that refers to an organism having
EITHER two dominant factors OR two recessive factors for a trait
Ex. A plant that is pure tall have two dominant factors for
height- one from the mother and one from the father
Ex. A plant that is pure short has two recessive factors for
height - one from the mother and one from the father
✤ Hybrid: a term meaning that an organism has one dominant and
one recessive factor for a trait
5/22/2015
5
Mendelian terminology cont...
✤ Homozygous: a term meaning that an organism has the SAME
alleles for a gene (both are dominant or both are recessive). Same
as pure breed
✤ Heterozygous: a term meaning an organism has DIFFERENT alleles
for a gene (one is dominant and one is recessive). Same as hybrid
✤ Genotype: the genetic makeup of an organism. It is combination of
alleles for an organism.
✤ Phenotype: the physical characteristics displayed by an organism. It
is the appearance of a trait in an organism.
Mendelian terminology cont...
✤ Punnet square: a square
used to help determine
the possible/probable
outcomes of a cross
between two individuals
✤ Cross: another way of
saying that a female has
been mated with a male
5/22/2015
6
Mendel's Laws
✤ 1. Principle of Dominance: a principle proposed by Mendel that
stated that the dominant form of a gene (trait) will ALWAYS be
expressed
✤ 2. Law of Segregation: a law proposed by Mendel that stated that
a pair of factors for a trait separated (segregated) during the
formation of gametes (sperm and egg) and then recombined
during fertilization
✤ 3. Law of Independent Assortment: a law stating that during
meiosis, genes for different traits are separated and distributed to
gametes independently of one another.
The single factor cross
✤ Single factor cross: a cross that is
done for ONE TRAIT.
✤ Ex. Straight hair is dominant to curly
hair. Mary is homozygous for
straight hair (HH), while Patrick is
heterozygous for straight hair (Hh).
What are the possible genotypic and
phenotypic ratios of a cross between
these two individuals?
✤ Genotypic ratio: 1/2 HH: 1/2 Hh
✤ Phenotypic ratio: all straight hair
H H
H HH HH
h Hh Hh
5/22/2015
7
The Monohybrid Cross
✤ Monohybrid cross: a single factor cross where the
parents are HETEROZYGOUS (HYBRID) for the trait
in question.
✤ Ex. In pea plants, tall (T) is dominant to short (t).
What would be the genotypic and phenotypic ratios of
a cross between two heterozygous pea plants?
The Monohybrid Cross
✤ Genotypic ratio: 1/4 TT:1/2 Tt:
1/4 Tt
✤ Phenotypic ratio: 3 tall: 1 short
✤ Note: In a MONOHYBRID
CROSS, these ratios are
ALWAYS the same.
✤ Genotypic: 1:2:1
✤ Phenotypic: 3:1
T t
T TT Tt
t Tt tt
5/22/2015
8
The product rule
✤ Product rule: the probability, or chance, that two or more
independent events will occur together is the product of
their individual probabilities occurring alone.
✤ Ex. What are the chances of a couple have 3 girls in a
row?
✤ Chance of one girl =1/2
✤ So for three in a row it would be 1/2 x 1/2 x 1/2 = 1/8
Test cross
✤ Test cross: a cross in which an individual of an unknown genotype is
crossed with a homozygous recessive individual
✤ This is done to determine the genotype of the unknown individual.
✤ Two crosses are performed (on paper) before mating
Cross a homozygous dominant individual with a homozygous recessive
individual
Cross a heterozygous individual with a homozygous recessive individual
✤ Compare results of mating with crosses to identify unknown individual
5/22/2015
9
Test cross- Homozygous Dominant
✤ Parents: TT x Tt
✤ Genotypic ratio: 100%
Tt
✤ Phenotypic ratio: all
tall
T T
t Tt Tt
t Tt Tt
Test cross -Heterozygous
✤ Parents: Tt x tt
✤ Genotypic ratio: 1/2 Tt:
1/2 Tt
✤ Phenotypic ratio: 1/2 tall:
1/2 short
T t
t Tt Tt
t tt tt
5/22/2015
10
Two factor crosses
✤ Two factor cross: a cross performed where two traits are involved
at the same time
✤ Ex. A cow that is homozygous dominant for brown coat (BB) and
homozygous dominant for split hoof (HH) is crossed with a cow
that is homozygous recessive for brown hair (bb) and
homozygous recessive for non-split hoof (hh). What are the
possible genotypic and phenotypic ratios for the offspring?
✤ Parents: BBHH x bbhh
✤ Gametes: BH, BH, BH, BH and bh, bh, bh, bh
Two factor crosses (cont)
BH BH BH BH
bh BbHh BbHh BbHh BbHh
bh BbHh BbHh BbHh BbHh
bh BbHh BbHh BbHh BbHh
bh BbHh BbHh BbHh BbHh
Genotypic ratio: 100 % BbHh
Phenotypic ratio: all black coat with split hoof
5/22/2015
11
Dihybrid cross
✤ Dihybrid cross: a two factor cross where both parents are
HETEROZYGOUS for BOTH traits
✤ Ex. In cows, the gene for black coat colour (B) is dominant to the
gene for white coat colour (b). The gene for split hoof (H) is
dominant to the gene for solid hoof (h). What are the expected
genotypic and phenotypic ratios for a dihybrid cross between two
cows?
✤ Parents: BbHh x BbHh
✤ Gametes: BH, Bh, bH, bh and BH, Bh, bH, bh
Dihybrid cross
BH Bh bH bh
BH BBHH BBHh BbHH BbHh
Bh BBHh BBhh BbHh Bbhh
bH BbHH BbHh bbHH bbHh
bh BbHh Bbhh bbHh bbhh
5/22/2015
12
Dihybrid cross
✤ Genotypic ratio: 1/16 BBHH: 2/16 BBHh: 2/16 BbHH: 4/16 BbHh: 1/16
BBhh: 2/16 Bbhh: 1/16 bbHH: 2/16 bbHh: 1/16 bbhh
✤ Phenotypic ratio:
9 black coat/split hoof
3 black coat/solid hoof
3 white coat/split hoof
1 white coat/solid hoof
✤ Note: these ratios remain constant for any dihybrid cross.