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Introduction to Genetics
Remember…
• DNA RNA ProteinTraits
• DNA contains the code for proteins
(protein synthesis…remember?)
• Proteins determine our traits
Remember…
Gregor Mendel 1822-1884
• Father of Genetics
– Studied characteristics garden plants
– Curious about inheritance patterns
Mendel’s pea plants
• Pea plants can:
– Self fertilize
– Cross fertilize
• Have easily observed contrasting traits (ex. tall or short)
Pea traits
Mendel’s experiments
Parent generation (P generation starting point)
Pure Green Pea X Pure Yellow Pea
Hypothesis: yellow-green offpsring
Genetic Crossing Parent (green) X Parent (yellow)
First Generation (F1 - offpsring)
**ALL YELLOW**
P P
F1 F1 F1 F1
Mendel’s Second Generation – F2
Mendel self-fertilized the F1:
F1 F1
F2 F2 F2 F2 F2 Green trait re-
appeared in F2
(second)
generation
3:1 ratio
Mendel’s First Law: The Law of Segregation
All individuals have two
copies of each allele (i.e.
Ss); each gamete receives
one copy of every allele
during gamete formation.
Hybrid Cross - mating between individuals
who have different alleles at one locus of interest
SS Ss
Ss ss
S
s
S s
Parent(mom) plant:
Ss
Parent
(dad)Plant:
Ss
Possible gametes passed down
Principle of Dominance
DOMINANT TRAIT – Traits that is
expressed; what you see (ex. T)
RECESSIVE TRAIT – Trait that is hidden;
(ex. t)
Each parent will either pass down an allele
that is dominant or an allele that is
recessive
Each parent
passes down only
1 of 2 possible
alleles
• Offspring will inherit two alleles, one
from mom and one from dad
• These alleles will either be dominant
or recessive
• Offspring inherit 2 alleles, one ‘S’
from each parent
– Possibilities:
• SS
• Ss or sS
• ss
Common Vocabulary
• Genotype
• Phenotype
• Homozygous
• Heterozygous
Common Vocabulary cont.
• Genotypic description:
– Homozygous dominant: SS
– Homozygous recessive: ss
– Heterozygous: Ss
• Remember: the phenotype is ‘what
you see’… SS and Ss - same
phenotype, different genotype
Test cross – to determine what
parent genotype was
• Cross between a dominant
phenotype and a recessive individual
S ? X ss
• Parent genotype can be determined
by looking at the offspring
Monohybrid Cross • Cross involving only one trait
Dihybrid Cross
• Cross involving two traits – Ex. Shape and Color
• Round (R) X Yellow (Y) (for peas)
• RrYy x RrYy
Each allele sorts independently from other alleles
in gamete formation
Mendel’s Second Law: The Law of Independent
Assortment
The two alleles for one gene segregate
(assort) independently of the alleles for
other genes during gamete formation
(meiosis).
2nd Law only refers for dihybrid crosses
involving 2 traits – the alleles for EACH trait will
separate independent of each other.
Summer squash – dihybrid ex.
• W (white) is dominant to w (yellow)
• D (disk shape) is dominant to d (sphere shape)
WWDD (white, disk-shaped fruit) X wwdd (yellow, sphere-shaped fruit)
Genotype
Phenotype
P (parent) Generation
All offspring for F1 : WwDd X WwDd
white, disk-shaped yellow, sphere-shaped
F1
P
Cross all the possibilities for the
offspring to determine F2
F1 possibilities
F1 possibilities
F1
F2 possibilities
What would the
genotype and
phenotype ratios
be?
You try it…monohybrid test cross
• In humans, brown eyes (B) are dominant
over blue (b)*. A brown-eyed man marries
a blue-eyed woman and they have three
children, two of whom are brown-eyed and
one of whom is blue-eyed. Draw the
Punnett square that illustrates this
marriage. What is the man’s genotype?
What are the genotypes of the children?
Try again.
• In dogs, there is an hereditary deafness caused by a
recessive gene, “d.” A kennel owner has a male dog that
she wants to use for breeding purposes. The dog can hear,
so the owner knows his genotype is either DD or Dd. If the
dog’s genotype is Dd, the owner does not wish to use him
for breeding so that the deafness gene will not be passed
on. This can be tested by breeding the dog to a deaf female
(dd). Draw the Punnett squares to illustrate these two
possible crosses. In each case, what percentage/how many
of the offspring would be expected to be hearing? deaf?
How could you tell the genotype of this male dog?
Dihybrid Cross • In humans, there is a gene that controls formation (or lack thereof)
of muscles in the tongue that allow people with those muscles to
roll their tongues, while people who lack those muscles cannot
roll their tongues. The ability to roll one’s tongue is dominant over
non-rolling. The ability to taste certain substances is also
genetically controlled. For example, there is a substance called
phenylthiocarbamate (PTC for short), which some people can
taste (the dominant trait), while others cannot (the recessive trait).
To people who are tasters, the paper tastes very bitter, but to non-
tasters, it just tastes like paper.
• Let’s let R represent tongue-rolling, r represent a non-roller, T
represent ability to taste PTC, and t represent non-tasting.
•
• Suppose a woman who is both a homozygous tongue-roller and a
non-PTC-taster marries a man who is a heterozygous tongue-
roller and is a PTC taster, and they have three children: a
homozygous tongue-roller who is also a PTC taster, a
heterozygous tongue-roller who is also a taster, and a
heterozygous tongue-roller who is a non-taster. If these parents
would have a bunch more children so that they had 12 in all, how
many of those 12 would you expect to be non-tasters who are
homozygous for tongue-rolling?
• If the first child (the homozygous tongue-roller who is also a PTC
taster) marries someone who is heterozygous for both traits, draw
the Punnett square that predicts what their children will be.
Sex linked Traits
• Found only on the X
chromosome
• Women - 2 X
chromosomes (XX)
• Men - 1 X
chromosome (XY)
• Any genes found on
the X chromosomes
are referred to as
sex-linked genes.
Designating Sex linked Traits
• Xr - Recessive trait
• XR - Dominant trait
What is the genotype for a female that is homozygous
dominant?
What is the genotype for a woman that is Homozygous
recessive?
What is the genotype for a woman that is Heterozygous?
What is the genotype for a male that is has the dominant trait?
What is the genotype for a man with the recessive trait?
Fruit Flies – gene for eye color • Eye color gene is carried on the X
chromosome (sex-linked)
• Red eyes – dominant - XR
• White eyes – recessive - Xr
• Try it…
If a white-eyed female
fruit fly is mated with a
red-eyed male, predict
the possible offspring.
Incomplete Dominance -
Blending • two different phenotypes produce a
third phenotype that is a blending of
the parental traits.
4 o’clock flowers, Red is
not completely
dominant over white RR – Red flowers
RW – PINK flowers
WW– White flowers
Incomplete Dominance
? ?
?
?
Cross a Red flower with a
White flower. Predict the
ratio of offspring?
Incomplete Dominance
? ?
?
?
Cross a Red flower with a
Pink flower, what is the
ratio of offspring?
Codominance - Together
• two different phenotypes produce a third
phenotype where both parental traits
appear together.
• For Example: Cattle can be red (RR =
all red hairs), white (WW = all white
hairs), or roan (RW = red & white
hairs together).
Codominance
• Cross a Red cow with
a white cow. Predict
the ratio of offspring.
? ?
?
?
Multiple Alleles – trait controlled
by more than two alleles
• Blood Type –
controlled by 3
alleles, not 2
• A, B, O alleles
• 4 blood types
– A, B, AB, O
Polygenetic Inheritance – many
genes affecting one trait
Environmental Factors
• Can affect whether or not a trait is
expressed (turn on the trait or turn it
off)
– Chemicals
– Radiation
– Diet
– Why twins look different – due to their
environment!