Download Introduction to Genetics

Introduction to Genetics
Remember…
• DNA RNA ProteinTraits
• DNA contains the code for proteins
(protein synthesis…remember?)
• Proteins determine our traits
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 (P) generation (generation starting
point)
Pure Green Pea X Pure Yellow Pea
Hypothesis: yellow-green offpsring
Genetic Crossing
Parent (green) X
Parent (yellow)
P
P
F1
F1
F1
F1
First Generation (F1 - offpsring)
**ALL YELLOW**
Mendel’s Second Generation – F2
Mendel self-fertilized the F1:
F1
F1
F2
F2
F2
3:1 ratio
F2
F2
Green trait reappeared in F2
(second)
generation
Before we can figure out what
happened….
• We need to learn some genetics!
Mendel’s First Law: The Law of Segregation
1. All individuals
have two copies
of each allele (i.e.
Ss)
2. Each gamete
receives one copy
of every allele
during gamete
formation (S or s)
In other words, they
SEGREGATE
Hybrid Cross - mating between individuals
who have different alleles at one locus of interest
Possible allele from gamete
passed down to offspring
Parent
(dad)Plant:
Ss
Parent(mom) plant: Ss
S
S
SS
s
Ss
s
Ss
ss
Possible
combinations for
alleles of offspring
after fertilization
Vocabulary
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, either
dominant or
recessive
Which allele is dominant? Which is recessive?
Parent(mom) plant: Ss
Parent
(dad)Plant:
Ss
S
S
SS
s
Ss
s
Ss
ss
In each square (offspring
trait), which parent is
passing down the
dominant trait? The
recessive trait?
Possible
combinations for
alleles of offspring
after fertilization
• 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
From Dad (only 1 gamete)
From Mom (only 1 gamete)
Common Vocabulary
• Phenotype
• Genotype
Phenotype
• The observable traits of an organism
(what you see!)
Genotype – the genetic make up
combination of alleles located on
homologous chromosomes that determines
a specific characteristic or trait.
Offspring can have genotypes
that are:
Homozygous dominant: SS
Homozygous recessive: ss
Heterozygous: Ss
**Remember on allele from mom and one
from dad**
You Try It
• Answer #7 on your PowerPoint Notes
Monohybrid Cross
• Cross involving only one trait
• Used to find traits of offspring
• Can be used to find the traits of
parents
You Try It
• In humans, brown eyes (B) are
dominant over blue (b). What are the
chances that a blue eyed man and a
homozygous brown eyed woman will
have children with blue eyes?
You try it…monohybrid cross
• 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?
You Try It
• If a heterozygous purple flower is mated
with another heterozygous purple
flower, what ratio of offspring will be
white in color? (Use P for purple and p
for white)
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 (Dd x dd and DD x dd). 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?
You try it…
• Genetics Protocol
• Monohybrid practice problems
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, so, color and shape sort
independently. i.e. round doesn’t necessarily go
to the same gamete as yellow
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 more than one trait– the alleles for
EACH trait will separate independent of each
other.
Summer squash – dihybrid example
• W (white) is dominant to w (yellow)
• D (disk shape) is dominant to d (sphere shape)
• Cross a white, disk shape with a yellow,
sphere shape
Genotype
WwDd (white, disk-shaped fruit) X wwdd (yellow, sphere-shaped fruit)
Phenotype
Let’s try it together: Cross WwDd x
wwdd
What are all the possible allele
combinations for WwDd? wwdd?
What would the genotype and
phenotype ratios be for this cross?
(WwDd x wwdd)
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 nontasters, 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.
•
•
Part A: Suppose a woman who is both a homozygous tongueroller 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?
•
Part B: 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.
You try it…
Non- Mendelian Genetics
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
• For male, xry , xRy
• For female, xrxr , xRxr , xR xR
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.
Male Pattern Baldness
• Male-pattern baldness is a sex-linked
trait in which affected people become
bald.
• XB = No baldness (dominant)
• Xb = Male-pattern baldness (recessive)
• If a Male with XbY genotype mated with
a Female with XB Xb genotype.
• What percent of male children will
have male-pattern baldness?
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.
?
?
?
?
Blood Type – a type of
codominance
• There are three alleles of this gene:
A, B, and O.
• A person’s blood type is determined
by which allele he/she inherits from
each parent.
AB, A, B, O type blood
• A and B genes are “co-dominant”
• if both an A and B allele are inherited, both are expressed
• O is a recessive allele
• If an A or B gene is inherited along with the O gene, the A
or B gene determines the person’s blood type.
• A person is type O only if he/she inherits two O genes.
Polygenic Inheritance – many
genes affecting one trait
• 3 genes affect skin color
Epistasis
• Occurs when two or more genes are
involved in determining traits (i.e.
skin color)
• When gene at one locus affects the
expression of a gene at another
locus
Epistasis example - Mice
• Black coat (B) is dominant to brown
(b)
• Must have bb to have brown coat
• A color gene (C) must be present for
the color to be deposited on the hair
• Must have Cc or CC for color, if cc,
no color and mouse is white
Linked Genes
• Genes that are inherited together
• Located near each other on the same
chromosome
• Chances of them crossing over to
another chromosome during meiosis
is small
• Ex. Eye color and wing color in fruit
flies are inherited together, not
independently sorted during meiosis
Environmental Factors
• Can affect whether or not a trait is
expressed (turn on the trait or turn it
off)
– Chemicals
– Radiation
– Diet
– Stress
– Why twins look different – due to their
environment!