Download Inheritable Traits

Heredity
What is inheritance?
► Heredity
or inheritance is the passing of
something from one generation to another
 Perhaps you will inherit a large fortune one day!
► In
this case, we are talking about inheriting
traits
 Ex. If your parents both have dimples, it is likely
that you will inherit dimples from them.
► Genetics
is the science of heredity
What are traits?
► Hair
Color
► Eye Color
► Freckles
► Skin Color
► Widow’s Peak
► Hitchhiker’s Thumb
► Height
Either/Or Traits
► Only
two versions
 Freckles or no freckles
 Widow’s peak or straight hairline
 Brachydactyl or normal finger length
► Traits
that ARE NOT either/or would be
things like
 Eye Color
 Hair Color
 Height
Gregor Mendel
► The
father of modern genetics
► Mendel was a monk who worked with pea
plants
► He fertilized the egg of one flower with the
sperm from the pollen of another flower.
This is called cross-fertilization.
 He wondered what color flower you would get
from crossing a purple flower with a white
flower
Why Pea Plants?
► Reproduce
quickly
► Many offspring
► Can easily control reproduction
► Many either/or observable traits
What Mendel Discovered
► Mendel
realized from his work with pea
plants that some traits are dominant over
others
► Principle of Dominance: a dominant trait
masks or prevents expression of a recessive
trait
 Dominant trait is represented by a capital letter
 Recessive trait is represented by a lower case
letter
Example: Dimples
► Dimples
are dominant, represented by D
► Lack of dimples is recessive, represented by
d
► If your mom gave you D and your dad gave
you d:
 You would have Dd
► The
capital D would be
dominant over the
lowercase d
 You would have dimples
Mendel
► Mendel
had to use several generations of pea
plants to test his hypotheses
 P = parent generation
 F1 = offspring of the P generation
 F2 = offspring of the F1 generation
► Principle
of Unit Characters: there are a pair of
factors (or “characters”) for every trait
 Characters = GENES!
 PAIR of genes = one from each parent
Mendel
► Law
of Segregation: two genes separate
(segregate) during gamete formation
(meiosis)
 Each gamete receives
only one gene for each trait
►Gametes
are haploid
Chromosome Theory of Heredity
► Walter
Sutton (early 1900s): Mendel’s
“factors” (genes) are carried on
chromosomes
► Chromosome Theory of Heredity: genes are
carried on chromosomes
 Each chromosome must carry hundreds of
genes
Important Vocabulary
► Allele:
Each alternative form of a gene for a
certain trait
 Ex. D or d
► Genotype:
Combination of alleles for a given
trait (i.e., what genes an organism has for a
trait)
 Ex. Dd, dd, or DD
► Phenotype:
Appearance of a trait (PHeno =
PHysique or PHysical appearance)
 Ex. Dimples or no dimples
Important Vocabulary
► Homozygous
SAME allele
(Pure): Having two of the
 Ex. DD or dd
 Homozygous dominant: having two of the
dominant allele
►Ex.
DD
 Homozygous recessive: having two of the
recessive allele
►Ex.
dd
► Heterozygous
(Hybrid): Having two
DIFFERENT alleles
 Ex. Dd
Example: Freckles
► Two
possible phenotypes for freckles are:
 Has Freckles (observable)
 No freckles (observable)
►A
genotype for freckles is indicated by two
versions of the freckle gene. The possible alleles
(versions) using the first letter of the trait “f” are:
 F (dominant) = Has Freckles
 f (recessive) = No freckles
► Question:
Using F and f, what are possible
genotypes of the allele pair for freckles?
Freckles: Genotypes & Phenotypes
►Question:
Using F and f, what are possible
genotypes of the allele pair for freckles?
Genotype
Phenotype
(genes)
(appearance)
FF
====== has freckles
Ff
====== has freckles
ff
====== no freckles
►One dominant allele (F) is sufficient for its trait
(has freckles) to be observable, but both alleles
have to be recessive (f) for the recessive trait (no
freckles) to be observable.
Other examples of either/or traits
in humans
► Hitchhiker’s
thumb
► Widow’s peak
► Brachydactylia
► Cleft chin
► Attached or free hanging
earlobes
► Interlocking fingers
Probability
► Probability
in genetics is the likelihood of an
offspring receiving a trait
► How
do we figure out the probability?
 Use a Punnett Square
►A
chart that helps predict the
possible combinations of
genes among offspring
 The gametes are written across
the top and down the side
Potter’s Hair
► Harry
has dark hair like his father, but his
mom had red hair. Using the genotypes of
rr (red hair) Rr (brown hair), RR (brown
hair), what possible genotypes does each of
the potters have?
► If Harry marries Ginny who has red hair,
what are the possible genotypes of their
children’s hair colors?
Probability
► Product
Law of Probability: the probability
of 2 or more events occurring
simultaneously is equal to the product of
each occurring separately
 There is a 1 in 2 chance of flipping a coin and it
landing on heads (½)
 Chance of flipping two coins and both of them
landing on heads = ½ x ½ = ¼ chance
Probability
► For
a Punnett square with traits Yy x Yy,
you expect:
 Genotype Ratio: 1:2:1
 Phenotype Ratio: 3:1
► The
expected does not always match the
actual
► The larger number of offspring, the more
likely it is for the actual to match the
expected
Incomplete Dominance
► Neither
allele is dominant or recessive
► When genes show incomplete dominance,
the traits are BLENDED
 Ex. Red flowers crossed with white flowers
make pink flowers
 RR – red
 Rr – pink
 rr - white
Harry Potter Example: Fire
Breathing Dragons
the dragons in Harry Potter have fire-power
alleles F (strong fire) and f (no fire) that follow
incomplete dominance, what are the phenotypes
for the following dragon-fire genotypes?
Genotypes
Phenotypes
► If
FF
Ff
ff
Strong Fire
Moderate Fire
No Fire
Codominance
► Both
traits are dominant and both are
expressed
 Ex. A red haired cow crossed with a white
haired cow makes a roan haired cow
► We
use two different letters for the different
alleles (both capitals)
 RR – red
 WW – white
 RW - roan
Harry Potter Example: Merpeople
► If
merpeople have tail color alleles B (blue) and G
(green) that follow the codominance inheritance
rule, what are possible genotypes and
phenotypes?
Genotypes
Phenotypes
BB
BG
GG
Blue
Blue and Green
Green
Two Traits
► Principle
of Independent Assortment: Genes for
different traits segregate independently during
gamete formation IF the genes are located on
different chromosomes
 Mendel formulated this principle, but the IF… was not
added until later
► For
two traits that assort independently, we can
use a 16 square Punnett Square
 A dihybrid cross (AaBb x AaBb)
will always produce 9:3:3:1 ratio
Multiple Alleles
► When
there are more than 2 alleles for a
given trait in a population
 Each individual still only carries 2 alleles for the
trait
► The
most common example of multiple
alleles is human blood type
Human Blood Type
►3
alleles used for human blood type
 A, B, O
►4
possible blood types in humans
 A, B, AB, O
► The
letters refer to antigens found on the
red blood cells (Type A has A antigens
present.)
Human Blood Type
► Both
the A and B alleles are dominant, but
O is recessive
► Possible genotypes and phenotypes for
human blood type:
Genotype
AA
AO
AB
BB
BO
OO
Phenotype
Type
Type
Type
Type
Type
Type
A
A
AB
B
B
O
Try a Punnett Square for Blood Type
► Cross
a person with type BO with a person
who has type AB
► What are the possible blood types of the
offspring?
Cross Matching
► Type
O is the “universal donor” because it
has no antigens. Therefore, no other blood
type will reject it
► Type AB is the “universal recipient” because
it has no antibodies. Therefore, it will not
reject any other blood type
Rh Factor
► Blood
types are not just represented by ABO, we
also say they are positive or negative
 A+, A-, etc.
► +/-
refers to the presence of Rh antigens on the
membrane of red blood cells.
 If Rh antigens are present, the blood type is +
 If Rh antigens are absent, the blood type is –
► Rh
is dominant so +/+ and +/- will be Rh +, while
only -/- will be Rh-
Try a Blood Type Cross
AB++ x Type OGenotypes:
List the gametes:
Now cross!
Offspring Genotypes & Percents:
Offspring Phenotypes & Percents:
Sex Determination
► Thomas
Hunt Morgan (1866-1945): discovered
that chromosomes of males and females are
slightly different
 Sex chromosomes: chromosomes that differ between
the sexes (X and Y)
 Autosomes: chromosomes that aren’t sex chromosomes
► Females
= XX
► Males = XY
► He worked with Drosophila, or fruit flies
 Fruit flies have the same sex chromosomes as humans!
Sex-Linked Inheritance
► Sex-linked
characteristics (sometimes called Xlinked characteristics) are traits that are linked
with gender
 Determined by an allele carried on the X chromosome
 The X chromosome is much larger than the Y
chromosome
► Most
sex-linked traits are recessive.
 A male who carries a particular recessive allele on the X
chromosome will have the sex-linked condition.
 In order for a female to express the condition, she must
have two X chromosomes both with the recessive allele.
Hemophilia
► An
example of a sex-linked condition is
hemophilia.
 Normal blood clotting (N) is dominant to
hemophilia (n).
 XN represents an X chromosome with normal
blood clotting
 Xn represents an affected X chromosome
carrying hemophilia
Hemophilia
► In
order to express hemophilia, a male only
has to have one Xn chromosome.
► A female with one Xn chromosome and one
XN chromosome will be a carrier of
hemophilia, but will not express the
condition.
► A female must have two Xn chromosomes
(XnXn) to express hemophilia.
Hemophilia
Genotypes
XNY
XnY
XNXN
XNXn
XnXn
Phenotypes
Non-affected Male
Affected Male
Non-affected Female
Carrier Female
Affected Female
Try a Sex-Linked Cross
Cross a mother who is a carrier for hemophilia
with a father who has hemophilia
Genotypes:
List the gametes:
Now cross!
Offspring Genotypes & Percents:
Offspring Phenotypes & Percents:
Polygenic Inheritance
► Polygenic
Inheritance: pattern of inheritance
in which many genes affect a single trait
► Many traits are controlled by multiple genes
 Hair color, height, skin color, eye color
► Continuous
Variation: presence of many
phenotypes from one extreme to another,
due to polygenic inheritance
 Think of how many different heights there are!
Gene Expression
► Modifier
genes: influence the amount,
intensity & distribution
 Ex. Eye color is produced by melanin, blue eyes
are a lack of melanin production, hazel & green
are variations in amount, intensity, &
distribution
► Developmental
Environment: the
environment in which an organism develops
may influence the expression of genes
 If a tree has genes to be tall, but is not given
enough nutrients, it cannot express that trait
Pedigree Chart
►A
chart that shows a family tree, including
genotypes and phenotypes of each
individual
► Helps to determine
genotypes and
phenotypes of other
family members