Chapter 14
• “Blending hypothesis” – proposes that the genetic material from two parents blend together (like yellow and blue paint to produce green)
• This would eventually lead to a uniform population
• Gregor Mendel – published his data in1863
• “Particulate” theory of inheritance – parents pass on discrete units of heredity that retain their separate identities in offspring
• Genes are sorted and passed on generation after generation in undiluted form
• Mendel was one of the earliest scientists to apply
a quantitative approach to the evaluation of
scientific data
• Mendel discovered the basic principles of heredity
by breeding garden peas in carefully planned
experiments.
• Pea plants have several advantages for genetic study
• Distinct characteristics
• Ability to control mating
• Short generation time
• Mendel chose only those
characteristics that could
be described as
“either – or”• Tall or short
• Purple or white
• Mendel could use pollen
from one plant to
fertilize another
When doing genetic crosses, we call
the original pair of organisms the
P, or parental, generation.
Their offspring (kids) are called the
F1, or “first filial,” generation.
• Mendel started his experiments with varieties that were true-breeding.
• In a typical experiment Mendel would cross-pollinate (hybridize) two contrasting true-breeding pea varieties.
• The true-breeding plants are the P generation and the hybrid offspring are the F1
generation
• Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation
• Mendel did experimental crosses for seven
different traits
• Each parent had a different version of each trait
• One of the traits “disappeared” in the F1
generation
• Did the recessive alleles
go away?
• NO! They come back in
the F2 Generation
• Mendel repeated his experiments multiple times and discovered a consistent ratio of three to one, purple to white flowers in the F2 generation.
• Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids
• Mendel called the purple flower color a dominant trait and the white flower color a recessive trait.
• Mendel found the same pattern of inheritance in the other pea characteristics
• What Mendel called a “heritable factor’ is what we now call a gene
• Chromosomes carry GENES (most have at least 1,000 genes)
• Gene: a section of a chromosome that codes for a specific trait (ex. hair color)
• Example: FLOWER COLOR, hair color, etc.
Each gene can have different forms/options
The alleles determine an organism’s TRAIT
TRAIT: physical appearance (what it looks like!)
Example: purple OR white flowers
Allele: different forms of a gene
1)Flower color looks purple
2) DNA segment coding for
flower color
3) DNA segment coding for
purple flower color
• Alternative versions of genes account for
variations in inherited characters.
• The alternative versions of a gene are called alleles
• For each character, an organism inherits two
alleles, one from each parent.
• Each diploid organism has a pair of homologous
chromosomes
• The two alleles on the homologous pair of
chromosomes may be identical (homozygous) or
different (heterozygous)
• If the two alleles at a locus differ, then one, the
dominant one, determines the organism’s
appearance. The other the recessive allele, has
no noticeable effect on the organism’s
appearance.
• Mendel’s Law of Segregation states that two
alleles for a heritable character separate and
segregate during gamete production and end
up in different gametes
• This accounts for the 3:1 ratio that Mendel saw
• A chart used to predict the results of a genetic cross between individuals of known genotype• Genotype =
genetic make up (PP or Pp)
• Phenotype = physical trait (Purple)
• Shows all possible combinations of gametes
• Allows for random fertilization
• In Chinchillas, Grey is a
recessive fur color.
• What would the alleles
of the grey chinchilla
be?
• In Chinchillas, Grey is a
recessive fur color.
• If two grey chinchillas
had babies, what color
would they be?
• In Chinchillas, Beige is a
dominant fur color.
• What would the alleles of the
beige chinchilla be?
• In Chinchillas, Beige is a
dominant fur color.
• If two Beige chinchillas had
babies, what color would
they be?
• Used to determine the genotype of an organism
when it exhibits the dominant phenotype.
• The organism in question is bred with an
individual exhibiting the recessive phenotype
• If any of the offspring display the recessive
phenotype, the parent in question must be
heterozygous.
• In Chinchillas, Grey is a
recessive fur color.
• Genotype?
• Phenotype?
• In Chinchillas, Ebony is a
dominant fur color.
• Genotype?
• Phenotype?
• In Chinchillas, White is a dominant fur color
• Homozygous White (FF) is LETHAL, so there are no
Homozygous
White Chinchillas.
• Genotype?
• Phenotype?
Having a cleft chin (N) is dominant to
not having a cleft chin (n).
A man with a cleft chin and a woman
without a cleft chin have children. They
have 2 children with a cleft chin and 1
child without.
1. What are the possible genotypes for the parents?
2. What alleles can the woman pass down in her eggs?
3.What alleles can the man pass down in his sperm?
4.What are the genotypes of the children?
5. Is the man homozygous dominant or heterozygous?
Probability
We can represent probability two ways: RATIOS and PERCENTAGES.
Ratio = a comparison between two things Example: 1:2 or 3:4
Percentage = amount out of 100. Example: 50% or 75%
Ratio =
The ORDER of the comparison matters!
Always reduce to simplest form!
number of ways
it could happen
number of
ways it will
not happen:
Example Question: You have 4 marbles. 2 are GREEN and 2 are BLUE.
What is the probability that you will pick
1 GREEN marble?
2 chances out of 4 = ½
½ of 100 = 50%
In ratio form: There are 4 possibilities TOTAL: 2 Green and 2 Blue
In percentage form:
number of ways
it could happen
number of ways it
will not happen:
2 : 2 1 : 1
1. What is the probability (in a ratio form) of
flipping a coin to heads?
2. What is the probability (in a percentage form)
of flipping a coin to heads?
3. What is the probability (in ration form) of it
being Monday?
4. What is the probability (in percentage form) of
it being Monday?
• Example 1: What will the offspring of a black
rabbit and white rabbit look like?
• Will it be black, white, or both?
Black Coat White Coat
• Punnett squares are a tool used to predict the
probability that offspring carry certain genetic
traits
• In a Punnett square, we consider all the possible
genotypes that could result from mixing two
parents’ DNA during fertilization
• In rabbits, the allele for white fur (F) is dominant
over the allele for black fur (f) color.
• If a female homozygous white rabbit mates
with a male homozygous black rabbit, what
color fur will their offspring have?
• Step 1) Identify the genotypes of EACH parent.
• Genotype for female:
• Genotype for male:
• Phenotype of the female:
• Phenotype of the male:
• Step 2) Write 1st genotype along the TOP
• Step 3) Write 2nd genotype along the SIDE
• Step 4) Drop the top alleles down
• Step 5) Move the side alleles across
• Each box of the square represents a 25% chance that
the offspring will get the trait
• Must represent results of the PHENOTYPE AND
GENOTYPE as a ratio and/or percentage
25% chance
25% chance
25% chance
25% chance
1. What is the phenotypic ratio
of white fur to black fur of the
offspring? ____: ______
2. What percentage of the
offspring will have black fur?
_______
3. What percentage will have
white fur? ____
• Each box of the square represents a 25% chance that
the offspring will get the trait
• Must represent results of the PHENOTYPE AND
GENOTYPE as a ratio and/or percentage
25% chance
25% chance
25% chance
25% chance
4. What is the genotypic ratio?
• Widow’s peak is a recessive trait while straight
hairline is a dominant trait.
• The mom is recessive while the dad is
heterozygous for this trait. What is the probability
their offspring will have a widow’s peak?
• Step 1:
•Genotype of the mother: _____
•Genotype of the father: ______
•Phenotype of the mother: _____________
•Phenotype of the father: _______________
• Steps 2-5: Draw the punnett square
• Step 6: Analyze and Record Results
What is the phenotypic
ratio of no widow’s peak to
widow’s peak?
• Steps 2-5: Draw the punnett square
• Step 6: Analyze and Record Results
What is the genotypic
ratio?
• Mendel’s first experiments investigated a single
character at a time.
• A cross that follows a single character and involves
two heterozygote parents is called a monohybrid
cross
• Mendel then followed two characters at a time.
• A cross that follows two characters and involves two
parents that are heterozygous for each character is
called a dihybrid cross
• Using a dihybrid cross, Mendel developed the
law of independent assortment
• This states that each pair of alleles segregates
independently of other pairs of alleles during
gamete formation.
• Genes located near each other on the same
chromosome tend to be inherited together.
• Mendel’s laws reflect the rules of probability
• The probability scale ranges from 0 (an event with no chance of occurring) to 1 (an event that is certain to occur)
• The probability of tossing heads with a normal coin is ½
• The probability of rolling a 3 with a six-sided die is 1/6; the probability of rolling any other number is 1-1/6 = 5/6
• Each event is independent of others
• When tossing a coin, the outcome of one toss does not impact on subsequent tosses.
• This rule states that the probability that two or
more independent events will occur together is
the product of their individual probabilities.
• Multiply the individual probabilities to obtain
the overall probability of these events occurring
together.
• Determine that event A and event B will occur
• AND Multiply
• Probability that two coins tossed at the same
time will both land on heads is
• ½ X ½ = ¼
• The probability that a heterozygous pea plant
(Pp) will self-fertilize to produce a white
flowered offspring (pp) is the chance that a
sperm with a white allele will fertilize an ovum
with a white allele
• ½ X ½ = 1/4
• For a heterozygous parent (YyRr) the
probability of producing a YR gamete is:
• ½ X ½
• We can use this to predict the probability of
producing a particular F2 genotype without
constructing a Punnett square
• The probability that a plant from the F2
generation will have a YYRR genotype is 1/16
• (¼ chance for YR ovum and ¼ chance for YR sperm)
• This is used to predict the probability of an
event that can occur two or more different
ways.
• Probability of event A or event B
• The probability is the sum of the separate
probabilities
• OR Add
• There are two ways that F1 gametes can
combine to form a heterozygote.
• Tt x Tt
• The dominant allele could come from the sperm and
the recessive from the ovum (probability = ¼)
• Or the dominant allele could come from the ovum
and the recessive from the sperm (probability = ¼)
• The probability of a heterozygote is ¼ + ¼ = ½
• Determine the probability of an offspring having two recessive phenotypes for at least two of three traits resulting from a trihybrid cross between pea plants that are PpYyRr and Ppyyrr.• Probability of producing a ppyyRr offspring
• Probability of producing pp =
• Probability of producing yy =
• Probability of producing Rr =
• Probability of all three being present =
• Probability for ppYyrr =
• Probability for Ppyyrr =
• Probability for PPyyrr =
• Prbability for ppyyrr =
• Therefore the chance that a given offspring will have at least two recessive traits =
• Alleles show different degrees of dominance
• Complete dominance = Mendelian trait
• Co-dominance
• Two alleles affect phenotype in separate and distinguishable ways.
• Both alleles are fully expressed
• Example: Blood types
• Incomplete dominance
• Heterozygotes show a distinct intermediate phenotype not seen in homozygotes
• Example: flower color in snap dragons – red x white → pink
• Heterozygous chickens –speckled with black AND white feathers
• Heterozygous cows “Roan”: white and red hairs
• Heterozygous blood type: alleles A and B are expressed together (AB)
• Incomplete Dominance
•Traits are MIXED
•Black + White = Grey
• Codominance
•Traits are BOTH SEEN
•Black + White = Black and White Spots or
Stripes
• Multiple Alleles: more than two alternative forms of alleles
• Example: Human ABO blood groups
• Individuals with type A blood have type A carbohydrates on the surface of their red blood cells.
• Individuals with type B blood have type B carbohydrates on the surface of their red blood cells
• Individuals with type AB blood have both A and B carbohydrates on the surface of their red blood cells
• Individuals with type O blood have neither A nor B carbohydrates on the surface of their red blood cells
• alleles = IA, IB, I
• IA and IB are codominant
• i = recessive
• Mother is Type A (IAIA)
• Father is Type O (ii)
• Mother is Type A (IAi)
• Father is Type AB (IAIB)
• The dominant allele does not interfere with the
activity of the recessive allele.
• Differences between the alleles is due to
differences in nucleotide sequences.
• The two alleles do not interact with each other
• The dominant allele is not necessarily more
common in a population than the recessive
allele.
• Polydactyly results form a dominant allele but is
not the most common phenotype
• The character of these relationships depend on the level at which we examine the phenotype
• Example: Individuals with Tay-Sachs disease lack a functioning enzyme to metabolize certain lipids. These lipids accumulate in the brain, damaging brain cells, and ultimately leading to death.
• Organism Level: Children with the disease have two recessive alleles; complete dominance.
• Molecular Level: heterozygotes produce equal numbers of normal and dysfunctional enzyme molecules; co-dominance
Autosomal Recessive
• Phenylketonuria
(PKU)
• Cystic fibrosis
• Tay-sachs disease
• Sickle Cell Disease
Autosomal Dominant
• Huntington’s disease
•Homozygous Lethal
• Polydactyly
• Acondroplasia
•Homozygous Lethal
• Epistasis: a gene at one locus alters the
phenotypic expression of a gene at a second
locus.
• Example: coat color in mice depends on two genes.
• The epistatic gene determines whether pigment
will be deposited in hair or not
• The other gene determines whether the pigment to
be deposited is black or brown.
• Polygenic inheritance: the additive effects of two or more genes on a single phenotypic character.• This results in a population with a
range of phenotypic characteristics.
• Example: skin color in humans is controlled by at least three independent genes.
• Each gene has two alleles, dark and light
• The genes are incompletely dominant
• AABBCC is very dark; aabbcc is very light.
• Multifactorial traits: environment contributes to the phenotype
• Example: nutrition influences height in humans
• Example: a single tree may have leaves that vary in size, shape, and greenness, depending on exposure to wind and sun.
• Example: identical twins accumulate phenotypic differences as a result of their unique experiences.
• Many human disorders have multifactorial basis including: heart disease; diabetes; cancer; alcoholism
• Western White Butterfly
• Butterflies hatching in springtime have more
pigment in their wings than those hatching in the
summer.
• The environment influences the expression of
their genes for wing coloration.
SpringSummer
• In order to fly effectively, the body
temperature of the Western white butterfly needs to
be 28–40°C.
• More pigment allows a butterfly to reach the warm
body temperature faster.
• In the hot summer months, less pigment prevents the
butterflies from overheating.
• The distribution of a phenotypic trait is mapped
on a family tree.
• Phenotypes of family members and knowledge of
dominant/recessive relations between alleles allow
researchers to predict the genotypes of members of
a family.