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.