Mendel’s Legacy

Genetics is everywhere these days – and it will continue as a dominant force in

biology and society for decades to come.

Wouldn’t it be nice if people understood it better?

The Fundamental Question

What is the relationship between genes (genotype) and observable

characteristics (phenotype)?

The answer?

Phenotype = Genotype + Environment.

Genes and Environment Determine Characters

Genetically identical hydrangeas growing in soils of different acidity

(different environments).

The phenotype = genotype + environment principle applies equally to

human traits.

A Mendelian Genetic Primer

Genes come in pairs that separate in the formation of gametes.

The members of the pair may be identical (homozygous) or non-identical

(heterozygous).

Each form of a particular gene is an allele.

Genes, Alleles, and Chromosomes

A Mendelian Genetic Primer

One allele is dominant over another (or so Mendel

believed).

Only two alleles of a given gene are possible in an

individual although many alleles of a gene are possible

within a population.

How did Mendel assist us in coming to these

underatandings?

Why was Mendel’s use

of the Garden Pea ideal?

• They were

commercially

available.

• Easy to grow and

matured quickly.

• Enclosed sex organs

in the flower.

• ***Seven different

traits with ONLY two

possible variations.

The Reality of “Round and Wrinkled” – Two Alternative Traits of

the Seed Shape Character

Note: each seed is a

new individual of a

different generation

– seeds are not of

the same generation

as the plant that

bears them.

Mendel’s Monohybrid Cross

– P to F1

• Mendel created purebred

plants for each of the traits he

studied and called them the

Parent generation (P).

• Crossing these parents he

created the First Filial (F1)

generation, that all possessed

the same trait variation!!!

• WHY??????

• Mendel then continued crosses

to the F2 (second filial, the

grandchildren).

What was learned?

The green trait was not lost or

altered, even though it

disappeared in the F1.

What this meant???

• One trait must have been

dominant to the other in

the expression of its trait.

What do the numbers tell???

• Calculating the reappearance of the recessive trait in

only ¼ of the F2 suggests that the genes must come in

pairs which separated in the formation of sex cells.

• These genes from different sex cells must then reform a

new pair of genes in the new individual.

• NOTE: During Mendel’s time there was no knowledge

of DNA/genes, so Mendel called them inheritance

factors.

Monohybrid Crosses and the Principle of Segregation

A cross between individuals differing

in a single character is a monohybrid

cross.

The analysis of monohybrid

crosses allowed Mendel to deduce

the Principle of Segregation ....

Genes come in pairs that separate

in the formation of sex cells (and

these sex cells unite randomly at

fertilization).

Principle of Segregation Demystified

NOW WE KNOW:

The principle of segregation is explained by the

behavior of homologous chromosomes at meiosis.

Segregation

A Punnett Square is a Handy Way of Analyzing Crosses

In a Punnett square for a monohybrid cross,

the Principle of Segregation is applied.

Possible offspring

Different Genotypes Can Produce the Same Phenotype

Consistency

is Good

Characters

investigated by

Mendel

No matter what

the character,

Mendel observed

a 3:1 ratio of

characters in the

F2.

Monohybrid Crosses Yielded Consistent Results

Therefore, the Principle of Segregation indeed

is a general principle of genetics.

What Works for Peas Also Works for Humans

An albino woman

In the cross Aa x Aa, where A is a

dominant allele for wild type (standard)

pigmentation and a is a recessive allele

for no pigmentation (albinism), ¾ of

offspring will be wild type and ¼ will be

albino.

Do this monohybrid cross

• In pea plants, round seeds are dominant to

wrinkled seeds. If a homozygous round seed

plant is crossed with a heterozygous round

seed plant, what is the expected phenotypic

and genotypic ratios in the F1 ?

Answer

R R

R

r

RR RR

Rr Rr

Phenotypic Ratio: All Round

Genotypic ratio: 2 RR : 2 Rr

P – RR x Rr

Alleles produced for gametes are R or R from one parent

and R or r from the other parent

Try This one !

Fruit fly wing length is controlled by a

dominant allele for long wings (L), short is

recessive. If a heterozygous long winged fly

is mated to a homozygous short winged fly,

what is the expected phenotypic and

genotypic ratio in their offspring?

• Do both sheets from webpage.

– Monohybrid Practice

– Spongebob genetics

Then Pg, 535 # 10,11,12 and 14.

Understand these concepts

pg 529-530

• Principle of Dominance –

• Law of Segregation –

• Unit Theory of Inheritance –

Probability & Product Rule

• Probability – is the chance of an event

occurring, usually represented by a ratio.

• Product Rule – states the chance that two or

more independent events will occur

TOGETHER.

– This is calculated by finding the product of

their individual probabilities occurring alone.

Flip a coin

• Probability of Heads?? - -

• ½ (50%)

• Probability of flipping Two (2) heads Together?

Try this

• What is the probability of rolling a regular die

and getting a 6?

• What is the probability of rolling a pair of dice

and getting “snake-eyes” (a pair of 1’s)?

Test Cross

• To determine the UNKNOWN genotype of

an organism that shows the dominant

phenotype, we perform a TEST CROSS.

• This is done by taking the organism with

the unknown genotype (TT or Tt) and cross

it with a purebred (homozygous) recessive

and look at the results.

• If any of the

offspring show

the recessive

condition then

the unknown

parent MUST

have been

heterozygous.

Extensions to Mendel: (The Exceptions)

Complexities in Relating Genotype to Phenotype

Extensions to Mendel’s analysis

• In Single-gene inheritance: – Sometimes pairs of alleles show deviations from complete

dominance and recessiveness

– Sometimes more than two different variations (alleles) exist

for a gene

– Sometimes one gene may determine more than one trait

• In Multifactorial inheritance:

• the phenotype expressed arises from the

interactions between one or more genes, the

environment, and chance events.

Dominance is not always complete

• Crosses between true-breeding strains can produce hybrids with phenotypes different from both parents

• Ex.

• Incomplete Dominance

• Co-Dominance

• Multiple Alleles

Incomplete dominance

• This occurs in the case where two pure breeding (homozygous) parents that show two completely different phenotypes have F1 hybrids that show a completely different phenotype from either parent.

• The F1 expresses an intermediate phenotype. Neither allele is dominant or recessive to the other.

• Phenotypic ratios are same as genotypic ratios

Incomplete Dominance for Flower Color in Snapdragon

Codominance

• Co-Dominance occurs when parents have different traits and the alleles for these traits are equally dominant to the other. The F1 hybrid offspring of a cross between two purebreds will express the phenotype of both parents equally.

• Phenotypic ratios are same as genotypic ratios

Codominance of Spotted and Dotted Coat Pattern Alleles

Three Different Forms of Dominance

Multiple Alleles

• In many species a gene may have more than

two possible alleles for any given gene.

– Ex. Human Blood Groups (A, B, and O)

– (A & B are co-dominant and O is recessive)

Codominance of IA and IB Blood Group Alleles

There Are Often More Than Two Alleles of a Gene

Blood groups are determined by one gene with three alleles.

Also that the blood groups show both complete dominance and

codominance.

Multiple Alleles Can be Grouped into a Dominance Series

Dominance series for lentil bean coat color.

Try:

-Practice Problem from Pg 542

-Thinking Lab pg 543.

- Then pg 544 - #2, 5, 6 a & b,

Do variations on dominance relations

negate Mendel’s law of segregation?

• Dominance relations only affect the relationship between genotype and phenotype

• Alleles still segregate randomly and unite randomly

• Gene products (plus external factors) control the expression of phenotypes

What about Multiple traits?

• Does being tall (dominant) give you more

of a chance at also being yellow (dominant).

• Mendel performed a series of experiments

like before, but tracked two different traits

at the same time.

• Two purebred

parents when

crossed still

produced a

heterozygous F1

• Note: Notice the

numbers for the

phenotype and

genotype of the

F2

• Anything similar?

• Phenotype:

– 9:3:3:1

• Genotype 1:1:2:2:4:2:2:1:1

Step by step!!

• #1 - Make a legend

• #2 – Show parents 4 dashes times 4 dashes.

• ____ ____ ____ ____ x ____ ____ ____ ____

– NOTE- Remember two dashes for each trait!!

• #3 Perform “FOIL” on each parent to make

gametes.

• #4 make your Punnett square.

• Tip- Keep Same letter together in the boxes

and always put the capital letter first (when

possible)

Try it

• Tall is dominant over short.

• Yellow is dominant over green.

• Cross a homozygous tall, homozygous

green plant with a plant that is heterozygous

for both traits.