Chapter 14: Mendel & The Gene Idea

The History of Your Inheritance

• Identify one or two physical characteristics that seem to be found in most of your family members. Why do you think these particular characteristics are so common?

• Are there some inherited characteristics that you have that your parents do not?

• Do you have any characteristics that are unique to you?

http://www.youtube.com/watch?v=6NvESo3mG90&feature=related

Character vs. Trait

• Some people use these two words synonymously, when they are, in fact, different.– Characteristic (Character)

• a heritable feature that varies among individuals (e.g.eye colour, flower colour, etc.)

– Trait• Each variant of a characteristic (e.g. blue vs.

brown eyes, purple vs. white flowers)

Gregor Mendel• Modern genetics began in the

mid-1800s, when monk named Gregor Mendel documented inheritance in peas– used quantitative analysis

• collected data & counted them– excellent example of scientific

method– Developed his theory decades

before chromosomes were observed under the microscope!

Why the Pea? Reason 1

• Peas are available in many varieties, each with easily distinguishable characteristics and traits– These varieties are easy

to count

Pea Plant Anatomy

filamentanther

stamen

stigmastyle

ovules inovary

carpel

Stamen: male organ Carpel: female organ

Where pollen (male gametes) produced

Sticky structure that receives pollen during fertilization

Female gametes

Why the Pea? Reason 2

• Can strictly control which plants mated with which

Mendel started with True-breeding Plants

• True-breeding plants– are those that produce offspring that are of

the same variety when they self-pollinate– E.g. a plant with purple flowers is true-

breeding if the seeds produced by self-pollination all give rise to plants that also have purple flowers

Pollen transferred from white flower to stigma of purple flower

anthersremoved

all purple flowers result

Mendel’s work• Bred pea plants

– cross-pollinate true breeding parents (P)• P = parental

– raised seed & then observed traits (F1)• F = filial

– allowed offspring to self-pollinate & observed next generation (F2)

F1

P

F2

self-pollinate

F2generation

3:175%purple-flower peas

25%white-flower peas

Looking closer at Mendel’s work

P

100%F1generation(hybrids)

100%purple-flower peas

Xtrue-breedingpurple-flower peas

true-breeding white-flower peas

self-pollinate

Where didthe whiteflowers go?

Whiteflowers cameback!

Hybridization!

What did Mendel’s findings mean?

• Traits come in alternative versions– purple vs. white flower color– alleles

• different alleles vary in the sequence of nucleotides at the specific locus of a gene

– some difference in sequence of A, T, C, G

purple-flower allele & white-flower allele are two DNA variations at flower-color locus

different versions of gene at same location on homologous chromosomes

Traits are inherited as discrete units

• For each characteristic, an organism inherits 2 alleles, 1 from each parent– diploid organisms inherit

2 sets of chromosomes, 1 from each parent

• homologous chromosomes

Key

Maternal set ofchromosomes (n = 3)

Paternal set ofchromosomes (n = 3)

2n = 6

Pair of homologouschromosomes(one from each set)

What did Mendel’s findings mean?

• Some traits mask others – purple & white flower colors are separate

traits that do not blend • purple x white ≠ light purple• purple masked white

– dominant allele • functional protein• masks other alleles

– recessive allele • allele makes a

malfunctioning proteinhomologouschromosomes

I’ll speak for both of us!

wild typeallele producingfunctional protein

mutantallele producingmalfunctioningprotein

paternal maternal

Genotype vs. phenotype• Difference between how an organism

“looks” & its genetics– phenotype

• description of an organism’s trait• the “physical”

– genotype • description of an organism’s

genetic makeup

Explain Mendel’s results using…dominant & recessive …phenotype & genotype F1

P X

purple white

all purple

Making crosses• Can represent alleles as letters

– flower color alleles P or p– true-breeding purple-flower peas PP– true-breeding white-flower peas pp

PP x pp

PpF1

P X

purple white

all purple

F2generation

3:175%purple-flower peas

25%white-flower peas

????

Looking closer at Mendel’s work

PX

true-breedingpurple-flower peas

true-breeding white-flower peas

PP pp

100%F1generation(hybrids)

100%purple-flower peas

Pp Pp Pp Pp

phenotype

genotype

self-pollinate Pp x Pp

Punnett squaresPp x Pp

P pmale / sperm

P

pfem

ale

/ egg

s

PP

75%

25%

3:1

25%

50%

25%

1:2:1

%genotype

%phenotype

PP Pp

Pp pp pp

Pp

Pp

F1generation(hybrids)

Aaaaah,phenotype & genotypecan have different ratios

Genotypes • Homozygous = same alleles = PP, pp

• Heterozygous = different alleles = Pp

homozygousdominant

homozygousrecessive

heterozygous

Phenotype vs. genotype• 2 organisms can have the same

phenotype but have different genotypes

homozygous dominantPPpurple

Pp heterozygouspurple

How do you determine the genotype of an individual withwith a dominant phenotype?

Can’t tellby lookin’at ya!

Test cross• Breed the dominant phenotype —

the unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele

ppis itPP or Pp?

x How does that work?

PP pp

How does a Test cross work?

p p

P

P

p p

P

p

Pp pp

x x

Pp

Pp Pp

Pp

100% purple

Pp

pp

Pp

50% purple:50% white or 1:1

pp

Am I this?

Or am I this?

Mendel’s 1st law of heredity• Law of segregation

– during gamete formation, alleles segregate

• homologous chromosomes separate during meiosis

– each allele for a trait is packaged into a separate gamete

PP

P

P

pp

p

p

Pp

P

p

Law of Segregation

• Which stage of meiosis creates the law of segregation?

Whoa!And Mendel

didn’t even knowDNA or genes

existed!

Metaphase 1

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Chapter 14

Probability & Genetics

2005-2006

Genetics & Probability

• Mendel’s laws reflect same laws of probability that apply to tossing coins or rolling dice

2005-2006

Probability & genetics• Calculating probability of

making a specific gamete is just like calculating the probability in flipping a coin– probability of tossing

heads? 50%– probability making a P

gamete…PP

P

P

Pp

P

p

50%

100%

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Probability & genetics• Outcome of 1 toss has no

impact on the outcome of the next toss (i.e. they are independent events!)– probability of tossing heads each

time?– probability making a P gamete

each time?Pp

P

p

50%

50%

2005-2006

Calculating probabilityPp x Pp

P pmale / sperm

P

p

fem

ale

/ eg

gs PP

Pp pp

Pp

sperm egg

1/2 1/2

offspring

=x 1/4P P PP

1/2 1/2 =x 1/4P p Pp

1/2 1/2 =x 1/4p p pp

1/2 1/2 =x 1/4p P Pp

2005-2006

• Chance that 2 or more independent events will occur together (AND)– probability that 2 coins tossed at the

same time will land heads up

– probability of Pp x Pp pp

Rule of multiplication (AND)

1/2 x 1/2 = 1/4

1/2 x 1/2 = 1/4

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Rule of addition (OR)

• Chance that an event can occur 2 OR more different ways – sum of the separate probabilities

– probability of Pp x Pp Pp

sperm egg

1/2 1/2

offspring

=x 1/4P p Pp

1/2 1/2 =x 1/4p P Pp

1/4

1/4+

1/2

2005-2006

Sample Problem

• In humans the allele for albinism is recessive to the allele for normal skin pigmentation. If two heterozygotes have children, (a) What is the genotypic and phenotypic ratios

for their offspring?

(b) What is the chance that a child will have normal skin pigment?

(c) What is the chance that the child will be albino?

Solution (a)• Let A be dominant allele for normal skin pigmentation, let a be the

recessive allele for albinism

Aa x Aa

AA

75% or3/4

25% or1/4

3:1

25% or1/4

50% or1/2

25% or1/4

1:2:1

%genotype

%phenotype

aa

Aa

Aa

A (1/2) a (1/2) sperm

A(1/2)

a(1/2)

eggs

AA(1/2 x 1/2 =1/4)

Aa(1/2 x 1/2 =1/4)

Aa(1/2 x 1/2 =1/4)

aa(1/2 x 1/2 =1/4)

Solution (b) and (c)• Let A be dominant allele for normal skin pigmentation, let a be the

recessive allele for albinism

Aa x Aa

A (1/2) a (1/2) sperm

A(1/2)

a(1/2)

eggs

AA(1/2 x 1/2 =1/4)

Aa(1/2 x 1/2 =1/4)

Aa(1/2 x 1/2 =1/4)

aa(1/2 x 1/2 =1/4)

Chances that the child will have normal skin pigmentation (i.e. they must be AA OR Aa OR Aa):

Chance of AA + Chance of Aa= 1/4 + 1/4 + 1/4= 3/4 or 75%

Chances that the child will be albino (i.e. they must have aa genotype)

Chance of aa = 1/4

Challenge:• If the child is normal, what is the chance that it is a carrier for the

albino allele?

A (1/2) a (1/2) sperm

A(1/2)

a(1/2)

eggs

AA(1/2 x 1/2 =1/4)

Aa(1/2 x 1/2 =1/4)

Aa(1/2 x 1/2 =1/4)

aa(1/2 x 1/2 =1/4)

Normal individuals are either AA or Aa

For it to be a carrier, it must have Aa genotype:

2/3 or 67%

2005-2006

Any Questions?

Practice

• Pg. 135 # 1-9, 13, 16

• Worksheet