Mendelian Genetics

What is Heredity?

The passing on of traits from one generation to the next

X

Female normal wing

Male double wing

P- generation

F1 – normal wing

Early Ideas about Heredity

• Blending Theory (19th century):– Each parent contributed “factors” that were blended in

the offspring

• What was the problem with this theory?– All individuals of a population would eventually look

the same.– Once blended, traits would never separate and show

up in later generations.

Chapter 9, Section 1: Pre-Mendel

Gregor Mendel (1822 – 1884)• Augustinian monk who

studied pea plants• Established the

particulate theory of heredity

• Significance:– Developed pure lines– Counted results and kept

statistical notes (data)

• His work remained undiscovered until 1903.

Chapter 9, Section 1: Mendel

Character Dominant Trait Recessive Trait

Flower Color

Flower Position

Seed Color

Seed Shape

Pod Shape

Pod Color

Stem Length

Purple White

Side Top

Yellow Green

Round Wrinkled

Inflated Constricted

Green Yellow

Tall Dwarf

Mendel’s Research

Why did he research pea plants?

1. They are normally self-pollinated, but can be cross-pollinated.

2. They have several qualitative traits that are easy to distinguish

i.e.,Tall vs. Short

Self Pollination

Cross Pollination

Mendel’s Research

1. Removed stamens from purple flower.

2. Transferred pollen from stamens of white flower to pistil of purple flower.

3. Pollinated flower matured into a pod.

4. Planted seeds from pod.5. Examined offspring:

All purple flowers…

Watch this video clip, and seeif you can explain why…

Mendel’s Conclusions

1. Law of Segregationa. Factors (genes) for a particular trait occur in

pairs

b. For each trait, an organism inherits two genes, one from each parent.

c. Dominant alleles mask recessive onesi. Exception 1: Incomplete Dominance

ii. Exception 2: Co-dominance

d. Two alleles for each trait segregate (separate) during gamete production

Homologous pair of Chromosomes

Locusfor

FlowerColorGene

Allele for White Flowers (p)

Allele for Purple Flowers (P)

Law of Segregation:

Factors for a particular trait occur in pairs

Back to Mendel’s Conclusions

Genes: The “factors” that control traits.

Alleles: Different forms of a gene.

• Each individual is diploid– Diploid: Containing a double-set of chromosomes (2n)

• Each gamete is haploid– Gamete: Reproductive cell (egg or sperm)– Haploid: Containing a single-set of chromosomes (n)

Back to Mendel’s Conclusions

Law of Segregation:

One version of each gene is inherited from each parent

Ww ww

W

w

orw

w

or

W = widow’s peak

w = no widow’s peak

P (Parent) Generation True-breeding parents (PP x pp)

F1 (1st Filial) Generation Hybrid Offspring (Pp)

F2 (2nd Filial) Generation

What is the F2 ratio?

705 plants had purple flowers

224 plants had white flowers

705:224 3:1

What happened to the recessive traits?

Dominant alleles mask recessive ones

P pPP pp

P p

Pp

PP

Pp Pp

pp

P P

p p

P Generation

F1 Generation

F2 Generation

Phenotype (Appearance) Purple Flowers White FlowersGenotype (Genetic Makeup)Gametes

Phenotype (Appearance)

Gametes

Purple FlowersGenotype (Genetic Makeup)

3: 1

What is the Genotypic Ratio of the F2 Generation?

PP:Pp:pp 1:2:1

What is the Phenotypic Ratio of the F2 Generation?

Dominant alleles mask recessive ones

Punnett Square

Genotype Phenotype

Purple

Purple

Purple

White

PP

Pp

Pp

pp

(homozygous)

(heterozygous)

(heterozygous)

(homozygous)

1

1

2

3

1

Ratio = 1:2:1 Ratio = 3:1

Dominant alleles mask recessive ones

Homozygous: same alleles

Heterozygous: different alleles

We will now play “Got Gametes?” in order to practice understanding the following

new terms: alleles, genotype, phenotype, homozygous, heterozygous

Vocabulary Practice

Vocabulary Practice

Each of you are haploid gametes carrying single alleles – for a trait.

Your single letter can be combined with another single letter (i.e., Hh) to form a genotype in a diploid organism.

You will observe a series of faces. Come to the front of the classroom if you think you have the right genotype to match the phenotype shown. Make sure to find the corresponding allele for the trait you represent!

How do you set up a Punnett square?

Example 1:

Heterozygous short hair (____) X heterozygous short hair (____)

HhHh

Hh Hh

HH

hh HH

Hh Hh

hhGenotypic Ratio HH:Hh:hh

1:2:1

Phenotypic Ratio Short hair:long hair

3:1

Now you try!

Dominate alleles mask recessive ones

Dominant phenotype, unknown genotype:

PP or Pp?

Recessive phenotype, known genotype:

pp

If PP, then all offspring purple:

If Pp, then ½ offspring purpleand ½ offspring white:

What would your hypothesis be if the genotype was PP?

P p

P p p

P p

pPp

Pp

Pp

Pp

Pp

Pp

pp

pp

What would your hypothesis be if the genotype was Pp?

This is called a test cross

If you have a dominant phenotype (like purple flowers) how would you

determine if it was homozygous (PP) or heterozygous (Pp)? What

experiment would you design?

Back to Mendel’s Conclusions

Exception to Dominant Alleles Masking Recessive Alleles

Incomplete Dominance: Pink Snapdragons

Use Root Letter “C” to designate incomplete dominance interaction

CR

CR

CW

CW

CWCW

CR CR

CRCW CRCW

CRCR

CWCW

P Generation

F1 Generation

F2 Generation

Phenotype(s): Red and White

Phenotype: Pink

Genotype(s): CRCR and CWCW

Genotype: CRCW

Gamete of Red flower: CR Gamete of White flower: CW

Gametes: CR and CW

Back to Mendel’s Conclusions

Exception to Dominant Alleles Masking Recessive Alleles:

Co-dominance: Blood Types (video)

Use root letter “I” for dominant alleles of equal strength and “i” for recessive

Back to Mendel’s Conclusions

This occurs in a process called meiosis:Specifically it is “crossing over,” which occurs very early during Prophase I of Meiosis and separation

in Anaphase I and II

Law of Segregation:

Two alleles for each trait segregate (separate) during gamete production

Back to Mendel’s Conclusions

Mendel’s Conclusions

The law of segregation followed one single trait at a time, such as seed color.

What if two traits were followed, such as seed color and seed shape?

Are these genes somehow connected (linked) and inherited together?

Following two traits:Seed shape and seed color

Hypothesis: If the genes for seed shape and color are connected in some way,

then the dominant R and Y alleles and the recessive r and y alleles will be matched sets in the gametes.

YR yr

YyRr

P

Gametes

F1

F2

YR YR

yr yr

Ova Sperm

For the traits of seed shape and color, this hypothesis is NOT supported by experimental

evidence

What did the data support instead?

YYRR yyrr

YR yr

What are the genotypes of the P Generation?

What does the P Generation pass on to F1?

F1 Phenotype?

YyRrF1 Genotype?

Gametes

What does the F1 Generation pass on to F2?

YR YR

Yr Yr

yR yR

yryr

SpermOva

YYRRCombine the sperm and ova to produce the offspring in F2

What do you expect the phenotype will be when YR and YR are combined?

What will the Genotype be when YR and YR are combined?

Now try to do it on your own!

YYRr YYRr

YyRR YyRR

YyRrYyRr YyRrYyRr

yyRryyRr

yyRRYyrr

YYrr

Yyrr

yyrr

Phenotypic Ratios

Yellow & Round

Green & Round

Yellow & Wrinkled

Green & Wrinkled

9/16

3/16

3/16

1/16

What are the phenotypic ratios?

Law of Independent Assortment

Mendel’s Conclusion!

Law of Independent Assortment:If the genes are not connected, then they

should segregate independently. The alleles are randomly packaged into different gametes during meiosis

(For example, genes for seed shape and color were not inherited together.)

YyRr YR Yr yR yr

FOIL

Significance of Mendel’s Research

Punnett Square was introduced as a tool to predict or determine the probability of an event

Now, let’s practice dihybrid crosses!

White board exercises

One-trait cross

The allele for the hairy trait is “H,” while the

allele for the hairless trait is “h.”

Make a cross between two homozygotes for each of these traits.

Punnett Square ExerciseOne-trait cross

In a population of Wisconsin fast plants, purple color is created by the pigment anthocyan. The gene that codes for this pigment is dominant (A), and without this pigment, the plant is green recessive (a).

Predict the offspring from the cross of a purple

homozygous plant and a green plant. What if the purple plant was heterozygous? How would the offspring be different?Green Purple

Purple plant: AA

Green plant: aa

A aaA Aa

AaAa

Aa100% Purple

Purple plant: Aa

Green plant: aa

A aaa Aa

Aaaa

aa

50% Purple

50% Green

Punnett Square ExerciseOne-trait cross

In populations of hamsters, brown fur is dominant (B), and white fur is recessive (b).

Predict the offspring from the cross of a white hamster and a brown hamster if the brown hamster's mother was white.

Brown hampster: Bb

White hampster: bb

B bbb Bb

Bbbb

bb

Offspring

50% brown mice

50% white mice

Punnett Square ExerciseOne-trait cross

Henry VIII divorced six of his wives (two of whom were

executed) for not bearing him any sons. Use Mendelian Genetics to prove to King Henry that it wasn’t his wives’ fault.Remember:

Females are XX and Males are XY.Go to Test Crosses

White-board exercises

Two-trait cross

Following two traits:

Kernel shape: Plump vs. withered (P and p)

and Kernel color Red vs. yellow (R and r) Predict the cross between a homozygousrecessive corn plant and a homozygous dominant corn plant.

Punnett Square Exercise

Two-trait cross

Now take the offspring from that cross, and self-pollinate that plant.

What is the phenotypicratio of this cross?

Punnett Square ExerciseTwo-trait cross

My pet guinea pigs (Joni and Chachi) are going to have little guinea pigs.

What is the possibility that their offspring will have long hair, if I know that Joni and Chachi are heterozygous for the following traits?

Black fur (B) White fur (b)

Long hair (L) Short hair (l)