Early Ideas about Heredity

• The Blending Theory:– Main theory of inheritance until the late 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

• 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

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 flowersAll 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 during gamete production

Homologous pair of Chromosomes

Locusfor

FlowerColorGene

Allele for White Flowers

Allele for Purple Flowers

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

P (Parent) Generation True-breeding parents

F1 (1st Filial) Generation Hybrid Offspring

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)

3:3: 11

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

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 design an experiment to determine

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

Back to Mendel’s Conclusions

Exception to Dominant Alleles Masking Recessive Alleles:

Incomplete Dominance: Pink Snapdragons

CCRR

CCRR

CCWW

CCWW

CCWWCCWW

CCRR CCRR

CCRRCCWW CCRRCCWW

CCRRCCRR

CCWWCCWW

P Generation

F1 Generation

F2 Generation

Phenotype(s): Red and White

Phenotype: Pink

Genotype(s): CRCR and CWCW

Genotype: CRCW

½ ½

Gametes: CR and CW

Gametes: CR and CW

½

½ ½

½

Exception to Dominant Alleles Masking Recessive Alleles:

Co-dominance: Blood Types

Back to Mendel’s Conclusions

This is a process called meiosis:

Specifically it is “crossing over,” which occurs very early during Prophase I of Meiosis

Law of Segregation:

Two alleles for each trait segregate during gamete production

Back to Mendel’s Conclusions

Mendel’s Conclusions

2. Law of Independent Assortment

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

What if two traits were followed?

Two hypotheses were written and experimented on to determine if traits were:– Dependent (linked), or – Independent (not linked)

If the genes are not connected, then they should segregate independently.

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 segregated as matched sets in the gametes.

YR yr

YyRr

P

Gametes

F1

F2

YR YR

yr yr

Ova Sperm

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

Significance of Mendel’s Research

1. Probability was introduced as a biological concept:

The number of times an event occurs The number of trials

2. Punnett Square was introduced as a tool

Probability:

The number of times an event occursThe number of trials

Back to Mendel’s Principles

Segregation of alleles into ova

Segregation of alleles into sperm

P P

pp½

½ ½

½ ½ x ½ =

¼

P P

Pp x Pp

¼

P p

¼

P p

¼

p pHow do you get a genotypic ratio of 1:2:1 from these numbers?

Punnett Square Exercise

One-trait crossOne-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 Exercise

One-trait crossOne-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.

Punnett Square Exercise

One-trait crossOne-trait cross

In a 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.

Punnett Square Exercise

Two-trait crossTwo-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 crossTwo-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 crossTwo-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)