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PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 14

Mendel and the Gene Idea

Gregor Mendel

experiments with garden peas

Figure 14.1

Mendel’s Experimental Approach

Why peas?

= available in many varieties

= could strictly control mating

Mendel’s Experimental Approach

Stamens (Male)

Carpel (Female)

Mendel’s Experimental Approach

Genetics Vocabulary

Alternative versions of genes = Alleles

Organism inherits 2 alleles:

1 from mom, 1 from dad

A genetic locus is represented twice

Genetics Vocabulary

If the two alleles at a locus differ…
Dominant allele = determines appearance
Recessive allele = no noticeable effect on appearance

Genetic Vocabulary: Homozygous vs. Heterozygous

Homozygous for a particular gene

Identical pair of alleles for that gene

Ex: PP (2 purple flower alleles)

True-breeding

- Homozygous dominant (PP)

- Homozygous recessive (pp)

Homozygous vs. Heterozygous

Homozygous for a particular gene

Identical pair of alleles for that gene

Ex: PP (2 purple flower alleles)

True-breeding

Heterozygous for a particular gene

Has a pair of alleles that are different for that gene

Ex: Pp (1 purple allele, 1 white allele)

Homozygous or Heterozygous?

Genetics Vocabulary

An organism’s genotype (EX: Pp, PP, pp)

genetic makeup

An organism’s phenotype (Ex: Purple or white)

physical appearance

Phenotype versus genotype

Mendel used :
Characters that varied in an “either-or” manner
Varieties that were “true-breeding”

Typical Mendelian Experiment

Parental Generation

Hybridization

F1 Generation

F1 self-pollinate

F2 generation

All Purple

Hybrids

3:1

Purple : White

Does Mendel’s segregation model account for the 3:1 ratio observed in the F2 generation?

We can answer this question using a Punnett square

Other pea plant characters

Construct a Punnett Square for the following crosses:
Seed color: Y = Yellow, y = green

YY X Yy

Expected ratio observed in offspring?

Seed shape: R = Round, r = wrinkled

Rr X rr

Expected ratio observed in offspring?

The Testcross

In pea plants with purple flowers

Genotype is not obvious (Pp or PP)?

= Perform testcross

The testcross

Monohybrid Cross

Mendel Followed a single trait (ex: flower color)

The P = true-breeding (PP or pp)
The F1 offspring = monohybrids (heterozygous for one character) (Pp)

Dihybrid Cross

Mendel followed 2 characters at the same time
P generation = Cross two, true-breeding parents differing in two characters

YYRR X yyrr

Character 1

Y =YELLOW

y =green

Character 2

R=ROUND

r = wrinkled

Mendel followed 2 characters at the same time
P generation = Cross two, true-breeding parents differing in two characters

YYRR X yyrr

F1 generation = Produces dihybrids (heterozygous for both characters)

YyRr

2. Independent Assortment of Chromosomes

Homologous orient randomly at metaphase I of meiosis

How are two characters transmitted from parents to offspring?

1. As a package? (Ex: yellow and round YR)

=Dependent Assortment

2. Independently?

=Independent Assortment

A dihybrid cross

Only YR and yr as inherited from P generation?

YR Yr yR yr ?

Make a punnett square for each case

Concept 14.2: The rules of probability govern Mendelian inheritance
Multiplication Rule
Addition Rule

The Multiplication and Addition Rules Applied to Monohybrid Crosses

The multiplication rule

Probability that two or more independent events will occur together

Ex: coin toss

Heads ½ X Heads ½ = ¼

Ex: Probability in a monohybrid cross

Rule of Addition

Probability that any one of two or more exclusive events will occur

Ex: Heterozygotes:

¼Rr + ¼rR = ½

A multi-character cross

= two or more independent monohybrid crosses occurring simultaneously

Calculate the chances for various genotypes:

1. Consider each character separately

2. Go back to question being asked

3. Multiply individual probabilities together

4. Use Rule of addition (if necessary)

3 characters = trihybrid cross

Purple flowers (Pp), Yellow (Yy), Round (Rr)

Purple flowers (Pp), green (yy), wrinkled (rr)

PpYyRr X Ppyyrr

Question: What percentage of the offspring from this cross would be predicted to have purple flowers and green and wrinkled seeds?

1. Consider each character separately (make a punnett square for each character)

PpYyRr X Ppyyrr:

Pp X Pp =

Yy X yy =

Rr X rr =

1. Consider each character separately (make a punnett square for each character)

PpYyRr X Ppyyrr

Pp X Pp = ¼ PP, ½ pP, ¼ pp

Yy X yy = ½ Yy, ½ yy

Rr X rr = ½ Rr, ½ rr

2. Go back to the original Question

PpYyRr X Ppyyrr

Pp X Pp = ¼ PP, ½ pP, ¼ pp

Yy X yy = ½ Yy, ½ yy

Rr X rr = ½ Rr, ½ rr

Question: What percentage of the offspring from this cross would be predicted to have purple flowers and green and wrinkled seeds?

Start by listing all genotypes that fulfill this condition:

2. Go back to the original Question

PpYyRr X Ppyyrr

Pp X Pp = ¼ PP, ½ Pp, ¼ pp

Yy X yy = ½ Yy, ½ yy

Rr X rr = ½ Rr, ½ rr

Question: What percentage of the offspring from this cross would be predicted to have purple flowers and green and wrinkled seeds?

Start by listing all genotypes that fulfill this condition:

Ppyyrr, PPyyrr

3. Calculate probability for each genotype

Pp X Pp = ¼ PP, ½ pP, ¼ pp

Yy X yy = ½ Yy, ½ yy

Rr X rr = ½ Rr, ½ rr

Ppyyrr ½ X ½ X ½ = 2/16
Ppyyrr

3. Calculate probability for each genotype

Pp X Pp = ¼ PP, ½ pP, ¼ pp

Yy X yy = ½ Yy, ½ yy

Rr X rr = ½ Rr, ½ rr

Ppyyrr ½ X ½ X ½ = 2/16
PPyyrr ¼ X ½ X ½ =1/16

4. Rule of addition

2/16 Ppyyrr

+1/16 Ppyyrr

3/16

= chance that the offspring from this cross would have purple flowers and green and wrinkled seeds

A multi-character cross #2

= two or more independent monohybrid crosses occurring simultaneously

Calculate the chances for various genotypes:

1. Consider each character separately

2. Go back to question being asked

3. Multiply individual probabilities together

4. Use Rule of addition

3 characters = trihybrid cross #2

white flowers (pp), Yellow (Yy), wrinkled (rr)

Purple flowers (Pp), green (yy), Round (Rr)

ppYyrr X PpyyRr

Question: What percentage of the offspring from this cross would be predicted to have white flowers and green and wrinkled seeds?

1. Consider each character separately (make a punnett square for each character)

ppYyRr X Ppyyrr:

pp X Pp = ½ Pp, ½ pp

Yy X yy = ½ Yy, ½ yy

rr X Rr = ½ Rr, ½ rr

2. Go back to the original Question

ppYyRr X Ppyyrr:

pp X Pp = ½ Pp, ½ pp

Yy X yy = ½ Yy, ½ yy

rr X Rr = ½ Rr, ½ rr

Question: What percentage of the offspring from this cross would be predicted to have white flowers and green and wrinkled seeds?

Start by listing all genotypes that fulfill this condition:

2. Go back to the original Question

ppYyRr X Ppyyrr:

pp X Pp = ½ Pp, ½ pp

Yy X yy = ½ Yy, ½ yy

rr X Rr = ½ Rr, ½ rr

Question: What percentage of the offspring from this cross would be predicted to have white flowers and green and wrinkled seeds?

Start by listing all genotypes that fulfill this condition:

ppyyrr

3. Calculate probability for each genotype

ppYyRr X Ppyyrr:

pp X Pp = ½ Pp, ½ pp

Yy X yy = ½ Yy, ½ yy

rr X Rr = ½ Rr, ½ rr

ppyyrr ½ pp X ½ yy X ½ rr = 1/8

Concept 14.3

Inheritance patterns are often more complex than predicted by simple Mendelian genetics
The relationship between genotype (Ex: Pp) and phenotype (Ex: purple) is rarely simple

The Spectrum of Dominance

Complete dominance

Phenotypes of the heterozygote and dominant homozygote are identical

Codominance

Two dominant alleles affect the phenotype in separate, distinguishable ways

Ex: human blood group MN

MM = RBC with M molecules

NN = RBC with N molecules

MN = ?

Incomplete dominance

F1 hybrid phenotype is between the phenotypes of the two parental varieties

Figure 14.10

Dominance and Phenotype

Dominant and recessive alleles

Do not “interact”

Different alleles = synthesis of different proteins that produce a phenotype

Dominance and Phenotype

Dominant and recessive alleles

Do not “interact”

Different alleles = synthesis of different proteins that produce a phenotype

Ex: flower color

White (W) vs. Red (R)

W= protein that produces white pigment

R = protein that produces red pigment

Multiple Alleles

Most genes exist in populations

In more than two allelic forms

The ABO blood group in humans

Is determined by multiple alleles:

3 different alleles for enzyme I

IA = attaches the A carbohydrate

IB = attaches the B carbohydrate

i = attaches neither A nor B

Table 14.2

Complex inheritance patterns

Codominance

Incomplete dominance

Multiple alleles

Mendel’s fundamental laws still apply!

Concept 14.4:

Human traits follow Mendelian patterns of inheritance
Humans = not convenient subjects for genetic research

How can we study Human Genetics?

Concept 14.4:

Human traits follow Mendelian patterns of inheritance
Humans = not convenient subjects for genetic research

How can we study Human Genetics?

= Pedigree analysis

Male =

Female =

Follow Attached earlobe = ff

Carriers?
Disease condition = aa
No disease symptoms = Aa or AA

Mating of Close Relatives

Mating between relatives

Can increase the probability of the appearance of a genetic disease

Albinism- recessive phenotype
Only aa

Albinism- recessive phenotype

Human achondroplasia phenotype
The phenotype is determined by a dominant allele = AA or Aa

Human achondroplasia: Dominant allele disease

PHENYLKETONURIA - [PKU] pp

Autosomal recessive disorder
Gene for phenylalanine hydroxylase (PAH), found on chromosome 12 mutated
PAH converts the amino acid phenylalanine to tyrosine
No PAH = concentration of phenylalanine in the body can build up to toxic levels

PKU: Recessive disease (pp)

Pp/PP

Review!

Mendel’s Pea experiments:
Experimental method
Typical Mendelian experiment:

P, F1, F2

Monohybrid cross vs. Dihybrid cross
Law of Segregation and Law of Independent assortment

Review!

Solving Multi-hybrid crosses with probability
More complex inheritance patterns:

Co-dominance, Incomplete dominance, Multiple alleles

Pedigree Analysis