chapter 14

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 14Chapter 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



Stamens (Male)Carpel (Female)


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


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 PurpleHybrids

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 1Y =YELLOWy =green

Character 2R=ROUNDr = 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


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 =



– 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




Start by listing all genotypes that fulfill this condition:



PpYyRr X Ppyyrr

– Pp X Pp = ¼ PP, ½ Pp, ¼ pp





Ppyyrr, PPyyrr


3. Calculate probability for each genotype

– Pp X Pp = ¼ PP, ½ pP, ¼ pp



• Ppyyrr ½ X ½ X ½ = 2/16

• Ppyyrr



– Pp X Pp = ¼ PP, ½ pP, ¼ pp



• 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?



ppYyRr X Ppyyrr:

– pp X Pp = ½ Pp, ½ pp


– rr X Rr = ½ Rr, ½ rr



ppYyRr X Ppyyrr:







2. Go back to the original QuestionppYyRr X Ppyyrr:






ppyyrr



ppYyRr X Ppyyrr:




• 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 dominanceF1 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

1

2

31 2


• 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

Cc CC

Cc Cc

cc


• 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

Pp Pp Pp

PpPp

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

chapter 14

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