Mendelian Genetics

Chapter 11Part 1

pp. 189-197, 202-204

Genetic Theories

1. Blending Theory - traits were like paints and mixed

evenly from both parents.2. Incubation Theory - only one parent controlled the traits

of the children.Ex: Spermists and Ovists

3. Particulate Model -

parents pass on traits as discrete units that retain their identities in the offspring.

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INTRODUCTION TO GENETICSINTRODUCTION TO GENETICS

Fruit and Flower of the Garden Pea

55

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a.

Flower Structure

filament

antherstamen

stigma

style

ovules inovary

carpel

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• Mendel crossed pea plants that differed in certain characteristics and traced the traits from generation to generation

Figure 9.2C

• This illustration shows his technique for cross-fertilization that results in hybrid offspring

1 Removed stamensfrom purple flower

White

Stamens

Carpel

PurplePARENTS(P)

OFF-SPRING

(F1)

2Transferred pollen from stamens of white flower to carpel of purple flower

3Pollinated carpel matured into pod

4 Planted seeds from pod

Self-pollination

• One flower as both parents.

• Natural event in peas.

• Results in pure-bred offspring where the offspring are identical to the parents.

Results - Summary

• In all crosses, the F1 generation showed only one of the traits regardless of which was male or female.

• The other trait reappeared in the F2 at ~25% (3:1 ratio).

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Mendel’s 4 Conclusions: 1. Various forms of genes exist

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GENE LOCI

Figure 9.4

P a B

DOMINANTallele

RECESSIVEallele

P a b

GENOTYPE: PP aa Bb

HOMOZYGOUSfor thedominant allele

HOMOZYGOUSfor therecessive allele

HETEROZYGOUS

Mendel’s 4 Conclusions: 2. For each characteristic, we inherit 2 alleles (one from each parent)3. If alleles are different, in complete dominance, there is a dominant and recessive allele

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GENETIC MAKEUP (ALLELES)

P PLANTS

F1 PLANTS(hybrids)

F2 PLANTS

PP pp

All P All p

All Pp

1/2 P 1/2 p

EggsP

p

P

PPp

Sperm

Pp Pp

pp

Gametes

Gametes

Phenotypic ratio3 purple : 1 white

Genotypic ratio1 PP : 2 Pp : 1 pp

Figure 9.3B

Mendel’s 4 Conclusions:

4. Law of Segregation: alleles separate during

meiosis and rejoin during fertilization of

sperm and egg. Remember Anaphase I (separation of tetrads)

MENDEL’S LAW OF SEGREGATIONMENDEL’S LAW OF SEGREGATION

During the formation of gametes (eggs or sperm), the During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each two alleles responsible for a trait separate from each other.  Alleles for a trait are then "recombined" at other.  Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of fertilization, producing the genotype for the traits of the offspring.the offspring.

Rr

R r

1010

We can show Mendel’s idea in the form of punnet

squares:1.What is the probability that a couple will have a boy?

1010

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Modern Genetics View

• Each trait in a pea plant is controlled by two alleles (alternate forms of a gene)

• Dominant allele (capital letter) masks the expression of the recessive allele (lower-case)

• Alleles occur on a homologous pair of chromosomes at a particular gene locus

– Homozygous = identical alleles

– Heterozygous = different alleles

1616

Genotype versus Phenotype

• Genotype

– Refers to the two alleles an individual has for a specific trait

– If identical, genotype is homozygous

– If different, genotype is heterozygous

• Phenotype

– Refers to the physical appearance of the individual

1111

Patterns of Inheritance

• Complete dominance (Mendel’s focus)

• Incomplete dominance• Codominance• Multiple Alleles• Pleiotropy• Epistasis• Polygenic Traits

1111

Fig. 14-UN2

Degree of dominance

Complete dominanceof one allele

Incomplete dominanceof either allele

Codominance

Description

Heterozygous phenotypesame as that of homo-zygous dominant

Heterozygous phenotypeintermediate betweenthe two homozygousphenotypes

Heterozygotes: Bothphenotypes expressed

Multiple alleles

Pleiotropy

In the whole population,some genes have morethan two alleles

One gene is able toaffect multiplephenotypic characters

CRCR CRCW CWCW

IAIB

IA , IB , i

ABO blood group alleles

Sickle-cell disease

PP Pp

Example

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Examples of COMPLETE DOMINANCE

1. Class Example: Brown eyes are dominant over blue.What is the probability that Mr. and Mrs. Brooks will have a child with brown eyes if both are hybrids?

2. Class Example:The ability to roll the tongue is dominant to the lack of this ability. Mr. Brooks is recessive and Mrs. Brooks is heterozygous.

3. Class Example: Right handedness is dominant to left. Tommy is right-handed but not a hybrid and his brother is left handed.

What is the phenotypic ratio? If Tommy has a child in the future, what handedness will this child be? What was the probability that Tommy would have the genotype

he has? What was the probability that Tommy would have the phenotype

he has?10

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Punnett Square Showing Earlobe Inheritance Patterns

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Parents

Ee Ee

eggs

spem

Punnett

sq

uare

Offspring

E e

E

e

Ee

EeEE

e e

Allele key Phenotypic Ratio

unattached earlobes3

1

E = unattached earlobese = attached earlobes attached earlobes

Mendel’s Law of Probability

Multiplication• Allows us to easily calculate probability, of

genotypes and phenotypes among the offspring• Punnett square in next slide shows a 50% (or ½)

chance – The chance of E = ½– The chance of e = ½

• An offspring will inherit:– The chance of EE =½x½=¼ – The chance of Ee =½x½=¼ – The chance of eE =½x½=¼ – The chance of ee =½x½=¼

Mendel’s Law of Probability: Addition

• Rule of Addition is that the probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways….applies to heterozygous being produced (Ee)

• An offspring will inherit:– The chance of EE =½x½=¼

– The chance of Ee =½x½=¼ – The chance of eE =½x½=¼ – The chance of ee =½x½=¼

The probability that a heterozygous offspring will be produced is 1/4 + 1/4 = 1/2.

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EXAMPLESMonohybrid using Rules of multiplication and addition

AA x Aa……Probability of Aa offspring?

Aa x Aa…..Probability of Aa offspring?

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What if you don’t know each parent’s

genotype?

TEST CROSS – designed to reveal the

genotype of an organism when

you do not know

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Example using a TEST CROSS

• The ability to roll the tongue is dominant to the lack of this ability. Mr. Smith is a tongue roller while Mrs. Smith is not. What are the possible probabilities of the next generation?

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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• By looking at two characteristics at once, Mendel found that the alleles of a pair segregate independently of other allele pairs during gamete formation

– This is known as the law of independent assortment

Mendel’s Law: The LAW of INDEPENDENT ASSORTMENT is revealed by tracking 2 traits

DIHYBRIDSAnalysis of two traits simultaneously

• Trait: Handedness (right vs. left) and Eye Color (brown vs. blue)

• Use ‘H’ for handedness and ‘E’ is for eye color. Right handedness is dominant to left and brown eyes are dominant to blue. Mom is heterozygous for both traits and dad is a hybrid for both traits.

–WHAT IS THE PHENOTYPIC RATIO?

Rules of Probability Applies to Dihybrid

Crosses too• For a dihybrid cross, HhEe x

HhEe, what is the probability of an F2 having the genotype

»HHEE?»HhEe?»hhee?

Let’s Try a Trihybrid

• Determine the probability that the offspring would produce recessive phenotypes for at least 2 of the 3 characters.

– Heterozygous purple flowers and yellow seeds and round seeds crossed with heterozygous purple and recessive green and wrinkled.

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Fig. 14-UN1

Incomplete Dominance

Class Example: A cross between a red four o clock and a white four o clock produces a pink one. Cross a red four o clock with a pink four o clock.

Codominance• Two alleles affect the phenotype in separate and distinguishable ways. • Neither allele can mask the other and both are expressed in the

offspring and not in an “intermediate” form.• In a study conducted by the University of Michigan, pitch black

feathered birds crossed with gray feathered birds produced a spotted (black and gray), bird. Show a cross between 2 speckled birds.

• 1) In cattle, roan coat color (mixed red and white hairs) occurs in the heterozygous (RW) offspring of red (RR) and white (WW) homozygotes. When two roan cattle are crossed, the phenotypes of the progeny are found to be in the ratio of 1 red:2 roan:1 white. Which of the following crosses could produce the highest percentage of roan cattle?

• A) roan x roan • B) red x white • C) white x roan • D) red x roan • E) All of the above crosses would give the same

percentage of roan.

Multiple AllelesABO Blood System

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Multiple Allelic Traits

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Rh Factor associated with Blood Types demonstrates

Complete Dominance

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Pleiotropy

• Most genes have multiple phenotypic effects. The ability of a gene to affect an organism in many ways is called pleiotropy.

EpistasisEpistasis• Epistasis occurs when a

gene at one locus alters or influences the expression of a gene at a second loci. In this example, C is for color and the dominate allele must be present for pigment (color) to be expressed.

•  In a certain breed of plants, B produces blue flowers and is dominant over b which produces green flowers.  Another gene determines in which cells the pigment will be synthesized.  Allele M allows complete synthesis of the pigment throughout the flowers but the mutant allele m prevents pigment production. 

You cross two plants that are BbMm. 

Polygenetic Inheritance• Qualitative variation

usually indicates polygenic inheritance. This occurs when there is an additive effect from two or more genes. Pigmentation in humans is controlled by at least three (3) separately inherited genes.

40

18

Fig. 14-UN2

Degree of dominance

Complete dominanceof one allele

Incomplete dominanceof either allele

Codominance

Description

Heterozygous phenotypesame as that of homo-zygous dominant

Heterozygous phenotypeintermediate betweenthe two homozygousphenotypes

Heterozygotes: Bothphenotypes expressed

Multiple alleles

Pleiotropy

In the whole population,some genes have morethan two alleles

One gene is able toaffect multiplephenotypic characters

CRCR CRCW CWCW

IAIB

IA , IB , i

ABO blood group alleles

Sickle-cell disease

PP Pp

Example

Fig. 14-UN3

DescriptionRelationship amonggenes

Epistasis One gene affectsthe expression ofanother

Example

Polygenicinheritance

A single phenotypiccharacter isaffected bytwo or more genes

BbCc BbCc

BCBC

bC

bC

Bc

Bc

bc

bc

9 : 3 : 4

AaBbCc AaBbCc