Transmission of Genes From Generation to Generation

Chapter 3

3.1 Heredity: How are Traits Inherited?

Why do we begin examining inheritance by discussing Gregor Mendel and pea plants?

Before Mendel there was no clear understanding of how traits were inherited and passed from one generation to the next

Mendel used experimental genetics to uncover the fundamental principles of inheritance

3.2 Mendel’s Experimental Design

Mendel’s experiments defined the ideal properties of an experimental organism: It should have a number of different traits

that can be studied The plant should be self-fertilizing and

have a flower that reduces accidental pollination

Offspring of self-fertilized plants should be fully fertile so more crosses can be made

http://www.youtube.com/watch?v=Mehz7tCxjSE

ANIMATION: Crossing garden pea plants

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Mendel’s Experiments in the Monastery Garden

Traits Selected for Study by Mendel

3.3 Crossing Pea Plants: Single Traits

Mendel’s initial crosses studied the inheritance of a single trait such as seed shape or seed color When plants with smooth peas were crossed with plants

having wrinkled peas, the offspring all produced smooth peas

3.3 Crossing Pea Plants: Single Traits P1: Smooth X wrinkled

F1: Smooth

Self-fertilize F1 plants

5,474 Smooth

1,850 wrinkledF2:

Total peas in F2: 7,324

• When the plants from these smooth seeds were crossed with each other, the offspring yielded 7,324 peas

5,474 were smooth

and 1,850 were wrinkled

ANIMATION: Monohybrid cross

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INTERACTION: Test Cross

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Mendel’s Terminology

P1 = parental generation F1 = first filial (generation after parent generation)

generation F2 = second generation

So in this experiment P1: smooth x wrinkled F1: offspring all smooth F2: offspring smooth (5,474) and wrinkled (1,850)

Mendel’s Conclusions

Only one of the parental traits was expressed in the F1 plants Genetic factors can be hidden (unexpressed) Despite identical appearance, P1 and F1 plants

must have different genetic factors F1 plants must carry hidden genetic factors

The trait not expressed in the F1 plants reappeared in 25% of the F2 plants F1 plants must carry two factors, one from each

parent. (these factors are now referred to as genes)

Traits were not blended as they passed though parents

Traits

Dominant Trait - Trait expressed in the F1 (heterozygous) condition

Recessive Trait - Trait repressed in the F1 but re-expressed in some members of the F2 generation

Phenotype - Observable properties of an organism

Genotype - The specific genetic constitution of an organism

Phenotype and Genotype

P1 :

Genotypes SS ss

Meiosis

Phenotypes Smooth wrinkled

Gametes S S s s

Fertilization

F1 :Genotype Ss

Phenotype Smooth

X

Mendel’s Principle of Segregation

Pairs of factors (genes) separate from each other during gamete formation and exhibit dominant/recessive relationships

https://www.youtube.com/watch?v=-deWjUJBxGk

Gene Pairs Segregate during Gamete Formation

Genes - Fundamental units of heredity

Segregation - Members of a gene pair separate from each other during gamete formation

Ss SsSs Ss

Smooth SmoothS s S s

F1 cross Gamete formation by F1 parents

Combinations of Gene Forms (Alleles)

Allele - One possible alternative form of a gene Usually distinguished by its phenotypic effects

Homozygous - Having identical alleles for one or more genes

AA or aa

Heterozygous - Having two different alleles for one or more genes

Aa

ANIMATION: Genetic terms

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Punnett Square

Ss Ss S sSs Ss

SSmooth Smooth

S s S s s

F1 cross Gamete formation by F1 parents

Set up Punnett square

Genotype PhenotypeS s

1 SS 3/4 Smooth2 Ss S

sS Smooth

Ss Smooth

1 ss1/4

wrinkleds

F2 ratioGamete combinations

represent random fertilization

ss wrinkled

SS Smooth

3.4 The Principle of Independent Assortment

Members of one gene pair separate or segregate independently of other gene pairs

https://www.youtube.com/watch?v=QyY8yb9S--s

Mendel’s Principle of Independent Assortment

Independent assortment - The random distribution of alleles into gametes during meiosis Leads to formation of all possible combinations of

gametes with equal probability in a cross between two individuals

http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html

3.4 The Principle of Independent Assortment

P1 cross

Smooth Yellow wrinkled green

F1: All Smooth Yellow

F1 x F1: Smooth YellowSmooth Yellow

F2:9/16 Smooth

Yellow3/16 Smooth

green

3/16 wrinkled Yellow

1/16 wrinkled green

x

x

ANIMATION: Dihybrid cross

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A Dihybrid Cross

F1: Smooth Yellow X Smooth Yellow

F2: Of all offspring Of all offspring Combined probabilities

if Smooth

3/4 are Yellow( 3/4 )( 3/4 ) = 9/16

Smooth Yellow3/4 are Smooth

1/4 are green ( 3/4 )( 1/4 ) = 3/16 Smooth green

and if wrinkled

1/4 are green ( 1/4 )( 1/4 ) = 1/16 wrinkled green

1/4 are wrinkled3/4 are Yellow

( 1/4 )( 3/4 ) = 3/16 wrinkled Yellow

Punnett square Illustrates Independent Assortment in a Dihybrid Cross

P1 cross

SSYY ssyy ssYY SSyy

Smooth Yellow xwrinkled green wrinkled Yellow Smooth green

Gamete formation

Gamete formation

SY sy sY SyFertilization Fertilization

SsYy

F1 = Smooth Yellow

F1 cross

SsYy

SsYy

P1 cross

x

Punnett square Illustrates Independent Assortment in a Dihybrid Cross

SY Sy sY sy

SY SSYY Smooth Yellow

SSYy Smooth Yellow

SsYY Smooth Yellow

SsYy Smooth Yellow

Sy SSYy Smooth Yellow

SSyy Smooth green

SsYy Smooth Yellow

Ssyy Smooth green

F2

GeneratIon

sY SsYY Smooth Yellow

SsYy Smooth Yellow

ssYY wrinkled Yellow

ssYy wrinkled Yellow

sy

SsYy Smooth Yellow

Ssyy Smooth green

ssYy wrinkled Yellow

ssyy wrinkled green

F2 Genotypic ratios F2 Phenotypic ratiosSSYY

9/16 Smooth YellowSSYySsYY

4/16 SsYy

1/16 SSyy 3/16 Smooth green2/16 Ssyy

1/16 ssYY 3/16 wrinkled Yellow2/16 ssYy

1/16 ssyy 1/16 wrinkled green

SsYyF1 cross SsYyx

1/16

2/162/16

Exploring Genetics: Evaluating Results: The Chi Square Test

A statistical test to determine whether the observed distribution of individuals in phenotypic categories is as predicted or occurs by chance

X2= ∑ d2/E

Acceptable probability depends on the number of phenotypic classes

df=n-1

Chi-Square Analysis of Mendel’s Data

Mendel’s Contribution

Mendel’s principle of segregation and principle of independent assortment are fundamental to our understanding of the science of heredity (genetics)

3.5 Meiosis Explains Mendel’s Results

Genes pairs (alleles) are located on chromosome pairs The position occupied by a gene on a chromosome is

referred to as a locus The behavior of chromosomes in meiosis causes

segregation and independent assortment of alleles

ANIMATION: Independent assortment

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Genes, Chromosomes, and Meiosis

A a a A Aa a

Meiosis I

B B b b bB B

A A a a A A a a

Metaphase II

B B b b b b B B

Gametes

A B A B a b a b A b A b a B a B

A

b

Segregation and Independent Assortment in Meiosis

3.6 Mendelian Inheritance in Humans

Inheritance of certain human traits is predictable based on Mendelian inheritance patterns

Aa Aa

Aa Aa

A a A a

Genotype Phenotype

A a

1 AA3/4 normal

2 AaA

AA normal

Aa normal

1 aa 1/4 albinoa

Aa normal

aa albino

Segregation of Albinism

AaDd AaDd

AD Ad aD ad

AADD Pigment Hearing

AADd Pigment Hearing

AaDD Pigment Hearing

AaDd Pigment Hearing

AD

AADd Pigment Hearing

AAdd Pigment

Deaf

AaDd Pigment Hearing

Aadd Pigment

DeafAd

AaDD Pigment Hearing

AaDd Pigment Hearing

aaDD Albino

Hearing

aaDd Albino

HearingaD

AaDd Pigment Hearing

Aadd Pigment

Deaf

aaDd Albino

Hearing

aadd Albino Deaf

ad

Independent Assortment of Two Traits Can be Followed

Pedigree

A diagram listing the members and ancestral relationships in a family

Used in the study of human heredity Uses a standard form and symbols Uses family history to show how a trait is inherited and

estimate risk for family members Provides genetic counseling for those at risk of having

children with genetic disorders

Symbols Used in Pedigree Construction

Male Aborted or stillborn offspring

FemaleDeceased offspring

Mating

Mating between relatives (consanguineous)

or Unaffected individual

IParents and children. Roman numerals symbolize generations. Arabic numbers symbolize birth order within generation (boy, girl, boy)

or Affected individual

II or1 2 3

Proband; first case in family that was identified

P P

Monozygotic twins or Known heterozygotes

Carrier of X-linked recessive traitDizygotic twins

Offspring of unknown sex Infertility

Numbering System used in Pedigrees

Each generation is identified by a Roman numeral

Each individual within a generation is identified by an Arabic number

Proband: The first affected family member who seeks medical attention for a genetic disorder

Following a Trait Through a Pedigree

ANIMATION: Pedigree diagrams

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3.8 Variations from Mendel

Alleles can interact in ways other than dominant/recessive Incomplete dominance Codominance Multiple alleles

Different genes can interact Epistasis

Incomplete Dominance

The expression of a phenotype that is intermediate to those of the parents.

An example is the inheritance of flower color in snapdragons: R1R1 (red) x R2R2 (white) = R1R2 (pink)

x

INTERACTION: Incomplete dominance

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Codominance

Full phenotypic expression of both members of a heterozygous gene pair Blood Types - If a person is type AB this means that

both the A allele and the B allele are equally expressed and are therefore codominant.

Roan fur in cattle. Many breeds of cattle are dominant for red hair or white hair

Multiple Alleles

Genes that have more than two alleles The inheritance of the ABO blood types in humans Rabbit coat color

Multiple Alleles

A B

A A B B

A A B B

A IAi IA or IAi

B IBIB or IBi

A

B B

A A

AB IAIB

B O ii

Genes Can Interact to Produce Phenotypes

Epistasis - A form of gene interaction in which one gene masks or prevents expression of another gene. Lab coat color Albino gene is an epistatic gene. When the albino

condition occurs, the genes that determine skin color are present, but not expressed.