Chapter 9:Patterns of Inheritance

3. Multiple Gene Inheritance

2. Single Gene Inheritance

4. Sex-linked Inheritance

1. Genetic Concepts

1. Basic Genetic Concepts

The Nature of Inheritance (Genetics)

Genetics is the study of how genes areinherited AND how they influence the physical characteristics of each individual.

1) the distribution of genes into haploid gametes

2) how the interaction of gene pairs in a diploidindividual influences physical characteristics

Genetics relates to 2 basic processes:

• i.e., by meiosis

haploid

diploid

gene 1

gene 2

gene 3

gene 4• all genes have a characteristic chromosomallocation or locus

Genes, Alleles & Chromosomes

• diploid organisms (e.g.,humans) have 2 copies of eachromosome, hence 2 copiesof each gene

• genes can come in different versions called alleles

Important Genetic TermsTrait (or character)

• physical characteristic (e.g., eye color)

• individual’s observable traits (e.g., brown eyes)Phenotype

• the alleles an individual has for a given gene(e.g., AA or Aa or aa)

Genotype

• different forms or versions of the same geneAllele

Dominant allele (designated by capital letter: A)• 1 copy of the allele determines phenotype (A-)

Recessive allele (designated by lower-case letter: a)• affects phenotype only when homozygous (aa)

• the 2 alleles for a gene are identical (AA or aa)Homozygous

• the 2 alleles for a gene are different (Aa)Heterozygous

Homozygous dominant = AAHomozygous recessive = aa

Heterozygous = Aa

2. Single Gene Inheritance

Gregor MendelDeduced the basicsof inheritance byanalyzing pea plantcrosses:

• examined a numberof traits exhibitingdominant & recessivephenotypes

P generation(true-breedingparents)

F1 generation

F2 generation

Purple flowers White flowers

×

All plants havepurple flowers

Fertilizationamong F1 plants(F1 × F1)

of plantshave purple flowers

of plantshave white flowers

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The Inheritance of Flower Color

• one “allele” isdominant overthe other(e.g., purple over white)

• offspring inherit 2 “alleles”, 1 from ea parent

Based on resultssuch as this for various traits, Mendel correctly deduced that:

Basis of Mendel’s results

In Mendel’s F1 crosses½ of the gametes fromeach parent contained“P”, ½ contained “p”:

Fertilization results in 4possible combinations, 3 of which result in purple flowers

The Punnett Square…is useful for determiningexpected results of a cross:

2) indicate all possible gametesfor each parent on either axisof a Punnet square

3) fill in all possible fertilizations(i.e., combinations of gametes)

4) determine the expected proportions of ea phenotype

1) write out the cross (i.e., the genotypes of each parent)

Pp x Pp

P

p

P p

Pp

Pp

PP

pp

ExpectedGenotypic Ratio:

1 PP : 2 Pp : 1 pp

ExpectedPhenotypic Ratio:

3 purple : 1 white

Parents

Offspring

SpermD d

Eggs

D

d

DDNormal

DdNormal(carrier)

DdNormal(carrier)

ddDeaf

DdJoshuaLambert

DdAbigailLinnell

D ?JohnEddy

D ?HepzibahDaggett

D ?Abigail

Lambert

ddJonathanLambert

DdElizabeth

Eddy

Dd Dd dd Dd Dd Dd dd

Female MaleDeafHearing

Pedigrees & Human GeneticsPedigrees are extremely useful for deducing

patterns of human genetic inheritance…

**Inheritance patternshows this form of

deafness to be recessive**

Test Cross

• cross the unknown witha homozygous recessive:

Test crosses are used to reveal an unknowngenotype

• if all offspring displaydominant phenotype: homozygous dominant (PP)

• if ½ recessive, ½dominant phenotype: heterozygous (Pp)

P? x pp

Incomplete Dominance

• heterozygotes have an intermediate phenotype

Dominant allele is expressed to lesser degree when only 1 allele is present (heterozygous):

BloodGroup(Phenotype) Genotypes

AntibodiesPresent inBlood

Reaction When Blood from Groups Below Is Mixed withAntibodies from Groups at Left

O A B AB

O

A

B

AB

ii

IAIAorIAi

IBIBorIBi

IAIB

Anti-AAnti-B

Anti-B

Anti-A

—

Codominance & Blood Type• “A” & “B” alleles are codominant (both alleles expressed)• “O” allele is recessive

IAi x IBi

IA

i

IB i

IAi

IBi

IAIB

ii

ExpectedPhenotypic Ratio:

¼ type AB¼ type A¼ type B¼ type O

3. Multiple Gene Inheritance

Inheritance of 2 TraitsThe results of a cross involving 2 genes canalso be predicted using a Punnet square.

First the possible gametes produced by eachparent with regard to both genes must bedetermined:

e.g. Aa Bb x Aa Bb• the # of possible gametes is the product of the # of different alleles for each gene

2 “A” alleles (A & a) x 2 “B” alleles (B & b) = 4

The 4 possible gametes for each parent in this cross (Aa Bb x Aa Bb) are:

Aa Bb

AB

Ab

aB

ab

AB Ab aB abAABB AABb AaBB AaBb

AABb AAbb AaBb Aabb

AaBB AaBb aaBB aaBb

AaBb Aabb aaBb aabb

Expected Ratios:

9 A-B-3 A-bb3 aaB-1 aabb

• crossed plants that breedtrue for 2 different traits,then crossed F1 plants:

One of Mendel’s “Dihybrid” Crosses

e.g., plants with smooth,yellow seeds (SSYY) X wrinkled, green seeds (ssyy)

(dihybridcross)

SSYY x ssyy = all SsYy

SsYy x SsYy = 9:3:3:1

All round yellow seeds(RrYy)

Metaphase Iof meiosis

(alternative arrangements)

Anaphase Iof meiosis

Metaphase IIof meiosis

Gametes

F1 generation

1

4RY

1

4ry

R

R

R

R RR

R

y

Y

Y

Y

Y Yy

yY Y y y

r

r

y

R

Y

r

y

R r r r r

y

r

Y

r

y

r

Y

R

y

r

Y

R

y

1

4rY

1

4Ry

• all possible gametes will be produced in equalproportions if each gene is on a different chromosome

Gamete Production by Meiosis

4. Sex-linked Inheritance

Sex-linked Inheritance

Sex-linked inheritance involves genes on theX (or Y) chromosome:

Human SexDetermination

XX = femaleXY = male

• human females have 2X chromosomes, hence2 alleles for each X-linked gene

• human males have 1X chromosome, hence1 allele for each X-linked gene

X-linked Genes

XH

Xh

XH Y

XHXh x XHYGenes on the Xchromosome have a unique inheritancepattern in males:• only 1 allele, so nomasking of recessive alleles

• X-linked alleles are always inheritedfrom the mother (carrier)

XHXH

XHXh

XHY

XhYe.g., Hemophilia iscaused by a recessiveX-linked allele (h):

An “X-linked” Pedigree

The inheritance patternshows that color-blindness

is recessive & X-linked

Key Terms for Chapter 9

• sex-linked, X-linked

• dominant, recessive, codominant

• test cross, Punnet square

• trait, phenotype, genotype, allele

• true-breeding, monohybrid, dihybrid

• homozygous, heterozygous

• pedigree

Relevant Review Questions: 1-7, 10-12, 15-17