Chapter 12Patterns of Inheritance

• 12.1 What is the physical basis of inheritance?

• 12.2 How did Gregor Mendel lay the foundation for modern genetics?

• 12.3 How are single traits inherited?• 12.4 How are multiple traits on different

chromosomes inherited?• 12.5 How are genes located on the same

chromosome inherited?

Chapter 12Patterns of Inheritance, cont.

• 12.6 How is sex determined, and how are sex-linked genes inherited?

• 12.7 Do the Mendelian rules of inheritance apply to all traits?

• 12.8 How are human genetic disorders investigated?

• 12.9 How are human disorders caused by single genes inherited?

• 12.10 How do errors in chromosome number affect humans?

12.1 What is the physical basis of inheritance?

• Inheritance is the process by which the characteristics of individuals are passed to their offspring

• Genes encode these characteristics

Genes

• A gene is a unit of heredity that encodes information for the form of a particular characteristic

• The location of a gene on a chromosome is called its locus

Alleles

• Homologous chromosomes carry the same kinds of genes for the same characteristics

• Genes for the same characteristic are found at the same loci on both homologous chromosomes

Alleles

• Genes for a characteristic found on homologous chromosomes may not be identical

• Alternate versions or forms of genes found at the same gene locus are called alleles

Alleles

• Each cell carries two alleles per characteristic, one on each of the two homologous chromosomes

• If both homologous chromosomes carry the same allele (gene form) at a given gene locus, the organism is homozygous at that locus

Alleles

• If two homologous chromosomes carry different alleles at a given locus, the organism is heterozygous at that locus (a hybrid)

12.2 How Did Gregor Mendel Lay the Foundation for Modern Genetics?

• Gregor Mendel studied many subjects (including botany and math) in the 2 years at the Univ. of Vienna

• He settled down as a monk in the monastery of St. Thomas (in what is now Brno in the Czech Republic)

• He did groundbreaking research on peas, despite no knowledge of genes or chromosomes

Portrait painted about 1888

12.1 How Did Gregor Mendel Lay the Foundation for Modern Genetics?

• Doing It Right: The Secrets of Mendel’s Success

• 3 steps to doing an experiment right1. Choosing the right organism2. Designing and performing the experiment

correctly3. Analyzing the data properly

12.2 How Did Gregor Mendel Lay the Foundation for Modern Genetics?

• Each flower has pollen grains (male gametes) which fertilize the eggs (female gametes)

• Peas are usually self-fertilized• A plant which is homozygous for a trait is

called true-breeding• Peas can easily be cross-fertilized• The traits Mendel looked at (like color) only

had one gene that coded for that trait.

Section 12.3 Outline

• 12.3 Inheritance of Single Traits– The Language of a Genetic Cross – Mendel’s Flower Color Experiments– Alleles of a Gene Are Dominant or Recessive– How Meiosis Separates Genes: Segregation– Understanding the Results of Mendel’s Flower

Color Experiments– “Genetic Bookkeeping”– Practical Application: The Test Cross

The Language of a Genetic Cross

• The parents used in a cross are part of the parental generation (known as P)

• The offspring of the P generation are members of the first filial generation (F1)

• Offspring of the F1 generation are members of the F2 generation, etc.

true-breeding,purple-flowered

plant

true-breeding,white-flowered

plant

cross-fertilize

pollen

pollen

all purple-flowered plants

Parental generation (P)

First-generationoffspring (F1)

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3/4 purple 1/4 white

Second- generationoffspring (F2)

First- generationoffspring (F1)

self-fertilize

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12.3 How are single traits inherited?

• After that, Mendel allowed the F2 plants to self-fertilize a third generation (F3)

• The white plants always produced white plants – no matter how many generations

• The purple-flowered F2 plants were of 2 types– 1/3 true-breeding purple– 2/3 were hybrids and produced purple and white

flowered peas

• F2 generation ratio was ¼ true breeding white : ½ hybrid : ¼ true breeding purple

Inheritance of dominant & recessive• Each trait is determined by a pair of

discrete units now called genes• The pairs separate during gamete

formation (meiosis). This is called Mendel’s Law of Segregation

• Which allele goes in which gamete is random (otherwise the math doesn’t work)

• One type of allele can mask the other allele (dominant and recessive) but the dominant does not alter the recessive allele

homozygous parent

A A AA

gametes

True-breeding organisms have two of the same allele for a given gene. This is also called homozygous.

heterozygous parent

A a aA

gametes

Hybrids organisms have two of the different alleles for a given gene. This is also called heterozygous.

Capital letters for dominant and lower case for recessive

PP P P

purple parent

all P sperm and eggs

pp p

white parent

all p sperm and eggs

p

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F1

offspring

p

p

Pp

Pp

sperm eggs

P

P

or

ppp

Ppp

p

gametes fromF1 plants

eggs

F2 offspring

p

PP

P

P

PP

sperm

Pp

1/4

1/4

1/4

1/4

Dominant and Recessive Alleles

• The particular combination of the two alleles carried by an individual is called the genotype (PP, Pp, or pp)

• The physical expression of the genotype is known as the phenotype (e.g. purple or white flowers)

Genetic Bookkeeping

• Punnett Square Method predicts offspring genotypes from combinations of parental gametes

1. First assign letters to the different alleles of the characteristic under consideration (uppercase for dominant, lowercase for recessive)

2. Determine the gametes and their fractional proportions (out of all the gametes) from both parents…

Genetic Bookkeeping

3. Write the gametes from each parent, together with their fractional proportions, along each side of a 2 x 2 grid (Punnett square)

4. Fill in the genotypes of each pair of combined gametes in the grid, including the product of the fractions of each gamete (e.g. ¼ P with ½ p = 1/8 Pp)…

P p

PP

P

p

pp

Pp

eggs

Ppself-fertilize

pP

12

12

12

12

14

sper

m

14

14

14

P P PP PP

P p Pppurple

P P pP

Pp

P p pp pp white

sperm eggsoffspring

genotypesgenotypic

ratio(1:2:1)

phenotypicratio(3:1)

12

12

12

12

12

12

12

12

14

14

14

14

14

24

14

14

34

A test cross

• Take a dominant phenotype (with unknown second allele)

• Cross it with a known recessive homozygous

• And you will get one of two results...

allsperm

pp

P

pp

Pp

ppall eggs

PP or Ppsperm unknown

if PP if Pp

egg egg

pollen

p

12

12

12

P

p

12

all Pp

sper

m

12.4 How Are Multiple Traits on Different Chromosomes Inherited?• Mendel hypothesized that genes on different

chromosomes are inherited independently

• He looked at many pea traits

• If these traits were linked in some way, then the ratios shouldn’t agree with hypothesis

• Remember that for the math to work out, you have to do lots of replicates – and Mendel also did that correctly.

Seedshape

Seedcolor

Podcolor

Podshape

Flowercolor

Flowerlocation at leaf

junctionsat tips ofbranches

tall(1.8 to 2 meters)

dwarf(0.2 to 0.4meters)

Plantsize

smooth

Dominant formTrait Recessive form

wrinkled

yellow

yellow

inflated

green

green

white

constricted

purple

Traits Are Inherited Independently

• Punnett Square from SSYY x ssyy crossGametes

¼sy ¼sy ¼sy ¼sy

¼SY SsYy SsYy SsYy SsYy F1: All SsYy Smooth

yellow peas

¼SY SsYy SsYy SsYy SsYy

¼SY SsYy SsYy SsYy SsYy

¼SY SsYy SsYy SsYy SsYy

1

16

1

16

1

16

1

16

1

16

1

16

1

16

1

16

1

16

1

16

1

16

1

16

1

16

1

16

1

16

1

16

SY

SY

SSYY SsYY

ssYY

ssyY

SsyY

SSYy SsYy

SsYy

ssyy

SsyySSyy

sSyY sSyy

sSYY sSYy

SSyY

sY

sY

sy

sy

Sy

Sy

eggs

self-fertilize

ssYy

14

14

14

14

14

14

14

14

sper

m

116

116

116

116

116

116

116

116

116

116

116

116

116

116

116

116

seed shape seed color phenotypic ratio(9:3:3:1)

smooth yellow smooth yellow

smooth green

wrinkled yellow

wrinkled green

yellow

green

green

smooth

wrinkled

wrinkled

34

34

34

34

14

14

14

14

316

316

916

116

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And of course, his experiments came out just like this. This idea is called the Law of Independent Assortment

The End of the Mendel Story

• In 1865, Gregor Mendel presented his work and published the following year

• He was pretty much ignored

• In 1900, 3 biologists were about to publish on the same subject

• They did a literature search and found Mendel’s work, and gave him all the credit

• He had died in 1884

12.5 How are genes located on the same chromosome inherited?

• Genes on the same chromosome tend to be inherited together.

• This is called linkage.

• One of the first pairs of linked genes was found on the sweet pea (a different species from Mendel’s garden pea)

Genes on the Same Chromosome

• Not all genes independently assort

• Mendel’s Law of Independent Assortment only works for genes whose loci are on different chromosomes

SS s ss s

Y

S

Y Y Yy y y

S

y

independent assortment produces four equallylikely allele combinations during meiosis

SY sy Sy sY

meiosis II

meiosis I

S

S

S

S

s

s

s

s

Y

Y

Y

Y

y

y

y

y

chromosomesreplicate

SS

ss

Y

Yy

y

replicated homologuespair during metaphaseof meiosis I,orienting like this

or like this

pairs of alleles on homologouschromosomes in diploid cells

Ss

Yy

flower color gene pollen shape gene

purpleallele, P

redallele, p

longallele, L

roundallele, l

12.5 How are genes located on the same chromosome inherited?

• Despite being on the same gene, crossing over can occur

• Thereby creating new linkages.• Exchanging DNA between homologous

chromosomes is genetic recombination• The farther apart genes are on a

chromosome, the more likely crossing over will occur between them

• Mendel wasn’t just a good scientist, he was really lucky he didn’t find linked traits

flower color gene

purple allele, P long allele, L

red allele, p round allele, l

pollen shape gene

sister chromatids

homologouschromosomes(duplicated)at meiosis I

sister chromatids

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crossing over

Copyright © 2005 Pearson Prentice Hall, Inc.

recombinedchromatids

P L

p L

P l

p l

Copyright © 2005 Pearson Prentice Hall, Inc.

P L

p L

P l

p l

12.6 How Is Sex Determined, and How Are Sex-Linked Genes Inherited?

• Females have two identical sex chromosomes XX

• Men have two different sex chromosomes XY

• That means the men control the sex of the offspring

X chromosome

Y chromosome

12.6 How Is Sex Determined, and How Are Sex-Linked Genes Inherited?

• Sex linked genes are only found on the X or the Y gene

• But the Y gene is small, so most sex linked problems are found on the X

• Women have XX, so they can have dominant and recessive X alleles

• Men only have one X, and so whatever is on their X is expressed

X1

X1

X2

X2

Xm

Xm

XmXmX1

X2

Y

Y

Y YX1 X2

eggs

female parent

female offspring

male offspring

male parent

sper

m

all the F2

femaleshave red eyes

half the F2 maleshave red eyes,half have whiteeyes

XRX

rX

r

XR

r

XR

R

R

Xr

r

YY

XR

R

XR

R

eggs

XR

R

XR

R

Xr

r

XR

R

XR

R

Y Xr

r

Y

XRY

female parent

female offspring

male offspring

male parent

sper

m

12.7 Do the Mendelian Rules of Inheritance Apply to All Traits?

• Incomplete dominance: The phenotype of heterozygotes is intermediate between the phenotypes of the homozygotes

• In other words, neither allele is completely dominant or recessive

• This is more like sharing or blending

RR

eggsF2:

RR

P:

F1:

RR

R

R

RR RR

R

RR’

RR R’R

R12

12

12

12

14

14

14

14

sper

m

12.6 Do the Mendelian Rules of Inheritance Apply to All Traits?

• A single gene may have multiple alleles

• Eye color in Drosophilia has more than a 1,000 possible alleles

• There are hundreds of alleles for Marfan syndrome and cystic fibrosis

• Blood type has multiple alleles with dominant and recessive and codominance

Polygenic Inheritance

• Phenotypes produced by polygenic inheritance are governed by the interaction of more than two genes at multiple loci

• Human skin color is controlled by at least 3 genes, each with pairs of incompletely dominant alleles

Polygenic Inheritance

• Phenotypes produced by polygenic inheritance are governed by the interaction of more than two genes at multiple loci

• Human skin color is controlled by at least 3 genes, each with pairs of incompletely dominant alleles

R1R1R2R2

eggs

R1R1R2R2

R1R1R2R2R1R1R2R2

R1R1R2R2R1R1R2R2

R1R1R2R2R1R1R2R2

R1R1R2R2

R1R1R2R2

R1R1R2R2

R1R1R2R2

R1R1R2R2

R1R1R2R2

R1R1R2R2

R1R1R1R2

R1R1R2R2

sper

m

R1R2

R1R2

R1R2

R1R2

R1R2

R1R1R2R2

R1R2R1R2R1R2

Pleiotropy

• Some alleles of a characteristic may create multiple phenotypic effects (pleiotropy)

– Mendel’s rules specify only one phenotype possible for any allele

• Example: The SRY gene in male humans– SRY gene stimulates development of gonads

into testes, which in turn stimulate development of the prostate, seminal vesicles, penis, and scrotum

Environmental Influence

• The environment can module how genes are expressed. Examples:

• Intelligence and height is based on genetics AND environment

• Himalayan rabbit– Himalayan rabbits have the genotype for black

fur all over the body– Black pigment is only produced in colder

areas of the body: the nose, ears, and paws– The Himalayan rabbit has dark skin below 34º

C (93º F)

12.8 How Are Human Genetic Disorders Investigated?

• Many experiments on humans are not allowed (not ethical)

• Human geneticists study medical, historical and family records

• Family pedigrees are records extending across several generations

• These help figure out which diseases are genetic, and how they are passed

12.9 How Are Human Disorders Caused by Single Genes Inherited?

• Some human genetic disorders are caused by recessive alleles

• Heterozygous individuals are called carriers and often don’t show any symptoms (although sometimes they show some)

• Related couples are more likely to express a recessive genetic disorder

Human Rattlesnake Wallaby

Albinism is a single, recessive allele

12.9 How Are Human Disorders Caused by Single Genes Inherited?

• Sickle-cell anemia is caused by a defective allele for hemoglobin synthesis

• The heterozygous have some abnormal hemoglobin, but not enough to cause much problems

• The heterozygous also help protect against malaria

12.8 How Are Human Disorders Caused by Single Genes Inherited?

• Some human genetic disorders are caused by dominant alleles– Wikipedia Huntington's Disease

• Some human genetic disorders are sex-linked– Color blindness and hemophilia

• Queen Victoria and Hemophilia

III

IV? ?

or = colorblind

= heterozygous carrier female, normal color vision

or = normal color vision (not carrier)

maternalgrandfather

mother father

sister G. Audesirk T. Audesirk

daughter

aunts

II

I

unaffected male

unaffected female

EdwardDuke of Kent

Victoria Princess of

Saxe-Coburg

AlbertPrince

of Saxe-Coburg-Gotha

VictoriaQueen

of England

Louis IVGrand Duke of

Hesse-Darmstadt

AlicePrincessof Hesse

Victoria Mary

Elizabeth Alexandra Tsarina

Frederick Ernest Mary Victoria

Irene

Olga Tatiana Maria Anastasia Alexis Tsarevitch

Edward VIIKing of

England

Alexandra of Denmark

Leopold Duke

of Albany

HelenPrincess of

Waldeck-Pyrmont

Henry Prince of Battenburg

Beatrice

present Britishroyal family(unaffected)

AlexanderAlbert

AlfonsoXII

Victoria Queen of Spain

Leopold Maurice

AlfonsoCrown Prince

Juan Beatrice died in

infancy

Marie Jaime Gonzalo

carrierdaughter

and hemophiliac

grandson

several unaffected

chidren

carrier female

hemophiliac male

?

Nicholas IIof Russia

?? ? ? ? ? ?

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12.9 How Do Errors in Chromosome Number Affect Humans?

• Some genetic disorders are caused by abnormal numbers of sex chromosomes

• Nondisjunction means errors in meiosis which cause a different number of chromosomes

• Most embryos of this type would abort

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