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CHAPTER 11 GENETICS

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Gregor MendelCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

© Ned M. Seidler/Nationa1 Geographic Image Collection

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Gregor Mendel

•The garden pea:• Organism used in Mendel’s experiments

• A good choice for several reasons:

• Easy to cultivate

• Short generation

• Normally self-pollinating, but can be cross-pollinated by hand

• True-breeding varieties were available

• Simple, objective traits

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Garden Pea Anatomy

stamenanther

a.

Flower Structure

filament

stigma

style

ovules inovary

carpel

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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All peas are yellow whenone parent produces yellowseeds and the other parentproduces green seeds.

Brushingon pollenfrom anotherplant

Cutting awayanthers

Garden Pea AnatomyCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Garden Pea AnatomyTrait *Dominant

Characteristics*Recessive

Stem length

Pod shape

Seed shape

Seed color

Flower position

Flower color

Pod color

b.

Green

Purple

Axial

Yellow

Round

Inflated

Tall Short

Constricted

Wrinkled

Green

Terminal

White

Yellow

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Figure 14.5 Genotype versus phenotype

Figure 14.6 A testcross

Figure 14.2 Mendel tracked heritable characters for three generations

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11.2 Mendel’s Laws

•Mendel performed cross-breeding experiments• Used “true-breeding” (homozygous) plants• Chose varieties that differed in only one trait

(monohybrid cross)• Performed reciprocal crosses

• Parental generation = P

• First filial generation offspring = F1

• Second filial generation offspring = F2

• Formulated the Law of Segregation

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Mendel’s Laws

•Law of Segregation:

• Each individual has a pair of factors (alleles) for each trait

• The factors (alleles) segregate (separate) during gamete (sperm & egg) formation

• Each gamete contains only one factor (allele) from each pair of factors

• Fertilization gives the offspring two factors for each trait

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Mendel’s Laws•C

lassical Genetics and Mendel’s Cross:

• 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

Figure 14.4 Mendel’s law of segregation (Layer 2)

Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross

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Mendel’s Laws

•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

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Mendel’s Laws•A

dihybrid cross uses true-breeding plants differing in two traits•M

endel tracked each trait through two generations.• Started with true-breeding plants differing in two traits• The F1 plants showed both dominant characteristics• F1 plants self-pollinated• Observed phenotypes among F2 plants

•Mendel formulated the Law of Independent Assortment• The pair of factors for one trait segregate independently of the factors for other traits• All possible combinations of factors can occur in the gametes

•P generation is the parental generation in a breeding experiment.

•F1 generation is the first-generation offspring in a breeding experiment.

•F2 generation is the second-generation offspring in a breeding experiment

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Classical View of Homologous Chromosomes

Replication

alleles at agene locus

sister chromatids

b. Sister chromatids of duplicated chromosomes have same alleles for each gene.

a. Homologous chromosomes have alleles for same genes at specific loci.

G

R

S

t

G

R

S

t

G

R

S

t

g

r

s

T

g

r

s

T

g

r

s

T

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Figure 15.1 The chomosomal basis of Mendel’s laws

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Mendel’s Laws•P

unnett Square• 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 =½ ½=¼ • The chance of Ee =½ ½=¼ • The chance of eE =½ ½=¼ • The chance of ee =½ ½=¼

Rules of Probability•P

robability scale ranges from 0 to 1•T

he possibilities of all outcomes must add up to 1•I

n every fertilization involving gametes, the ovum has a ½ chance of carrying a dominant allele and ½ chance of carrying a recessive allele

•Two basic laws of probability can help are the rule of multiplication and the rule of addition

Rule of Multiplication•H

ow can we determine the chance that two or more independent events will occur together in some combination?

• Compute the probability for each independent event, then multiply these individual probabilities to obtain the overall probability of these events occurring together

Rule of Addition•T

he probability of an event that can occur in two or more different ways is the sum of the separate probabilities of those ways

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Mendel’s Laws•G

enetic disorders are medical conditions caused by alleles inherited from parents

•Autosome - Any chromosome other than a sex chromosome (X or Y)

•Genetic disorders caused by genes on autosomes are called autosomal disorders • Some genetic disorders are autosomal dominant

• An individual with AA has the disorder• An individual with Aa has the disorder• An individual with aa does NOT have the disorder

• Other genetic disorders are autosomal recessive• An individual with AA does NOT have the disorder• An individual with Aa does NOT have the disorder, but is a carrier• An individual with aa DOES have the disorder

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Autosomal Recessive PedigreeCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

I

II

III

IVKey

Gen

erat

ion

s

Autosomal recessive disorders• Most affected children have unaffected parents.

• Heterozygotes (Aa) have an unaffected phenotype.• Two affected parents will always have affected children.• Close relatives who reproduce are more likely to have affected children.

• Both males and females are affected with equal frequency.

A?

aa A?

A? Aa Aa A?

Aa*

Aa A?

A?aaaaaa = affectedAa = carrier (unaffected)AA = unaffectedA? = unaffected (one allele unknown)

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Autosomal Dominant Pedigree Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

• Affected children will usually have anaffected parent.

• Heterozygotes (Aa) are affected.• Two affected parents can produce an unaffected child.• Two unaffected parents will not have affected children.• Both males and females are affected with equal frequency.

AA = affectedAa = affectedA? = affected (one allele unknown)aa = unaffected

I

II

III Aa

aa

Aa

Aa

*

Aa

A?

aa

aa aa aa

aaaaaa

Aa

Key

Gen

erat

ion

s

Autosomal dominant disorders

Non-single gene genetics

•Incomplete dominance: appearance between the phenotypes of the 2 parents. Ex: snapdragons

•Codominance: two alleles affect the phenotype in separate, distinguishable ways. Ex: Tay-Sachs disease• Multiple alleles: more than 2 possible alleles for a gene. Ex:

human blood types

Figure 14.9 Incomplete dominance in snapdragon color

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Blood Types•S

ome traits are controlled by multiple alleles (multiple allelic traits)

•The gene exists in several allelic forms (but each individual only has two alleles)

•ABO blood types

• The alleles:

• IA = A antigen on red blood cells, anti-B antibody in plasma

• IB = B antigen on red blood cells, anti-A antibody in plasma

• i = Neither A nor B antigens on red blood cells, both anti-A and anti-B antibodies in plasma•T

he ABO blood type is also an example of codominance• More than one allele is fully expressed• Both IA and IB are expressed in the presence of the other

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ABO Blood Type

GenotypeIAIA, IAiIBIB, IBiIAIB

ii

PhenotypeABABO

Non-single gene genetics

•Pleiotropy: genes with multiple phenotypic effect. Ex: sickle-cell anemia

•Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Ex: mice coat color

•Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character Ex: human skin pigmentation and height

Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote

Figure 14.11 An example of epistasis

Figure 14.12 A simplified model for polygenic inheritance of skin color

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Extending the Range of Mendelian Genetics

•X-Linked Inheritance•In mammals•The X and Y chromosomes determine gender•Females are XX •Males are XY

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Extending the Range of Mendelian Genetics

•X-Linked Inheritance• The term X-linked is used for genes that have nothing to

do with gender• X-linked genes are carried on the X chromosome. • The Y chromosome does not carry these genes • Discovered in the early 1900s by a group at Columbia

University, headed by Thomas Hunt Morgan. • Performed experiments with fruit flies

• They can be easily and inexpensively raised in simple laboratory glassware

• Fruit flies have the same sex chromosome pattern as humans

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X – Linked InheritanceCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

XrY

XRY

XR

Xr

Y

XrY

Xr XRY

Offspring

eggs

sp

erm

P generation

P gametes

F2 generation

F1 generation

F1 gametes

Allele Key==

XR

Xr red eyes white eyes

Phenotypic Ratioall red-eyedred-eyedwhite-eyed

females:males: 1

1

XRY

XRXrXRXR

XR

XRXr

XRXR

X – Linked Inheritance

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Extending the Range of Mendelian Genetics•S

everal X-linked recessive disorders occur in humans:• Color blindness

• The allele for the blue-sensitive protein is autosomal• The alleles for the red- and green-sensitive pigments are on the X chromosome.

• Menkes syndrome• Caused by a defective allele on the X chromosome • Disrupts movement of the metal copper in and out of cells.• Phenotypes include kinky hair, poor muscle tone, seizures, and low body temperature• Muscular dystrophy• Wasting away of the muscle• Caused by the absence of the muscle protein dystrophin• Adrenoleukodystrophy

• X-linked recessive disorder• Failure of a carrier protein to move either an enzyme or very long chain fatty acid into

peroxisomes. • Hemophilia

• Absence or minimal presence of clotting factor VIII or clotting factor IX

• Affected person’s blood either does not clot or clots very slowly.

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X-Linked Recessive PedigreeCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

XBXB XbY grandfather

daughterXBXbXBY XBY XbXb

XbY

XBXb grandsonXBY XBXB XbY

KeyXBXB = Unaffected femaleXBXb = Carrier femaleXbXb = Color-blind femaleXbY = Unaffected maleXbY = Color-blind maleX-Linked Recessive

Disorders

• More males than females are affected.

• An affected son can have parents who have the normal phenotype.

• For a female to have the characteristic, her father must also have it. Her mother must have it or be a carrier.

• The characteristic often skips a generation from the grandfather to the grandson.

• If a woman has the characteristic, all of her sons will have it.

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Expressivity refers to variations in a phenotype among individuals carrying a particular genotype.

Penetrance is the proportion of individuals carrying a particular variant of a gene (allele or genotype) that also express an associated trait (phenotype).

Chromosomal errors, I•N

ondisjunction: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II

•Aneuploidy: chromosome number is abnormal• Monosomy ~ missing chromosome• Trisomy ~ extra chromosome (Down syndrome)• Polyploidy~ extra sets of chromosomes

Figure 15.11 Meiotic nondisjunction

Chromosomal errors, II•A

lterations of chromosomal structure: •D

eletion: removal of a chromosomal segment•D

uplication: repeats a chromosomal segment•I

nversion: segment reversal in a chromosome•T

ranslocation: movement of a chromosomal segment to another

Figure 15.13 Alterations of chromosome structure

Figure 15.14 Down syndrome

Figure 15.x2 Klinefelter syndrome

Figure 15.x3 XYY karyotype

Chromosomal Disorders•D

own syndrome

•XXY Klinefelter syndrome

•XYY

•XXX

•XO Turner syndrome

Extranuclear Genes•N

ot all genes are located on the nuclear chromosomes. These genes do not exhibit Mendelian genetics.

•Mitochondrial DNA which is given from the mother only, can in some rare cases cause some disorders. If the Mitochondrial DNA is defected this would reduce the amount of ATP made.