Chapter 12 - Inheritancekissanek/Handouts/Handouts/6... · 2009. 2. 27. · Inheritance of Human...

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Chapter 12

Patterns of InheritanceWho is the Father of Modern Genetics?

Answer: Johann Gregor Mendel

Chapter 12: Inheritance

Definitions:

Character - a heritable feature

Trait - each variant for a character

Definitions:

• True/pure breeding

�consistently gives rise to

offspring with same trait

when selfed (or bred with

same type).

• Hybrid cross

�cross between parents true-

breeding for different trait(s)

�Black lab with a golden lab.

x

Stamen

(pollen)

Carpel(eggs)

1) Self-fertilization:

• Same flower providespollen and eggs

2) Cross-fertilization:

• Different flowers providepollen and eggs (by hand)

• True-breeding (homozygous)


2

P (parental)

Generation

F1 (filial 1)

Generation

F2 (filial 2)

Generation

Progeny (offspring)

Intercross or self

F1 offspring

Controlled genetic crossesInheritance of Single Traits:

• Mendel raised true-breeders

for different forms of a traitand then cross-fertilized the

forms:

• Mendel then allowed F1

to self-fertilize:



Modern Genetic Concepts:

Locus

Locus: Specific location of a gene ona chromosome

Alleles: Alternate forms of a nucleotide sequence for a single gene

Locus

Heterozygous (“different pair”)

• Homologous chromosomes havedifferent alleles for a gene

Modern Genetic Concepts:

Locus

Locus: Specific location of a gene ona chromosome

Alleles: Alternate forms of a nucleotide sequence for a single gene

Heterozygous (“different pair”)

• Homologous chromosomes havedifferent alleles for a gene

Homozygous (“same pair”)

• Homologous chromosomes havesame allele for a gene

Locus


Mendel’s Hypothesis:

1) Traits are determined by pairs of discrete physical units

• Physical units = genes

• One allele of each gene / homologous chromosome

2) Alleles of a gene segregate from one another during meiosis

• Law of Segregation

• Which allele enters which gamete occurs by chance

3) Differing alleles can mask one another if occupying same cell

• Dominant Allele: Allele expressed (observed)

• Recessive Allele: Allele mask (not observed)

4) True-breeding individuals have two of the same alleles

• Hybrids have two different alleles for the gene


purple

purple

One may be

dominant to

the other.

Mendel’s model:

1. Hereditary units (genes) come in different forms (alleles)

� Gene = hair color

� Alleles = blonde hair, red hair, black hair, brown hair.

2. Two hereditary units for each trait (per individual)

One from each parent

white

3

Inheritance of Single Traits:

Phenotype:

• Outward expression of

a trait (appearance, behavior)

Genotype:

• Actual combination of alleles

carried by an individual


B. The law of segregation

• Dominant trait - The trait exhibited by the F1 generation

• Recessive trait - the trait NOT seen in the F1 generation

xP

F1

Individual 1 = Pp

Individual 2 = Pp

Inheritance of Single Traits:


Punnett Square Method: (“Genetic Bookkeeping”)

Keep in Mind:Predicted proportionswill fluctuate in real life

Phenotype:3 purple : 1 white

Genotype:1 PP : 2 Pp : 1 pp


Mendel’s hypothesis can be used to predict cross outcomes:

Crossing a dominate phenotype individual with a homozygous recessive individual to determine what the genotype of the dominate individual

Test Cross:

PP or Pp

pp

P P

p

p

Pp Pp

Pp Pp

P p

p

p

Pp

Pp

pp

pp

100% Purple 50% Purple / 50% White


Inheritance of Multiple Traits:

• Mendel then allowed F1

to self-fertilize:

YySs

YYSS x yyss

YySs x YySs

• Mendel raised true-breeders for different forms of two traits

and then cross-fertilized the

forms:


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Inheritance of Multiple Traits:

• Resulting ratio of self-fertilization: 9 Yellow, smooth seeds3 Yellow, wrinkled seeds3 Green, smooth seeds1 Green, wrinkled seeds

9 : 3 : 3 : 1

Yy = Seed Color

Y y

S s

y

Y

Ss = Seed Texture

s

S

YY Yy

Yy yy

SS Ss

Ss ss

3/4 yellow

1/4 green

3/4 smooth

1/4 wrinkled

9/16 Yellow, smooth seeds3/16 Yellow, wrinkled seeds3/16 Green, smooth seeds1/16 Green, wrinkled seeds

9 : 3 : 3 : 1


Law of Independent Assortment:

• The alleles for one trait may be distributed to the gametes independently of the alleles for other traits

• Occurs via random assortment of chromosomes during Meiosis I

• Traits located on separate chromosomes


Law of independent assortment

Each pair of alleles segregates independently

of other allele pairs.

as long as they are

unlinked

i.e., not on the same

chromosome

Punnett Square (Multiple Traits):

YySs x YySsSperm

Egg

YS

Ys

yS

ys

YS Ys yS ys


Independent Assortmentof Alleles:

(Figure 12.7)

Punnett Square (Multiple Traits):

YySs x YySsSperm

Egg

YS

Ys

yS

ys

YS Ys yS ys


YYSS

5

Punnett Squarefor Two Traits:

(Figure 12.6)

9 Yellow, smooth seeds3 Yellow, wrinkled seeds3 Green, smooth seeds1 Green, wrinkled seeds

9 : 3 : 3 : 1

Genes on the Same Chromosome Tend to be Inherited Together

• Linked Genes = Genes on same chromosome


Linked genes may be separated by crossing over events:

• Crossing Over = Exchange of segments of homologous chromosomes

• Occurs during metaphase of meiosis I

GeneticRecombination


Sex Chromosomes:

• Determine sex of individual:

• Humans = X and Y chromosomes

• Female = XX : Male = XY

• Birds/Reptiles = W and Z chromosomes

• Female = WZ : Male = ZZ

• Only one pair per species:

• Humans = 23 chromosome pairs (1 pair sex)

• Dogs = 39 chromosome pairs (1 pair sex)

• Autosomes = All other chromosome pairs other thansex chromosomes (usually same size)


Sex-linked Genes:

• Genes present on one sex chromosome but not the other

• Human Y chromosome ∼ 80 genes

• Human X chromosome ∼ 1500 genes

• Color vision genes

• Blood clotting factors

• Females = Normal dominant/recessive relationshipsexist for alleles on X chromosomes

• Males = Fully express alleles on X chromosome


Cross:

female Bb x male B

A difference in phenotypic ratios

between male and female

progeny always indicates sex

linkage.

� i.e., more males have the disorder than

females.

XB Xb

XB

Y

XB XB

XB Y

XB Xb

XbY

X

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Example of Sex-linkedTrait:

(Figure 12.10)

Variations on the Mendelian Theme:

• Assumptions so far:

• Heterozygous phenotype is intermediate betweenhomozygous phenotypes

1) One allele is completely dominant over another allele

Fact: Alleles can display Incomplete Dominance:

Flower Color in Snapdragons(Figure 12.11)




• Most genes have > 10 alleles

• Alleles arise from mutation

2) There are only two possiblealleles for each gene

Fact: Genes can have multiple alleles:

Remember: An individual willhave only a maximum of twounique alleles for a gene(diploid)

AA or AO BB or BO

AB O

Codominance:

• Both phenotypes expressedequally in heterozygotes




3) Each trait is completely controlled by a single gene

Fact: Many traits are influenced by several genes

• Polygenic Inheritance:

• Interaction of 2 or more genes contribute to a single phenotype

• Skin Color = 3 or 4 genes

• Eye Color = 2 genes


Example of PolygenicInheritance:

(similar idea to Figure 12.12)

Additional Variations on the Mendelian Theme:

• Pleiotrophy: Single genes may have multiple phenotypic effects

• SRY gene: Codes for protein that activates otherproteins in male cascade

Fertilization Sex Determining Region of the Y

Gonad FormationHormones

Sexual Differentiation of phenotype


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Pleiotropy

• Marfan’s syndrome

�Pleiotropic trait that causes

�Tall, lanky bodies

�Detached retinas

�Collapsed lungs

�Aortic dissections

�Nearsightedness (myopia)

Additional Variations on the Mendelian Theme:

• The Environment influences Gene Expression:

• Temperature in Himalayan Rabbits

• Enzyme deactivated at high temperature

• Skin color (exposure)

• Height (nutrition)

• Intelligence (?)

Humans:


Human Genetic Disorders:

How are Human Genetic Disorders Investigated?

Answer: Pedigrees

• Diagrams which show the genetic relationship amongrelated individuals

• Can reveal whether trait is dominant, recessive, orsex-linked

Chapter 12: InheritancePedigrees:

(Figure 12.14)

Dominant Trait

Recessive Trait

Inheritance of Human Disorders (Single Gene):

1) Recessive Alleles:

• Disease if individual inherits two recessive alleles

• Carrier = Heterozygous individual (not affected)

Sickle-cell Anemia (defective hemoglobin)

Albinism (no pigmentation; pink eyes)


Inheritance of Human Disorders (Single Gene):

2) Dominant Alleles:

• Potential Effects:

• Produce abnormal protein

• Carry out toxic reactions

• Produce “overactive” proteins

• Huntington Disease (deterioration of brain)

• Abnormal protein production

3) Sex-linked:

• Genes associated with sex chromosomes

• Frequently in males… often skip generations

• Color blindness (defective color receptors)

• Hemophilia (defective clotting proteins)


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Royal Families of Europe:

Errors in Chromosome Number can Affect Humans:

• Nondisjunction: Homologous pairs don’t separate during meiosis (affects gamete chromosome counts)

1) Sex Chromosomes:

• XO (Female) = Turner Syndrome (Infertile; lack 2°sexual char.)

• XXX (Female) = Trisomy X (Fertile; usually tall; potential ↓ IQ)

• XXY (Male) = Klinefelter Syndrome (Infertile; often unnoticed)

• XYY (Male) = Fertile; Excess testosterone (violent?); ↓ IQ)


Errors in Chromosome Number can Affect Humans:

• Nondisjunction: Homologous pairs don’t separate during meiosis (affects gamete chromosome counts)

2) Autosomal Chromosomes:

• Down Syndrome (Trisomy 21)

• Physical characteristics

• Mental retardation

• Heart malformations

• Frequency increase with ageat pregnancy


Chapter 12 - Inheritancekissanek/Handouts/Handouts/6... · 2009. 2. 27. · Inheritance of Human...

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