Chapter 12 Patterns of Inheritance - wou.edulemastm/Teaching/BI102/Chapter 12 - Inheritance (2).pdf1 Chapter 12 Patterns of Inheritance Chapter 12: Inheritance Modern Genetic Concepts:

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Chapter 12

Patterns of Inheritance

Chapter 12: Inheritance

Modern Genetic Concepts:

Locus

Locus: Specific location of a gene on a

chromosome

Allele: Alternate form of a nucleotide

sequence for a single geneLocus

Heterozygous (“different pair”)

• Homologous chromosomes having

different alleles for a given gene

What is Inheritance?

Answer: The passage of genes from parent to offspring

Homozygous (“same pair”)

• Homologous chromosomes having

the same alleles for a given gene

Locus

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Who is the Father of Modern Genetics?

Answer:

Johann Gregor Mendel


• Same flower provides pollen and eggs

• True-breeding (homozygous)


Pea Plant Ideal for Genetic Work:

1) Normally undergo self-fertilization

• Different flower provides pollen and eggs

• Controlled experimentation

2) Can undergo cross-fertilization (manually)

Figure 12.3 – Audesirk2 & Byers

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• Mendel raised true-breeders

for different forms of a trait

and then cross-fertilized the

forms:

• Mendel then allowed F1 generation

to self-fertilize:


Inheritance of Single Traits:

Figure 12.4 / 12.5 – Audesirk2 & Byers

1) Traits are determined by pairs of discrete physical units

• Physical units = genes

• One allele of each gene / homologous chromosome

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

• Law of Segregation

• Which allele enters which gamete occurs by chance

2) 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


Inheritance of Single Traits – Mendel’s Findings:

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Phenotype:

Outward expression of a trait(e.g., appearance, behavior)



Genotype:

Actual combination of alleles

carried by an individual

Figure 12.8 / 12. 9 – Audesirk2 & Byers

=



Arrgghh!


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Keep in Mind:

Predicted proportions

will fluctuate in real life

Phenotype:

3 purple : 1 white

Genotype:

1 PP : 2 Pp : 1 pp


Punnett Square Method: (“Genetic Bookkeeping”)


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

Crossing a dominant phenotype individual with a homozygous recessive

individual to determine the genotype of the dominant 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


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• 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:


Inheritance of Multiple Traits:


Resulting ratio of self-fertilization:

9 Yellow, smooth seeds

3 Yellow, wrinkled seeds

3 Green, smooth seeds

1 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 seeds

3/16 Yellow, wrinkled seeds

3/16 Green, smooth seeds

1/16 Green, wrinkled seeds

9 : 3 : 3 : 1



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Law of Independent Assortment:

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

of the alleles for other traits

• Traits located on separate

chromosomes



• Occurs via random assortment

of chromosomes during

meiosis I

YySs x YySsSperm

Egg

YS

Ys

yS

ys

YS Ys yS ys


Punnett Square (Multiple Traits):

YYSS

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9 Yellow, smooth seeds

3 Yellow, wrinkled seeds

3 Green, smooth seeds

1 Green, wrinkled seeds

9 : 3 : 3 : 1

Punnett Square (Multiple Traits):


Genes on the Same Chromosome Tend to be Inherited Together:

Linked Genes:

Genes on same chromosome


Linked genes may be separated via crossing over:

Crossing Over:

Exchange of homologous chromosome segments(occurs during metaphase of meiosis I)

Genetic

Recombination

Figure 12.17-12.20 – Audesirk2 & Byers

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• 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 individual in a species

Autosomes:

All other chromosome pairs other

than sex chromosomes


Sex Chromosomes:

Sex-linked Genes: Genes present on one sex chromosome but not the other

• Human Y chromosome 20 unique genes

• Human X chromosome 1500 unique genes

• Examples: Color vision genes / Blood clotting factors

• Females = Normal dominant / recessive relationships exist for

alleles on X chromosomes

• Males = Fully express alleles on X chromosome

Example of Sex-linked Trait:

Chapter 12: InheritanceFigure 12.23 – Audesirk2 & Byers

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Heterozygous phenotype is

intermediate between

homozygous phenotypes

Assumption:

One allele is completely dominant

over another allele

Fact:

Alleles can display

incomplete dominance

Flower Color in SnapdragonsChapter 12: Inheritance

Variations on Mendelian Theme:

Assumption:

There are only two possible alleles

for each gene

Fact:

Genes can have multiple alleles



AA or AO BB or BO

AB O

Table 12.1 – Audesirk2 & Byers

Codominance:

Both phenotypes expressed

equally in heterozygotes

• Most genes have > 10 alleles

• Alleles arise from mutation

Remember:

An individual will have only a

maximum of two unique alleles

for a gene (diploid organisms)

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Assumption:

Each trait is completely controlled

by a single gene

FactA:

Many traits are influenced by

several genes



Polygenic Inheritance:

Interaction of two (2) or more genes

contribute to a single phenotype


Assumption:


by a single gene

FactB:

Single genes may have multiple

phenotypic effects (Pleiotrophy)



Fertilization Sex Determining Region of the Y

Gonad FormationHormones

Sexual Differentiation of phenotype

SRY gene:

Codes for protein that

activates other

proteins in male cascade

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Assumption:


by a single gene

FactC:

The environment can influence

gene expression



Example:

Temperature in Himalayan rabbits

• Enzyme deactivated at

high temperature

• Skin color (exposure)

Humans:

• Height (nutrition)

• Intelligence (?)

How are Human Genetic Disorders Investigated?

Diagrams showing the genetic relationship

among related individuals:

Can reveal whether trait is

dominant, recessive, or sex-linked


Answer: Pedigrees

Dominant Trait

Recessive Trait

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1) Recessive Alleles:

Sickle-cell Anemia(defective hemoglobin)


Inheritance of Human Disorders (single gene):

Albinism (no pigmentation; pink eyes)

Carrier:

Heterozygous individual (not affected)

• Disease if individual inherits two recessive alleles

2) Dominant Alleles:


Inheritance of Human Disorders (single gene):

• Potential effects: A) Produce abnormal proteins

B) Perform toxic reactions

C) Produce “overactive” proteins Huntington Disease:

Deterioration of brain(abnormal protein production)3) Sex-linked Alleles:

• Genes associated with the sex chromosomes

• Frequently in males; often skips a generation

Example 1:

Color blindness(defective receptors)

http://go2.wordpress.com/?id=725X1342&site=billnoble.wordpress.com&url=http://billnoble.files.wordpress.com/2010/04/4-17-10-albino-buckeye-seedling-stebbins-cold-canyon.jpg&sref=http://billnoble.wordpress.com/

http://en.wikipedia.org/wiki/File:Huntington.jpg

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Royal Families of Europe:


Example 2:

Hemophilia(defective clotting proteins)

Nondisjunction:

Homologous pairs do not

separate during meiosis

1) Sex Chromosomes:


Errors in Chromosome Number can Occur:

XO

Turner Syndrome

(♀; Infertile; lack 2° sexual char.)


XXX

Trisomy X

(♀; Fertile; usually tall; potential IQ)

XXY

Klinefelter Syndrome

(♂; Fertile; often unnoticed)

XYY

Jacob Syndrome

(♂; Fertile; excess testosterone (violent?))

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Nondisjunction:

Homologous pairs do not

separate during meiosis

2) Autosomal Chromosomes:


Errors in Chromosome Number can Occur:


• Physical characteristics

• Mental retardation

• Heart malformations

Down Syndrome(Trisomy 21)

Frequency increase with

age at pregnancy

Documents

Chapter 12 Patterns of Inheritance - wou.edulemastm/Teaching/BI102/Chapter 12 - Inheritance (2).pdf1 Chapter 12 Patterns of Inheritance Chapter 12: Inheritance Modern Genetic Concepts: