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Chromosomal Inheritance II. Outline. Incomplete Dominance, Codominance , and Multiple Allelism Interaction of genes Pedigree Studies Genetics and Ethics. Extending Mendel’s Rules. Incomplete dominance heterozygotes have an intermediate phenotype Codominance

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outline
Outline
  • Incomplete Dominance, Codominance, and Multiple Allelism
  • Interaction of genes
  • Pedigree Studies
  • Genetics and Ethics
extending mendel s rules
Extending Mendel’s Rules
  • Incomplete dominance
    • heterozygotes have an intermediate phenotype
  • Codominance
    • Heterozygotes displays the phenotype of both alleles
  • multiple allelism
    • Multiple distinct genes versions (i.e., alleles) are present in the population
  • polymorphism
    • Multiple distinct phenotypes are present in a population
multiple alleles and polymorphism

Carbohydrate

Allele

Phenotype

(blood group)

Red blood cell

appearance

Genotype

Multiple Alleles and Polymorphism

IA

A

B

IB

i

none

(a) The three alleles for the ABO blood groups

and their associated carbohydrates

  • ABO blood group in humans are determined by three alleles : IA, IB, and i.

IAIA or IA i

A

B

IBIB or IB i

AB

IAIB

ii

O

(b) Blood group genotypes and phenotypes

pleiotropy
Pleiotropy
  • A gene that influences many traits rather than just one is pleiotropic.
    • Marfan Syndrome (FBN1): defective fibrillinlimbs, spinal chord, heart
    • Cystic fibrosis (CFTR): defective salt transportlungs, pancreas, sebacious glands, etc.

Lung(s)

pancreas

healthy

CF

antagonistic pleiotropy
Antagonistic pleiotropy
  • Some effects are good; some are bad
  • Sickle cell anemia (hemoglobin B)
    • Codominant trait
    • HBB/HBB; HBB/hbb; hbb/hbb

HBB/hbb

Malaria protection

Mild sickle cell disease

Healthy

Unhealthy

???

slide7

Fig. 14-UN2

Degree of dominance

Example

Description

Heterozygous phenotype

same as that of homo-

zygous dominant

Complete dominance

of one allele

PP

Pp

Heterozygous phenotype

intermediate between

the two homozygous

phenotypes

Incomplete dominance

of either allele

CRCW

CRCR

CWCW

Codominance

Heterozygotes: Both

phenotypes expressed

IAIB

Multiple alleles

In the whole population,

some genes have more

than two alleles

ABO blood group alleles

IA , IB , i

One gene is able to

affect multiple

phenotypic characters

Sickle-cell disease

Pleiotropy

epistasis

Fig. 14-12

Epistasis

BbCc

BbCc

Sperm

  • A gene at one locus alters the phenotypic expression of a gene at a second locus
  • Coat color in mice
    • pigment color (B for black; b for brown)
    • Pigment deposit (C for color; c for no color)

1/4

1/4

1/4

1/4

BC

bC

Bc

bc

Eggs

1/4

BC

BBCc

BBCC

BbCC

BbCc

1/4

bC

bbCC

bbCc

BbCC

BbCc

1/4

Bc

BBcc

Bbcc

BBCc

BbCc

1/4

bc

BbCc

bbCc

Bbcc

bbcc

: 4

9

: 3

discrete vs quantitative traits
Discrete vs. Quantitative Traits
  • Discrete traits.
    • seed color in peas—no intermediate phenotypes
  • Quantitative traits
    • Traits that fall into a continuum
  • Frequencies
    • form a bell-shaped curve (normal distribution) for a population.

A phenotype distribution that forms a bell-shaped curve.

Normal distribution—bell-shaped curve

quantitative traits result from the action of many genes
Quantitative Traits Result from the Action of Many Genes

Wheat kernel color is a quantitative trait.

Hypothesis to explain inheritance of kernel color

Parental

generation

aa bb cc

(pure-line white)

AA BB CC

(pure-line red)

F1

generation

Aa Bb Cc

(medium red)

Self-fertilization

F2

generation

20

15

15

6

6

1

1

polygenic inheritance
Polygenic Inheritance

AaBbCc

AaBbCc

Sperm

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

  • Traits that vary in the population along a continuum
  • Additive effect of 2+ genes on a single phenotype
  • Skin color in humans is an example of polygenic inheritance

1/8

1/8

1/8

1/8

Eggs

1/8

1/8

1/8

1/8

Phenotypes:

1/64

6/64

15/64

20/64

15/64

1/64

6/64

Number of

dark-skin alleles:

2

6

0

3

4

5

1

slide12

Relationship among

genes

Example

Description

Epistasis

One gene affects

the expression of

another

BbCc

BbCc

BC

bC

Bc

bc

BC

bC

Bc

bc

: 3

: 4

9

A single phenotypic

character is

affected by

two or more genes

Polygenic

inheritance

AaBbCc

AaBbCc

applying mendel s rules to humans
Applying Mendel’s Rules to Humans
  • Humans  terrible genetic models

– Generation time is too long

– Parents produce relatively few offspring

– Breeding experiments are frowned upon

  • Human disorders follow 5 patterns

1) Autosomal dominant 2) Autosomal recessive

3) X-linked recessive 4) X-linked dominant

5) Y-linked

  • Pedigrees (family trees)
    • analyze the human crosses that already exist.
human pedigree reports
Human Pedigree Reports

Key

Mating

Male

Affected

male

Offspring, in

birth order

(first-born on left)

Female

Affected

female

fig 14 15b

1st generation

(grandparents)

Ww

Ww

ww

ww

2nd generation

(parents, aunts,

and uncles)

Fig. 14-15b

Ww

Ww

ww

ww

Ww

ww

3rd generation

(two sisters)

WW

ww

or

Ww

Widow’s peak

No widow’s peak

Is a widow’s peak a dominant or recessive trait?

autosomal recessive traits
Autosomal Recessive Traits

I

Carrier male

Carrier female

Carriers (heterozygotes) are indicated with half-filled symbols

  • If a phenotype is due to an autosomal recessive allele
    • trait = homozygous
    • parents (w/o trait) = heterozygous carriers.
  • Carriers carry the allele and transmit it even though they do not exhibit the phenotype.

Each row represents a generation

II

Affected

male

III

Affected

female

IV

autosomal or sex linked trait
Autosomal or Sex-Linked trait?
  • Equally often in males and females
    • likely to be autosomal.
  • Males more likely to have the trait
    • usually X-linked.
  • Hemophilia is an example of an X-linked trait resulting from a recessive allele.

I

Prince Albert

Queen Victoria

Female carrier of hemophilia allele

II

Affected male

III

IV

slide18

Frequency of Dominant Alleles

  • Not necessarily more common (NOT always “WT”)
    • one baby out of 400 in the United States is born with extra fingers or toes
    • Dominant allele; uncommon occurrence
  • In this example, the recessive allele is far more prevalent than the population’s dominant allele
slide20

Fetal Testing

  • Tests to determine in utero if a child has a disorder.
    • 14th to 16th week of pregnancy
    • Blood or amniocentesis
  • Fetal tests can reveal a serious disorder
    • Trisomy 21, 18, etc.
  • Some testing after birth
    • eg Type I diabetes

http://www.youtube.com/watch?v=qA25_fiyh_E&feature=related

slide21

Eugenics

  • Science of “improving the genetic stock” of humans
    • Old Testament
    • Plato’s Republic(description of the ideal society )
  • Francis Galton
    • “National Eugenics Laboratory”
    • Experimental studies of heredity
      • Twins
  • Karl Pearson
    • The higher birth rate of the poor
    • Supplant by "higher" races
us propaganda and policy
US Propaganda and Policy
  • The Immigration Act of 1924
    • quota for different nationalities
    • perceived tendencies towards crime etc.
  • Forced Sterilization
eugenics and the third reich
Eugenics and the Third Reich
  • Nazi Germany
    • The Aryan Nation and the Holocaust
  • Human races
    • Ill-founded concept
    • Populations with overlapping gene pools.
    • No major difference in the genome sequence