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Chapter 4: Modification of Mendelian Ratios. Allele. *Wild-type allele. *Mutant allele. Conventional symbols for alleles: recessive allele - initial letter of the name of the recessive trait, lowercased and italicized dominant allele - same letter in uppercase . P Generation.

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slide1

Chapter 4: Modification of Mendelian Ratios

Allele

*Wild-type allele

*Mutant allele

Conventional symbols for alleles:

recessive allele - initial letter of the name of the recessive trait, lowercased and italicized

dominant allele - same letter in uppercase

slide2

P Generation

White

Red

RR

rr

Gametes

r

R

F1 Generation

Pink

Rr

Gametes

r

R

Sperm

F2 Generation

r

R

R

RR

Rr

Eggs

1

1

1

1

1

1

r

rr

Rr

2

2

2

2

2

2

In incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents.

slide3
Hypercholesterolemia
  • Dangerously high levels of cholesterol in the blood.
  • Is a human trait that is incompletely dominant.
  • Heterozygotes have blood cholesterol levels about 2X normal.
  • Homozygotes have blood cholesterol levels about 5X normal.

Hh

HH

hh

Homozygous

for ability to make

LDL receptors

Heterozygous

Homozygous

for inability to make

LDL receptors

GENOTYPE

LDL

LDL

receptor

PHENOTYPE

Cell

Normal

Mild disease

Severe disease

slide4

Codominance:

Condition in which the phenotypic effects of a genes alleles are fully and simultaneously expressed in the heterozygote.

Example:

MN Blood group - red blood cells contain a transmembraneglycoprotein (glycophorin); two different forms of this protein exist, M and N

slide5

Multiple Alleles

Blood

Group

(Phenotype)

Examples:

*Table 4.1: over 100 alleles at a given locus in Drosophila

*ABO Blood group in humans

Red Blood Cells

Genotypes

Carbohydrate A

IAIA

or

IAi

A

Carbohydrate B

IBIB

or

IBi

*Characterized by the presence of glycoprotein antigens on the surface of red blood cells

*Distinct from the M and N antigens

*Also exhibits codomiance

B

AB

IAIB

ii

O

slide6

Lethal Alleles:

Example: Coat color in mice

*A = agouti = wild-type allele

*AY = yellow = mutant allele

slide8

Epistasis

Example:

In Drosophila, gene:eyeless

*Black (B) is dominant to brown (b)

*Second gene responsible for allowing pigment to be deposited in hair,

C = presence

c = absence (colorless)

slide10
Pleiotropy is the impact of a single gene on more than one character.

Pleiotropy

Multiple traits

(e.g., sickle-cell

disease)

Single

gene

slide12

Polygenic

inheritance

Single trait

(e.g., skin color)

Multiple genes

  • Polygenic inheritance is the additive effects of two or more genes on a single phenotype.
slide13

P Generation

aabbcc

(very light)

AABBCC

(very dark)

F1 Generation

AaBbCc

AaBbCc

Sperm

F2 Generation

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

Eggs

1

1

20

15

15

6

6

64

64

64

64

64

64

64