*Unit factors in pairs- genetic characteristics are controlled by unit factors that exist in pairs in individual organisms

*Principle of Dominance and Recessiveness- one factor is dominant over the recessive one

*Law of Segregation- During gamete formation the paired unit factors segregate randomly so that each gamete receives one or the other

*Law of Independent Assortment – During gamete formation, segregating pairs of unit factors assort independently of each other

Review:

Dihybrid cross

F1

All yellow, round

GGWW ggww

Example:

P1

yellow, round

green, wrinkled

X

GgWw

Dihybrid cross con’t:

F1

All yellow, roundGgWw

F1

All yellow, roundGgWw

X

F2

The Trihybrid Cross:

Trihybrid cross

Example:

Theoretical gene pairs represented by the symbols A, B, and C

P1 AABBCC aabbccX

Gametes:

ABC abc

F1 AaBbCc

Gametes:

ABC ABc

aBC

AbC

aBc

Abc

abC abc

The Forked-Line Method (branch diagram):

Recall:

*The F1 that result from a monohybrid cross (AA x aa) all have the genotype Aa and the phenotype represented by A

*The F2 that result from a cross between 2 individuals from the F1, have a phenotypic ratio of 3:1

The Forked-Line Method (branch diagram):

*NOTE: We are assuming that independent assortment of these 3 gene pairs is a random process!

The Forked-Line Method (branch diagram):

Mendel Rediscovered:Why did Mendel’s work go unnoticed for so long?

*1879 Walter Flemming

Charles Darwin and Alfred Russel Wallace:

continuous variation – offspring were a blend of parents’ phenotypes

*early 20th century

*Hugo de Vries*Karl Correns*Erich Tcshermak

*1902 Walter Sutton and Theodor Boveri

Chromosomal Theory of Inheritance

How do we account for genetic variation?

*Independent assortment *Crossing over *Random fertilization

Independent Assortment:

Cross over:

Human PedigreesPedigree

= Female

= Male

=Unknown

*Proband (p)

Chapter 4: Modification of Mendelian RatiosAllele

*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

Genetic nomenclature is extremely diverse!

Incomplete or Partial DominanceCross between parents

with contrasting traits: Red flowers or white

flowers

Offspring with an intermediate

phenotype: pink flowers

Codominance:

Example:

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

Examples:

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

*ABO Blood group in humans

Multiple Alleles:

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

*Distinct from the M and N antigens

*Also exhibits codomiance