Download Brooker Chapter 8

Genetics:
At the Crossroads
(Quantitative, Evolution, &
Development)
(CHAPTERs 23, 24 & 26- Brooker Text)
December 11, 2007
BIO 184
Dr. Tom Peavy
Molecular
Structure
Cellular
Function
Organismal
Evolution
Population
• Quantitative genetics (the study of traits that
can be described numerically) is important for
two reasons
– 1. Most of the key characteristics considered by
plant and animal breeders are quantitative traits
– 2. Many of the traits that allow a species to adapt
to its environment are quantitative traits
Discontinuous vs. continuous traits
(discrete states) (continuum of variation)
POLYGENIC INHERITANCE
• Most quantitative traits are polygenic and exhibit a
continuum of phenotypic variation
• Polygenic inheritance refers to the transmission of
traits that are governed by two or more genes
• The locations on chromosomes that affect the
outcome of quantitative traits are called quantitative
trait loci (QTLs)
– QTLs may contain many genes
• Some or all of which may affect quantitative traits
QTLs Are Now Mapped by
Linkage to Molecular Markers
• Molecular markers, such as RFLPs, are now being
used as reference points along chromosomes
• These genetic markers have been used to
construct detailed genomic maps
– These maps make it easier to determine the number of
genes that affect a quantitative trait
• Detailed genomic maps have been obtained from
– Model organisms
– Organisms of agricultural importance
• The data are analyzed
by computer programs
that can statistically
associate the phenotype
(e.g. fruit size) with the
molecular marker
• Molecular genetics has greatly facilitated our
understanding of speciation and evolution
• Differences in nucleotide sequences are quantitative
– They can be analyzed using mathematical principles in
conjunction with computer programs
• Thus, evolutionary changes at the DNA level can be
objectively compared among different species
– This will establish evolutionary relationships
– Furthermore, this approach can be used to compare any
two existing organisms
• No matter how greatly they differ in their morphologies
• E.g., Humans and bacteria; or plants and fruit flies
Homologous Genes are Derived
from a Common Ancestral Gene
• Two genes are said to be homologous if they are
derived from the same ancestral gene
• Genes can exhibit interspecies homology and
intraspecies homology
– Orthologous genes or orthologs are homologous genes
found in different species
– Paralogous genes or paralogs are homologous genes
found within a single species
• A gene family consists of two or more copies of homologous genes
within the genome of a single organism
Figure 26.9
Evolution of paralogous and orthologous genes
The current model links all life on our planet
Figure 26.10
Three main evolutionary
branches
Developmental Genetics
Bithorax Mutation
Evolution of Development (Evo-Devo)
Homeobox Genes first discovered in Drosophila
(Gehring and Kaufman, 1983)
HOMEOBOX = DNA sequence found within genes involved
in the regulation of development (morphogenesis) of
animals, fungi and plants.
(Homeobox gene family).
A particular subgroup of homeobox genes are the Hox genes,
= Hox cluster (also called Hox complex).
= function in patterning the body axis
(determine where limbs and other body segments will
grow in a developing fetus or larva)