(CHAPTERs 23, 24 & 26- Brooker Text)

Genetics:

At the Crossroads

(Quantitative, Evolution, & Development)

December 11, 2007BIO 184

Dr. Tom Peavy

Molecular

Cellular

Organismal

Population

Structure

Function

Evolution

• 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)

• 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

POLYGENIC INHERITANCE

• 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

QTLs Are Now Mapped by Linkage to Molecular Markers

• 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

• 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

Homologous Genes are Derived from a Common Ancestral Gene

Figure 26.9 Evolution of paralogous and orthologous genes

Figure 26.10

The current model links all life on our planet

Three main evolutionary branches

Developmental Genetics

Bithorax Mutation

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)

Evolution of Development (Evo-Devo)

Homeobox Genes first discovered in Drosophila(Gehring and Kaufman, 1983)