A key mechanism of gene duplication in sexually reproducing organisms involves unequal crossing over between homologous chromosomes during meiosis

In first phase of meiosis (meiosis I), hmologous chromosomes form pairs, with each chromosome containig two siste chromatids. 

Crossing over is a common phenomenon, and may contribute to genetic variation in natural populations

In most cases crossing over involves equal lengths of non-sister chromatids, with the breaks in the chromatids occuring at the same point. 

Unequal crossing over

Unequal crossing over occurs during meiosis involves chance mistakes on the part of the proteins tha tmanage recombination and constitutes a form of chromosomal mutation. 

Because unequal crossing over occurs during meiosis, the resulting mutaitons are transmitted to the gametes and thus are heritable (germ line mutations)

By chance, a transposon may replicate not only itself but also adjecent gene. By inserting a new copy of the gene in another pert of the genome, the transposon causes the gene to be duplicates

many gene duplications tht arise by this mechanism are likely to be somatic mutations. 

polyploidy is also known as genome duplication, becasue all of the organism's genes are duplicated at the same time

Polyploidy is rare in animals but common in plants

Tetraploid (4n)

In plants, chance errors during meiosis may lead to the production of diploid rather than haploid gamete. If two diploid gametes fuse, the resulting zygote will be tetraploid. 

If the tetraploid individual can self fertilize, or if it can mate with another tetraploid then a population of tetraploid individuals may become established. 

Recent estimates indicate that more than 50% of all angiosperm species are polyploid. Some are tetraploid, others are ocotoploid, and still others have higher ploidy levels. 

Polyploidy involves germ line mutations

1. Both copies of the gene may retain their original function, with the result that the organism may simply produce larger quantities of the encoded polypeptide

2. One copy of the gene may retain its original function, while the second copy accumulates nunsynonymous mutations that lead to a divergence in its function. 

3. One copy of the gene may retain its original function, while the second copy is incapacitates by the accumulation of deleterious mutations, such that it becomes a functionless pseudogen. the deleterious mutations may arise either within the protein-coding region or the promoter region of the gene

Successive rounds of duplication and dicergence may lead to evolution of gene families

In many gene families, the genes are arrayed in sets along one or more chromosomes. 

An example is the globin gene family in vertebrates

Globin genes encode polypeptides of the oxygen-binding proteins myogloin and hemoglobin

Myoglobin is the primary oxygen storage protein in muscles

Hemoglobin carries oxygen in the blood, binding it in the lungs or gils and transporting it to various body tissues. 

In humans there are three functional members of the a-globin cluster and five functional members of the b-globin cluster

The functional genes within each cluster are seperates by noncoding spacer regions

Each cluster also contains one or more pseudogenes

During human development, different members of the b-globin cluster are expressed at different times and in different tissues

For example, just before birth there is a major switch from G-globin and A-globin expresion to B-globin expression

the G-globin and A-globin polypeptides bind oxygen more tightly than does the B-globin polypeptide

The presence of the G-globin and A-globin polypeptides ensure that the oxygen will be transferred from the maternal blood to the fetal blood. 

Hox genes encode polypeptides that function as transcription factors, in that they bind to the promoter regions of other genes and control their transcription

Hox genes control differentiation along the anterior-posterior body axis in arthropods and verterbrates, and thus play a key role in regulating development