segregation of chromosomes

Above: Children generated by sex have mixture of chromosomes from both parents. Since there are a huge number of combinations of each of the two chromosomes there is a great variety among the children (but also similarity because the chromosome pairs are not that different).

clonal growth

Above: In clonal growth there is no mixture of chromosomes because there are not two parents. However, there can be considerable mutation, recombination, and "mistakes"during the creation of clones.

Essay- December 2013: Diversity within and among individuals

Most of Life on the Dock (LOD), concerns itself with marine biology, after all it is all about life on the dock. However, there are general principles that help understand life, on or off the dock. Chapter 15: The Family, is about the most basic. How are the animals and plants related to each other.

A central concept is that organisms are clumped into species. Members of one species look like each other, and can breed with each other to produce children that are hybrids of the parents. The traits passed on by parents are stored in DNA molecules that are the functional components of chromosomes (see pg 129 - 139 of LOTD). It is assumed in introductory discussions that the DNA in all the cells of one individual are the same. However, that is turning out to be more of an approximation than was originally thought.

Sex, reproduction, and diversity

Let's use humans as an exampled (see diagram on right). Our cells contain 23 pairs of chromosomes. Thus, when a human fertilized egg is produced it contains 23 pairs of chromosomes. One member of each pair comes from the father (the sperm), and one from the mother (the unfertilized egg). But which chromosome from each of the father's pairs are selected and which chromosome from the mother's pairs? It's more or less random, so there are many, many possible combinations that end up in the child. While there are other processes that generate diversity, e.g. mutation and recombination, chromosome distribution is the main reason children can look somewhat different than the parents.

Clonal reproduction

Colonial ascidians are described in detail starting at Chapter 10, pg 67 of LOTD. However, an adult human is also colony of cells generated by repeated division of the original fertilized egg. Both organisms arise from clonal reproduction (diagram on right). It had been assumed that only rare mutations would cause a very small fractions of cells in the adult to be different. However, in [2] (summarized in [1]), McConnell et al. show that in the brain "13 to 41% of neurons have at least one megabase-scale" ...(duplicated or deleted segment) ..."deletions are twice as common as duplications, and a subset ... have highly aberrant genomes...". Thus not even identical twins have identical brains. Maybe the cellular heterogeneity described in [2] results in behavioral diversity?


[1] Our fallen Genomes, Macosko and McCarroll, Science vol 342, pg 564 ( 1 November 2013)

[2] Mosaic copy number variation in human neurons, Science vol 342, pg 632 (1 November 2013)