Is is common to encounter news accounts of some new discovery
that some trait is "Genetic". Well what does it mean to say that
something has a genetic basis? When we say that some trait has a
genetic basis, we may mean any one of four very different things,
that is any of four different levels of understanding, anywhere
from the very simple sense that the trait is inherited in some
way, to a detailed understanding of the molecular basis of the
trait.
Meanings of "Genetic"
- Genetic = some portion of the trait is inherited in some way.
- Genetic = the trait is controlled by some combination of named
Mendelian genes, which may be may be identified as being located
on a particular chromosome, and near some other genes.
- Genetic = The length of DNA of the gene responsible for a
trait has been identified and its sequence determined.
- Genetic = the gene responsible for a trait has been
sequenced, and we know the molecular biology of how the gene
product works and how mutations in it produce different results.
Heritability
Children tend to look like their parents, but not exactly, and
not like a mixture halfway between each parent. Children often
inherit distinct characteristics, some from one parent, some from
another, and some from a grandparent. Yet in other
characteristics, children seem distinct. A phenotype is the
appearance of an individual (what it looks like) whereas the
genotype represents the genes carried by that individual.
Genetics is the study of how those characteristics (both
phenotype and genotype are passed from parents to their
children. The phenotype of an individual is the result of the
interaction between the individuals genotype and its environment.
Heritability is the extent to which the phenotype is
determined by the genetic makeup of the individual. Some traits
are purely genetic, and these traits have a high heritability.
Other traits are strongly influenced by the individual's
environment, and their heritability is low.
Mendelian Genetics
Part of the study of Genetics involves being able to predict
the appearance (Phenotype) and genetics (Genotype) of the
offspring of animals or people. Sometimes this is just for fun
such as breeding dogs to get a certain color, but sometimes it
is very serious, as when dealing with diseases that can be passed
(that is, inherited) from parents to children.
Without any understanding of the molecular basis of how some
gene works, we can still identify that some trait is genetic.
Indeed, genetics had its origins in the late 1800s, well before
any concept of the molecular basis of heredity.
The basic unit of inheritance is the Gene. A gene is a
length of DNA on a chromosome that does something particular for
an organism. A gene can come in more than one form. These
flavors of genes are called Alleles. An allele represents one or
more DNA sequences in a gene that produces a particular phenotypic result. Several different mutations might cause a
gene to produce a defective protein. All of these mutations
might be lumped into a single allele - the gene in a form that
produces a defective protein. There isn't necessarily a one to
one mapping between mutations to the DNA sequence of a gene and
different alleles for that gene. Alleles are just broad variants
of a gene that can produce different results.
Genes come in two copies per individual
The different alleles of a gene may be dominant and recessive
with respect to each other.
The basic tool for Mendelian genetics is the Punnett Square.
Examples of Genes
It is rare that an aspect of an organism will be controlled by
a single gene that has just two alleles. Most real world
phenotypes are produced by complex interactions between sets of
genes.
1) A simple Mendelian dominant-recessive: tasting PTC (phenylthiocarbamide)
2) Two genes that both influence a phenotype:
Human Eye Color Genes
3) Two genes that interact, one of which has multiple alleles and
incomplete dominance:
Shetland Sheep Dog Coat Color
Genes
Packaging Genes
Bacteria are normal - mammals are strange....
Two basic designs for DNA packaging
Bacteria - 'chromosome' + plastids
Everyone else - chromosomes
Haploid-Diploid-Polyploid
Puzzles of how genes work - [separate page]
Mechanism - level one - genes at loci on chromosomes.
Observation - genes seem like particles
Explanation - genes are pieces of information stored in DNA
Observation - genes come in pairs of copies
Explanation - chromosomes come in pairs
(note - not from DNA double helix - sense&antisense copies on one
strand - other chromosome has second set of sense&antisense)
Observation - both parents and children have just one pair of
each gene, and children get one of their pair from each parent.
Explanation - chromosomes duplicate and split up in mitosis so
that gametes get one of each chromosome each.
Haploid-diploid. Chromosomes come in pairs.
Mitosis - normal cell division
Meiosis - making gametes
Observation - two different genes, even on the same chromosome
tend to be inherited independently
Explanation - recombination during miosis.
Observation - most genes act like they are inherited
indpendently. The allele on gets for one gene from one parent
seems independent of the alleles for any other genes.
Paradox - since genes get packaged on chromosomes, how could this
be so? Genes on the same chromosome should be inherited all
together.
Recombination
Mitosis and Miosis
To give a specific example, if genes for Agouti and Merle (to use
shelties) happened to be on the same chromosome. If one parent
was sable and trifactored at the Agouti locus and Merle and
not-merle at the merling locus, then that parent should only be
able to produce gametes that were sable-merle or
tricolor-notmerle, but never gametes that were sable-notmerle.
Thus breedings to a bicolor dog that isn't merled should only be
able to produce sable merles and tricolors, but never blue
merles. But we observe that breeding a trifactored sable merle
to a bicolor produces sable merles, trifactored sables,
tricolors, bicolors, and blue merles - combinations that couldn't
occur if the genes were on the same chromosome.
Examples of how genes work:
Mechanism - level 2 -
DNA stores information, transcribed to RNA, RNA translated to
protein. Proteins do things: make structures, help catalyse
reactions, regulate other genes. Mutations change gene product.
Three kinds of genes at work:
1) Structrural - Type I Collagen.
Protiens have shapes. Proteins can assemble in groups to
build larger structures.
2) Doing Something - Hemoglobin
Proteins don't do it alone. Single base pair changes can
alter a protien's function, or not, depending on where they are.
Allele can map onto several mutations.
3) Regulating Activity -
Genes can regulate other genes.
Sources:
Copyright © 2000 Athro, Limited. All Rights Reserved.
Written by Paul J. Morris mole@morris.net
Maintained by Athro Limited
Date Created: 6 Jan 2000
Last Updated: 6 Jan 2000