Hello
This takes me back. I remember sitting in school wondering what
pea-plants and dog hair length had to do with me. ( It must have
clicked eventually, as I went on to do a Biological Sciences degree. )
Everyday experience tells us that in sexually reproducing organisms,
characteristics are derived from both parents. Therefore, in this
instance a zygote must contain a contribution of genetic material (an
allele) for a character such as hair length from both the male and
female parents.
We start by denoting the short haired allele by A and the long
straight haired allele by a. We’ve been told they are homozygous,
that means they both have the same allele eg short haired dogs have
the alleles AA, long straight haired aa. This is known as being
homozygous in relation to the alleles of the gene. (A zygote with the
combination Aa is called a heterozygote, which we’ll deal with later)
(The genetic constitution of an organism in relation to one or more
pairs of characteristics is known as its genotype; thus the genotype
of a long haired dog is aa. The appearance of the organism with
respect to this characteristic is referred to as its phenotype; thus
the phenotype of a dog with aa zygote is long-haired. The phenotype
of dogs with the genotype AA is short haired.)
We can construct a diagrammatic model of our next generation using 2 x
2 tables, (known as a Punnett square) as diagrammatic models of
breeding. The following Punnett square depicts the possible outcomes
of a cross between short haired dogs AA and long haired dogs aa
Male Parent gametes
A A
Female parent gametes a Aa Aa
a aA aA
(Note that there are 4 possible combinations of alleles. Thus the
probability of each combination arising is = 0.25.)
The pups are all heterozygous, i.e have a differnt allele from each
parent, with a zygote of Aa. As the A allele is dominant, all the pups
will be short haired So the expected proportion of long, straight
haired pups is this generation (usually called "f1" is 0 )
In the next generation, we assume that the pups are interbreeding with
pups of the same genotype as them. The Punnet square for the next
generation looks like this:
Male parent gametes
Female parent A a
gametes A AA Aa
a aA aa
So, you can see that in this second generation ("f2"), even although
all the parents are short haired, 1 in 4 of their pups will have the
genotype aa, so 1 in 4 of the population will be long haired.
This is all standard Mendelian genetics. You can read a short bio of
him here (http://www.biopoint.com/engaging/MENDEL/MENDEL.HTM )
Hope this helps
Willie
A very good overview of simple genetics
Basic Principles of Genetics: Mendels Genetics
http://anthro.palomar.edu/mendel/mendel_1.htm
Google seach terms
mendel genetics inheritance
mendel genetics "hair length" |
