In last screencast, we looked at the data Mendel collected, and the laws he derived
from his data. Remember that Mendel did not know what chromosomes and genes were, but
he used inductive reasoning to derive his laws, which are the foundation of modern genetics.
Recall from our cell division activity that genes are located on chromosomes. Chromosomes
come in pairs, and each chromosome in a pair comes from a different parent. These pairs
are called homologous chromosomes, and have genes for certain traits, although the form
of the gene, called an allele, may be different on each of the homologous chromosomes. We
now know that a pea plant has 14 chromosomes, which means it has 7 homologous pairs. Recall
that when the chromosomes are present in pairs, the cell is called diploid. So here we have
a diploid set of chromosomes, which are replicated. That is, they have sister chromatids. Remember
that a chromosome is always a chromosome, whether it is single or replicated. We can
only observe real chromosomes during cell division, when they are replicated and denser.
If a pea plant's phenotype is homozygous for round seeds, we can denote its genotype with
2 capital R's. The phenotype is the physical or physiological description of a trait and
the genotype is the genetic make-up for that trait. So here the capital R stands for the
dominant allele. Now chromatids before crossing-over will be genetically identical, because one
was made from the other. So each chromatid will have an uppercase R. The other chromosome
in the pair will also have an uppercase R, since the cell is homozygous for round seeds.
When meiosis occurs, alleles on homologous chromosomes will end up in different gametes.
So a gamete has only one of each chromosome, which makes it haploid, or n. Each parent
will only give one R. Remember that Mendel crossed the round seed plants with wrinkled
seed plants. Since wrinkled is recessive to round, we represent the gamete with the wrinkled
seeds with a lower case r. We now represent genetic crosses using Punnett squares. Working
after Mendel, Reginald Punnett wrote the first text book on genetics, and used boxes to represent
the potential offspring of a genetic cross. In our first example, each parent can pass
on only one kind of gamete to the offspring: one parent passes on the round seed trait
and the other parent passes on the wrinkled seed trait
The offspring inside the box, the F1 generation, thus have one of each
allele in their genotype, which is big R, small r. Because the round seed trait is dominant
to the wrinkled seed trait, we know that all the offspring, shown inside the box, will
have the phenotype round seeds, but will be heterozygous. Recall the cross of F1 organisms
produces the F2 generation. This generation will have one big R on one of its homologues,
and a small r on the other, which means it is hybrid, or heterozygous. This cross is
a monohybrid cross, since each parent is heterozygous, or hybrid for the trait, shown by the gametes
on the outside of the box as big R and little r. These letters stand for the products of
meiosis. The letters on the top represent the gametes of one parent, and the letters
on the side represent the gametes of the other parent. A monhybrid organism will produce
gametes that have a 50% chance of having either allele. Inside the box now are three possible
genotypes: big R big R, big R small r, and small r small r.
The recessive trait has reappeared in the F2 generation. These
These two qualities are shown here. The recessive trait has reappeared in the f2 generation.
These three possibilities are shown here because the heterozygous condition has two possible
ways of forming, it represents 50% of the total potential offspring.
Thus the genotype ratio
is 1:2:1, or 1/4 homozygous dominant, 2/4 heterozygous, and 1/4 homozygous recessive.
Or a ratio of 3 round to 1 wrinkle.
Recall that Mendel's data showed this 3:1 ratio. The reappearance of the recessive trait led Mendel to develop
his Law of Segregation.