Check Your Understanding
- What is a visual representation of a genetic cross?
- What is stated in Mendel’s law of segregation?
Introduction
Although Mendel laid the foundation for modern genetics, there were still a lot of questions
left unanswered. How is inheritance determined for traits that do not seem to follow a simple
dominant-recessive pattern? What exactly are the hereditary factors that determine traits
in organisms? And how do these factors work? One of the great achievements of this past
century was the discovery of DNA as the genetic material. And it is the DNA that makes up
the hereditary factors that Mendel identified. By applying our modern knowledge of DNA
and chromosomes, we can explain Mendel’s findings and build on them.
Traits, Genes, and Alleles
Interpreting Mendel’s discoveries through the eye of modern genetics, we now know that
Mendel’s hereditary factors are made up of DNA. Recall that our DNA is wound into chro-
mosomes. Each of our chromosomes contains a long chain of DNA that encodes hundreds,
if not thousands, of genes. Each of these genes can have slightly different versions from
individual to individual. These variants of genes are calledalleles.For example, remember
that for the height gene in pea plants there are two possible alleles, the dominant allele for
tallness (T) and the recessive allele for shortness (t).
Genotype and Phenotype
Genotyperefers to the combination of alleles that an individual has for a certain gene.
For each gene, an organism has two alleles, one on each chromosome of a homologous pair
of chromosomes. The genotype is often referred to with the letter combinations that were
introduced in the previous lesson, such asTT,Tt, andtt. When an organism has two of the
same alleles for a specific gene, it ishomozygousfor that gene. An organism can be either
homozygous dominant (TT) or homozygous recessive (tt). If an organism has two different
alleles (Tt) for a certain gene, it is known asheterozygous. Genes have a specific place
on a specific chromosome, so in the heterozygous individual these alleles are in the same
location on each homologous chromosome.
Phenotyperefers to the visible traits or appearance of the organism, as determined by the
genotype. For example, the phenotypes of Mendel’s pea plants were either tall or short, or
were purple-flowered or white-flowered. Keep in mind that plants with different genotypes
can have the same phenotype. For example, both a pea plant that is homozygous dominant