PG 78 CHAPTER 3 Mendelian Genetics: How Are Traits Inherited?
When Mendel crossed pea plants, he counted thousands of offspring. Because there
were so many, the ratios of the different phenotypes were very close to the theoretical
values that his laws of segregation and independent assortment predicted. They were
close, but never exactly right on.
Half Is Enough The examples we have considered thus far apply to organisms, like humans,
that spend most of their life history carrying two alleles for each gene. Only the gametes carry
single alleles. Some organisms, however, such as algae and mosses, spend much of their lives
carrying only a single copy of each gene. How do Mendel's laws apply to these creatures?
Piecing It Together
Mendel's principles, derived from his experiments with peas, are true of all sexually
reproducing organisms-those that produce gametes that join at fertilization. Here, we
summarize Mendel's conclusions about the inheritance of traits, as well as some of the
insights that have emerged from his work:
1. Hereditary characteristics are passed from parent to offspring as units or particles.
We refer to the basic unit of inheritance for a given trait as a gene. The different
forms of a gene are called alleles. Mendel's factors correspond to alleles.
2. Individuals carry two alleles for every gene.The two alleles for a given trait may be
identical, in which case the individual is said to be homozygous for that trait. Alternatively,
the two alleles may differ, and hence, the individual is said to be heterozygous
for that trait.
3. Prior to reproduction, pairs of alleles are separated so that specialized reproductive
cells called gametes contain only one allele from each pair. At fertilization, gametes
fuse, each contributing one allele for each trait to the new offspring.
4. Some genes show dominance; that is, heterozygous individuals may express only
one allele-the dominant allele-while the other, the recessive allele, is masked.
Thus, the phenotype-the traits that are expressed in an individual-may not always
reveal the genotype, or the full complement of alleles that the individual carries.
Recessive alleles remain hidden in the genes.
5. Mendel's law of segregation states that heterozygous parents are equally likely to
pass either of their two alleles on to their offspring. In other words, gametes combine
at fertilization without regard to which alleles they carry. Because the alleles that an
individual inherits are purely a matter of chance, the rules of probability can be used
to determine the likelihood that any given allele is passed on.The Punnett square is
a tool that illustrates the law of segregation.
6. Mendel's law of independent assortment applies when two or more genes are
considered simultaneously. It states that the alleles of one gene are passed to offspring
independently of the alleles for other genes.The Punnett square illustrates this law,
3-2 Why Aren't Members of the Same Species Identical?
Like begets like.When pea plants are crossed, the resulting offspring are never roses or
geraniums: They are always peas.When dogs are bred, the result is puppies, and when
racehorses are bred, inevitably they bear foals. Each species is a particular combination