Section 2 Review
Differentiatebetween alleles and genes. 6AApplythe terms homozygous, heterozygous,
dominant,or recessiveto describe plants with
the genotypes PPand Pp. 6DIdentifythe phenotypes of rabbits with the
genotypes Bband bb,where B= black coat and
b= brown coat. 6DDeterminewhether the rabbits in item 3 are
heterozygous or homozygous. 6DCritical ThinkingCritiquing Explanations
Review Mendel’s two laws according to their
strengths and weaknesses in terms of our mod-
ern understanding of meiosis. 3A 3FIf a pea plant is heterozygous
for a particular trait, how can the alleles that control
the trait be characterized? 6D
Atwo recessive Cone dominant, one recessive
Btwo dominant Dthree dominant, one recessiveTAKS Test PrepTAKS Test PrepThe Laws of Heredity
Mendel’s hypotheses brilliantly predicted the results of his crosses
and also accounted for the ratios he observed. Similar patterns of
heredity have since been observed in countless other organisms.
Because of their importance, Mendel’s ideas are often referred to as
the laws of heredity.
The Law of Segregation
The first law of heredity describes the behavior of chromosomes
during meiosis. At this time, homologous chromosomes and then
chromatids are separated. The first law, the ,
states that the two alleles for a trait segregate (separate) when
gametes are formed (as shown in Figure 4).
The Law of Independent Assortment
Mendel went on to study whether the inheritance of one trait (such
as plant height) influenced the inheritance of a different trait (such
as flower color). To study how different pairs of genes are inherited,
Mendel conducted dihybrid crosses. A dihybrid cross is a cross that
considers two pairs of contrasting traits. For example, a cross that
considers both plant height and flower color is a dihybrid cross.
Mendel found that for the traits he studied, the inheritance of one
trait did not influence the inheritance of any other trait. The
states that the alleles of different genes
separate independently of one another during gamete formation. For
example, the alleles for the height of the plant shown in Figure 7sep-
arate independently of the alleles for its flower color. We now know
that this law applies only to genes that are located on different chro-
mosomes or that are far apart on the same chromosome.
The search for the physical nature of Mendel’s “factors” domi-
nated biology for more than half a century after Mendel’s work was
rediscovered in 1900. We now know that the units of heredity are
portions of DNA called genes,which are found on the chromosomes
that an individual inherits from its parents.
of independent assortmentlawlaw of segregationSECTION 2Mendel’s Theory 169
Figure 7 The law of
independent assortment.
Mendel found that the
inheritance of one trait, such
as plant height, did not
influence the inheritance of
another trait, such as flower
color.