Heredity ❮ 101does not interfere with inheritance of another trait. For example, if an individual is BbRr
for two genes, gametes formed during meiosis could contain BR, Br, bR, or br. The B and
b alleles assort independentlyof the R and r alleles.
The law of dominance.Also based on Mendel’s work, this states that when two opposite
pure-breeding varieties (homozygous dominant vs. homozygous recessive) of an organism
are crossed, all the offspring resemble one parent. This is referred to as the dominanttrait.
The variety that is hidden is referred to as the recessivetrait.
It is time for you to answer a question for us (of course, we have no way of knowing
whether or how you will answer this question): Can the phenotype of an organism be deter-
mined from simple observation? Yes—just look at the organism and determine whether it is
tall or short, has blue eyes or brown eyes, and so on. However, the genotype of an organism
cannotalways be determined from simple observation. In the case of a recessive trait, the
genotype is known. If a person has blue eyes (recessive to brown), the genotype is bb. But if
that person has brown eyes, you cannot be sure if the genotype is Bb or BB—the individual
can be either homozygous dominant or heterozygous dominant. To determine the exact
genotype, you must run an experiment called a test cross.Geneticists breed the organism
whose genotype is unknown with an organism that is homozygous recessive for the trait.
This results in offspring with observable phenotypes. If the unknown genotype is heterozy-
gous, probability indicates one-half of the offspring shouldexpress the recessive phenotype.
If the unknown genotype is homozygous dominant, allthe organism’s offspring should
express the dominant trait. Of course, such experiments are not done on humans.Remember me!
Mendel discovered many statistical laws of heredity. He learned that a monohybrid cross
such as Yy ×Yy will result in a phenotype ratio of 3:1 in favor of the dominant trait. He
learned that a dihybrid cross, such as YyRr ×YyRr, will result in a phenotype ratio of
9:3:3:1 (9 RY, 3 rY, 3 Ry, 1 ry). These two ratios, when they appear in genetic analysis
problems, imply mono- and dihybrid crosses.Intermediate Inheritance
The inheritance of traits is not always as simple as Mendel’s pea experiments seem to indi-
cate. Traits are not always dominant or recessive, and phenotype ratios are not always 9:3:3:1
or 3:1. Mendel’s experiments did not account for something called intermediate inheri-
tance,in which an individual heterozygous for a trait (Yy) shows characteristics not exactly
likeeitherparent. The phenotype is a “mixture” of both of the parents’ genetic input. There
are two major types of intermediate inheritance:- Incomplete dominance or “blending inheritance”
- Codominance
Incomplete Dominance (“Blending Inheritance”)
Inincomplete dominance(“blending inheritance”) the heterozygous genotype produces
an “intermediate” phenotype rather than the dominant phenotype; neither allele dominates
the other. A classic example of incomplete dominance is flower color in snapdragons—
crossing a snapdragon plant that has red flowers with one that has white flowers yields
offspring with pink flowers.Marcy (college
freshman):
“Understanding
this concept is
worth 2 points
on the exam.”
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