Study Guide for Chapter 13
1. What were the positive advantages of Mendel’s experimental system and why is it
important to the success of an experiment to choose an appropriate experimental system?
2. Know the genetics vocabulary: true-breeding, homozygous, heterozygous, dominant,
recessive, alleles, genotype, phenotype, F
generation, test cross, pedigree, genetic
recombination, wild type, linkage, nondisjunction
transmitting unchanged from generation to generation.
plants with white flowers, for example, when crossed with each other, produced
only offspring with white flowers, regardless of the number of generations.
– When two haploid gametes containing exactly the same allele of a
factor fuse during fertilization to form a zygote, the offspring that develops from
that zygote is said to be homozygous.
When two haploid gametes contain different alleles, the
individual offspring is heterozygous.
– Mendel referred to the form or trait expressed in the F1 plants as
– to the alternative form that was not expressed in the F1 plants as
– In modern terms, the alternative forms of a factor, leading to alternative
forms of a character are called alleles.
– To distinguish between the presence of an allele and its expression,
modern geneticists refer to the totality of alleles that an individual contains as the
– and to the physical appearance of that individual as its phenotype.
– Employing these simple symbols, we can now go back and
reexamine the crosses Mendel carried out.
Because a white flowered parent (pp)
can produce only p gametes, and a pure purple flowered parent (PP) can produce
only P gametes, the union of an egg and a sperm from these parents can produce
only heterozygous pP offspring in the F1 generation.
They will all be purple.
– To test his hypothesis further, Mendel devised a simple and
powerful procedure called the testcross.
Consider a purple flowered plant.
impossible to tell whether such a plant is homozygous or heterozygous siply by
looking at its phenotype.
To learn its genotyple, you must cross it with some
What kind of cross would prvide the answer?
If you cross it with a
homozygous dominant individual, all of the progeny will show the dominant
phenotype whether the test plant is homozygous or heterozygous.