Mendel and Inheritance for AP Biology
Practice problems for these concepts can be found at: Heredity Review Questions for AP Biology
Terms Important in Studying Heredity
The following is a list of terms that will help in your understanding of heredity:
Allele: a variant of a gene for a particular character. For example, the two alleles for eye color discussed later in the chapter are B (dominant) and b (recessive).
F1: the first generation of offspring, or the first "filial" generation in a genetic cross.
F2: the second generation of offspring, or the second "filial" generation in a genetic cross.
Genotype: an organism's genetic makeup for a given trait. A simple example of this could involve eye color where B represents the allele for brown and b represents the allele for blue. The possible genotypes include homozygous brown (BB), heterozygous brown (Bb), and homozygous blue (bb).
Heterozygous (hybrid): an individual is heterozygous (or a hybrid) for a gene if the two alleles are different (Bb).
Homozygous (pure): an individual is homozygous for a gene if both of the given alleles are the same (BB or bb).
Karyotype: a chart that organizes chromosomes in relation to number, size, and type.
Nondisjunction: the improper separation of chromosomes during meiosis, which leads to an abnormal number of chromosomes in offspring. A few classic examples of nondisjunction- related syndromes are Down, Turner, and Klinefelter syndromes.
P1: the parent generation in a genetic cross.
Phenotype: the physical expression of the trait associated with a particular genotype. Some examples of the phenotypes for Mendel's peas were round or wrinkled, green or yellow, purple flower or white flower.
Mendel and His Peas
The person whose name is most often associated with heredity is Gregor Mendel. Mendel spent many years working with peas. It was a very strange hobby, indeed, but it proved quite useful to the world of science. He mated peas to produce offspring and recorded the phenotype results in order to determine how certain characters are inherited. A character is a genetically inherited characteristic that differs from person to person.
Before he began his work in the 1850s, the accepted theory of inheritance was the "blending" hypothesis, which stated that the genes contributed by two parents mix as colors do. For example, a blue flower mixed with a yellow flower would produce a green flower. The exact genetic makeup of each parent could never be recovered; the genes would be as inseparable as the blended colors. Mendel used plant experiments to test this hypothesis and developed his two fundamental theories: the law of segregation and the law of independent assortment.
When Mendel was observing a single character during a mating, he was doing something called a monohybrid cross—a cross that involves a single character in which both parents are heterozygous (Bb ???????Bb). A monohybrid cross between heterozygous gametes gives a 3 : 1 phenotype ratio in the offspring (Figure 10.1). As you can see in Figure 10.1, an offspring is three times more likely to express the dominant B trait than the recessive b trait.
Mendel also experimented with multiple characters simultaneously. The crossing of two different hybrid characters is termed a dihybrid cross (BbRr × BbRr.) A dihybrid cross between heterozygous gametes gives a 9 : 3 : 3 : 1 phenotype ratio in the offspring (Figure 10.2).
From his experiments, Mendel developed two major hereditary laws: the law of segregation and the law of independent assortment.
The law of segregation. Every organism carries pairs of factors, called alleles, for each trait, and the members of the pair segregate (separate) during the formation of gametes. For example, if an individual is Bb for eye color, during gamete formation, one gamete would receive a B, and the other made from that cell would receive a b.
The law of independent assortment. Members of each pair of factors are distributed independently when the gametes are formed. Quite simply, inheritance of one trait or characteristic does 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 independently of 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 dominant trait.
The variety that is hidden is referred to as the recessive trait. It is time for you to answer a question for me (of course, I have no way of knowing whether or how you will answer this question): Can the phenotype of an organism be determined 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 cannot always 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 heterozygous, probability indicates one-half of the offspring should express the recessive phenotype. If the unknown genotype is homozygous dominant, all the organism's offspring should express the dominant trait. Of course, such experiments are not done on humans.
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