Chapter 11 Observable Patterns of Inheritance
I. Sickled Cells and Garden Peas
A. Gregor Mendel observed patterns of inheritance in peas that provided clues about what the genes were doing.
B. Sickle-cell anemia is a genetic disease in which an altered gene leads to many health difficulties.

II. Mendel’s Insights Into the Patterns of Inheritance
A. Introduction
1. By the late nineteenth century, natural selection suggested that a population could evolve if members showed variation in heritable traits. Variations that improved survival chances would be more common in each generation—in time, the population would change or evolve.
2. The theory of natural selection did not fit with the prevailing view of inheritance—blending.
a. Blending would produce uniform populations—such populations could not evolve.
b. Many observations did not fit blending—for example, a white horse and a black horse did not produce only gray offspring.
3. Gregor Mendel used experiments in plant breeding and a knowledge of mathematics to form his hypotheses.
B. Mendel’s Experimental Approach
1. Mendel used the garden pea in his experiments.
a. This plant can fertilize itself; true-breeding varieties were available to Mendel.
b. Peas can also be cross-fertilized by human manipulation of the pollen.
2. Mendel cross-fertilized true-breeding garden pea plants having clearly contrasting traits (example: white vs. purple flowers).
C. Some Terms Used in Genetics
1. Genes are instructions for producing a trait.
2. Each gene has a locus on a chromosome.
3. Diploid cells have two genes (a gene pair) for each trait—each on a homologous chromosome.
4. Alleles are various molecular forms of a gene for the same trait.
5. If homozygous, both alleles are the same.
6. If heterozygous, the alleles differ.
7. When heterozygous, one allele is dominant (A), the other is recessive (a).
8. Thus, homozygous dominant = AA, homozygous recessive = aa, and heterozygous = Aa.
9. Genotype is the sum of the genes, and phenotype is how the genes are expressed (what you observe).
D. The Concept of Segregation
1. Mendel’s first experiments were monohybrid crosses.
a. Monohybrid crosses have two parents that are true-breeding for contrasting forms of a trait.
b. One form of the trait disappears in the first generation (F1), only to show up in the second generation.
c. We now know that all members of the first generation are heterozygous because one parent could produce only an A gamete and the other could produce only an a gamete.
2. Results of the F2 generation required mathematical analysis.
a. The numerical ratios of crosses suggested that genes do not blend.
b. For example, the F2 offspring showed a 3:1 phenotypic ratio.
c. Mendel assumed that each sperm has an equal probability of fertilizing an egg. This can be seen most easily by using the Punnett square.
d. Thus, each new plant has three chances in four of having at least one dominant allele.
3. The Mendelian principle of segregation states that 2n organisms inherit two genes per trait, and each gene segregates from the other during meiosis such that each gamete will receive only one gene per trait.
E. Testcrosses
1. To support his concept of segregation, Mendel crossed F1 plants with homozygous recessive individuals.
2. A 1:1 ratio of recessive and dominant phenotypes supported his hypothesis.
F. The Concept of Independent Assortment
1. Mendel also performed experiments involving two traits—a dihybrid cross.
a. Mendel correctly predicted that all F1 plants would show both of the dominant alleles (example: all purple flowers and all tall).
b. Mendel wondered if the genes for flower color and plant height would travel together when two F1 plants were crossed.
2. We now know that genes located on nonhomologous chromosomes segregate independently of each other and give the same phenotypic ratio as Mendel observed—9:3:3:1.
3. The Mendelian principle of independent assortment states that each gene of a pair tends to assort into gametes independently of other gene pairs located on nonhomologous chromosomes.
G. Mendel’s Impact
1. He reported his ideas on heredity to the Brunn Society in 1865 and published them a year later.
2. Few people understood his principles or took note of them.
3. He died in 1884 unaware of the revolutionary impact his ideas would have.

III. Variations on Mendel’s Themes
A. Dominance Relations
1. In incomplete dominance, a dominant allele cannot completely mask the expression of another (example: snapdragons).
2. In codominance, both alleles are expressed in heterozygotes (example: ABO blood groups).
3. Whenever more than two forms of alleles exist at a given locus, it is called a multiple allele system. In this instance it results in four blood types: A, B, AB, and O.
B. Interactions Between Different Gene Pairs
1. One gene pair can influence other gene pairs, with their combined activities producing some effect on phenotype.
2. Comb shape in poultry is determined by the allelic combinations of two gene pairs that cooperate to produce phenotypes that neither can alone.
3. Hair color in mammals is determined by the masking effects of one gene pair on another (called epistasis).
a. In Labrador retrievers, one gene pair codes for the quantity of melanin produced while another codes for melanin deposition.
b. Still another gene locus determines whether melanin will be produced at all—lack of any produces an albino (recessive).
C. Multiple Effects of Single Genes
1. Pleiotropy occurs when a single gene affects unrelated aspects of the phenotype.
2. The gene for sickle-cell anemia codes for a variant form of hemoglobin. The altered hemoglobin in turn affects the shape of the red blood cells, which clump together and block capillaries. Impaired gas flow damages tissues.
D. Environmental Effects on Phenotype
1. Fur on the extremities of certain animals will be darker because the enzyme for melanin production will operate at cooler temperatures but is sensitive to heat on the rest of the body.
2. The buttercup plant will develop leaves of different shape depending on whether or not its leaves are submerged in water.
3. The internal environment can influence gene expression as when testosterone causes a deepening of the voice during male puberty.
E. Variable Gene Expression in a Population
1. A given phenotype can vary, by different degrees, from one individual to the next in a population.
a. This is the result of interactions with other genes, and environmental influences.
b. Eye color and hair color are examples.
2. Most traits are not qualitative but show continuous variation and are transmitted by quantitative inheritance.