A. Gregor Mendel�s Discoveries
1. Mendel brought an experimental and quantitative approach to genetics.
a. Around 1857, Mendel began breeding garden peas to study inheritance.
b. Pea plants have several advantages for genetics.
1. Pea plants are available in many varieties with distinct heritable features (characters) (color) with different variants (traits) (purple/white).
2. Some of the characters are true-breeding.
3. Some of the characters have either/or traits (no blending).
4. Another advantage of peas is that Mendel had strict control over which plants mated with which. (Fig. 38.2)
a. Each pea flower has male (stamens) and female (carpel) sexual organs.
b. In nature, pea plants typically self-fertilize, fertilizing ova with their own sperm.
c. However, Mendel could also move pollen from one plant to another to cross-pollinate plants.
5. In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties. (Fig. 14.1)
a. The true-breeding parents are the P generation and their hybrid offspring are the F1 generation.
b. Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation.
c. It was mainly Mendel�s quantitative analysis of F2 plants that revealed the two fundamental principles of heredity: the law of segregation and the law of independent assortment.
2. By the law of segregation, the two alleles for a character are packaged into separate gametes.
a. Monohybrid crosses
1. In a cross of a plant with true-breeding purple flowers with a plant with true-breeding white flowers the F1 hybrids all have purple flowers. (Fig. 14.2)
a. When Mendel allowed the F1 plants to self-fertilize, the F2 generation included both purple-flowered and white-flowered plants.
b. The white trait, absent in the F1, reappeared in the F2.
c. Based on a large sample size, Mendel recorded 705 purple-flowered F2 plants and 224 white-flowered F2 plants from the original cross.
d. This cross produced a three purple to one white ratio of traits in the F2 offspring.
2. Mendel reasoned that the heritable factor for white flowers was present in the F1 plants, but it did not affect flower color.
a. Purple flower is a dominant trait and white flower is a recessive trait.
b. The reappearance of white-flowered plants in the F2 generation indicated that the heritable factor for the white trait coexisted with the purple-flower factor in F1 hybrids but was not expressed.
3. Mendel found similar 3 to 1 ratios of two traits among F2 offspring when he conducted crosses for six other characters, each represented by two different varieties. (Table 14.1)
a. For example, when Mendel crossed two true-breeding varieties, one of which produced round seeds, the other of which produced wrinkled seeds, all the F1 offspring had round seeds, but among the F2 plants, 75% of the seeds were round and 25% were wrinkled.
4. Mendel developed a hypothesis to explain these results that consisted of four related ideas.
a. Idea 1: Alternative versions of genes (different alleles) account for variations in inherited characters.
1. Different alleles vary somewhat in the sequence of nucleotides at the specific locus of a gene.
2. The purple-flower allele and white-flower allele are two DNA variations at the flower-color locus. (Fig. 14.3)
b. Idea 2: For each character, an organism inherits two alleles, one from each parent.
1. A diploid organism inherits one set of chromosomes from each parent.
2. Each diploid organism has a pair of homologous chromosomes and therefore two copies of each gene (2 alleles).
3. These homologous alleles may be identical, as in the true-breeding plants of the P generation.
4. Alternatively, the two alleles may differ.
5. In the flower-color example, the F1 plants inherited a purple-flower allele from one parent and a white-flower allele from the other.
c. Idea 3: If an organism has 2 different alleles, then one, the dominant allele, is fully expressed in the organism�s appearance.
1. The other, the recessive allele, has no noticeable effect on the organism�s appearance.
2. Mendel�s F1 plants had purple flowers because the purple-flower allele is dominant and the white-flower allele is recessive.
d. Idea 4: The two alleles for each character segregate (separate) during gamete production.
1. This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis.
2. If an organism has identical alleles for a particular character, then that allele exists as a single copy in all gametes.
3. If different alleles are present, then 50% of the gametes will receive one allele and 50% will receive the other.
4. The separation of alleles into separate gametes is summarized as Mendel�s law of segregation.
5. A Punnett square predicts the results of a genetic cross between individuals of known genotype.
a. Draw purple/white cross-ask what gametes can be made from PP
b. Have someone do seed shape on board
b. Genetics vocabulary
1. An organism with two identical alleles for a character is homozygous for that character.
2. Organisms with two different alleles for a character is heterozygous for that character.
3. A description of an organism�s traits is its phenotype.
4. A description of its genetic makeup is its genotype.
c. Testcrosses (Fig. 14.5)
1. Two organisms can have the same phenotype but have different genotypes if one is homozygous dominant and the other is heterozygous.
2. For flower color in peas, both PP and Pp plants have the same phenotype (purple) but different genotypes (homozygous and heterozygous).
3. The only way to produce a white phenotype is to be homozygous recessive (pp) for the flower-color gene.
4. A testcross, breeding a homozygous recessive with dominant phenotype, but unknown genotype, can determine the identity of the unknown allele.
3. By the law of independent assortment, each allele of a pair segregates into gametes independently.
a. Mendel conducted other experiments in which he followed the inheritance of two different characters, a dihybrid cross.
b. In one dihybrid cross experiment, Mendel studied the inheritance of seed color and seed shape.
1. The allele for yellow seeds (Y) is dominant to the allele for green seeds (y).
2. The allele for round seeds (R) is dominant to the allele for wrinkled seeds (r).
3. Mendel crossed true-breeding plants that had yellow, round seeds (YYRR) with true-breeding plants that had green, wrinkled seeds (yyrr).
c. One possibility is that the two characters are transmitted from parents to offspring as a package. (Fig. 14.7)
1. The Y and R alleles and y and r alleles stay together.
2. If this were the case, the F1 offspring would produce yellow, round seeds.
3. The F2 offspring would produce two phenotypes in a 3:1 ratio, just like a monohybrid cross.
4. This was not consistent with Mendel�s results.
d. An alternative hypothesis is that the two pairs of alleles segregate independently of each other.
1. The presence of one specific allele for one trait has no impact on the presence of a specific allele for the second trait.
2. The F1 offspring would still produce yellow, round seeds.
3. However, when the F1 plants produced gametes, genes would be packaged into gametes with all possible allelic combinations.
4. Four classes of gametes (YR, Yr, yR, and yr) would be produced.
5. These combinations produce four distinct phenotypes in a 9:3:3:1 ratio.
6. This was consistent with Mendel�s results.
e. Mendel repeated the dihybrid cross experiment for other pairs of characters and always observed a 9:3:3:1 phenotypic ratio in the F2 generation.
f. The independent assortment of each pair of alleles during gamete formation is now called Mendel�s law of independent assortment.
B. Extending Mendelian Genetics
1. The relationship between genotype and phenotype is rarely simple.
a. In the 20th century, geneticists have extended Mendelian principles not only to diverse organisms, but also to patterns of inheritance more complex than Mendel described.
b. Mendel chose a system that was relatively simple genetically.
1. Each character is controlled by a single gene.
2. Each gene has only two alleles, one of which is completely dominant to the other.
2. Incomplete dominance
a. The heterozygous F1 offspring of Mendel�s crosses always looked like one of the parental varieties because one allele was dominant to the other (complete dominance).
b. However, some alleles show incomplete dominance where heterozygotes show a distinct intermediate phenotype, not seen in homozygotes.
1. Offspring of a cross between heterozygotes will show three phenotypes: both parentals and the heterozygote.
2. The phenotypic and genotypic ratios are identical, 1:2:1. (Fig. 14.9)
3. What is a dominant allele?
a. Dominant alleles do not somehow subdue a recessive allele.
1. It is in the pathway from genotype to phenotype that dominance and recessiveness come into play.
2. For example, wrinkled seeds in pea plants with two copies of the recessive allele are due to the accumulation of monosaccharides and excess water in seeds because of the lack of an enzyme that converts sugar to starch.
a. The swollen seeds wrinkle when they dry.
b. Both homozygous dominants and heterozygotes produce enough enzyme to convert all the monosaccharides into starch and form smooth seeds when they dry.
b. Because an allele is dominant does not necessarily mean that it is more common in a population than the recessive allele.
1. For example, polydactyly, in which individuals are born with extra fingers or toes, is due to an allele dominant to the recessive allele for five digits per appendage.
2. However, the recessive allele is far more prevalent than the dominant allele in the population.
a. 399 individuals out of 400 have five digits per appendage.
c. Dominance/recessiveness relationships have three important points.
1. They range from complete dominance, though various degrees of incomplete dominance, to codominance.
2. They reflect the mechanisms by which specific alleles are expressed in the phenotype and do not involve the ability of one allele to subdue another at the level of DNA.
3. They do not determine or correlate with the relative abundance of alleles in a population.
d. Codominance is a type of inheritance in which two alleles affect the phenotype in separate, distinguishable ways.
1. For example, the A, B, and AB blood groups of humans are due to the presence of two different carbohydrates on the surface of red blood cells.
2. People of group A have one type of carbohydrate on their red blood cells, people of group B have the other type, and people of group AB have both carbohydrates present.
4. Multiple alleles
a. Most genes have more than two alleles in a population.
b. The ABO blood groups in humans are determined by three alleles, IA, IB, and i.
1. Both the IA and IB alleles are dominant to the i allele
2. The IA and IB alleles are codominant to each other.
c. Because each individual carries two alleles, there are six possible genotypes and four possible blood types.
1. Individuals that are IA IA or IA i are type A and have type A carbohydrates on the surface of their red blood cells.
2. Individuals that are IB IB or IB i are type B and have type B carbohydrates on the surface of their red blood cells.
3. Individuals that are IA IB are type AB and have both type A and type B carbohydrates on the surface of their red blood cells.
4. Individuals that are ii are type O and have neither carbohydrate on the surface of their red blood cells.
d. If the donor�s blood has an A or B oligosaccharide that is foreign to the recipient, antibodies in the recipient�s blood will bind to the foreign molecules, cause the donated blood cells to clump together, and can kill the recipient.
5. Pleiotropy
a. The genes covered so far affect only one phenotypic character.
b. However, most genes are pleiotropic, affecting more than one phenotypic character.
1. For example, the wide-ranging symptoms of sickle-cell disease are due to a single gene. (Fig. 14.15)
6. Polygenic inheritance
a. Some characters do not fit the either/or type that Mendel studied but vary in a population along a continuum.
b. These are usually due to polygenic inheritance, the additive effects of two or more genes on a single phenotypic character.
1. For example, skin color in humans is controlled by at least three different genes.
2. An AABBCC individual is dark and aabbcc is light.
3. A cross between two AaBbCc individuals (intermediate skin shade) would produce offspring covering a wide range of shades. (Fig. 14.12)
4. Individuals with intermediate skin shades would be the most likely offspring, but very light and very dark individuals are possible as well.
5. The range of phenotypes forms a normal distribution.
7. Phenotype depends on environment and genes.
a. A single tree has leaves that vary in size, shape, and greenness, depending on exposure to wind and sun.
b. For humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests.
c. Even identical twins, accumulate phenotypic differences as a result of their unique experiences.
8. Phenotype can be used to describe single characters, but it is also used to describe all aspects of an organism.
a. Genotype can refer not just to a single genetic locus, but also to an organism�s entire genetic makeup.
b. An organism�s phenotype reflects its overall genotype and unique environmental history.