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Chapter 14: Mendel and the Gene Idea. Phenotypic features of organisms are characters ; each can have a variety of traits (states, forms). e.g., eye color: blue, brown, etc.; blood type: A, B, O, etc.; height: tall, short, etc.
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Phenotypic features of organisms are characters; each can have a variety of traits (states, forms). e.g., eye color: blue, brown, etc.; blood type: A, B, O, etc.; height: tall, short, etc. The states of each are controlled to varying extents by genes (often in combination with environmental effect). For now, just consider the genetic part.
How are traits of organisms inherited (passed on from parents to offspring)? Mendel: European monk: experimented on inheritance of states of different traits in sweet pea plants in the 1860s. No one knew ANYTHING about DNA, alleles, function of chromosomes, meiosis, etc.
Inheritance was thought to be due to “blending”: some kind of “hereditary material” from the parents was thought to mix together to produce offspring. So: Expect phenotypes of offspring to be intermediate between those of parents. e.g., one parent short, one parent tall: children “should” be intermediate height. Sometimes works out that way; sometimes doesn’t.
All of Mendel’s studies were based on examining the phenotypes of his pea plants, and making inferences about the rules that govern inheritance, tracing inheritance of traits across generations. So really, he was tracking the inheritance of the genotypes (combinations of alleles) that determine phenotypes. Note: He didn’t use this terminology.
Mendel’s work showed that inheritance is “particulate”: hereditary material doesn’t simply blend together. Instead, there are “factors” (we call these genes) that can have different forms (we call these alleles) that are distinct units and maintain their identities across generations.
Mendel’s peas: Lots of variation in traits of different characters. e.g., can have purple or white flowers, round or wrinkled seeds, etc. And: Mendel started with “true breeding” strains of peas: ones that reliably produce only purple flowers, or only round seeds, etc.
Pea flowers contain male and female parts; can either self-pollinate (fertilize themselves) or pollinate others. Can control the crosses (which one pollinates which): Cut off the stamens (male parts) of one flower: now can’t produce the pollen (which contain sperm); collect pollen from another and put on the carpel (female part that contains eggs).
After fertilization, a zygote is formed inside a seed; divides and divides, seed germinates and grows into a plant. AND: Can cross those offspring to each other, look at the next generation, etc.
Mendel only looked at characters with discrete traits: take one form or another; no in between (e.g., purple vs. white flowers, round vs. wrinkled seeds, etc.). In other words: Traits that can be assigned to a limited number of categories (unlike a character like height, which varies continuously).
Mendel hybridized peas from different strains that were “true breeding” for some trait of a character: crossed them to each other. When studying crosses involving only traits of one character: monohybrid cross (two characters: dihybrid cross, etc.).
In a cross, the original ones bred together originally are the parental (P) generation. The offspring are the F1 generation. Their offspring are the F2 generation. And so on. Mendel did his crosses through to the F2 generation (at least).
By following to the F2 generation, Mendel figured out two key rules of inheritance: Law of Segregation 2) Law of Independent Assortment
Developing the Law of Segregation: If inheritance occurred by blending, then in a cross between true-breeding purple and white flowered plants (P generation) should produce F1 that have light purple (intermediate) flowers. BUT: purple X white gave F1 that were all just as purple as the purple parent.
Next, Mendel crossed the F1 offspring to each other. Result: About 3/4 of the F2 had purple flowers, but about 1/4 had white. Somehow the white trait kept its identity across generations and “re-emerged”. Same was true for various other characters and their traits: seed shape, seed color, pod form, etc.
Figure 14.2 Mendel tracked heritable characters for three generations
Mendel recognized that factors that have different forms (here, flower color) stay intact across generations (“particulate” inheritance). Now we know that the factors represent genes that have specific locations (loci; singular is locus) on specific chromosomes. The different forms (traits) are due to different alleles of the gene.
Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants
Mendel developed a 4-part hypothesis: Variations in inherited characters are due to “factors” that may produce different versions of the same characters. (Today: The character is encoded by a gene on a chromosome, and the different traits are due to different alleles of the gene).
2) For each character, an individual inherits one factor from the male parent and one from the female. Could get one each of the same version of the factor from each parent, or two different ones from the two parents.
Today: An individual gets one of each pair of homologous chromosomes from one parent, and one from the other. Both homologues may carry the same allele of the gene, or different ones.
3) If two different versions of the factor are present, one is dominant and controls the trait observed. The other version is recessive and can’t show its effects if a dominant factor is present. The trait produced by the recessive factor is only seen if two copies are present.
Today: One allele of a gene may be dominant over another. If an allele is dominant, then as long as at least one copy is present, the trait that it produces will be seen in the phenotype. Only see the trait associated with the recessive allele if there are two copies present.
NOTE: Inheritance doesn’t have to follow a dominant-recessive pattern, but all of the characters/traits that Mendel studied were simple dominant-recessive situations. This is how he was able to figure out his rules. We’ll discuss other possibilities later.
4) The two factors present in each parent segregate during gamete production. Each egg (ovum) or sperm cell gets only one of the two copies of the factor present in that parent. If both versions of the factor in a parent are the same, then all the gametes will have the same kind of factor. If they’re different, then half the gametes get one version of the factor and half get the other.
Today: The two copies of each gene segregate into gametes at meiosis, so each gamete gets only one copy of the gene. If that parent has two copies of the same allele (is homozygous for that gene) then all the gametes carry that allele. If that parent is heterozygous (has two different alleles), then half the gametes have one allele and half have the other.
Mendel’s “true breeding” strains of peas (e.g., produce only white flowers in every generation) were all homozygous for the gene controlling the character of interest. So they can only produce one type of gamete, carrying the “white” allele. But crossing them to another true-breeding strain produces F1 offspring that are heterozygous for the gene controlling the traits of the character.
The Law of Segregation describes the separation of genes and their alleles into separate gametes, now known to be the result of meiosis.
Note about terminology: When describing alleles of genes, usually the first letter of one version of the trait is used. If dominant vs. recessive, the first letter of the trait produced by the dominant allele is used and capitalized. The same letter is used for the recessive allele, but in lower case. So: P = purple allele, p = recessive
How dominance works in this case: The “purple” gene is involved in a biochemical pathway that produces purple coloration. The P (purple) allele produces a functional protein; the p (white) allele has a mutation that makes it nonfunctional. As long as there is at least 1 P allele, there is enough purple pigment to make the flower purple; if only 2 p alleles, get white.
How dominance works in this case: The “purple” gene is involved in a biochemical pathway that produces purple coloration. The P (purple) allele produces a functional protein; the p (white) allele has a mutation that makes it nonfunctional. As long as there is at least 1 P allele, there is enough purple pigment to make the flower purple; if only 2 p alleles, get white.
Note that a dominant allele does NOT have to be the most common allele in a population. For example, the allele for polydactyly (multiple fingers and toes) is dominant to the recessive allele for normal number. Even one copy causes polydactyly. But, the allele is quite rare in human populations.
Suppose that you have a pea plant with purple flowers. The phenotype is purple, but you don’t know if the genotype is PP or Pp (because P is dominant). How to figure this out? Testcross. Cross with a white flowered plant. For this one, you know the genotype has to be pp. If the purple parent is PP, all offspring will be Pp and have the purple phenotype. If Pp: about half the offspring will be Pp (purple phenotype) and half pp (white).
Mendel’s Law of Independent Assortment: Mendel tested whether traits of two DIFFERENT characters are inherited together, e.g., seed color AND seed shape. This involves a dihybrid cross: plants that differ in the traits of two characters.
For pea seeds: Yellow (Y) is dominant to green (y). Round (R) is dominant to wrinkled (r). So if a plant produces yellow round seeds, its genotype for the two genes is YYRR or YyRr; if green wrinkled, yyrr. Cross plants from true-breeding yellow round to true-breeding green wrinkled: YYRR X yyrr (next figure)
Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross
So, the genes that control color versus shape (and their alleles) are distributed independently of one another into the gametes (independent assortment). (Now know that the genes are on different chromosomes). The 9:3:3:1 F2 phenotypic ratio is characteristic of a dihybrid cross involving dominant and recessive alleles.
Segregation of alleles in gametes and fertilization follow basic laws of probability. Consider a coin flip: the chances of getting heads vs. tails are 50% : 50% Each flip is an independent event. The chances of two independent events happening together are multiplicative.
Flip coin once: chance of head = 50% Flip again: 50% Chance of getting two heads in two flips = 50% X 50% = 25% Similarly, for a heterozygous individual (say, Yy), the chances of producing Y versus y ova (eggs) or sperm are 50% : 50% Chance of Y sperm cell fertilizing Y ovum from another heterozygote is 50% X 50% = 25%
What if there is more than one way to get the same result? Then the probabilities are also additive. Coin flip: chance of one head + one tail in two flips = (50% X 50%) + (50% X 50%) = 25% + 25% = 50% (Because you could flip one head, then one tail OR one tail, then one head)
