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4.1 Early Beliefs and MendelThe idea that biological traits are inherited existed long before the mechanismsof inheritance and gene interaction were understood. Stone tablets crafted by theBabylonians 6000 years ago show the pedigrees of successive generations ofchampion horses. Other carvings from the same period show the artificial cross-pollination of date palms. Early records kept by Chinese farmers provide evidenceof methods used for improving different varieties of rice. The selection of desiredtraits was based on keen observation and, to a great extent, trial and error.

Early naturalists made assumptions about some incredible cross-specieshybrids. The giraffe, for example, was thought to have been a cross between aleopard and a camel. (The fact that camels and leopards are not compatible didnot seem to deter the theory.) The banana was thought to have been a hybrid ofthe acacia and the palm.

Humans also mated selectively to produce desirable traits. The ancientEgyptians encouraged the intermarriage of royalty to preserve bloodlines. Forexample, Cleopatra married her younger brother. Plato, a Greek philosopher ofthe early 4th century B.C., called for the segregated mating of the elite. To main-tain a line of strong warriors, the ancient Spartans practised infanticide, killingbabies with undesirable characteristics.

Pioneer of Genetics: Gregor Mendel

One of the classic scientific experiments on inheritance was performed by anAustrian monk named Gregor Mendel (1822–1884) during the mid-19th century(Figure 1). Mendel’s work with garden peas not only explained the mechanism ofgene inheritance in plants, but provided a basis for understanding heredity in gen-eral. When Mendel’s work was rediscovered many years later, it provided themissing piece in the theory of how organisms survive and reproduce.

Why did Mendel choose the garden pea on which to perform his work? First,he observed that garden peas have a number of characteristics that are expressedin one of two ways (see Figure 2). For example, some garden peas produce green

hybrids: offspring that differ from theirparents in one or more traits. Interspecifichybrids result from the union of two differentspecies.

Figure 1

Gregor Mendel was an Austrian monk whoseexperiments with garden peas laid the founda-tion for the science of genetics.

characteristics dominant recessive dominant recessivetrait trait trait trait

seed shape

seed colour

pod shape

pod colour

flower colour

round

yellow

inflated

green

purple

wrinkled

green

constricted

yellow

white

flowerposition

stemlength

side of stem

tall

end of stem

short

Figure 2

The seven characteristics Mendel studiedin his experiments with garden peas. Flowercolour and seed colour are correlated. Plantswith white flowers produce seeds that areyellow, and plants with violet-purple flowersproduce seeds that are green.

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seeds (peas), while others produce yellow seeds. Some plants are tall, while othersare short. Mendel also noticed different flower positions on the stem and dif-ferent flower colours. The fact that there were only two ways for each trait to beexpressed would make it easy to see which traits had been inherited from gener-ation to generation.

A second reason for using garden peas is the way the plant reproduces.Garden peas are both self-fertilizing and cross-fertilizing. Fertilization occurswhen pollen produced by the stamen, the male part of the plant, attaches to thepistil, the female part (Figure 3). The pistil consists of the stigma, style, andovary. The pollen grains fertilize the egg cells in the ovary. This process is calledpollination. In self-fertilization, pollination occurs within one flower and thetraits of the offspring are easily predicted. Mendel cross-pollinated the peaplants rather than allowing them to self-pollinate. He made sure to use pure-breeding plants, that is, plants that always produce identical offspring. Forexample, tall plants produce only tall plants. If any offspring in any generationwas not tall, then Mendel did not consider the parent plant to be pure and didnot use it in his experiment. He transferred the pollen from one plant to thepistil of another plant, thus combining the male and female sex cells of differentplants. To ensure that the recipient plant didn’t pollinate itself, he first removedits anthers. The pollen present then had to originate from the donor plant,resulting in seeds produced from cross-pollinated plants (Figure 4).

Mendel’s Experiments

Mendel’s predecessors had hypothesized that the crossing of different traitswould create a blend. According to this theory, crossing a plant that producedround seeds with one that produced wrinkled seeds would result in slightly wrin-kled seeds. However, Mendel proved that this was not the case. When he crossedthe pollen from a plant that produced round seeds with the eggs of one that pro-duced wrinkled seeds, the offspring were always round. Did this mean that thepollen determines the seed coat? To test this idea, Mendel crossed the pollen froma wrinkled seed plant with the eggs from a round seed plant. Once again, all theoffspring were round. In fact, the round trait dominated, regardless of whetherthe trait came from the male (pollen) parent or the female (seed) parent.

Mendel repeated the procedure for other characteristics. He discovered thatone trait always dominated another, whether the sex cell came from the male orfemale part of the plant. Tall plants produced tall offspring when cross-pollinatedwith short plants; likewise, plants that had yellow seeds produced offspring withyellow seeds when cross-pollinated with plants that had green seeds. Mendel rea-soned that things called factors control the traits of a plant. The factors were later

filament anther pollen

style ovary stigma

pistil

Figure 3

The structure of a flower

transfer pollen from pollenparent to seed parent

remove anthersfrom seed parent

Figure 4

The donor plant is also known as the pollenparent and the recipient is also known as theseed parent.

stamen

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called genes. He assumed that the genes control the inheritance of particulartraits, such as seed colour and plant stem height. He also realized that there arealternate forms of a gene. Today, the alternate forms of a gene are called alleles.Green and yellow are expressions of the different alleles for seed colour. Tallstems and short stems are expressions of the different alleles for stem height. Ingarden peas, the traits that were expressed most often were considered to bedominant and those expressed less frequently were recessive. The allele for ayellow seed is dominant over the allele for a green seed; the allele for tall stemsis dominant over the allele for short stems.

Mendel cross-pollinated many plants and kept track of all the results. Foreach type of cross, he recorded the number of offspring that exhibited the domi-nant trait versus the recessive trait. He created a system of symbols to show whattraits were passed to offspring. In this system, letters are used to represent traits.Uppercase letters stand for dominant traits, and lowercase letters stand for reces-sive traits. For the dominant trait of yellow seeds, Y represents the allele for yellowseeds; y represents the allele for green seeds, the recessive trait. Today, Mendel’ssystem is still in use.

Mendel continued his experimentation by crossing two hybrid plants withround seeds from the first generation. He referred to the first generation as filialgeneration one, or F1 generation. The word filial comes from the Latin for son.Both of these F1 plants contain R and r alleles, one from each of their parents.This makes them hybrids. Remember, the R represents the round allele, while ther represents the wrinkled allele.

You might predict an equal number of round and wrinkled offspring in thesecond, or F2, generation. However, this is not the ratio Mendel discovered whenthe two hybrids were crossed. He was astonished to find that 75% of the offspringexpressed the dominant round trait, while only 25% expressed the wrinkled trait.How can these results be explained?

Figure 5 shows what happens when the sex cells, or gametes, from the F1generation recombine to form an F2 generation. All members of the F1 genera-tion are round, but wrinkled offspring appear during the F2 generation. Anymembers of the F2 generation with an R allele will be round because the roundallele is dominant over the wrinkled allele. To be wrinkled, the offspring must

alleles: two or more alternate forms of agene. The alleles are located at the sameposition on one of the pairs of homologouschromosomes.

dominant: alleles of this type determinethe expression of the genetic trait in offspring

recessive: alleles of this type are overruledby dominant alleles, which determine thegenetic trait

Rr Rr

R

RR Rr Rr rr

round round round wrinkled

Meiosis occurs.Each gamete has one

of the homologouschromosomes.

F2 generation inheritsalleles from the gametes

of the F1 generation.

r R r

Figure 5

The result of crossing two hybrid pea plantswith round seeds from the first generation

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Creating a Personal ProfileTable 1 lists human traits controlled by dominant and recessive alleles.

Try ThisActivity

Table 1

Trait Dominant Recessiveeye colour brown or black or green blue or greyhair colour brown or black blonde or redhairline pointed on forehead straight across foreheadfreckles present absentearlobe suspended attached to headhair texture curly straighteyesight near or farsighted normal visioneyelashes long shortnose line convex tip concave or straightfingers 6 fingers 5 fingersRh blood factor positive Rh factor negative Rh factorear rim curled rim not curled rimthumb joint last joint bends out last joint is straightfinger hair present absentfolded hands left thumb over right right thumb over lefttongue rolling can be rolled into U shape cannot be rolledclenched fist two wrist cords three wrist cordschin dimple dimple in middle no dimpleblood type type A, B, AB type Oeyes astigmatism no astigmatism

Copy Table 2. Use information in Table 1 to complete your personalprofile. Which additional traits can you include?

Table 2

Trait (use the Appearance or Dominant or Possibleletter indicated) physical condition recessive genetic makeupeye colour E/e ? ? ?hairline L/l ? ? ?earlobe T/t ? ? ?ear rim R/r ? ? ?freckles F/f ? ? ?thumb joint J/j ? ? ?finger hair P/p ? ? ?tongue rolling Y/y ? ? ?folded hands D/d ? ? ?nose line N/n ? ? ?hair colour H/h ? ? ?chin dimple G/g ? ? ?clenched fist K/k ? ? ?