Reading Essentials for Biology Glencoe (794133), страница 26
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When pollen is transferred from the malereproductive organ to the female reproductive organ, it is calledpollination. Garden peas are self-pollinators. That means thepollen from a flower pollinates the female sex cells within thatsame flower. The seeds that develop carry the traits of that plant.This was important for Mendel. When he wanted to have thegametes of different plants unite, Mendel opened the petals of aflower and removed the male reproductive organs on one plant,and dusted the female reproductive organ with the pollen from adifferent plant. This is cross-pollination. The seeds that developfrom cross-pollination have traits of the two different plants.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Why Mendel SucceededNameDateClassSection10.1Mendel’s Laws of Heredity, continuedHow did Mendel proceed?To get accurate results, Mendel needed to carefully control hisexperiment.
He studied one trait at a time to control variables.He decided to study how height in pea plants is passed from parent to offspring. He used plants that were true breeding. Thatmeant the plants always passed the same trait from parent to offspring. He took pollen from a true-breeding tall pea plant andcross-pollinated a true-breeding short pea plant.Mendel’s Monohybrid CrossesCrossing a tall pea plant with a short pea plant produced offspring called hybrids.
A hybrid is the offspring of parents thathave different forms of a trait, such as tall and short height. Thefirst hybrids that Mendel produced are known as monohybridcrosses. Mono means one. Since the parent plants that Mendelused differed from each other by only one trait—height—theoffspring are called monohybrids.2. What is a monohybrid?________________________________________________________________________Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What were Mendel’s results?The results of Mendel’s experiment were interesting.
Mendelcross-pollinated a six-foot tall pea plant with a pea plant less thantwo feet tall. When he planted the seeds from this cross, all of theoffspring grew as tall as the taller parent plant. The short trait didnot appear at all.Mendel allowed the offspring, known as the first generation,to self-pollinate. Mendel planted the seeds from the first generation. There were more than 1000 plants in the second generation.Three-fourths of the plants were as tall as the tall parent plant.One-fourth of the plants were as tall as the short parent plant.The short trait had reappeared. The ratio of tall to short plantsin the second generation was three tall plants for every oneshort plant.How do you identify the generations?In genetics, abbreviations are used for the generations.
Theoriginal parents are known as the P1 generation. P stands for“parent.” The offspring of the parents are called the F1 generation.The F stands for “filial,” which means son or daughter. Whenyou cross two F1 plants with each other their offspring are theF2 generation, the second filial generation.READING ESSENTIALS3. List the abbreviations foreach generation and tellwhat they stand for.________________________________________________________________________Chapter 10101NameDateClassSection10.1Mendel’s Laws of Heredity, continuedMendel did similar monohybrid crosses with seven pairs oftraits. He used traits such as whether a seed was wrinkled orround, yellow or green.
In every case he found that one trait ofa pair did not appear in the F1 generation. Then the trait reappeared in one fourth of the F2 plants. For example, when hecrossed a plant that produced round seeds with a plant that produced wrinkled seeds, all of the offspring (F1) had round seeds.But when the plants of the F1 generation self-pollinated, onefourth of their offspring (F2) had wrinkled seeds.From these results Mendel determined that each organism hastwo factors that control each of its traits. Today we know thatthese factors are genes. Genes exist in alternative forms, such as,tall and short, round and wrinkled. The alternative forms arecalled alleles (uh LEELZ). Mendel’s pea plants had two allelesfor height.
A plant could have two alleles for tallness, two allelesfor shortness, or one allele for tallness and one allele for shortness. The organism receives or inherits one allele from the femaleparent and one allele from the male parent.________________________________________________________________________102Chapter 10But why did the offspring of a short plant crossed with a tallplant all grow into tall plants? Mendel called the observed traitdominant and the trait that disappeared recessive.
A dominantallele will mask, or cover up, a recessive allele. The allele for tallplants is dominant to the allele for short plants. The plants thathad a tall and short allele were tall because the tall allele is dominant and the short allele is recessive. The plants with two allelesfor tallness were tall. The plants with two alleles for shortnesswere short. In the F1 generation of plants each plant had one tallallele and one short allele.
That is why the offspring of a tall andshort plant were all tall.When the results of crosses are written down, scientists use thesame letter for different alleles of the same trait. An uppercaseletter is used for the dominant allele, and a lowercase letter isused for a recessive allele. So in writing down the results ofMendel’s experiment, T is used for the dominant allele for tallness, while t is used for the recessive allele for shortness. Thedominant allele is always written first.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What is dominance?4.
If a plant has a dominantallele for height and arecessive allele for height,which allele will determine the height of theplant?NameDateClassSection10.1Mendel’s Laws of Heredity, continuedMendel took the facts that he learned from his experiments tocreate rules or laws to explain heredity. The first of his laws iscalled the law of segregation.
This law says that every organismhas two alleles of each gene and when gametes are produced thealleles separate. Each gamete receives one of these alleles. Duringfertilization, these gametes randomly pair to produce four combinations of alleles.5. Explain the differencebetween phenotype andgenotype.________________________________________________Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Phenotypes and GenotypesYou cannot always tell by looking at anorganism what genes it might pass on.Sometimes tall plants crossed with each otherproduce both tall and short offspring.Sometimes a short plant and a tall plant produce all tall offspring.Two organisms can look alike but have different allele combinations. The way an organismlooks and behaves is called its phenotype(FEE noh tipe).
The phenotype of a tall plant istall. The plant could have an allele combinationof TT or Tt. The allele combination of anorganism is called the genotype (JEE noh tipe).The genotype cannot always be determinedeven if you know the phenotype.________________________P1 generationShort plant(homozygous)Tall plant(homozygous)TTttF1 generationAll tall plantsTtWhat are homozygous and heterozygous alleles?An organism is homozygous (hoh moh ZI gus) for a trait ifthe two alleles for the trait are the same. So a plant with two alleles for tallness (TT ) would be homozygous for the trait of height.Remember that since tallness is dominant, a TT plant is homozygous dominant for height.
A short plant always has two alleles forshortness (tt). Therefore, a short plant would be homozygousrecessive for height.What if the two alleles are not the same? An organism is heterozygous (heh tuh roh ZI gus) for a trait if its two alleles for thetrait are different from each other. A tall plant that has one allelefor tallness and another allele for shortness (Tt) is heterozygousfor the trait of height.READING ESSENTIALS6. Explain the differencebetween homozygousand heterozygous.________________________________________________________________________Chapter 10103NameDateClassSection10.1Mendel’s Laws of Heredity, continuedIn Mendel’s first set of experiments he used monohybridcrosses.
He used plants that differed from each other in only onetrait. Later, Mendel used pea plants that differed from each otherin two traits. A cross involving two different traits is called a dihybrid cross. Di means two. Mendel wanted to know whether, in adihybrid cross, the two traits would stay together in the next generation, or whether they would be passed on independently ofeach other.In this experiment Mendel observed the traits for seed colorand seed shape.
He knew from previous experiments that theyellow seed color and round seed shape were dominant. He crosspollinated true-breeding round, yellow seeds (RRYY ) with truebreeding wrinkled, green seeds (rryy). This created a dihybridcross. Mendel discovered that in the F1 generation all the plantsproduced round yellow seeds. That was not surprising sinceround and yellow are dominant traits.When the F1 plants self-pollinated and produced offspringthere were plants with round yellow seeds and plants with wrinkled green seeds. That was not surprising either. But Mendel alsofound plants producing two other seed types, round green, andwrinkled yellow. When Mendel sorted and counted the plants ofthe F2 generation he found a ratio of phenotypes—9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green.What is the law of independent assortment?7.
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