©CNRI/Photo and the other allele is not.Alleles are often

©CNRI/Photo Researchers, Inc.MAIN IDEA Genes influence the development of traits.6AYou may have heard about the Human Genome Project. Its goal was to find out the sequence of the 3 billion nucleotide pairs that make up a human’s genome.

A genome is all of an organism’s genetic material. Unless you have an identical twin, you have a unique genome that determines all of your traits. Some of your traits, such as the color of your eyes, can be seen.

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Other traits, such as the exact chemical makeup of your eyeball, cannot be seen.In genetics, we often focus on a single trait or set of traits. A genome is all of an organism’s genes, but a genotype (JEHN-uh-TYP) typically refers to the genetic makeup of a specific set of genes. The genotype of a pea plant includes both of the genes that code for flower color, even if one of these genes is masked. In contrast, the physical characteristics, or traits, of an individual organism make up its phenotype (FEE-nuh-TYP). A pea plant with purple flowers has a phenotype for purple flowers. The plant might have a hidden gene for white flowers, but that does not matter to its phenotype.Dominant and Recessive Alleles If an organism is heterozygous for a trait, which allele will be expressed? That is, if a plant has one allele for purple flowers and one for white flowers, what color will the flowers be? As Mendel learned, one allele may be dominant over another allele.

A dominant allele is the allele that is expressed when two different alleles or two dominant alleles are present. A recessive allele is the allele that is expressed only when two copies are present. In Mendel’s experiments, the allele for purple flowers was dominant to the allele for white flowers. All F 1 plants were purple even though they had only one allele for purple flowers.Sometimes the word dominant is misunderstood. A dominant allele is not necessarily better or stronger than a recessive allele.

It does not necessarily occur most often in the population. An allele is dominant in a heterozygote simply because it is expressed and the other allele is not.Alleles are often represented on paper with individual letters. An organ- ism’s genotype for a trait can be shown with two letters—one per allele. Uppercase letters are used for dominant alleles, and lowercase letters are used for recessive alleles. For example, the dominant allele for height in pea plants is written as T , for tall. The recessive allele for short plants is written as t .

CONNECT TO EXCEPTIONS TOMENDEL’S LAWS Mendel’s theory of inheritance cannot explain all patterns of inheritance. As you will learn in the chapterExtending Mendelian Genetics, incomplete dominance, codominance, polygenic traits, and environmental influences all provide exceptions.FIGURE 4.1 Polydactyly is the condition of having more than the typical number of fingers or toes. The allele for polydactyly is dominant.

VISUAL VOCAB A dominant allele is expressed when two different alleles are present.A recessive allele is expressed only when two copies are present.wrinkledrecessivegenotype phenotyperounddominantgenotyperecessivewrinkled wrinkledrecessivephenotypeChapter 6: Meiosis and Mendel 175176177Aalleles aA alleles aA A A aA aa a©John Innes ArchivesTraits and Probability KEY CONCEPT The inheritance of traits follows the rules ofVOCABULARYPunnett squaremonohybrid crosstestcrossdihybrid crosslaw of independentassortmentprobabilityprobability.MAIN IDEAS Punnett squares illustrate genetic crosses. A monohybrid cross involves one trait.

A dihybrid cross involves two traits. Heredity patterns can be calculated with probability.Connect to Your World If you have tried juggling, you know it can be a tricky thing. Keeping three flaming torches or clubs in motion at the same time is a challenge. Trying to keep track of what organism has which genotype and which gamete gets which allele can also be a lot to juggle. Fortunately, R. C. Punnett developed a method to keep track of all of the various combinations graphically.

MAIN IDEA Punnett squares illustrate genetic crosses.3F, 6F, 6GShortly after Mendel’s experiments became widely known among scientists, a poultry geneticist named R. C. Punnett, shown in FIGURE 5.1, developed the Punnett square. A Punnett square is a grid system for predicting all possible genotypes resulting from a cross.

The axes of the grid represent the possible gamete genotypes of each parent. The grid boxes show all of the possible geno- types of offspring from those two parents. Because segregation and fertilization are random events, each combination of alleles is as likely to be produced as any other.

By counting the number of squares with each genetic combination, we can find the ratio of genotypes in that generation. If we also know how the genotype corresponds to the phenotype, we can find the ratio of phenotypes in that generation as well. Let’s briefly review what you’ve learned about meiosis and segregation to examine why the Punnett square is effective.

Both parents have two alleles for each gene. These alleles are represented on the axes of the Punnett square. During meiosis, the chromosomes—and, therefore, the alleles—are separated.

R. C. Punnett FIGURE 5.1 R. C. Punnett devel- oped the Punnett square as a way to illustrate genetic crosses.VISUAL VOCAB The Punnett square is a grid systemfor predicting possible genotypes ofoffspring.

possiblegenotypesof offspringParent 2Parent 13F, 6F, 6G3F research and describe the history of biology and contributions of scientists; 6F predict possible outcomes of various genetic combinations such as monohybrid crosses, dihybrid crosses and non-Mendelian inheritance; 6G recognize the significance of meiosis to sexual reproductionChapter 6: Meiosis and Mendel 1776.5178179180181182183184185186187CHAPTERReviewINTERACTIVE ReviewHMDScience.comGO ONLINE Review Games • Concept Map • Section Self-Checks CHAPTER VOCABULARYReviewing Vocabulary Visualize VocabularyFor each term below, use simple shapes, lines, or arrows to illustrate its meaning. Below each picture, write a short caption. Here’s an example for the term diploid: 1. gene2. fertilization3.

crossing over 4. genetic linkage5. haploidGREEK WORD ORIGINS 6. The word meiosis comes from a Greek word meaning “to diminish,” or make less. How does this word’s origin relate to its meaning?7. The word haploid comes from the Greek word haplous, which means “single.” The word diploid comes from the Greek word diplous, which means “double.” Explain how these two terms’ meanings relate to their origins.

8. The Greek prefix homo- means “one and the same.” How does this relate to the words homologous and homozygous ?Compare and ContrastDescribe one similarity and one difference between the two terms in each of the following pairs.9. monohybrid cross, dihybrid cross10. heterozygous, homozygous 11.

genotype, phenotypeReviewing MAIN IDEAS 12. Each of your cells has a set of chromosomes, including autosomes and sex chromosomes. Explain the main differences between these two types of chromosomes.13. A fruit fly has diploid cells with 8 chromosomes. Explain how many chromosomes are in its haploid gametes.14.

Meiosis is a continuous process, but we can think of it as taking place in two stages, meiosis I and meiosis II. How do the products of meiosis I differ from those of meiosis II? 6G15. The foundation for our modern study of genetics began with Gregor Mendel, who studied pea plants. What were Mendel’s two main conclusions about how traits are passed between generations? 3F16. How did Mendel’s use of purebred plants—for example, purebred white- and purebred purple-flowered peas— contribute to his understanding of inheritance? 6F17. How does the homozygous condition differ from the heterozygous condition? In your answer, use the terms gene, homologous chromosome, and allele.

18. What does each of the following parts of a Punnett square represent: (a) the entries on each axis of the grid and (b) the entries in the four squares within the grid?19. How did the results of Mendel’s dihybrid crosses lead him to formulate the law of independent assortment? 3F, 6F20. How does crossing over during meiosis I increase genetic diversity? 6GREADING TOOLBOXDiploid cells have two copies ofeach chromosome.Chapter 6: Meiosis and Mendel 18766.1 somatic cellgametehomologous chromosomeautosomesex chromosomesexual reproductionfertilizationdiploidhaploidmeiosis6.2 gametogenesisspermeggpolar body6.3 traitgeneticspurebredcrosslaw of segregation6.

4 geneallelehomozygousheterozygousgenomegenotypephenotypedominantrecessive6.5 Punnett squaremonohybrid crosstestcrossdihybrid crosslaw of independent assortmentprobability6.6 crossing overgenetic linkage188189190CHAPTER(t) ©2006 Jupiterimages; (c) ©Diana Koenigsberg/Getty Images; (b) ©Gazimal/Getty ImagesExtending Mendelian GeneticsBIG IDEAMendel’s laws of inheritance do not account for the expression of all traits, which may be influenced by the number of genes involved, linkages with other genes, or the environment in which the organism lives.

ONLINE BIOLOGYHMDScience.comONLINE Labs?QuickLabSex-Linked Inheritance?Codominance?Pedigree Analysis?Incomplete Dominance?Human Heredity?Examining Banding Patterns in Polytene Chromosomes?Video LabPlant Genetics190Unit 3: Genetics77. 1Chromosomes and Phenotype7.2Complex Patterns of inheritance6F7. 3gene Linkage and Mapping 3F, 6HData Analysis CONSTRUCTING BAR GRAPHS 2G7. 4Human Genetics and Pedigrees 6F, 6h191192193194195196ContentsBookmarksNotesPage ViewSearchMore