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Patterns of Inheritance. Chapter 13. LEARNING OBJECTIVE 1. Define the following terms relating to genetic inheritance: dominant and recessive ; homozygous and heterozygous ; genotype and phenotype. KEY TERMS. DOMINANT Said of an allele that is always expressed when it is present
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Patterns of Inheritance Chapter 13
LEARNING OBJECTIVE 1 • Define the following terms relating to genetic inheritance: dominantand recessive; homozygousand heterozygous; genotypeand phenotype
KEY TERMS • DOMINANT • Said of an allele that is always expressed when it is present • RECESSIVE • Said of an allele that is not expressed in the presence of a dominant allele
KEY TERMS • HOMOZYGOUS • Possessing a pair of identical alleles for a particular gene • HETEROZYGOUS • Possessing a pair of unlike alleles for a particular gene
KEY TERMS • GENOTYPE • Genetic makeup of an individual • PHENOTYPE • Physical expression of an individual’s genes
(c) Anthers snipped from the flower. (a) Reproductive structures of flowers are enclosed by petals. (d) Pollen from a different flower brushed onto tip of carpel. Anther Carpel Carpel (b) Petals opened to reveal male and female reproductive structures. (e) Fertilized carpel produces seeds, which are planted. (f) Offspring are observed. Fig. 13-1, p. 256
Flower color Seed color Pod color Pod shape Yellow Green Purple Seed shape Round Wrinkled White Green Yellow Pinched Inflated Stem height Flower position Axial Terminal Tall Short Fig. 13-2, p. 257
Parents X Tall plant Short plant Fig. 13-3a, p. 258
Tall plant Tall plant Tall plant Tall plant First generation F1 offspring All tall plants Fig. 13-3b, p. 258
Tall plant Tall plant Tall plant Short plant Second generation F2 offspring 3 tall:1 short Fig. 13-3c, p. 258
LEARNING OBJECTIVE 2 • Distinguish among chromosomes, genes, and alleles
KEY TERMS • LOCUS • Location of a particular gene on a chromosome • ALLELE • One of two or more alternative forms of a gene
Genes • Genes occur in pairs and are inherited as parts of chromosomes • Alleles are carried on homologous chromosomes • may be alike (homozygous) or unlike (heterozygous)
Homologous chromosomes METAPHASE I T t t T METAPHASE II T T t t T T t t T t Fig. 13-4, p. 259
Homologous chromosomes METAPHASE I T T t t METAPHASE II t t T T T t t T T t Stepped Art Fig. 13-4, p. 259
Alleles controlling height: Gene loci A pair of alleles Short Tall Alleles controlling flower color: Red White These genes are not alleles (a) These chromosomes are nonhomologous. Each chromosome is made up of thousands of genes. A locus is a specific place on a chromosome where a gene is located. (b) These chromosomes are homologous. Alleles are members of a gene pair that occupy corresponding loci on homologous chromosomes. (c) Alleles govern the same character but do not necessarily contain the same information. Fig. 13-5, p. 260
LEARNING OBJECTIVE 3 • Solve simple genetics problems involving monohybrid and dihybrid crosses
Monohybrid Cross • A cross between two individuals in which only one character is being studied • A cross between one homozygous dominant and one recessive individual yields F1offspring that are all heterozygous • A cross between heterozygous individuals yields a 3:1 phenotypic ratio in F2offspring
F1 Phenotypes True-breeding short parent (recessive) tt Tall Tt Tall Tt True-breeding tall parent (homozygous dominant) t t t t T Tt T Tt TT Tall Tt Tall Tt T Tt Tt T (a) When a genetically pure tall garden pea is crossed with a genetically pure short garden pea, all the offspring are tall. Fig. 13-6a, p. 261
F2 Phenotypes Tt F1 plant Tall TT Tall Tt t T F1 plant t T T T TT Tt Tt Tall Tt Short tt t T Tt tt (b) When two individuals of the F1 generation are crossed, the F2 generation is produced. The phenotypic ratio of the F2 is 3:1. Fig. 13-6b, p. 261
Dihybrid Cross • A cross between two individuals in which two characters are being studied • A cross between one individual homozygous dominant for both characters and one recessive for both characters yields F1 offspring heterozygous for both characters • A cross between two heterozygous individuals yields a 9:3:3:1 phenotypic ratio in F2 offspring
rryy True-breeding parent F1 Phenotypes ry ry ry ry True-breeding parent RrYy RrYy RY RY RrYy RrYy RRYY RY RY RrYy RrYy RrYy RrYy All round yellow (a) When Mendel crossed a true-breeding plant having round and yellow seeds with a true-breeding plant having wrinkled and green seeds, the seeds produced by the F1 plants were all round and yellow. Fig. 13-8a, p. 263
F2 Phenotypes F1 plant RrYy RY Ry rY ry RrYY RrYy RRYY RRYy ry rY Ry RY F1 plant RRyy RrYy RRYy Rryy RrYy RY RRYY RrYY RRYy RY Rryy RrYy Ry RRYy RRyy Ry RrYy RrYY RrYy rrYY rrYy RrYy rrYy rY rY RrYY rrYY Rryy rryy RrYy rrYy ry ry RrYy Rryy rrYy rryy Phenotypic ratio is 9:3:3:1 (b) When two heterozygous individuals are crossed, the ratio of phenotypes is 9:3:3:1 (9 round yellow:3 round green: 3 wrinkled yellow:1 wrinkled green). Fig. 13-8b, p. 263
LEARNING OBJECTIVE 4 • Explain how a test cross is used to determine the genotype of an individual exhibiting a dominant phenotype
Test Cross • An experimental cross that determines if an individual with a dominant phenotype is homozygous or heterozygous • Dominant individual is crossed with a recessive individual (Example: tt) • If the dominant individual is heterozygous, some of offspring should have the recessive phenotype
Phenotypes of offspring Recessive tt Tall Tt Tall Tt t t Homozygous dominant t t T Tt T Tt TT Tall Tt Tall Tt T Tt Tt T (a) If a homozygous tall garden pea is crossed with a short one, all the offspring are tall. Fig. 13-7a, p. 262
Phenotypes of offspring Recessive tt Tall Tt Tall Tt t t Heterozygous t t T Tt T Tt Tt t t Short tt Short tt tt tt (b) If any of the offspring are short, the tall garden pea must be heterozygous. Fig. 13-7b, p. 262
LEARNING OBJECTIVE 5 • Define linkage • Explain why linked genes are sometimes not inherited together
KEY TERMS • LINKAGE • Grouping of genes on the same chromosome
Linked Genes 1 • Two linked genes tend to be inherited together because genes that occur on the same chromosome tend to remain with each other during meiosis
Linked Genes 2 • Sometimes linked genes are not inherited together • Failure of linked genes to stay together is the result of crossing overof segments of homologous chromosomes during prophase I of meiosis
B A A B b b B B A a b B Meiosis I a B a b A B A b Meiosis II Four haploid cells produced Parental type Recombinant type Recombinant type Parental type Fig. 13-9, p. 266
LEARNING OBJECTIVE 6 • Contrast incomplete dominance and polygenic inheritance
KEY TERMS • INCOMPLETE DOMINANCE • A condition in which neither member of a pair of contrasting alleles is completely expressed when the other is present • A heterozygote exhibits a phenotype intermediate between those of its two parents