Chapter 9

1. Chapter 9 Patterns of Inheritance

2. 9.1 What Is The Physical Basis Of Inheritance? • Inheritance occurs when genes are transmitted from parent to offspring. • The units of inheritance are genes, which are segments of DNA of variable length.

3. 9.1 What Is The Physical Basis Of Inheritance? • Genes are segments of DNA at specific locations on chromosomes. • A gene’s physical location on a chromosome is called its locus. • Each member of a pair of homologous chromosomes carries the same genes, located at the same loci. • Different versions of a gene at a given locus are called alleles.

4. 9.1 What Is The Physical Basis Of Inheritance? • The relationship among genes, alleles, and chromosomes a pair ofhomologouschromosomes Both chromosomes carry the same alleleof the gene at this locus; the organism ishomozygous at this locus gene loci This locus contains another gene for whichthe organism is homozygous Each chromosome carries a different alleleof this gene, so the organism isheterozygous at this locus the chromosomefrom the maleparent the chromosomefrom the femaleparent Fig. 9-1

5. 9.1 What Is The Physical Basis Of Inheritance? • Mutations are the source of alleles. • Differences in alleles at a given locus are due to mutations at that gene. • If a mutation occurs in the cells that become sperm or eggs, it can be passed on from parent to offspring.

6. 9.1 What Is The Physical Basis Of Inheritance? • An organism’s two alleles may be the same or different. • A diploid organism has pairs of homologous chromosomes with two copies of each gene at a given locus. • If both homologous chromosomes have the same allele at a locus, the organism is said to be homozygous.

7. 9.1 What Is The Physical Basis Of Inheritance? • An organism’s two alleles may be the same or different (continued). • If two homologous chromosomes have different alleles at a locus, the organism is heterozygous at that locus. • The gametes of a homozygous individual are all the same at a particular locus, while gametes of a heterozygous individual would contain half one allele and half the other allele.

8. 9.2 How Were The Principles Of Inheritance Discovered? • The patterns of inheritance were discovered by an Austrian monk, Gregor Mendel. Fig. 9-2

9. 9.2 How Were The Principles Of Inheritance Discovered? • Doing it right: the secrets of Mendel’s success • Mendel was the first geneticist to employ three key steps in his experimentation: • Choosing the right organism for the work • Designing and performing experiments correctly • Analyzing the data properly

10. 9.2 How Were The Principles Of Inheritance Discovered? • Mendel chose edible pea as the experimental subject for his experiments in inheritance. • Pea egg cells in a pea flower fertilized by sperm from the same flower is called self-fertilization. • When sperm from one organism fertilizes eggs from a different organism, the process is called cross-fertilization.

11. 9.2 How Were The Principles Of Inheritance Discovered? • Mendel chose edible pea as the experimental subject for his experiments in inheritance (continued). • Mendel studied individual characteristics of pea plants, such as flower color; these characteristics are called traits. • He followed the inheritance of these traits for several generations, counting the numbers of offspring with each type of trait.

12. 9.3 How Are Single Traits Inherited? • True-breeding traits of organisms, such as purple flower color, are always inherited by all of their offspring that result from self-fertilization. • In one experiment, Mendel cross-fertilized white-flowered plants with purple-flowered plants. • When he grew the resulting seeds, he found all the first-generation offspring, or the F1 generation, produced purple flowers. • What happened to the white color?

13. 9.3 How Are Single Traits Inherited? • Cross of pea plants that are true-breeding for white or purple flowers pollen Parentalgeneration (P) pollen cross-fertilize true-breeding,purple-floweredplant true-breeding,white-floweredplant First-generationoffspring (F1) all purple-floweredplants Fig. 9-4

14. 9.3 How Are Single Traits Inherited? • The F2 generation • Next, Mendel allowed the F1 flowers to self-fertilize, collected the seeds, and grew the second generation, called the F2 generation. • Flowers in the F2 generation were three-fourths purple and one-fourth white, in a ratio of 3 purple to 1 white. • This showed that the gene for white flowers was “hidden” in the F1 generation, but appeared again in the F2 generation.

15. 9.3 How Are Single Traits Inherited? • Cross of F1 plants with purple flowers First-generationoffspring (F1) self-fertilize Second-generation offspring (F2) 3/4 purple 1/4 white Fig. 9-5

16. 9.3 How Are Single Traits Inherited? PLAY Animation—Crosses Involving Single Traits

17. 9.3 How Are Single Traits Inherited? • All the white-flowered plants in the F2 generation only produced additional white-flowered plants. • Purple-flowered plants were of two types: • About ⅔ were true-breeding for purple, while ⅔ produced both purple- and white-flowered offspring (ratio 3 purple/1 white). • Therefore, the F2 generation included ¼ true-breeding purple plants, ½ hybrid purple, and ¼ true-breeding white plants.

18. 9.3 How Are Single Traits Inherited? • The inheritance of dominant and recessive alleles on homologous chromosomes can explain the results of Mendel’s crosses. • Mendel’s results allow us to develop a five-part hypothesis to explain the inheritance of single traits. • Each trait is determined by pairs of distinct physical units called genes. • There are two alleles for each gene, one on each homologous chromosome.

19. 9.3 How Are Single Traits Inherited? • When two different alleles are present in an organism, the dominant allele may mask the expression of the recessive allele; however, the recessive allele is still present. • The two alleles of a gene segregate (separate) from one another during meiosis; this is known as Mendel’s law of segregation. • Which allele ends up in any given gamete is determined by chance.

20. 9.3 How Are Single Traits Inherited? • True-breeding (homozygous) organisms have two copies of the same allele for a given gene; hybrid (heterozygous) organisms have two different alleles for a given gene.

21. 9.3 How Are Single Traits Inherited? • The distribution of alleles in gametes homozygous parent gametes A A A A (a) Gametes produced by a homozygous parent heterozygous parent gametes A a A a (b) Gametes produced by a heterozygous parent Fig. 9-6

22. 9.3 How Are Single Traits Inherited? • In pea plants, purple is dominant to white. • Letters can be used to describe the alleles (P is for the dominant allele; p is for the recessive allele). • Homozygous purple plants are PP; homozygous white plants are pp.

23. 9.3 How Are Single Traits Inherited? • Homozygous purple plants are PP; homozygous white plants are pp. purple parent PP P P + all P sperm and eggs white parent pp + p p all p sperm and eggs (a) Gametes produced by homozygous parents Fig. 9-7a

24. 9.3 How Are Single Traits Inherited? • F1 plants were produced by P and p gametes, making Pp F1 hybrid offspring. F1 offspring sperm eggs Pp + p P or Pp + p P (b) Fusion of gametes produces F1 offspring Fig. 9-7b

25. 9.3 How Are Single Traits Inherited? • Next, Mendel crossed two F1 hybrid plants (Pp x Pp). • This cross made three types of F2 offspring, with the following allele composition • ¼ were PP; ½ were Pp; ¼ were pp.

26. 9.3 How Are Single Traits Inherited? • Fusion of gametes from the F1 generation produces F2 offspring. gametes from F1Pp plants F2 offspring sperm eggs PP + P P Pp p + P Pp P p + pp p p + (c) Fusion of gametes from the F1 generation produces F2 offspring Fig. 9-7c

27. 9.3 How Are Single Traits Inherited? • Mendel’s hypothesis was that two plants may look alike, called its phenotype, but have a different allele composition, called its genotype. • In this case, purple plants had PP or Pp genotypes, but their phenotype (purple color) was the same. • The F2 generation could be described as having three genotypes (¼ PP, ½ Pp, and ¼ pp) and two phenotypes (¾ purple and ¼ white).

28. 9.3 How Are Single Traits Inherited? • Simple “genetic bookkeeping” can predict the genotypes and phenotypes of offspring. • The Punnett square method is a convenient way to predict the genotypes and phenotypes of offspring.

29. Pp self-fertilize p 1 1 P eggs 2 2 1 P 2 1 1 PP Pp 4 4 sperm 1 p 2 1 1 pP pp 4 4 9.3 How Are Single Traits Inherited? • The Punnett square method Fig. 9-8

30. 9.3 How Are Single Traits Inherited? PLAY Animation—Punnett Square

31. 9.3 How Are Single Traits Inherited? • Mendel’s hypothesis can predict the outcome of new types of single-trait crosses. • Mendel predicted the outcome of cross-fertilizing Pp plants with homozygous recessive plants (pp)—there should be equal numbers of Pp (purple) and pp (white) offspring.

32. pollen PP or Pp ppall eggs p sperm unknown if PP if Pp p p eggs eggs allsperm 1 P P 2 1 all Pp Pp 2 sperm 1 p 2 1 pp 2 9.3 How Are Single Traits Inherited? • A Punnet square shows how this “test cross” results in the predicted offspring. Fig. 9-9

33. 9.4 How Are Multiple Traits Inherited? • Mendel next crossed pea plants that differed in two traits, such as seed color (yellow or green) and seed shape (smooth or wrinkled). • He knew from previous crosses that smooth and yellow were both dominant traits in peas. • His first cross was a true-breeding plant with smooth, yellow seeds (SSYY) to a true-breeding plant with wrinkled, green seeds (ssyy).

34. 9.4 How Are Multiple Traits Inherited? • Traits of pea plants studied by Gregor Mendel Trait Dominant form Recessive form Seedshape smooth wrinkled Seedcolor green yellow Podshape inflated constricted Podcolor green yellow Flowercolor purple white Flowerloca-tion at leafjunctions at tips ofbranches Plantsize tall(about6 feet) dwarf(about 8 to16 inches) Fig. 9-10

35. 9.4 How Are Multiple Traits Inherited? • All the offspring of this cross (F1 generation) were SsYy and had smooth, yellow seeds (both dominant traits). • F1 plants were allowed to self-fertilize and produced F2 offspring in the phenotypic ratio 9:3:3:1.

36. 9.4 How Are Multiple Traits Inherited? • Mendel concluded that multiple traits are inherited independently. • Mendel realized that these results could be explained if the genes for seed color and seed shape were inherited independently. • The independent inheritance of two or more distinct traits is called the law of independent assortment. • Multiple traits are inherited independently because the alleles of one gene are distributed to gametes independently of the alleles of other genes.

37. 9.4 How Are Multiple Traits Inherited? • Predicting genotypes and phenotypes SsYy self-fertilize eggs 1 1 1 1 Sy sY SY sy 4 4 4 4 1 SY 4 1 1 1 1 SSYY SSYy SsYY SsYy 16 16 16 16 1 Sy 4 1 1 1 1 SSyy SSyY SsyY Ssyy 16 16 16 16 sperm 1 sY 4 1 1 1 1 sSYY sSYy ssYY ssYy 16 16 16 16 1 sy 4 1 1 1 1 sSyy ssyY ssyy sSyY 16 16 16 16 Fig. 9-11

38. 9.4 How Are Multiple Traits Inherited? PLAY Animation—Multiple Traits PLAY Animation—Multiple Traits

39. 9.4 How Are Multiple Traits Inherited? • Independent assortment of alleles S pairs of alleles onhomologous chromosomesin diploid cells s Y y chromosomes replicate replicated homologuespair during metaphaseof meiosis I,orienting like this or like this y Y S S s s y Y meiosis I Y Y y S y S s s S s s S y Y y Y meiosis II S S s s S S s s Y Y y y y Y y Y SY sy Sy sY independent assortment produces four equallylikely allele combinations during meiosis Fig. 9-12

40. 9.4 How Are Multiple Traits Inherited? • In an unprepared world, genius may go unrecognized. • Gregor Mendel presented his theories of inheritance in 1865. • His experiments made little impact on science during his lifetime. • It was not until 1900 that three biologists—Carl Correns, Hugo de Vries, and Erich Tschermak—rediscovered Mendel’s work and acknowledged its importance to science.