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Chapter 9

Chapter 9. Patterns of Inheritance. 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. 9.1 What Is The Physical Basis Of Inheritance?.

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Chapter 9

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  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? • 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. • Pea plants can do both.

  10. 9.2 How Were The Principles Of Inheritance Discovered? • Mendel studied individual characteristics of pea plants, such as flower color; these characteristics are called traits. • He used traits that varied in an “either or” fashion • He followed the inheritance of these traits for several generations, counting the numbers of offspring with each type of trait.

  11. 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. (b/c parent is homozygous) • 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?

  12. 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

  13. 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.

  14. 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

  15. 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 1/3 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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

  20. 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.

  21. 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

  22. 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

  23. 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.

  24. 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

  25. 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).

  26. 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.

  27. 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

  28. 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.

  29. 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

  30. 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).

  31. 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

  32. 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.

  33. 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.

  34. 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

  35. 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

  36. 9.6 How Is Sex Determined? • Offspring sex is determined by a special pair of chromosomes called the sex chromosomes. • In mammals, females have two X chromosomes and males have an X chromosome and a Y chromosome. • While the X chromosomes look alike, the Y chromosomes are much smaller than the X chromosomes.

  37. 9.6 How Is Sex Determined? • Photomicrograph of human sex chromosomes Y chromosome X chromosome Fig. 9-14

  38. 9.6 How Is Sex Determined? • During gamete formation, the sex chromosomes segregate so that each female gamete gets one X, but the male gametes get either an X or a Y. • All animals have one pair of sex chromosomes and a variable number of other chromosomes, called autosomes. • A Punnett square cross shows how offspring sex is determined from the segregation of the sex chromosomes.

  39. 9.6 How Is Sex Determined? • Sex determination in mammals female parent X2 X1 eggs X2 X1 X2 Xm X1 Xm male parent Xm female offspring Y sperm Xm Y X1 X2 Y Y male offspring Fig. 9-15

  40. 9.7 How Are Sex-Linked Genes Inherited? • Genes that are found on one sex chromosome but not on the other are called sex-linked. • Because females have two X chromosomes, they can be either homozygous or heterozygous for genes on the X chromosome. • Males only have one X chromosome, and therefore express all the alleles they have on their X chromosome.

  41. 9.7 How Are Sex-Linked Genes Inherited? • If a man inherits one defective, recessive allele on his X chromosome, he will show the defective phenotype. • A female, however, may be phenotypically normal because one of her two X chromosomes may display a functional, dominant allele.

  42. 9.7 How Are Sex-Linked Genes Inherited? • In humans, the most familiar genetic defects caused by recessive alleles on X-chromosome genes are hemophilia and red-green color blindness. Fig. 9-16

  43. 9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? • When a heterozygous phenotype is intermediate between the two homozygous phenotypes, the pattern of inheritance is called incomplete dominance. • Human hair texture is influenced by a gene with two incompletely dominant alleles, C1 and C2. • A person with two copies of the C1 allele has curly hair; two copies of the C2 allele produces straight hair; heterozygotes with C1C2 genotype have wavy hair.

  44. mother C1C2 C1 C2 eggs father C1 C1C1 C1C2 sperm C1C2 C2 C1C2 C2C2 9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? • Two wavy-haired people could have the following children: ¼ curly (C1C1), ½ wavy (C1C2), and ¼ straight (C2C2). Fig. 9-17

  45. 9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? • A single gene may have multiple alleles. • A single individual can have only two alleles for any gene, one on each homologous chromosomes. • However, within all the members of a species there could be dozens of alleles for every gene.

  46. 9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? • A single gene may have multiple alleles (continued). • Human blood types A, B, and O arise as a result of three different alleles of a single gene on chromosome 9; this gene codes for an enzyme that adds sugar molecules to recognition proteins on the surfaces of red blood cells. • A person may have one of six genotypes: AA, BB, AB, Ao, Bo, and oo.

  47. 9.8 Do Mendelian Rules Of Inheritance Apply To All Traits?

  48. 9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? • Alleles A and B are dominant to o. • People with type AB blood have both A and B proteins and have type AB blood. • When heterozygotes express phenotypes with both of the homozygotes, the pattern of inheritance is called codominance.

  49. 9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? • Alleles A and B are dominant to o (continued). • People with type AB blood have both A and B proteins and have type AB blood. • When heterozygotes express phenotypes with both of the homozygotes, the pattern of inheritance is called codominance.

  50. 9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? • A single trait may be influenced by several genes. • Many physical traits are governed not by single genes, but by interactions among two or more genes, a phenomenon called polygenic inheritance. • The more genes that contribute to a single trait, the greater the number of phenotypes and the finer the distinctions among them.

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