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Chapter 10

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Chapter 10

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  1. Chapter 10 • Patterns of Inheritance

  2. What Is the Physical Basis of Inheritance? • Inheritance is the process by which traits are passed to the offspring • Genesare sequences of nucleotides at specific locations on chromosomes • A gene is a unit of heredity that encodes information needed to produce proteins, cells, and entire organisms • The location of a gene on a chromosome is called its locus (plural, loci)

  3. DNA in Eukaryotic Chromosomes • Homologous chromosomes are usually not identical • Mutations - Changes in nucleotide sequence in DNA of one homologous chromosome • Mutations are the raw material for evolution • Mutations in gametes may become a part of the genetic makeup of the species • Mutation gives rise to new alleles, alternative gene forms that may produce differences in structure or function gene 1 gene 2 same alleles different alleles

  4. The Relationship Among Genes, Alleles, and Chromosomes a pair of homologous chromosomes Both chromosomes carry the same allele of the gene at this locus; the organism is homozygous at this locus gene loci This locus contains another gene for which the organism is homozygous Each chromosome carries a different allele of this gene, so the organism is heterozygous at this locus the chromosome from the male parent the chromosome from the female parent Fig. 10-1

  5. How Were the Principles of Inheritance Discovered? intact pea flower flower dissected to show its reproductive structures • Who was Gregor Mendel? • Pea plants have qualities that make it a good organism for studying inheritance • Pea flower petals enclose both male and female flower parts and prevent entry of pollen from another pea plant – self-fertilization Carpel (female, produces eggs) Stamen (male, produces pollen that contain sperm)

  6. How Are Single Traits Inherited? • The language of a genetic cross • A genetic cross is the mating of pollen and eggs (from same or different parents) • The parents used in a cross are part of the parental generation (known as P) • The offspring of the P generation are members of the first filial generation (F1) • Offspring of the F1 generation are members of the F2 generation

  7. Cross of Pea Plants True-Breeding for White or Purple Flowers pollen Parental generation (P) pollen cross-fertilize true-breeding, purple-flowered plant true-breeding, white-flowered plant First-generation offspring (F1) all purple-flowered plants Fig. 10-4

  8. Self-Fertilization of F1 Pea Plants with Purple Flowers First- generation offspring (F1) self-fertilize Second- generation offspring (F2) 3/4 purple 1/4 white Fig. 10-5

  9. 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 Fig. 10-6 (b) Gametes produced by a heterozygous dominant parent

  10. Segregation of Alleles and Fusion of Gametes purple parent P P PP + all Psperm and eggs white parent p pp p + all p sperm and eggs Fig. 10-7a (a) Gametes produced by homozygous parents 

  11. Segregation of Alleles and Fusion of Gametes F1 offspring sperm eggs + p P Pp or Pp + p P (b) Fusion of gametes produces F1 offspring Fig. 10-7b

  12. Segregation of Alleles and Fusion of Gametes gametes from F1Pp plants F2 offspring sperm eggs PP + P P p Pp + P Pp p + P p pp p + (c) Fusion of gametes from the F1 generation produces F2 offspring Fig. 10-7c

  13. How Are Single Traits Inherited? • The hypothesis explains Mendel’s results with peas • The particular combination of the two alleles carried by an individual is called the genotype • The physical expression of the genotype is known as the phenotype (for example, purple or white flowers) • Simple “genetic bookkeeping” can predict genotypes and phenotypes of offspring • The Punnett square method predicts offspring genotypes and phenotypes from combinations of parental gametes

  14. Determining the Outcome of a Single-Trait Cross Pp self-fertilize p 1 1 P eggs 2 2 genotypic ratio (1:2:1) phenotypic ratio (3:1) offspring genotypes eggs sperm 1 P 2 1 1 1 1   P P PP PP 4 2 4 2 1 1 PP Pp sperm 4 4 1 1 1 p   P Pp 2 4 2 3 purple 4 1 Pp 2 1 1 1 p   pP P 1 p 2 4 2 2 1 1 1 1 1 p   pp pp white p 1 1 4 4 2 4 2 pP pp 4 4 (a) Punnett square of a single-trait cross Using probabilities to determine the offspring of a single-trait cross Fig. 10-8

  15. Traits of Pea Plants Studied by Gregor Mendel Trait Dominant form Recessive form Seed shape smooth wrinkled Seed color yellow green Pod shape inflated constricted Pod color green yellow Flower color purple white Flower location at leaf junctions at tips of branches Plant size tall (about 6 feet) dwarf (about 8 to 16 inches) Fig. 10-10

  16. Predicting Genotypes and Phenotypes for a Cross between Parents That Are Heterozygous for Two Traits Fig. 10-11 SsYy self-fertilize eggs 1 1 1 1 sy Sy sY SY 4 4 4 4 1 SY 4 1 1 1 1 16 SSYY 16 SSYy 16 SsYY 16 SsYy 1 Sy 4 1 1 1 1 SSyY SSyy SsyY Ssyy 16 16 16 16 sperm seed shape seed color phenotypic ratio (9:3:3:1) 3 3 9 1 sY smooth  yellow smooth yellow  4 4 16 4 1 1 1 1 sSYY sSYy ssYY ssYy 3 1 16 16 16 16 3 smooth green   smooth green 4 4 16 1 3 3 1 sy wrinkled yellow wrinkled yellow   4 4 16 4 1 1 1 1 1 1 1 sSyY sSyy ssyY ssyy 16 16 16 16 green wrinkled wrinkled green   4 4 16 (a) Punnett square of a two-trait cross (b) Using probabilities to determine the offspring of a two-trait cross

  17. How Are Multiple Traits Inherited? • Mendel’s results supported his hypothesis that traits are inherited independently • The independent inheritance of two or more 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 for other genes • Independent assortment will occur when the traits being studied are controlled by genes on different pairs of homologous chromosomes

  18. Independent Assortment of Alleles S s pairs of alleles on homologous chromosomes in diploid cells Y y chromosomes replicate replicated homologous pair during metaphase of 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 Fig. 10-12 independent assortment produces four equally likely allele combinations during meiosis

  19. Do the Mendelian Rules of Inheritance Apply to All Traits? • Many traits do not follow simple Mendelian rules of inheritance • Not all traits are completely controlled by a single gene • Not all traits have only two possible alleles • A trait may not be completely dominant to another

  20. Do the Mendelian Rules of Inheritance Apply to All Traits? mother • Incomplete dominance • When the heterozygous phenotype is intermediate between the two homozygous phenotypes • Hair texture C1C2 C1 C2 eggs father C1 C1C1 C1C2 sperm C1C2 C2 C1C2 C2C2

  21. Do the Mendelian Rules of Inheritance Apply to All Traits? • A single gene may have multiple alleles • A species may have multiple alleles for a given characteristic • However, each individual still carries two alleles for this characteristic • The human blood types are an example (Codominance)

  22. Do the Mendelian Rules of Inheritance Apply to All Traits? • Polygenic inheritance • Some characteristics show a range of continuous phenotypes instead of discrete, defined phenotypes • Examples of this include human height, skin color, and body build, and in wheat, grain color • Phenotypes produced by polygenic inheritance are governed by the interaction of more than two genes at multiple loci

  23. Polygenic Inheritance of Skin Color in Humans Fig. 10-14 • Human skin color is controlled by at least three genes, each with pairs of incompletely dominant alleles

  24. Do the Mendelian Rules of Inheritance Apply to All Traits? • The environment influences the expression of genes • Newborn Siamese cats demonstrate the effect of environment on phenotype • A Siamese cat has the genotype for dark fur all over its body • However, the enzyme that produces the dark pigment is inactive at temperatures above 93°F (34°C)

  25. How Are Genes Located on the Same Chromosome Inherited? flower-color gene pollen-shape gene • Genes on the same chromosome tend to be inherited together • Mendel’s law of independent assortment works only for genes whose loci are on different pairs of homologous chromosomes • Characteristics whose genes tend to assort together are said to be linked purple allele, P long allele, L red allele, p round allele, l

  26. How Are Genes Located on the Same Chromosome Inherited? • Crossing over creates new combinations of linked alleles • Genes on the same chromosome do not always sort together • Crossing over, or genetic recombination, in prophase I of meiosis creates new gene combinations pollen-shape gene flower-color gene sister chromatids homologous chromosomes (duplicated) at meiosis I purple allele, P long allele, L L P P L sister chromatids p l p red allele, p round allele, l l (a) Replicated chromosomes in prophase of meiosis I (b) Crossing over during prophase I

  27. Crossing Over Recombines Alleles on Homologous Chromosomes P L P L p L p L recombined chromosomes unchanged chromosomes recombined chromatids l P unchanged chromatids l P p p l l (c) Homologous chromosomes separate at anaphase I (d) after meiosis II Each being a different gamete with different alleles Fig. 10-18c, d

  28. How Is Sex Determined? • Sex chromosomes that dictate gender • Females have two X chromosomes • Males have an X chromosome and a Y chromosome • A small section of the X and Y chromosomes is homologous, allowing them to pair in prophase I and segregate during meiosis • Autosomes - rest of the (non-sex) chromosomes • occur in identical pairs Y chromosome X chromosome

  29. Sex Determination in Mammals female parent X1 X2 eggs X1 X2 X2 Xm X1 Xm male parent Xm female offspring sperm Y Xm X1 X2 Y Y Y Fig. 10-20 male offspring

  30. How Are Sex-Linked Genes Inherited? • Sex-linked genes are found only on the X or only on the Y chromosome • Genes carried on one sex chromosome, but not on the other, are sex-linked • In humans, the X chromosome is much larger than the Y and carries over 1,000 genes • In contrast, the human Y chromosome is smaller and carries only 78 genes • During embryonic life, the action of the Y-linked gene SRY sets in motion the entire male developmental pathway • Under normal conditions, SRY causes the male gender to be linked 100 percent to the Y chromosome

  31. How Are Sex-Linked Genes Inherited? • Sex-linked genes are found only on the X or only on the Y chromosomes • Few of the genes on the X chromosome have a specific role in female reproduction • Most of the genes on the X chromosome have no counterpart on the Y chromosome • Some genes found only on the X chromosome are important to both sexes, such as genes for color vision, blood clotting, and certain structural proteins in muscles

  32. How Are Sex-Linked Genes Inherited? • Sex-linked genes are found only on the X or only on the Y chromosomes • Because females have two X chromosomes, recessive sex-linked genes on an X chromosome may or may not be expressed • Because males, with only one X chromosome, have no second copy to mask recessive genes, they fully express all the X-linked alleles they have, whether those alleles are dominant or recessive

  33. Sex-Linked Inheritance of Color Blindness Can’t distinguish red from green female parent XC Xc (a) Normal color vision (b) Red-green color blindness eggs XC Xc Xc XC XC XC XC male parent sperm female offspring Y XC XC Y Xc Y Y male offspring (c) Expected children of a man with normal color vision (CY), and a heterozygous woman (Cc) Fig. 10-21

  34. FamilyPedigrees - show the genetic relationships among relatives How are Human Genetic Disorders Inherited? How to read pedigrees = generations = male = female = parents (a) A pedigree for a dominant trait = offspring or = shows trait ? ? ? ? or = does not show trait or = known carrier (heterozygote) for recessive trait ? ? ? or = cannot determine the genotype from this pedigree ? ? (b) A pedigree for a recessive trait Fig. 10-22

  35. How Are Human Disorders Caused by Single Genes Inherited? • Some human genetic disorders are caused by recessive alleles • New alleles produced by mutation usually code for nonfunctional proteins • Alleles coding for nonfunctional proteins are recessive to those coding for functional ones • The presence of one normal allele may generate enough functional protein to enable heterozygotes to be phenotypically indistinguishable from homozygotes with two normal alleles

  36. How Are Human Disorders Caused by Single Genes Inherited? • Some human genetic disorders are caused by recessive alleles • Heterozygous individuals are carriers of a recessive genetic trait (but otherwise have a normal phenotype) • Recessive genes are more likely to occur in a homozygous combination (expressing the defective phenotype) when related individuals have children • Sickle-cell anemia • Mutation in hemoglobin structure • Malaria resistance in heterozygotes

  37. Albinism Fig. 10-22

  38. How Are Human Disorders Caused by Single Genes Inherited? • Some human genetic disorders are caused by dominant alleles • A dominant disease can be transmitted to offspring if at least one parent suffers from the disease and lives long enough to reproduce • Dominant disease alleles also arise as new mutations in the DNA of eggs or sperm of unaffected parents

  39. How Are Human Disorders Caused by Single Genes Inherited? • Some human genetic disorders are caused by dominant alleles • Huntington disease is a dominant disorder that causes a slow, progressive deterioration of parts of the brain • The disease results in a loss of coordination, flailing movements, personality disturbances, and eventual death • The disease becomes manifest in adulthood, ensuring its maintenance in the population

  40. How Are Human Disorders Caused by Single Genes Inherited? • Some human genetic disorders are sex-linked • The X chromosome contains many genes that have no counterpart on the Y chromosome • Because males have only one X chromosome, they have no other allele to exert dominance over a sex-linked (X-linked) allele causing disease • Consequently, sex-linked diseases tend to occur in males

  41. How Are Human Disorders Caused by Single Genes Inherited? • Some human genetic disorders are sex-linked • Sex-linked disorders caused by a recessive allele have a unique pattern of inheritance • A son receives his X chromosome from his mother and passes it on only to his daughters, since the gene doesn’t exist on his Y chromosome • Sex-linked genes typically skip generations because the affected male passes the trait to a phenotypically normal carrier daughter, who in turn bears affected sons • Several defective alleles for characteristics encoded on the X chromosome are known, including red-green color deficiency, muscular dystrophyand hemophilia

  42. unaffected male hemophiliac male unaffected female carrier female Edward Duke of Kent Victoria Princess of Saxe-Coburg Albert Prince of Saxe- Coburg-Gotha Victoria Queen of England Helen Princess of Waldeck-Pyrmont Henry Prince of Battenburg Edward VII King of England Alexandra of Denmark Leopold Duke of Albany Louis IV Grand Duke of Hesse-Darmstadt Alice Princess of Hesse several unaffected chidren Beatrice present British royal family (unaffected) Elizabeth Frederick Ernest Irene Victoria Queen of Spain Leopold Maurice Alexandra Tsarina Nicholas II of Russia Mary Victoria Alexander Albert Alfonso XII Victoria Mary carrier daughter and hemophiliac grandson ? ? ? ? ? ? Alexis Tsarevitch Olga Tatiana Maria Anastasia Alfonso Crown Prince Juan Beatrice Marie Jaime Gonzalo died in infancy Hemophilia Among the Royal Families of Europe Fig. 10-25

  43. How Do Errors in Chromosome Number Affect Humans? • The incorrect separation of chromosomes or chromatids in meiosis is known as nondisjunction • Nondisjunction causes gametes to have too many and too few chromosomes • Most embryos that arise from fusion of gametes with abnormal chromosome numbers spontaneously abort, but some survive to birth and beyond

  44. How Do Errors in Chromosome Number Affect Humans? • Some genetic disorders are caused by abnormal numbers of sex chromosomes (continued) • Turner syndrome (XO) occurs in females with only one X chromosome • At puberty, hormone deficiencies prevent XO females from menstruating or developing secondary sexual characteristics • Hormone treatment promotes physical development, but because affected women lack mature eggs, they remain infertile • More susceptible to recessive disorders such as red-green color blindness and hemophilia

  45. How Do Errors in Chromosome Number Affect Humans? • Some genetic disorders are caused by abnormal numbers of sex chromosomes • Trisomy X (XXX) results in a fertile “normal” woman with an extra X chromosome • Most affected women show no abnormal symptoms • There is an increased chance of learning disabilities and a tendency toward tallness • By some unknown mechanism that prevents an extra X chromosome from being included in their eggs, women with trisomy X bear normal XX and XY children

  46. How Do Errors in Chromosome Number Affect Humans? • Some genetic disorders are caused by abnormal numbers of sex chromosomes • Men with Klinefelter syndrome (XXY) have an extra X chromosome • Most afflicted males show no symptoms, although some may show mixed secondary sexual characteristics, including partial breast development, broadening of the hips, and small testes • XXY men are often infertile because of low sperm count but are not impotent

  47. How Do Errors in Chromosome Number Affect Humans? • Some genetic disorders are caused by abnormal numbers of sex chromosomes • Males with Jacob syndrome (XYY) have an extra Y chromosome • Have high levels of testosterone, tend to develop severe acne, and may be exceptionally tall, more susceptible to learning disabilities

  48. How Do Errors in Chromosome Number Affect Humans? • Some genetic disorders are caused by abnormal numbers of autosomes • Nondisjunction of autosomes can occur during meiosis in the father or mother, resulting in eggs or sperm that are missing an autosome or that have two copies of an autosome • Embryos with one or three copies of an autosome (trisomy) usually spontaneously abort; however, a small fraction of embryos with three copies of chromosomes 13, 18, or 21 survive to birth • The frequency of nondisjunction increases with the age of the parents

  49. How Do Errors in Chromosome Number Affect Humans? • Some genetic disorders are caused by abnormal numbers of autosomes • In trisomy 21 (Down syndrome), afflicted individuals have three copies of chromosome 21