Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 15 The Chromosomal Basis of Inheritance

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes The chromosome theory of inheritance states that: – Mendelian genes have specific loci (positions) on chromosomes – It is the chromosomes that undergo segregation and independent assortment

LE 15-2 P Generation Gametes Meiosis Yellow-round seeds (YYRR) Fertilization Green-wrinkled seeds (yyrr) All F 1 plants produce yellow-round seeds (YyRr) Meiosis Two equally probable arrangements of chromosomes at metaphase I Anaphase I LAW OF INDEPENDENT ASSORTMENT Metaphase II Fertilization among the F 1 plants F 2 Generation Gametes F 1 Generation LAW OF SEGREGATION

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan’s Experimental Evidence: Scientific Inquiry The first solid evidence associating a specific gene with a a specific chromosome came from Thomas Hunt Morgan, an embryologist.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan’s Choice of Experimental Organism Characteristics that make fruit flies a convenient organism for genetic studies: – They breed at a high rate – A generation can be bred every two weeks – They have only four pairs of chromosomes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan noted wild type, or normal, phenotypes that were common in the fly populations Traits alternative to the wild type are called mutant phenotypes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) – The F 1 generation all had red eyes – The F 2 generation showed the 3:1 red:white eye ratio, but only males had white eyes Morgan determined that the white-eye mutant allele must be located on the X chromosome Morgan’s finding supported the chromosome theory of inheritance

P Generation F 1 Generation F 2 Generation P Generation F 1 Generation Ova (eggs) Sperm F 2 Generation Ova (eggs) Sperm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 15.2: Linked genes tend to be inherited together because they are located near each other on the same chromosome Genes located on the same chromosome that tend to be inherited together are called linked genes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How Linkage Affects Inheritance: Scientific Inquiry Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters P Generation (homozygous) Parental-type offspring Double mutant (black body, vestigial wings) Recombinant (nonparental-type) offspring b b vg vg Double mutant (black body, vestigial wings) b b vg vg Ova Sperm TESTCROSS 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal Wild type (gray body, normal wings) b + b + vg + vg + F 1 dihybrid (wild type (gray body, normal wings) b + b vg + vg

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings From the results, Morgan reasoned that body color and wing size are usually inherited together in specific combinations (parental phenotypes) because the genes are on the same chromosome However, nonparental phenotypes were also produced

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recombination of Unlinked Genes: Independent Assortment of Chromosomes Parental types - Offspring with a phenotype matching one of the parental phenotypes. Recombinant types or Recombinants - Offspring with nonparental phenotypes (new combinations). – A 50% frequency of recombination is observed for any two genes on different chromosomes

LE 15-UN278-2 Gametes from green- wrinkled homozygous recessive parent (yyrr) Parental-type offspring Gametes from yellow-round heterozygous parent (YyRr) Recombinant offspring

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recombination of Linked Genes: Crossing Over Crossing over of homologous chromosomes explains how recombinants can sometimes occur to otherwise linked genes. Animation: Crossing Over Animation: Crossing Over

LE 15-6 Testcross parents Parental-type offspring Recombinant offspring Ova Sperm 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal Gray body, normal wings (F 1 dihybrid) Sperm Testcross offspring Recombination frequency 391 recombinants 2,300 total offspring  100 = 17% = Ova Gametes Replication of chromosomes Black body, vestigial wings (double mutant) Replication of chromosomes Meiosis I and II: No new allele combinations are produced. Meiosis I: Crossing over between b and vg loci produces new allele combinations. Recombinant chromosomes Meiosis II: Separation of chromatids produces recombinant gametes with the new allele combinations.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Linkage Mapping: Using Recombination Data: Scientific Inquiry The farther apart two genes are, the higher the probability that a crossover will occur between them, and therefore the higher the recombination frequency Recombination frequencies 9%9.5% 17% bcn vg Chromosome

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A linkage map is a genetic map of a chromosome based on recombination frequencies Distances between genes can be expressed as map units; one map unit, or centimorgan, represents a 1% recombination frequency Recombination frequencies 9%9.5% 17% bcn vg Chromosome

LE 15-8 Short aristae Black body Cinnabar eyes Mutant phenotypes Vestigial wings Brown eyes Wild-type phenotypes Red eyes Normal wings Red eyes Long aristae (appendages on head) Gray body III I IV X Y II

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 15.3: Sex-linked genes exhibit unique patterns of inheritance An organism’s sex is an inherited phenotypic character determined by the presence or absence of certain chromosomes In humans and other mammals, there are two varieties of sex chromosomes, X and Y

LE 15-9 The X-Y system The X-0 system The Z-W system The haplo-diploid system Parents Sperm Ova Zygotes (offspring)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance of Sex-Linked Genes The sex chromosomes have genes for many characters unrelated to sex A gene located on either sex chromosome is called a sex-linked gene Sex-linked genes follow specific patterns of inheritance

LE Sperm Ova Sperm Ova Sperm Ova

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some disorders caused by recessive alleles on the X chromosome in humans: – Color blindness – Duchenne muscular dystrophy – Hemophilia

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings X inactivation in Female Mammals In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character Early embryo: X chromosomes Allele for orange fur Cell division and X chromosome inactivation Allele for black fur Active X Inactive X Black fur Orange fur Active X Two cell populations in adult cat:

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Abnormal Chromosome Number In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis

LE Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n + 1 Number of chromosomes Nondisjunction of homologous chromosomes in meiosis I n + 1n – 1 n + 1n – 1nn Nondisjunction of sister chromatids in meiosis I

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Aneuploidy results from the fertilization of gametes in which nondisjunction occurred – Trisomic zygote has three copies of a particular chromosome – Monosomic zygote has only one copy of a particular chromosome – Polyploidy is a condition in which an organism has more than two complete sets of chromosomes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Aneuploidy Trisomy 21 (Down’s Syndrome) affects about one out of every 700 children born in the United States.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Alterations of Chromosome Structure Breakage of a chromosome can lead to four types of changes in chromosome structure: – Deletion removes a chromosomal segment – Duplication repeats a segment – Inversion reverses a segment within a chromosome – Translocation moves a segment from one chromosome to another

LE Deletion Duplication Inversion Reciprocal translocation A deletion removes a chromosomal segment. A duplication repeats a segment. An inversion reverses a segment within a chromosome. A translocation moves a segment from one chromosome to another, nonhomologous one.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Aneuploidy of Sex Chromosomes Nondisjunction of sex chromosomes produces a variety of aneuploid conditions Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Disorders Caused by Structurally Altered Chromosomes One syndrome, cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5 A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood Chromosomal Disorders Website