Campbell Biology in Focus Chapter 12: The Chromosomal Basis of Inheritance PDF

Campbell Biology in Focus Third Edition Chapter 12 The Chromosomal Basis of Inheritance Lecture Presentations...

Campbell Biology in Focus Third Edition Chapter 12 The Chromosomal Basis of Inheritance Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge, Simon Fraser University Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Overview: Locating Genes Along Chromosomes Mendel’s “hereditary factors” were genes; segments of DNA located along chromosomes The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Figure 12.1 Where in the Cell Are Mendel’s Hereditary Factors Located? Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Overview: Locating Genes Along Chromosomes Mitosis and meiosis were first described in the late 1800s The chromosome theory of inheritance states – Mendelian genes have specific loci (positions) on chromosomes – Chromosomes undergo segregation and independent assortment The behavior of chromosomes during meiosis can account for Mendel’s laws of segregation and independent assortment Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Figure 12.2 The Chromosomal Basis of Mendel’s Laws Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Morgan’s Choice of Experimental Organism Morgan selected a species of fruit fly, Drosophila melanogaster, as his research organism Several characteristics make fruit flies a convenient organism for genetic studies – They produce many offspring – A generation can be bred every two weeks – They have only four pairs of chromosomes Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Morgan’s Choice of Experimental Organism Morgan noted wild-type, or normal, phenotypes that were common in the fly populations Traits alternative to the wild type are called mutant phenotypes The first mutant phenotype he discovered was a fly with white eyes instead of the wild type red Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Figure 12.3 Morgan’s First Mutant Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved 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 F1 generation all had red eyes – The F2 generation showed the classical 3:1 red:white ratio, but only males had white eyes Morgan concluded that the eye color was related to the sex of the fly Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair Morgan determined that the white-eyed mutant allele must be located on the X chromosome Morgan’s finding supported the chromosome theory of inheritance Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Figure 12.4 Inquiry: In a Cross Between a Wild-Type Female Fruit Fly and a Mutant White-Eyed Male, What Color Eyes Will the F1 and F2 Offspring Have? Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Concept 12.2: Sex-Linked Genes Exhibit Unique Patterns of Inheritance The behavior of the members of the pair of sex chromosomes can be correlated with the behavior of the two alleles of the eye-color gene white Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved The Chromosomal Basis of Sex Humans and other mammals have two types of sex chromosomes: a larger X chromosome and a smaller Y chromosome Individuals who inherit two X chromosomes develop anatomy we associate with the female sex Properties considered “male” are associated with the inheritance of one X and one Y chromosome Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Figure 12.5 Human Sex Chromosomes Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved The Chromosomal Basis of Sex Only the ends of the Y chromosome have regions that are homologous with corresponding regions of the X chromosome These regions allow the X and Y chromosomes to pair and behave like homologs during meiosis in males Each ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome Other animals have different methods of sex determination Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Figure 12.6 The Mammalian X-Y Chromosomal System of Sex Determination Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved The Chromosomal Basis of Sex A gene that is located on either sex chromosome is called a sex-linked gene Genes on the X chromosome are called X-linked genes Genes on the Y chromosome are called Y-linked genes; there are few of these Because of the complexity of the process of sex determination, many variations exist Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Inheritance of X-Linked Genes Many Y-linked genes help determine sex The X chromosomes have genes for many characters unrelated to sex Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Inheritance of X-Linked Genes X-linked genes follow specific patterns of inheritance For a recessive X-linked trait to be expressed – A female needs two copies of the allele (homozygous) – A male needs only one copy of the allele (hemizygous) X-linked recessive disorders are much more common in males than in females Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Figure 12.7 The Transmission of X-Linked Recessive Traits Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved X Inactivation in Female Mammals In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development The inactive X condenses into a Barr body If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Concept 12.3: Linked Genes Tend to Be Inherited Together Because They Are Located near Each Other on the Same Chromosome Each chromosome has hundreds or thousands of genes (except the Y chromosome) Genes located on the same chromosome that tend to be inherited together are called linked genes Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Recombination of Unlinked Genes: Independent Assortment of Chromosomes Mendel observed that combinations of traits in some offspring differ from either parent Offspring with a phenotype matching one of the parental phenotypes are called parental types Offspring with nonparental phenotypes (new combinations of traits) are called recombinant types, or recombinants A 50% frequency of recombination is observed for any two genes on different chromosomes Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Recombination of Unlinked Genes: Independent Assortment of Chromosomes Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Recombination of Linked Genes: Crossing Over Morgan discovered that even when two genes were on the same chromosome, some recombinant phenotypes were observed He proposed that some process must occasionally break the physical connection between genes on the same chromosome That mechanism was the crossing over between homologous chromosomes Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved New Combinations of Alleles: Variation for Natural Selection Recombinant chromosomes bring alleles together in new combinations in gametes Random fertilization increases even further the number of variant combinations that can be produced This abundance of genetic variation is the raw material upon which natural selection works Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Concept 12.4: Alterations of Chromosome Number or Structure Cause Some Genetic Disorders Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders Plants tolerate such genetic changes better than animals do Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Abnormal Chromosome Number In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis I or sister chromatids do not separate during meiosis II As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Video: Nondisjunction Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Figure 12.13 Meiotic Nondisjunction Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Abnormal Chromosome Number Aneuploidy results from fertilization involving gametes in which nondisjunction occurred Offspring with this condition have an abnormal number of a particular chromosome Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Abnormal Chromosome Number A monosomic zygote has only one copy of a particular chromosome A trisomic zygote has three copies of a particular chromosome Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Abnormal Chromosome Number Polyploidy is a condition in which an organism has more than two complete sets of chromosomes – Triploidy (3n) is three sets of chromosomes – Tetraploidy (4n) is four sets of chromosomes Polyploidy is common in plants but not animals Spontaneous origin of polyploid individuals plays an important role in the evolution of plants Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved 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 orientation of a segment within a chromosome – Translocation moves a segment from one chromosome to another Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Figure 12.14 Alterations of Chromosome Structure Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Alterations of Chromosome Structure A diploid embryo that is homozygous for a large deletion is likely missing a number of essential genes; such a condition is generally lethal Duplications and translocations also tend to be harmful In inversions the balance of genes is normal, but phenotype may be influenced if the expression of genes is altered Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Human Disorders Due to Chromosomal Alterations Alterations of chromosome number and structure are associated with some serious disorders Some types of aneuploidy upset the genetic balance less than others, resulting in individuals surviving to birth and beyond These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Down Syndrome (Trisomy 21) Down syndrome is an aneuploid condition that results from three copies of chromosome 21 It affects about one out of every 830 children born in the United States The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained Some research points to an age-dependent abnormality in meiosis Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Figure 12.15 Down Syndrome Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Aneuploidy of Sex Chromosomes Aneuploid conditions involving sex chromosomes appear to upset the genetic balance less than those involving autosomes Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals About one in every 1,000 males is born with an extra Y chromosome (XYY) and does not exhibit any defined syndrome Females with trisomy X (XXX) have no unusual physical features except being slightly taller than average Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Aneuploidy of Sex Chromosomes Monosomy X, called Turner syndrome, produces X0 females, who are sterile It is the only known viable monosomy in humans Copyright © 2020, 2016, 2014 Pearson Education, Inc. All Rights Reserved Figure 12.16 Translocation Associated with Chronic Myelogenous Leukemia (CML) Copyright © 2020, 2016, 2014 Pearson Education, Inc. 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