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Biology A Guide to the Natural World Chapter 12 • Lecture Outline Units of Heredity: Chromosomes and Inheritance Fifth Edition David Krogh © 2011 Pearson Education, Inc. 12.1 X-linked Inheritance in Humans © 2011 Pearson Education, Inc. X-linked Inheritance • Certain human conditions, such as redgreen color blindness and hemophilia, are called X-linked conditions. • They stem from a variant form of gene (an allele) that is dysfunctional and that is located on the X chromosome. © 2011 Pearson Education, Inc. X-linked Inheritance © 2011 Pearson Education, Inc. Figure 12.1 X-linked Inheritance • Men are more likely than women to suffer from these conditions because men have only a single X chromosome. © 2011 Pearson Education, Inc. X-linked Inheritance • A woman with a dysfunctional bloodclotting allele on one of her X chromosomes usually will be protected from hemophilia by a functional allele on her second X chromosome. © 2011 Pearson Education, Inc. X-linked Inheritance • Hemophilia and red-green color blindness are examples of recessive genetic conditions, meaning conditions that will not exist in the presence of even a single functional allele. © 2011 Pearson Education, Inc. X-linked Inheritance • Given the nature of recessive genetic conditions, persons who do not themselves suffer from such conditions may still possess an allele for it, which they can pass on to their offspring. © 2011 Pearson Education, Inc. X-linked Inheritance mother not color-blind functional redgreen allelles X X nonfunctional redgreen allelles egg X father not color-blind XX XX daughters are not color-blind XY XY one son is color-blind sperm Y © 2011 Pearson Education, Inc. Figure 12.2 X-linked Inheritance • Such persons, referred to as carriers, are heterozygous for the condition. • The alleles they have for the trait differ: one is functional, the other is dysfunctional. © 2011 Pearson Education, Inc. X-linked Inheritance Animation 12.1: X-linked Recessive Traits © 2011 Pearson Education, Inc. 12.2 Autosomal Genetic Disorders © 2011 Pearson Education, Inc. Autosomal Genetic Disorders • Sickle-cell anemia is an example of an autosomal recessive disorder. • It is autosomal because the genetic defect that brings it about involves neither the X nor Y chromosome. © 2011 Pearson Education, Inc. Autosomal Genetic Disorders © 2011 Pearson Education, Inc. Figure 12.3 Autosomal Genetic Disorders • It is recessive because persons must be homozygous for the sickle-cell allele to suffer from the condition—they must have two alleles that code for the same sickle-cell hemoglobin protein. © 2011 Pearson Education, Inc. Autosomal Genetic Disorders • Some genetic disorders are referred to as dominant disorders, meaning those in which a single allele can bring about the condition regardless of whether a person also has a normal allele. © 2011 Pearson Education, Inc. (a) Sickle-cell anemia: transmission of a recessive disorder. mother not sick s S egg S father not sick SS Ss sperm Ss ss s Sickle-cell anemia is a recessive autosomal disorder; both the mother and father must carry at least one allele for the trait in order for a son or a daughter to be a sickle-cell victim. When both parents have one sickle-cell allele, there is a 25% chance that any given offspring will inherit the condition. 25% probability of inheriting the disorder (b) Huntington disease: transmission of a dominant disorder. mother not sick h h egg H Hh father sick Hh sperm h hh hh 50% probability of inheriting the disorder In Huntington disease, if only a single parent has a Huntington allele there is a 50% chance that a son or daughter will inherit the condition. © 2011 Pearson Education, Inc. Figure 12.4 Some Human Genetic Disorders Animation 12.2: Some Human Genetic Disorders © 2011 Pearson Education, Inc. 12.3 Tracking Traits with Pedigrees © 2011 Pearson Education, Inc. Pedigrees • In tracking inherited diseases, scientists often find it helpful to construct medical pedigrees, which are genetic familial histories that normally take the form of diagrams. • Pedigrees allow experts to make deductions about the genetic makeup of several generations of family members. © 2011 Pearson Education, Inc. Pedigrees I Aa Aa ? ? A? A? female male normal II ? aa A? ? Aa Aa carrier A? albino III ? ? A? A? ? aa A? © 2011 Pearson Education, Inc. Figure 12.5 © 2011 Pearson Education, Inc. 12.4 Aberrations in Chromosomal Sets: Polyploidy © 2011 Pearson Education, Inc. Polyploidy • Human beings and many other species have diploid or paired sets of chromosomes. • In human beings, this means 46 chromosomes in all: • 22 pairs of autosomes • And either an XX chromosome pair (for females) or an XY pair (for males) © 2011 Pearson Education, Inc. Polyploidy • The state of having more than two sets of chromosomes is called polyploidy. • Many plants are polyploid, but the condition is inevitably fatal for human beings. © 2011 Pearson Education, Inc. 12.5 Incorrect Chromosome Number: Aneuploidy © 2011 Pearson Education, Inc. Aneuploidy • Aneuploidy is a condition in which an organism has either more or fewer chromosomes than normally exist in its species’ full set. • Aneuploidy is responsible for a large proportion of the miscarriages that occur in human pregnancies. © 2011 Pearson Education, Inc. Aneuploidy • A small proportion of embryos survive aneuploidy, but the children who result from these embryos are born with such conditions as Down syndrome. © 2011 Pearson Education, Inc. Nondisjunction • The cause of aneuploidy usually is nondisjunction, in which homologous chromosomes or sister chromatids fail to separate correctly in meiosis • This leads to eggs or sperm that have one too many or one too few chromosomes. © 2011 Pearson Education, Inc. Nondisjunction Normal Abnormal Abnormal Nondisjunction in meiosis I Nondisjunction in meiosis II 23 23 23 23 100% of gametes get normal number of chromosomes 24 24 22 22 100% of gametes get abnormal number of chromosomes 23 23 50% normal © 2011 Pearson Education, Inc. 22 24 50% abnormal Figure 12.7 Aneuploidy • Aneuploidy can come about in regular cell division (mitosis) as well as in meiosis. © 2011 Pearson Education, Inc. Aneuploidy and Cancer • A number of cancer researchers believe that mitotic aneuploidy can be a cause of cancer rather than an effect of it, as previously believed. • Recent evidence indicates that, at the least, such aneuploidy appears prior to the initiation of some forms of cancer. © 2011 Pearson Education, Inc. Aneuploidy and Cancer © 2011 Pearson Education, Inc. Figure 12.9 © 2011 Pearson Education, Inc. 12.6 Structural Aberrations in Chromosomes © 2011 Pearson Education, Inc. Chromosomal Aberrations • Harmful aberrations can occur within chromosomes, with many of these aberrations coming about because of mistakes in chromosomal interactions. © 2011 Pearson Education, Inc. Chromosomal Aberrations • Chromosomal aberrations include: • • • • deletions inversions translocations duplications © 2011 Pearson Education, Inc. Chromosomal Aberrations Inversion Deletion © 2011 Pearson Education, Inc. Translocation Duplication Figure 12.11 © 2011 Pearson Education, Inc.