Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. BIOLOGY A GUIDE TO THE NATURAL WORLD FOURTH EDITION DAVID KROGH Units of Heredity: Chromosomes and Inheritance

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings X-linked Inheritance in Humans

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. X-linked Inheritance Certain human conditions, such as red-green 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.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. X-linked Inheritance Figure 12.1

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. X-linked Inheritance Men are more likely than women to suffer from these conditions because men have only a single X chromosome.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. X-linked Inheritance A woman with a dysfunctional blood-clotting allele on one of her X chromosomes usually will be protected from hemophilia by a functional allele on her second X chromosome.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 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.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 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.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. X-linked Inheritance Figure 12.2 X X X XYXY X daughters are not color-blind one son is color-blind nonfunctional red- green allelles XX XY Y mother not color-blind egg sperm functional red- green allelles father not color-blind

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 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.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. X-linked Inheritance PLAY Animation 12.1: X-linked Recessive Traits

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Autosomal Genetic Disorders

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 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.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Autosomal Genetic Disorders Figure 12.3

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 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.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 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.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Autosomal Genetic Disorders Figure 12.4 S Ss s H h Hh hh h h father not sick mother not sick father sick mother not sick 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 percent chance that any given offspring will inherit the condition. In Huntington disease, if only a single parent has a Huntington allele there is a 50 percent chance that a son or daughter will inherit the condition. egg sperm egg SSSs ss (a) Sickle-cell anemia: transmission of a recessive disorder. (b) Huntington disease: transmission of a dominant disorder. 50% probability of inheriting the disorder 25% probability of inheriting the disorder

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Some Human Genetic Disorders PLAY Animation 12.2: Some Human Genetic Disorders

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Tracking Traits with Pedigrees

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 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.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Pedigrees Figure 12.5 I II III Aa A? aa femalemale normal carrier albino ?? Aa A? aa ? ? ???

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Aberrations in Chromosomal Sets: Polyploidy

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 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)

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 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.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Incorrect Chromosome Number: Aneuploidy

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 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.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Aneuploidy A small proportion of embryos survive aneuploidy, but the children who result from these embryos are born with such conditions as Down syndrome.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 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.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Nondisjunction Figure 12.7 NormalAbnormal % of gametes get normal number of chromosomes 100% of gametes get abnormal number of chromosomes 50% normal50% abnormal Nondisjunction in meiosis II Nondisjunction in meiosis I

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Aneuploidy Aneuploidy can come about in regular cell division (mitosis) as well as in meiosis.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 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.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Aneuploidy and Cancer Figure XY

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Structural Aberrations in Chromosomes

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Chromosomal Aberrations Harmful aberrations can occur within chromosomes, with many of these aberrations coming about because of mistakes in chromosomal interactions.

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Chromosomal Aberrations Chromosomal aberrations include: –deletions –inversions –translocations –duplications

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Chromosomal Aberrations Figure Deletion Inversion Translocation Duplication