Standard 3—Genetics Mendel’s Law 1

3. A multi-cellular organism develops from a single zygote, and its phenotype depends on its genotype, which is established at fertilization. 2

3a. Know how to predict the probable outcome of phenotypes in a genetic cross from the genotypes of the parents and mode of inheritance (autosomal or X-linked, dominant or recessive. 3

Copyright Pearson Prentice Hall Sex-Linked Genes Sex-Linked Genes The ___ chromosome and the __ chromosomes determine sex. Genes located on these chromosomes are called ________________ genes. More than ____ sex-linked genetic disorders have now been mapped to the X chromosome. Copyright Pearson Prentice Hall 4

Copyright Pearson Prentice Hall Sex-Linked Genes X Chromosome Duchenne muscular dystrophy Melanoma The __ chromosome is much smaller than the __ chromosome and appears to contain only a few genes. X-inactivation center X-linked severe combined immunodeficiency (SCID)‏ Colorblindness Hemophilia Genes on X and Y chromosomes, such as those shown in the diagrams, are called sex-linked genes. Y Chromosome Testis-determining factor Copyright Pearson Prentice Hall 5

Copyright Pearson Prentice Hall Sex-Linked Genes Why are sex-linked disorders more common in males than in females? Copyright Pearson Prentice Hall 6

Copyright Pearson Prentice Hall Sex-Linked Genes For a __________ allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes. Males have just ______ X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive. Copyright Pearson Prentice Hall 7

Copyright Pearson Prentice Hall Sex-Linked Genes Colorblindness Three human genes associated with color vision are located on the ______ chromosome. In males, a defective version of any one of these genes produces ________________. Copyright Pearson Prentice Hall 8

Copyright Pearson Prentice Hall Sex-Linked Genes Possible Inheritance of Colorblindness Allele X-linked alleles are always expressed in males, because males have only one X chromosome. Males who receive the recessive Xc allele all have colorblindness. Females, however, will have colorblindness only if they receive two Xc alleles. Copyright Pearson Prentice Hall 9

Copyright Pearson Prentice Hall Sex-Linked Genes Hemophilia The ___ ______________ also carries genes that help control blood clotting. A recessive allele in either of these two genes may produce hemophilia. In ______________, a protein necessary for normal blood clotting is missing. Hemophiliacs can bleed to death from cuts and may suffer internal bleeding if bruised. Copyright Pearson Prentice Hall 10

Copyright Pearson Prentice Hall Sex-Linked Genes Duchenne Muscular Dystrophy Duchenne muscular dystrophy is a _______________ disorder that results in the weakening and loss of skeletal muscle. It is caused by a defective version of the gene that codes for a muscle protein. Copyright Pearson Prentice Hall 11

X-Chromosome Inactivation British geneticist Mary Lyon discovered that in female cells, one ___ chromosome is randomly switched off. This chromosome forms a dense region in the nucleus known as a Barr body. Barr bodies are generally not found in males because their single ___ chromosome is still active. Copyright Pearson Prentice Hall 12

Chromosomal Disorders What problems does nondisjunction cause? The most common error in meiosis occurs when homologous chromosomes fail to separate. This is known as ________________, which means, “not coming apart.” Copyright Pearson Prentice Hall 13

Chromosomal Disorders Nondisjunction Homologous chromosomes fail to separate. Meiosis I: Nondisjunction Nondisjunction causes gametes to have abnormal numbers of chromosomes. The result of nondisjunction may be a chromosome disorder such as Down syndrome. Meiosis II Copyright Pearson Prentice Hall 14

Chromosomal Disorders Down Syndrome If ______ copies of an autosomal chromosome fail to separate during meiosis, an individual may be born with three copies of a chromosome. Down syndrome involves three copies of chromosome _____. Copyright Pearson Prentice Hall 15

Chromosomal Disorders Down Syndrome Karyotype Down syndrome produces mild to severe mental retardation. It is characterized by: increased susceptibility to many diseases higher frequency of some birth defects This karyotype is from a person with Down syndrome. Down syndrome causes mental retardation and various physical problems. People with Down syndrome can, however, lead active, happy lives. Photo credit: ©Dr. Dennis Kunkel/CNRI/Phototake Copyright Pearson Prentice Hall 16

Chromosomal Disorders Sex Chromosome Disorders In females, nondisjunction can lead to Turner’s syndrome. A female with Turner’s syndrome usually inherits only one _____ chromosome (karyotype 45,X). Women with Turner’s syndrome are sterile. Copyright Pearson Prentice Hall 17

Chromosomal Disorders In males, nondisjunction causes Klinefelter’s syndrome (karyotype 47,_________). The extra X chromosome interferes with meiosis and usually prevents these individuals from _______________. Copyright Pearson Prentice Hall 18