Copyright Pearson Prentice Hall 14–1 Human Heredity 14-1 Human Heredity Photo credit: Richard Hutchings/Photo Researchers, Inc. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes Human Chromosomes Cell biologists analyze chromosomes by looking at karyotypes. Cells are photographed during mitosis. Scientists then cut out the chromosomes from the photographs and group them together in pairs. A picture of chromosomes arranged in this way is known as a karyotype. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes Human Karyotype These human chromosomes have been cut out of a photograph and arranged to form a karyotype. Photo credit: ©CNRI/Science Photo Library/Photo Researchers, Inc. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes Two of the 46 human chromosomes are known as sex chromosomes, because they determine an individual's sex. Females have two copies of an X chromosome. Males have one X chromosome and one Y chromosome. The remaining 44 chromosomes are known as autosomal chromosomes, or autosomes. Copyright Pearson Prentice Hall

Mendel’s rules of inheritance apply to autosomal genetic disorders where two alleles interact to produce a phenotypic trait. A heterozygote for a recessive disorder is a carrier. Disorders caused by dominant alleles are uncommon. (dominant)

Males and females can differ in sex-linked traits. Genes on sex chromosomes are called sex-linked genes. Y chromosome genes in mammals are responsible for male characteristics. X chromosome genes in mammals affect many traits.

Male mammals have an XY genotype. All of a male’s sex-linked genes are expressed because males have no second copies of sex-linked genes.

Female mammals have an XX genotype. Expression of sex-linked genes is similar to autosomal genes but one X chromosome in each cell is randomly “turns off” through X chromosome inactivation.

Chromosomal Disorders What problems does nondisjunction cause? Copyright Pearson Prentice Hall

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

Chromosomal Disorders If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result. An individual may be born with three copies of a chromosome or only 1 copy. Copyright Pearson Prentice Hall

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

Chromosomal Disorders Down Syndrome Karyotype Down syndrome produces mild to severe mental retardation. It is characterized by increased susceptibility to many diseases and higher frequency of other 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

Chromosomal Disorders Down Syndrome Down syndrome involves three copies of chromosome 21. This is also called Trisomy 21 and occurs in about 1 in 1000 births Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Edward’s Syndrome 50% of babies who are carried to term will be born alive, and baby girls will have higher rates of live birth than baby boys. Significant developmental delays that do not allow them to live independently without full time caregiving A Trisomy 18 error occurs in about 1 out of every 2500 pregnancies in the United States and 1 in 6000 live births Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Patau Syndrome Most fetuses with Patau are miscarried spontaneously. Trisomy 13 occurs in about 1 in 9,500 births Fetal or congenital malformation is usual, with holoprosencephaly (the failure of the forebrain to develop into two hemispheres), microphthalmia (small eye), cleft lip and palate, and polydactyly (extra digits), cardiac malformations, severe growth, mental retardation and kidney malformations are also associated with the condition. Copyright Pearson Prentice Hall

Chromosomal Disorders Sex Chromosome Disorders In females, nondisjunction can lead to Turner’s syndrome. A female with Turner’s syndrome usually inherits only one X chromosome (karyotype 45,X). Instead of an addition of a chromosome, there is a deletion. Women with Turner’s syndrome are sterile. Copyright Pearson Prentice Hall

Chromosomal Disorders Nondisjunction causes Klinefelter’s syndrome, an extra X on the 23rd pair. (karyotype 47,XXY for males or karyotype 47 XXX for females). The extra X chromosome interferes with meiosis and usually prevents these individuals from reproducing. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes How is sex determined? Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes All human egg cells carry a single X chromosome (23,X). Half of all sperm cells carry an X chromosome (23,X) and half carry a Y chromosome (23,Y). About half of the zygotes will be 46,XX (female) and half will be 46,XY (male). Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Chromosomes Males and females are born in a roughly 50 : 50 ratio because of the way in which sex chromosomes segregate during meiosis. In humans, egg cells contain a single X chromosome. Sperm cells contain either one X chromosome or one Y chromosome. In a population, approximately half of the zygotes are XX (female) and half are XY (male). Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Traits Pedigree Charts  A pedigree chart shows the relationships within a family. Genetic counselors analyze pedigree charts to infer the genotypes of family members. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Traits A square represents a male. A circle represents a female. A vertical line and a bracket connect the parents to their children. A horizontal line connecting a male and a female represents a marriage. A circle or square that is not shaded indicates that a person does not express the trait. A shaded circle or square indicates that a person expresses the trait. This drawing shows what the symbols in a pedigree represent. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes Human Genes The human genome includes tens of thousands of genes. In 2003, the DNA sequence of the human genome was published. In a few cases, biologists were able to identify genes that directly control a single human trait such as blood type. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes Recessive Alleles Many disorders are caused by autosomal recessive alleles. Both alleles must be present in order to inherit the disease. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes This table shows the major symptoms of some well-known genetic disorders. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes Dominant Alleles The effects of a dominant allele are expressed even when the recessive allele is present. Two examples of genetic disorders caused by autosomal dominant alleles are achondroplasia and Huntington disease. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes This table shows the major symptoms of some well-known genetic disorders. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes Codominant Alleles In codominant inheritance, two different versions (alleles) of a gene can be expressed, and each version makes a slightly different protein. Both alleles influence the genetic trait or determine the characteristics of the genetic condition. Sickle cell disease is a serious disorder caused by a codominant allele. Sickle cell is found in about 1 out of 500 African Americans. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Human Genes Copyright Pearson Prentice Hall

Interpreting a Pedigree Chart To determine whether the disorder is dominant or recessive If the disorder is dominant, one of the parents must have the disorder If the disorder is recessive, neither parent has to have the disorder because they can be heterozygous (hidden recessive gene). They are called carriers. Copyright Pearson Prentice Hall

Interpreting a Pedigree Chart To determine if the pedigree chart show an autosomal or X-linked disease If most of the males in the pedigree are affected, then the disorder is X-linked If it is a 50/50 ratio between men and women, then the disorder is autosomal Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Rules of Inheritance If a trait is autosomal dominant Appears in both sexes with equal frequency Both sexes transmit the trait to their offspring Does not skip generations Affected offspring must have an affected parents Unaffected parents do not transmit the trait Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Rules of Inheritance If a trait is autosomal recessive Both parents are carriers Appears in both sexes equally Affected offspring usually have unaffected parent Appears more frequently among children of consanguine (blood relatives) marriages Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Rules of Inheritance If a trait is sex-linked dominant Both males and females are affected Does not skip generations Affected daughter must have an affected parent Affected son must have an affected mother Affected fathers will always pass the trait to their daughter Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Rules of Inheritance If a trait is X-linked recessive More males than females are affected Affected sons are usually born to unaffected moms, therefore the trait skips generations It is never passed from father to son All daughters of affected fathers are carriers Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule From Gene to Molecule How do small changes in DNA cause genetic disorders? Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule In both cystic fibrosis and sickle cell disease, a small change in the DNA of a single gene affects the structure of a protein, causing a serious genetic disorder. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule Cystic Fibrosis Cystic fibrosis is caused by a recessive allele. Sufferers of cystic fibrosis produce a thick, heavy mucus that clogs their lungs and breathing passageways. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule The most common allele that causes cystic fibrosis is missing 3 DNA bases. As a result, the amino acid phenylalanine is missing from the CFTR protein. Cystic fibrosis is usually caused by the deletion of three bases in the DNA of a single gene. As a result, the body does not produce normal CFTR, a protein needed to transport chloride ions. Cystic fibrosis causes serious digestive and respiratory problems. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule Normal CFTR is a chloride ion channel in cell membranes. Abnormal CFTR cannot be transported to the cell membrane. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule The cells in the person’s airways are unable to transport chloride ions. As a result, the airways become clogged with a thick mucus. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule Sickle Cell Disease Sickle cell disease is a common genetic disorder found in African Americans. It is characterized by the bent and twisted shape of the red blood cells. These red blood cells contain the abnormal hemoglobin characteristic of sickle cell disease. Photo credit: ©Omikron/Photo Researchers, Inc. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule Hemoglobin is the protein in red blood cells that carries oxygen. In the sickle cell allele, just one DNA base is changed. As a result, the abnormal hemoglobin is less soluble than normal hemoglobin. Low oxygen levels cause some red blood cells to become sickle shaped. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule People who are heterozygous for the sickle cell allele are generally healthy and they are resistant to malaria. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule There are three phenotypes associated with the sickle cell gene. An individual with both normal and sickle cell alleles has a different phenotype—resistance to malaria—from someone with only normal alleles. Sickle cell alleles are thought to be codominant. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall From Gene to Molecule Malaria and the Sickle Cell Allele The map on the left shows where malaria is common. The map on the right shows regions where people have the sickle cell allele. Regions where malaria is common Regions where the sickle cell allele is common Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 14–1 Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 14–1 A chromosome that is not a sex chromosome is know as a(an) autosome. karyotype. pedigree. chromatid. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 14–1 Whether a human will be a male or a female is determined by which sex chromosome is in the egg cell. autosomes are in the egg cell. sex chromosome is in the sperm cell. autosomes are in the sperm cell. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 14–1 Mendelian inheritance in humans is typically studied by making inferences from family pedigrees. carrying out carefully controlled crosses. observing the phenotypes of individual humans. observing inheritance patterns in other animals. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 14–1 An individual with a blood type phenotype of O can receive blood from an individual with the phenotype O. A. AB. B. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 14–1 The ABO blood group is made up of two alleles. three alleles. identical alleles. dominant alleles. Copyright Pearson Prentice Hall

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