- Slides: 56
Ch 7 Extending Mendelian Genetics
Studying Human Genetics • A pedigree is a chart for tracing genes in a family. – Used to determine the chances of offspring having a certain genetic disorder. • Karyotype=picture of all of the chromosomes in a cell
Gene Expression • Related to whether a gene is located on an autosome or a sex chromosome – Sex chromosomes=determine an organism’s sex – Autosome=all the other chromosomes, determine everything but the sex of an organism
Autosomes-pairs 1 -22 Sex chromosomes-X and Y
Genes vs. Alleles • We have two of each chromosome – One from each parent • Both chromosomes have the same gene, but may be different alleles for those genes • Different alleles can produce different phenotypes – Ex: white or purple flowers
Phenotype can depend on interactions of alleles. • Incomplete dominance=neither allele is completely dominant nor completely recessive. • Codominant alleles =both alleles will be completely expressed. – Codominant alleles are neither dominant nor recessive. – The ABO blood types result from codominant alleles.
Blood Types • Example of codominant alleles – Three alleles: A, B, and O, at the ABO locus on chromosome – The expression of the O allele is recessive to that of A and B, which are said to be codominant. • Blood types: A, O, B, or AB • 40% of the population is type O+
Blood Types Blood Type Can Donate Blood To: Can Receive Blood From: A A, AB A, O B B, AB B, O AB AB AB, O O A, B, AB, O O
Mendel • All of the traits Mendel studied are determined by genes on autosomes. – Most traits are the result of autosomal genes, not sex linked genes. • Ex: hair color, hair texture, eye color are all affected by autosomal genes
Inheritance Patterns • Refers to how genes are passed from parents to their offspring. • Also referred to as Mendelian inheritance. • Four types of inheritance patterns: – Autosomal Dominant – Autosomal Recessive – Sex-Linked Dominant – Sex-Linked Recessive
Genetic Disorders • Many human genetic disorders are caused by autosomal genes. • These types of disorders are either caused by the dominant allele or the recessive allele. – Dominant genetic disorders are far less common than recessive disorders.
Autosomal Dominant Disorders • Dominant inheritance means an abnormal gene from one parent is capable of causing disease, even though the matching gene from the other parent is normal. – The abnormal gene "dominates" the pair of genes. – only need to get one abnormal gene from one parent in order for you to inherit the disease. – One of the parents may often have the disease. • If just one parent has a dominant gene defect, each child has a 50% chance of inheriting the disorder.
Disorders Caused by Dominant Alleles • Huntington’s disease – Named after the first physician to document the disorder – Damages nervous system and appears in adulthood • Affects movement – 50% chance that a child will have the disease even if only one parent has one of the alleles. – 75% chance of child of two heterozygous parents inheriting the disease
Disorders Caused by Dominant Alleles • Debbie Viau suffered her onset of Huntington's Disease 13 years ago. • Her family founded Twin Cities Hoop-a. Thon to raise money for research • Families’ grandmother and great-grandfather also had Huntington’s.
Disorders Caused by Dominant Alleles • Neurofibromatosis • Marfan Syndrome • Achondroplasia
Disorders Caused by Recessive Alleles • Some human genetic disorders are caused by recessive alleles on autosomes. – 2 copies of recessive alleles must be present for the trait to show in the individual
Disorders Caused by Recessive Alleles • Person who is homozygous for the recessive allele WILL have the disease • Disorders often appear in offspring with two heterozygous parents – Parents are carriers=does not show disease symptoms, but can pass on the disease-causing allele to their offspring – Each parent has the dominant “normal” allele that masks the disease-causing recessive allele
Disorders Caused by Recessive Alleles • Cystic Fibrosis – Recessive disorder that mainly affects the sweat and mucous glands • Affects the lungs and pancreas • Accumulation of thick mucous can lead to blockage of the airways and then infection • Chronic lung disease can lead to cardiac failure which requires heart-lung transplant – Most males with CF are sterile-they cannot reproduce – 1955 -life expectancy was less than 5 years…present life expectancy-30 years
Disorders Caused by Recessive Alleles • Cystic Fibrosis
Disorders Caused by Recessive Alleles • Sickle Cell Anemia – Blood disorder that affects hemoglobin-the protein found in red blood cells (RBCs) that helps carry oxygen throughout the body. – Affected individuals’ red blood cells are shaped like a crescent moon instead of flexible and discshaped • Sickle cells do not flow in vessels as easily which can lead to clogs
Disorders Caused by Recessive Alleles
Disorders Caused by Recessive Alleles
Disorders Caused by Recessive Alleles • More than 70, 000 Americans have sickle cell anemia. • About 2 million Americans — 1 in 12 African Americans — have sickle cell trait, which means they carry a single gene for the disease and can pass this gene along to their children, but do not have the disease itself.
Disorders Caused by Recessive Alleles • Phenylketonuria (PKU) – Deficiency of enzyme phenylalanine hydroxylase – If untreated, can result in severe mental retardation – PKU test performed on newborns during their first week of life
Sex Linked Genes • Genes that are located on the sex chromosomes • Females=XX genotype – Pass only X chromosome to offspring • Males=XY genotype – Can pass either X or Y to offspring • Y chromosome contains gene responsible for development of males • X chromosome has more influence over phenotype – Has many genes that affect many traits
Sex-Linked Disorders • X-linked diseases usually occur in males. Males have only one X chromosome. A single recessive gene on that X chromosome will cause the disease. • The Y chromosome is the other half of the XY gene pair in the male. However, the Y chromosome doesn't contain most of the genes of the X chromosome. – This is seen in diseases such as hemophilia and Duchenne muscular dystrophy.
Sex Linked Disorders • Hemophilia – bleeding disorder where the blood does not clot normally. – Persons with hemophilia may bleed for a longer time than others after an injury or accident. • They also may bleed internally, especially in the joints (knees, ankles, and elbows). – Carried on X chromosome so hemophiliac patients are usually male
Sex Linked Disorders • Hemophilia in Queen Victoria’s descendants
Sex Linked Disorders • 55 -year-old male hemophiliac with a large bruise (hematoma) on his leg, one week after falling from a bike.
Chromosome Disorders • Chromosome disorders are due to errors on any of the body's 23 pairs of chromosomes – Errors include broken or missing pieces or too many pieces or copies. • Karyotypes are used to show chromosomal disorders.
Karyotype • Chemicals are used to stain the chromosomes – Chemical stains produce a pattern of bands on the chromosomes • Band size and location are consistent for each chromosome • Bands differ greatly among different chromosomes which helps with identifying chromosomes XY
Chromosome Disorders • Classified as one of the following: – Monosomy-one member of homologous pair is lost – Trisomy-3 chromosomes instead of two – Triploidy-3 times the haploid number – Tetraploidy-2 times the diploid number
Chromosome Disorders • Down Syndrome – Most common genetic disorder in children – The cause of Down syndrome is due to one of the three: 1. Trisomy 21. 2. Mosaic Down syndrome. 3. Translocation Down syndrome.
Chromosome Disorders • Trisomy 21. More than 90 percent of cases of Down syndrome are caused by trisomy 21. • A child with trisomy 21 has three copies of chromosome 21 — instead of the usual two copies — in all of his or her cells. • This form of Down syndrome is caused by abnormal cell division during the development of the sperm cell or the egg cell.
Chromosome Disorders • Mosaic Down syndrome. – In this rare form of Down syndrome, children have SOME CELLS with an extra copy of chromosome 21. This mosaic of normal and abnormal cells is caused by abnormal cell division after fertilization.
Chromosome Disorders • Translocation Down syndrome. – Down syndrome can also occur when part of chromosome 21 becomes attached (translocated) onto another chromosome, before or at conception. – Children with translocation Down syndrome have the usual two copies of chromosome 21, but they also have additional material from chromosome 21 stuck to the translocated chromosome. – This form of Down syndrome is uncommon.
Chromosome Disorders • Symptoms of Down Syndrome – Lifelong mental retardation – Flattened facial features – Protruding tongue – Small head – Upward slanting eyes, unusual for the child's ethnic group – Unusually shaped ears
Chromosome Disorders • Inheritance of Down Syndrome – Most cases of Down syndrome aren't inherited. – They're caused by a mistake in cell division during the development of the egg, sperm or embryo. – Translocation Down syndrome is the only form of the disorder that can be passed from parent to child.
Chromosome Disorders • Risk factors of Down Syndrome – Advancing maternal age. • A woman's chances of giving birth to a child with Down syndrome increase with age because older eggs have a greater risk of improper chromosome division. By age 35, a woman's risk of conceiving a child with Down syndrome is 1 in 400. By age 45, the risk is 1 in 35. However, most children with Down syndrome are actually born to women under age 35 because younger women have far more babies. – Having had one child with Down syndrome. – Being carriers of the genetic translocation for Down syndrome.
Chromosome Disorders • Tests and Diagnosis of Down Syndrome – Ultrasound – Blood Test – Amniocentesis – Chorionic villus sampling (CVS) – Percutaneous umbilical blood sampling (PUBS)
Chromosome Disorders • • • Patau Syndrome (Trisomy 13) Angelman Syndrome (Chromosome 15) Prader-Willi Syndrome (Chromosome 15) Klinefelter Syndrome (47, XXY) Fragile X Syndrome Turner Syndrome
Fragile X Syndrome
Turner Syndrome Women have only one X chromosome
Pedigrees • A chart used to trace genes in a family. – Used to determine the chances of offspring having a certain genetic disorder.
Human Pedigree • Shows several types of information: – Boxes=males – Circles=females – Shaded shape=person shows trait – White shape=person does not carry the trait – Half-shaded shape=person is a carrier – Lines connect a person to his or her mate, and to their children.
Sample Pedigree http: //www. zerobio. com/drag_gr 11/pedigree_overview. htm
Genetic Counseling • Genetic counselors can help identify and interpret the risks of an inherited disorder, explain inheritance patterns, suggest testing, and lay out possible scenarios. – They refer you to a doctor or a laboratory for the actual tests. • The will explain the meaning of the medical science involved, provide support, and address and emotional issues raised by the results of the genetic testing.
Genetic Counseling • Who should seek genetic counseling? – Patient’s who’s genetic tests were abnormal. – Inherited disease or birth defect present in family – Mother-to-be has had 2 or more miscarriages – Mother-to-be will be 35 or older when the baby is born – Concerns about ethnic or race related disorders
Genetic Counselors • Masters degree required • Certification is obtained through successful completion of documented clinical experience and the American Board of Genetic Counseling’s examination • Areas of work: – Clinical – Commercial – Diagnostic Laboratories – Education and Public Policy – Research • Annual salary range: $40, 900 -$61, 700