Genetics 3 Flashcards
(12 cards)
Pedigree Analysis
• A pedigree is a family tree that describes the interrelationships of parents and children across generations
• Inheritance patterns of particular traits can be traced and described using pedigrees
• Pedigrees can also be used to make predictions about future offspring.
• We can use the multiplication and addition rules to predict the probability of specific phenotypes
Recessive inherited disorders
• Thousands of genetic disorders are known to be inherited as simple recessive traits.
• These disorders range in severity from relatively mild, such as albinism (lack of pigmentation, which results in susceptibility to skin cancer and vision problems), to life-threatening such as cystic fibrosis.
• Both copies of the same gene (one inherited from each parent) must be present for the condition to be apparent.
• The parents may not be affected themselves but parents are carriers of the condition
• If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low.
• Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele.
• Most societies and cultures have laws or taboos against marriages between close relatives.
• Albinism (lack of pigmentation, which results in susceptibility to skin cancer and vision problems
• Due to a mutation in a single gene (melanin).
• The change results in the absence or alteration of the corresponding protein.
• Both copies of the same gene (one inherited from each parent) must be changed for the condition to be apparent.
• The parents may not be affected themselves but parents are carriers of the condition
Cystic Fibrosis
• Cystic fibrosis is the most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent.
• Among people of European descent, one out of 25 (4%) are carriers of the cystic fibrosis allele.
• The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes
• The normal allele for this gene codes for a membrane protein that functions in the transport of chloride ions between certain cells and the extracellular fluid.
• These chloride transport channels are defective or absent in the plasma membranes of children who inherit two recessive alleles for cystic fibrosis.
• The result is abnormally high concentration of intracellular chloride.
• This in turn causes the mucus that coats certain cells to become thicker and stickier than normal.
• The mucus builds up in the pancreas, lungs, digestive tract, leading to multiple effects
• poor absorption of nutrients from intestines
• chronic bronchitis
• recurrent bacterial infections
• Untreated, cystic fibrosis can cause death by the age of 5.
• Treatment: daily doses of antibiotics, pounding on the chest to clear mucus from clogged airway
Sickle-cell disease
• Sickle-cell disease affects one out of 400 African-Americans
• The disease is caused by the substitution of a single amino acid in the haemoglobin protein in red blood cells
• Symptoms include physical weakness, pain, organ damage, and even paralysis
Dominantly inherited disorders
• Some human disorders are caused by dominant alleles
• Dominant alleles that cause a lethal disease are rare and arise by mutation
• A person needs only one copy of the nontypical gene to develop the disorder
• A lethal dominant allele often causes the death of afflicted individuals before they can mature and reproduce, and the allele is not passed on to future
generations.
• A lethal dominant allele may be passed on if the lethal disease symptoms first appear after reproductive age.
• In these cases the individual may already have
transmitted the allele to the children
Dominantly inherited disorders
Huntington’s Disease.
• This condition is a degenerative disease of the nervous system.
• The condition is caused by a lethal dominant allele that has no obvious phenotypic effect until the individual is about 35 to 45 years old.
• Mutated huntingtin (Htt) protein - neurodegeneration.
• A child born to a parent with Huntington’s disease allele has a 50% chance of inheriting the allele and disorder
Multifactorial Disorders
• Many people are susceptible to diseases that have a multifactorial basis.
• A genetic component plus a significant environmental influence are involved.
• Heart disease, diabetes, cancer, certain mental illnesses such as schizophrenia and bipolar disorder are all examples of multifactorial disorders.
• In this case, the hereditary component is polygenic.
• For example, many genes affect cardiovascular health, making some of us prone than others to heart attacks and strokes.
• No matter what our genotype, our lifestyle has a tremendous effect on phenotype for cardiovascular health and other multifactorial characters
X-linked inheritance and its specific characteristics
• Humans and other mammals have two types of sex chromosomes: X and Y.
• The Y chromosome is much smaller than the X chromosome.
• A person who inherits two X chromosomes, one from each parent, usually develops anatomy we associate with the «female» sex
• «male» properties are associated with the inheritance of one X chromosome and one Y chromosome
• In mammalian testes and ovaries, the two sex
chromosomes segregate during meiosis.
• Each egg receives one X chromosome.
• Sperm fall into two categories: half the sperm
cells a male produces receive an X chromosome, and half receive a Y chromosome.
• Both X-Linked dominant and X-Linked recessive
conditions are known.
• Most commonly inherited X-Linked disorders are
recessive
Hemophilia: the Royal Disease
• Hemophilia is an X-linked recessive disorder defined by the absence of one or more of the proteins required for blood clotting.
• In the 1800s, hemophilia was widespread among the royal families of Europe (Queen Victoria of England).
• Today, people with hemophilia are treated as needed with intravenous injections of the protein that is missing
Linked genes
• Linked genes tend to be inherited together because they are located near each other on the same chromosome
Genetic Linkage
• It is important to note that there is an exception to the law of independent assortment for genes that are located very close to one another on the same chromosome because of genetic linkage.
• When two genes are close together on the same chromosome, they do not assort independently and are said to be linked. Alleles of genes located on the same chromosome will be inherited together
Linked genes
• Genetic linkage describes the way in which two genes that are located close to each other on a chromosome are frequently inherited together.
• The closer two genes are to one another on a chromosome, the greater their chances are of being inherited together or linked.
• In contrast, genes located farther away from each other on the same chromosome are more likely to be separated during recombination, the process that recombines DNA during meiosis.
• The strength of linkage between two genes, therefore, depends upon the distance between the genes on the chromosome.
Alterations of Chromosome Number or Structure cause some Genetic Disorders.
• Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortion (miscarriage) of a fetus, and individuals born with these types of genetic defects commonly exhibit various developmental disorders.
Nondisjunction event: the members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister
chromatids fail to separate during meiosis II
• Aneuploidy results from the fertilization of gametes in which nondisjunction occurred.
• A monosomic zygote has only one copy of a particular chromosome.
• A trisomic zygote has three copies of a particular chromosome.
• One aneuploid condition, Down syndrome, affects one out of every 444 children born in the Ireland.
• Down syndrome is usually the result of an extra chromosome 21, so that each body cell has a total of 47 chromosomes. Because the cells are trisomic for chromosome 21, Down syndrome is often called trisomy 21.
• Down syndrome includes characteristic facial features, short stature, heart defects and developmental delays.
• The frequency of Down syndrome increases with the age of the mother.
Alterations of Chromosome Structure
• Breakage of a chromosome can lead to four types of changes in chromosome structure.
• Deletion removes a chromosomal segment.
• Duplication repeats a segment.
• Inversion reverses a segment within a chromosome.
• Translocation moves a segment from one chromosome to another.
