Biology~Chapter 12 Inheritance Patterns & Human Genetics
I. Chromosomes & Inheritance A.Thomas Hunt Morgan- early 1900s -experiments with fruit flies -observed 4 pairs of chromosomes -noticed that 3 pairs were the same in males & females but that 1 pair was different. - called these sex chromosomes.
B. Autosomes & Sex Chromosomes Sex chromosomes- contain genes that determine the gender of an individual. Autosomes- the remaining pairs of chromosomes that do not directly determine sex. In mammals - 2 X chromosomes= FEMALE (XX) - 1 X + 1 Y is a MALE (XY)
C. Males determine the sex of the offspring -each sperm has an equal chance of having an X or a Y - however- the only option for eggs is to receive an X chromosome. 1:1 male to female ratio
II. Linked & Sex Linked Genes are pairs of genes that tend to be inherited together.
A. Linked genes Genes which are close together on same chromosome. Linked genes do not exhibit Mendels law of independent assortment
B. Sex-Linked traits are traits that are coded for by alleles on a sex chromosome. Genes found on the X chromosome are X-linked genes Since the X chromosome is larger- there are more X-linked than Y- linked traits. NOTE: Since males have only 1 X- a male who carries the recessive allele will show the X-linked trait.
III. Mutations A. Mutation - a change in the nucleotide-base sequence of a gene or DNA 1. Germ cell- occurs in gametes - do not affect the organism itself 2. Somatic Cell-occur in organisms body cells - may affect the organism (ex-cancer) - NOT inherited 3. Lethal Mutations-cause death, often before birth 4. Beneficial Mutations- result in phenotypes that are beneficial.
B. Types of Chromosome Mutations 1. Deletion- loss of a piece of chromosome due to breakage. 2. Inversion a chromosome piece breaks off, flips around backwards & reattaches 3. Translocation a piece of chromosome breaks off & attaches to a non-homologous chromosome 4.Non-Disjunction a chromosomes fails to separate from its homologue during meiosis. Note human disorders that follow on next few slides:
Examples of Chromosomal Mutations: Deletion & Inversions http://staff.jccc.net/PDECELL/evolution/mutations/mutation.html
An example of a human disorder with a deletion error: Cri du chat syndrome- missing part of chromosome 5- cry of infants is similar to that of a meowing kitten, due to problems with the larynx and nervous system. About 1/3 of children lose the cry by age 2.
Types of non disjunction: Trisomy- an extra chromosome leads to an individual with an extra chromosome in every cell of his/her body. Monosomy- is a deficiency in number of chromosomes and is defined as only one copy of a chromosome that is normally present in two copies. When fertilized, the outcome is 45 chromosomes in total. Monosomies are less likely to survive when compared to trisomies.
What causes non-disjunction? The cause of non-disjunction is unknown. Non- disjunction seems to be a chance event. Nothing that an individual does or doesn't do during their reproductive years can cause these chromosomal changes. We do know that non-disjunction occurs more frequently in the eggs of women as they get older.
Non- Disjunction Disorders with an extra chromosome (these folks have 47 instead of the normal 46 chromosomes in people): Down syndrome- extra 21 Edwards Syndrome- extra 18 Patau syndrome extra 13
Examples Non disjunction: Extra #21= Downs Syndrome Extra #18= (most dont live beyond 1 st 7 months) This girl is now 6.) http://starbulletin.com/96/03/25/news/story2.html
Sex-chromosome abnormalities may also be caused by non-disjunction. Klinefelters Syndrome- XXY ( extra X) Turners Syndrome only 1 X- missing a second X (XO) Super males XYY Any combination (up to XXXXY) produces maleness. Males with more than one X are usually underdeveloped and sterile. XXX and XO women are usually sterile
C. Types of Gene Mutation 1. Point Mutation- the substitution, addition or removal of a single nucleotide, occurs within a single gene or segment of DNA 2. Substitiution- one nucleotide replaces another 3. Frameshift Mutation- if some nucleotides are deleted- whole segment moves 4. Insertion Mutation one or more nucleotides added- also causes a frameshift
Which type of gene mutation do you think would cause the most serious errors? Point mutation Substitution Frameshift
Example: substitution error Just 1 Amino Acid is a Substitution ERROR in Sickle Cell Hemoglobin Causes Hb to be sickle shaped instead of round- cant fit into red blood cells & changes their shape too.
Sickle Cell - Disease or Trait: Normally, a person inherits two genes (one from each parent) that produce normal hemoglobin (hemoglobin A). sickle cell traitA person with sickle cell trait inherits one normal beta-globin gene (hemoglobin A) and one defective gene (hemoglobin S) (this can actually be a good thing it provides some protection from malaria, which infects RBCs!) sickle cell diseaseA person with sickle cell disease inherits 2 bad copies- only makes HgS
IV. Human Genetics A. Inheritance of Traits 1. Pedigrees - be able to draw & read these! 2. Patterns - we learn about genetic disease by looking at patterns of inheritance over several generations. B. Genetic Traits & Disorders 1. Polygenic Inheritance 2. Complex Characters 3. Multiple Alleles 4. Incomplete Dominance 5. X-linked Traits 6. Sex- Influenced Traits 7. Single Allele Traits
Inheritance of Traits Pedigree Pedigree - a diagram that shows how a trait is inherited over several generations - see diagrams page 241 textbook
Reading a pedigree males are represented by squares females by circles. An individual who exhibits the trait, (for example, someone who suffers from Marfan syndrome), is represented by a filled symbol. A horizontal line between two symbols is a mating HE SHE DAD MOM
Reading a Pedigree The offspring : are connected to each other by a horizontal line above the symbols and to the parents by vertical lines.
Example pedigree: Is the mom or dad in generation I affected by a trait? How many offspring are shown in generation II? How many daughters & sons in generation II? How many have the trait? How many offspring does daughter #1 have? How many have the trait?
Dominant traits are passed on to a son or daughter EVEN if only one parent has it. Every affected individual has at least one affected parent Affected individuals who mate with unaffected individuals have a 50% chance of transmitting trait to each child Two affected parents may have unaffected children.
Recessive pedigrees:Recessive pedigrees: Where did the trait seen in generation III come from? Traits can be passed on to children if both parents, even if they may seem "normal, are carriers of the recessive trait.
Recessive traits are passed on to children from both parents, although the parents may seem perfectly "normal." Characteristics of recessive pedigrees are: An individual who is affected may have parents who are not affected; All the children of two affected individuals are affected; In pedigrees involving rare traits, the unaffected parents of an affected individual may be related to each other.
Recessive pedigree: If individuals 2 + 3 in generation 3 have more children- will they be affected? Can Individual #8 in gen. 3 have any unaffected children? Can individual #9 in gen 3 have affected children?
Example pedigree of cystic fibrosis www.cfscreening.com.au/.../CF/CFInherited.shtml
1.Polygenic Inheritance - traits influenced by several genes (most human characteristics) example- skin color is additive effect of 6 genes B. Genetic Traits & Disorders
2. Complex Characters traits influenced by both genes & environment Example- height B. Genetic Traits & Disorders
3. Multiple Alleles- genes with 3 or more alleles. Example- human blood type. IA, IB, i 3 alleles- both IA & IB are dominate over i, Codes for sugars that are on blood cell surface. four different blood types- A, B, AB, O B. Genetic Traits & Disorders
What are the 2 genotypes for type B? Which blood type can donate to all the others? (universal donor?) Which blood type makes antibodies (will clot against) both A & B blood types?
4. Incomplete Dominance there is an intermediate phenotype Human example- wavy hair is the intermediate between straight & curly hair (remember chapter 9 plant example: - pink 4 oclock flowers, In between red & white flowers) B. Genetic Traits & Disorders
5. X-linked Traits Traits that are on the X chromosome.Traits that are on the X chromosome. Since males only have one X chromosome- they are affected more than females Females have 2 chances to get a good copy of the gene but males only get 1 chance. B. Genetic Traits & Disorders
Examples of X-linked traits Hemophilia- blood clotting disorder- boys may bleed to death Duchene muscular dystrophy -affects cardiac and skeletal muscle, as well as some mental functions. Fragile-X syndrome- part of X chromosome has errors- most common cause of mental retardation in males Red-green color blindness- males cannot see these 2 colors.
6. Sex- Influenced Traits males & females show different phenotypes with same genotype- example- male pattern baldness B. Genetic Traits & Disorders
7. Single Allele Traits trait controlled by 1 allele- Dominant or Recessive Dominant Examples: Achondroplastic dwarfism- dwarf size Polydactly is the presence of a sixth digit. Huntington's disease (also called Woody Guthrie's disease)-progressive destruction of brain cells after age 30 Marfans Syndrome- progressive connective tissue disorder. (some cases are new mutations with no family history)
Single allele recessive traits: Albinism lack of pigmentation in skin, hair, and eyes, - Homozygous recessive (aa) people make no pigments PKU (Phenylketonuria) - lack enzyme to use the amino acid phenylalanine, buildups breakdown products leads to mental retardation. 1 in 15,000 infants has- treat with DIET! (no phenylalanine) Tay-Sachs Disease- degeneration of the nervous system. Children rarely survive past five years of age. Maple Syrup Urine Disease (MSUD) like PKU amino acids leucine, isoleucine, and valine
More single allele recessive examples: Sickel Cell Anemia- single amino acid substitution in hemoglobin causes lack of oxygen to body Cystic fibrosis- body creates sticky thick mucus chloride ions- detected in SWEAT TEST. - was once considered a fatal childhood disease- modern treatments can increase lifespan to 30 years plus.
C. Detecting Genetic Disease 1. Testing Amniocetesis- Dr removes some amniotic fluid, 14th-18th week of pregnancy, analyze fluid for proteins & look at chromosomes- some risk of pregnancy loss Chorionic Villi Sampling- clip cells from placental area- 8-14th week pregnancy 2. Treatments- vary with disease
Huntingtons Disease Downs Syndrome Cystic Fibrosis Duchenne muscular dystrophy Sickle Cell Anemia Trisomy 18 (Edwards) Tay-Sachs disease Color blindness PKU (Phenylketonuria) Fragile X syndrome Thalassemia Marfan syndrome Breast Cancer Hemophilia Cri du Chat Polydactyly Maple Syrup urine disease Turner Syndrome Klinefelter's syndrome, (XXY syndrome) Super males (XYY) Patau syndrome Albinism See me about other topics List of Human genetic disorders- Pick 1 for your project.