Chapter 14
Concept 14.1:
Mendel used the scientific approach to identify two laws of inheritance
Concept 14.4:
Many human traits follow Mendelian patterns of inheritance
Concept 14.3:
Inheritance patterns are often more complex than predicted by simple Mendelian genetics
Concept 14.2:
Probability laws govern Mendelian inheritance
Drawing from the Deck of Genes
~ The explanation of heredity most widely in favor during the 1800s was the concept called "blending hypothesis".
- the idea to 2 genetic material from 2 parents.
~ Alternative - "Participate Hypothesis"
- the gene idea; a collection of genes.
Mendel's Experiment, Quantitative Approach
~ Background: Small farming region in Austria - now the Czech Republic.
- Received agricultural training along with basic education.
- 1843, 21 yrs old, stayed at the Augustinian monastery.
- 1851, he left to pursue studies in physics and chemistry at the University of Vienna.
- Inspired by Christain Doppler (Physicist) and Franz Unger (Botanist).
- 1857, began the study of hereditary using garden peas.
The Law of Segregation
~ If blending model of inheritance were correct, F1 hybrids from cross between purple-flowered and white-flowered pea plants would have pale purple flowers, a trait intermediate between those of the P generation.
- 705 of F2 plants had purple; dominant.
- 224 of F2 plants had white; recessive.
Mendel's Model
~ Made a model to explain the 3:1 inheritance pattern to observe the F2 offspring.
- First: alternative versions of genes account for variations in inherited characters.
~ Alleles: Alternative versions of a gene.
Useful Genetics Vocabulary
~ Homozygote: An organism that has a pair of identical alleles for a gene encoding a character - Homozygous. (PP or pp)
~ Heterozygote: An organism that has 2 different alleles for a gene - Heterozygous. (Pp)
~ Phenotype: Organism's appearance/ observable traits.
~ Genotype: Genetic makeup.
The Testcross
The Law of Independent Assortment
~ Monohybrids: a genetic cross made to examine the distribution of one specific set of alleles in the resulting offspring.
~ Monohybrid Cross: a genetic mix between two individuals who have homozygous genotypes, or genotypes that have completely dominant or completely recessive alleles, which result in opposite phenotypes for a certain genetic trait.
The laws of segregation and independent assortment reflect the same rules of probability.
The Multiplication and Addition Rules Applied to Monohybrid Crosses
~ Multiplication Rule: to determine probability, multiply the probability of 1 event by the other event.
1/2 x 1/2 = 1/4
Solving Complex Genetics Problems with the Rules of Probability
~ Same sets of rules but to multiple of variables/characters.
- Dihybrid/other multi-character cross is equivalent to 2 or more independent monohybrid crosses occurring simultaneously.
Extending Mendelian Genetics for Two or More Genes
Epistasis
~ Definition: the interaction between genes that influences a phenotype.
- They can affect one another in such a way that, regardless of the allele of one gene, it is recessive to one dominant allele of the other.
Polygenic Inheritance:
~ Definition: The determination of a particular characteristic: height or skin color, by many genes (polygenes), each having a small effect individually.
- Characteristics controlled in this way show continuous variation.7
~ Quantitative Characters: a feature, that is either present or not present, depending on whether the gene responsible for that trait is present (or functional) or absent (or non-functional).
Nature and Nurture: The Environmental Impact on Phenotype
~ Multifactorial: The type of hereditary pattern seen when there is more than one genetic factor involved and, sometimes, when there are also environmental factors participating in the causation of a condition.
A Mendelian View of Heredity and Variation
~ The key to make the transition from the reductionist emphasis on single genes and phenotypic characters to the emergent properties of the organisms as a whole.
~ Character: Heritable features that varies among individuals.
~ Trait: Each variant for a character.
~ True-Breeding: Over many generations of self-pollination, these plants had produced only the same variety as the parent plant.
~ Hybridization: Mating/Crossing of true-breeding varieties.
~ P Generation: True-Breeding parents; Parental Generation.
~ F1 Generation: Hybrid offspring; First filial generation.
- Self-pollinate or cross with other F1 hybrids.
~ F2 Generation: Product of the F1 Generation.
~ 2nd: For each character, an organism inherits 2 copies (2 alleles) of a gene, 1 from each parent.
~ 3rd: 2 alleles at a locus differ.
- Dominant Allele: Determines the organism's appearance.
- Recessive Allele: has no noticeable effect on the organism's appearance.
~ 4th: The Final; The law states that 2 alleles for a heritable character segregate during gamete formation and end up in different gametes.
~ Dihybrids: individuals heterozygous for the 2 characters being followed in the cross.
~ Dihybrid Cross: a mating experiment between two organisms that are identically hybrid for two traits. A hybrid organism is one that is heterozygous, which means that is carries two different alleles at a particular genetic position, or locus.
Example: Coin Tossing
Scale: 0 - 1
Heads: 1/2 ; Tails: 1/2
Example: Deck of Cards
Goal: Ace of spades
Probability: 1/52 ; Other: 51/52
~ Addition Rule: probability that 2 or more mutually exclusive events will occur is calculated by adding their individual probabilities.
- 1/4 + 1/4 = 1/2
Extending Mendelian Genetics for a Single Gene
~ The inheritance of characters determined by a single gene deviates from simple Mendelian patterns when alleles are not completely dominant pr recessive, when a particular gene has more than 2 alleles.
Degrees of Dominance
~ Complete Dominance: a form of dominance in heterozygous condition wherein the allele that is regarded as dominant completely masks the effect of the allele that is recessive.
~ Incomplete Dominance: a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele.
- This results in a third phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles.
Co-Dominance: a relationship between two versions of a gene. Individuals receive one version of a gene, called an allele, from each parent.
- If the alleles are different, the dominant allele usually will be expressed, while the effect of the other allele, called recessive, is masked.
The Relationship Between Dominance and Phenotype
~ When a dominant allele coexists with a recessive allele in a heterozygote, they don't actually interact at all.
~ Tay-Sachs Disease: A genetic metabolic disorder caused by deficiency of the enzyme hexosaminidase A (hex-A) that results in a failure to process a lipid called GM2 ganglioside that accumulates in the brain and other tissues.
- Molecular Level: Co-Dominant.
Multiple Alleles
~ Only 2 alleles exist for the pea characters that Mendel studied, but most genes exist in more than 2 allelic forms.
Pleiotropy
Definition: mutation in one gene that causes a disease with a wide range of symptoms, phenomenon in which a single gene contribution to multiple phenotypic traits.
Example:
~ Dominant: Black
~ Recessive: Brown
~ E: Black/Brown Pigment
~ e: yellow coat
Pedigree Analysis
~ Pedigree: a diagram that depicts the biological relationships between an organism and its ancestors.
- A pedigree is used for different animals, such as humans, dogs, and horses.
Often, it is used to look at the transmission of genetic disorders.
Recessively Inherited Disorders
~ Range from Albinism to life threatening.
Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications
~ Sickle-Cell Disease: A genetic blood disease due to the presence of an abnormal form of hemoglobin, namely hemoglobin S. Hemoglobin is the molecule in red blood cells that transports oxygen from the lungs to the farthest areas of the body.
Genetic Testing and Counseling
~ Many hospitals have genetic counselors who can provide information to prospective parents concerned about a family history for a specific disease.
Multifactorial Disorders
~ A genetic component plus a significant environmental influence.
- heart disease
- diabetes
- cancer
- alcoholism
- certain mental illness
Dominantly Inherited Disorders
~ Huntington's Disease: an autosomal dominant pattern of inheritance with high penetrance (a high proportion of persons with the gene develop the disease). The characteristic findings of Huntington disease are caused by loss of neurons (nerve cells) in the brain.
Fetal Testing
~ Amniocentesis: a procedure used to diagnose fetal defects in the early second trimester of pregnancy. A sample of the amniotic fluid, which surrounds a fetus in the womb, is collected through a pregnant woman's abdomen using a needle and syringe.
~ Chorionic Villus Sampling: a prenatal test in which a sample of chorionic villi is removed from the placenta for testing. The sample can be taken through the cervix (transcervical) or the abdominal wall (transabdominal).
Counseling Based on Mendelian Genetics and Probability Rules
~ When using Mendel's laws to predict possible outcomes or matings, it is important to remember that each child represents an independent event in the sense that its genotype is unaffected be the genotypes of older siblings.
Tests for Identifying Carriers
~ Most children with recessive disorders are born to parents with normal phenotypes.
- The key: to accurately assess the genetic risk for a particular disease is therefore to find out whether the prospective parents are heterozygous carriers of the recessive allele.
Newborn Screening
~ Some genetic disorders can be detected at birth by simple biochemical tests that are now routinely performed in most hospitals in the United States.
- Identyfying carrriers
The Behavior of Recessive Alleles
~ Hoe can we account for the behavior of alleles that cause recessively inherited disorders?
- An allele can cause a genetic disorder; malfunctioning protein or no protein at all.
~ Carriers: an organism that carries two different forms (alleles) of a recessive gene (alleles of a gene linked to a recessive trait) and is thus heterozygous for that the recessive gene.
Cystic Fibrosis
~ Definition: A common grave genetic disease that affects the exocrine glands and is characterized by the production of abnormal secretions, leading to mucus buildup that impairs the pancreas and, secondarily, the intestine.
- Mucus buildup in lungs can impair respiration.
~ 1 out of 2,500 people
- European Decendents: 1 out of 25 (4%) are carriers.
~ Untreated: Death by the age of 5 years old.
~ Achondroplasia: a genetic disorder that is marked by abnormally slow conversion of cartilage to bone during development resulting in a form of dwarfism characterized by a usually normal torso and shortened limbs and that is usually inherited as an autosomal dominant trait.
Example:
~ Couple: John and Carol
- each had a brother that died of inherited lethal disease.
- both John and Carol are carriers to a recessive gene.
- Aa x Aa
- 2/3 of a chance that John and Carol could be the carrier.
- There is a 1/4 of a chance that their first born might have the disease.