Mendelian Genetics

Vocabulary:

• Gregor Johann Mendel: 1866, theorized about transmission of genetics as a result of experiments with garden peas. Monk, mathematician, scientist

• Pea: Model organism that Mendel used, easy to grow, has true-breeding strains, controlled matings (self/cross fertilization), grows in one season, and has observable characteristics with two distinct forms

• Transmission genetics: The study of how genes are transmitted from parents to offspring

• Gene: Fundamental unit of inheritance

• Allele: Different forms of a gene

• Locus: Location of a gene in the genome

• Homozygous: Two of the same allele

• Heterozygous: Two different alleles

• Genotype: The alleles an individual has, written in pairs

• Phenotype: The appearance of the individual

• Dominant: Expressed trait if at least one allele is present

• Recessive: Expressed trait if two alleles are present

• Monohybrid cross: Between parents that differ in one phenotypic characteristic

• True breed: A kind of organism that over many generations will have the same traits as the parents

• Filial generation (F): Offspring generation resulting from a parental generation

• Parental generation (P): Generation that provides genetic material for a new generation to form from

• Self fertilization: One plant is fertilized with its own genetics

• Monohybrid cross: A cross between two organisms that only differ in one trait

• Dihybrid cross: A cross between two organisms that differ in 2 traits, used to distinguish if the traits are inherited together or separately

• Reciprocal Cross: Not sex dependent, F1 and F2 patterns of inheritance are similar regardless of the P1 source, crosses made in both directions

• Particulate unit factors: What Mendel called basic units of heredity, passes unchanged from generation to generation, determine various traits expressed by each individual plant

• Homozygote: An organism that has 2 same alleles for a given trait

• Heterozygote: An organism that has 2 different alleles for a given trait

• Dominant allele: Determines the phenotype o the heterozygote, has a capital letter

• Recessive allele: No visible/observable effect on the phenotype of the heterozygote, has a lowercase letter, only in phenotype if homozygous for this

• Wild type allele (WT): The “standard” allele for a given trait or function of an organism, denoted with a lowercase italicized letter with a +, more abundant in nature and commonly but not always dominant

• Mutant allele: Opposite of wild type alleles, the uncommon/unusual, indicated by a lowercase and italicized letter

• Punnett square: A way to display and predict patterns of inheritance, from P to F1 generations. Shows all possible outcomes

• Product rule: The probability of two mutually exclusive events happening at the same time is the product of their individual probabilities

• Test cross: Mates the parent in question with a homozygous recessive individual, can distinguish between homozygous dominant and heterozygous parents, all progeny exhibit the possible combinations of traits in equal rations if the alleles are not linked

• Probability: The likelihood of the occurrence of a particular event, used to predict the outcome of genetic crosses

• Multiplication rule: Used to find the probability of two independent events both occurring, used to find phenotypic rations of dihybrid crosses

• Addition rule: Used to find the probability of one or another independent events occurring

• Product Law: Like the multiplication rule, calculates the probability of outcomes occurring together, ½ and ½ = ¼

• Sum Law: Like the addition rule, calculates the probability of outcomes independent of each other, ¼ or ¼ = ½. Used in genetics when more that one thing provide the same result

• Chromosomal theory of inheritance: Thought of by Walter Sutton and Theodor Bovert, states genetic material in living organisms is contained in chromosomes, and that they separate during meiosis

• Law of Segregation: Mendel’s first law, each individual has 2 alleles for each gene, these alleles segregate/separate during gamete formation so that each gamete receives only one allele

• Law of Independent Assortment: Mendel’s second law, genes for different traits are inherited independently of one another, assuming the genes are located on different chromosomes. Leads to extensive genetic variation

• Law of Dominance: Mendel’s third law, in a heterozygote one allele may dominate the expression of the other allele, the other one is recessive and masked unless both alleles are recessive

• Chi square analysis: Evaluates the influence of chance on genetic data

• Chance deviation: Chance events are subject to random fluctuations, and expected outcome is diminished by larger sample sizes

• Sample size: The amount of data that is available for a given analysis, average deviation decreases as this increases

• Null hypothesis: Assumes data will fit into a given ration, assumes there is no real different between measured values and predicted values. Attributes apparent difference to chance

• Chi Square: The goodness of the fit to the null hypothesis, used to evaluate observed vs. expected deviations

• Degree of freedom: The number of different categories into which data points may fall, in genetics this is # of phenotypes -1

• Probability value (P-value): Helps determine the significance of results, smaller indicates strong evidence against the null hypothesis due to chance (leads to its rejection), typically .05 in science

• Gene interaction: When a single phenotype is affected by more than one set of genes

• X-linkage: Genes that are on the X chromosome

• Mutation: The ultimate source of alleles, results in new genotypes and possibly new phenotypes. They may eliminate enzyme functionality, change enzyme efficiency, or overall impact function

• Loss of function mutation: A mutation resulting in a new phenotype, loss of wild-type function

• Gain of function mutation: A mutation that enhances the function of a wild type gene, quantity of the product increases

• Neutral mutation: A mutation that doesn’t change the phenotype, and therefore doesn’t impact evolutionary fitness

• Complete dominance: Occurs when phenotypes of the heterozygote and dominant homozygote are identical

• Incomplete dominance: When the phenotype of a heterozygote is somewhere between phenotypes of each homozygote (pink flower)

• Codominance: When two dominant alleles at a locus affect the phenotype of a heterozygote in separate, distinguishable ways (flower with white and red sections)

• No dominance: Occurs when there isn’t an allele that overpowers another, denoted with italic uppercase letters and superscripts for alternative alleles

• Tay-Sachs disease: A disease caused by incomplete dominance, the homozygous recessives affected by fatal lipid-storage disorder, fatal for neonates

• Threshold effect: Normal phenotypic expression results anytime a minimal level of a gene product is attained

• Multiple alleles: Gene interaction when there are 3+ alleles for the same gene, result in unique modes of inheritance, can only be studied in populations (like human blood, ABO)

• Blood types: Codominant system with multiple alleles, uses I^A, I^B, and i (for O). The i allele doesn’t produce an antigen, while the other two do, they are codominant. A and B are dominant to O

• Antibodies: Part of what determines blood type, proteins produced by the immune system to help fight disease, a B allele produces anti-B and the A allele anti-A, none are produced by the i allele

• Bombay phenotype: “Unexpected” with blood type inheritance, female has O type despite having a parent with AB, she ends up being homozygous for FUT1 (fucosyl transferase locus), preventing the synthesis of A or B, with a O phenotype. Will pass on like she is AB to future generations

• Essential genes: Absolutely required for basic survival, organisms cannot survive without at least one functional copy of the gene. Heterozygous organisms are okay

• Lethal allele: An allele that has the potential to cause death of the organism in certain combinations, can be dominant or recessive. Typically a result of mutations in essential genes, or combinations of other genes

• Dominant lethal allele: A lethal allele that will impact heterozygotes (like Huntington’s), quite rare as to persist the organism must survive long enough to reproduce

• Huntington’s disease: Caused by a dominant lethal allele, H. Onset is around 40, characterized by progressive degeneration of the nervous system which leads to early dementia and death

• Recessive lethal allele: A lethal allele that needs to be homozygous to have effect, may also impact a separate phenotype like in yellow mice

• Epistasis: When the expression of one gene depends on a presence of particular genes at a different locus

• Heterogenous trait: Involves mutations in different genes all leading to the same phenotype

Mendel started with 34 varieties of peas, and bred them for 2 generations to confirm that they were true bred. He then studied monohybrid crosses. One example was a P generation of homozygous round and wrinkled seeds crossed, resulting in all round peas (heterozygous) in the F1 generation. He self-fertilized them to form F2 generation, with a 3:1 ratio of round to wrinkled peas

Traits on the same chromosome are more likely to be inherited together, if traits are on different chromosomes, they will assort independently

If AaBbcc x AaBbCc, what proportion will be AABbcc?
AA= ¼, Bb= ¼ + ¼ = ½, cc= ½
¼ x ½ x ½ = 1/16

If using the chi square table, find the value you calculated and the given cell for the used p-value and degrees of freedom. If the calculated chi square is higher than the value, the null hypothesis is rejected

Pedigree: A ‘family tree’ with respect to a given trait, the trait is typically colored or highlighted. Reveals information about if a trait is recessive or dominant. Circles are female, squares male. Diamonds are unknown. Parents are connected by a horizontal line, offspring stem off the vertical line from parents. Double lines show related parents, like cousins (consanguineous). A weird looking triangle will show identical twins, and just branching is fraternal twins. A ‘P’ is assigned to the main individual in question