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University of Waterloo
BIOL 239
Christine Dupont

Genetics 139 Midterm Study Notes Slideshow 1: Mendelian Genetics Artificial Selection: purposeful mating between animals (domesticated) 1950 Belyaev: domesticated fox matings – Russia PreMendel  one parents contributes more to offspring inherited features, and that parental traits become mixed and forever changed in offspring BOTH WRONG Gregor Mendel: Augustinian monk and expert plant breeder (1822 -1884) Advantages 1. garden peas are easy to cross fertilize, large number of offspring and short growing season 2. clear cut alternative forms (round vs wrinkled, yellow vs. green) 3. establishment of pure breeding lines (parental traits remain constant from generation to generation) 4. carefully controlled breeding (reciprocal crosses and self fertilization) Phenotype: observable characteristic commonly called traits. They are controlled by genotype Genotype: genetic makeup, description of the genetic information carrier by the individual Mendel used selective breeding to produce two true breeding lines of peas (offspring always showed same phenotype as parents). Hybrids were produced when parents with antagonist traits were bred Monohybrid Crosses Matings between parents that differed wrt to one trait. All f1 generation resembled one parent, but the lost trait came back in the f2 generation. Discrete unit of inheritance are called alleles, they are alternative forms of a single gene Most human traits are determined by multiple genes with multiple alleles (skin) Homologous chromosomes (one from each parent) carry the same genes but different alleles Polymorphic: a gene that has several different alleles that normally occur in a population (one human can have a max of two alleles, we are diploid) Monomorphic: a gene that only has one allele normally present in a population Mendel’s Law of Segregation: the two alleles for each trait separate individually in the gametes and then unite randomly (one from each parent) at fertilization. Each parent begins with two alleles or versions of each gene Dominant and recessive are terms used to describe the phenotypic affect of different alleles. Different alleles have different nucleotide sequences, and they result in different amino acid production or levels of protein. In ww, there is a large amount of soluble starch. So water moves in when the pea is young and then leaves when it matures giving it a wrinkled appearance. In Ww, enough insoluble starch is made to prevent this, as the enzymes produced are reusable. PHENOTYPIC RATIO: 3:1 for simple D/R traits that result from a monohybrid cross. The GENOTYPIC RATIO is 1:2:1 To determine an unknown genotype, self fertilization is the easiest (cross against a homozygous recessive individual because this allows the trait of the unknown to show) Mendel’s Law of Product: the chance of two or more independent events occurring together is the product of each event occurring by itself Mendel’s Law of Sum: the chance that either of two mutually exclusive events occurring is the sum of their individual probabilities Dihybrid Crosses: mating between individuals that differ in two traits Even with simple D/R in these crosses new phenotypic combinations appear, result of independent assortment of gametes PHENOTYPIC RATIO: 9: 3:3:1 Law of Segregation – how different alleles of a gene behave//treat alleles of a single gene independently Mendel’s Law of Independent Assortment: during gamete formation, different pairs of alleles separate independently of each other Law of Independent Assortment – how different genes behave//treat genes separately [go over branching diagrams] Mutlihybrid Crosses: mating between individuals that differ in three or more traits. Punnet squares and even branching diagrams get a bit too complicated, so we use genetic ratios established for heterozygous crosses for each gamete independently to work out the probability of a given offspring Key Points • Mendel studies the inheritance of 7 difference traits in garden peas, each trait was controlled by a different gene • His research led him to three principals: a) the alleles of a gene are either D or R b) different alleles of a gene segregate from each other during the formation of gametes (segration) c) alleles of different genes assort independently (independent assortment) • outcomes of crosses can be predicted by systematically using the genotypes in a punnett square • when there are more than two genes involved, the fork lined or probability method are used to get outcomes Slideshow 2: Modifications of Mendelian Ratios Incomplete Dominance: offspring resembles neither purebred parent (pink instead of red or white) • one allele produces pigment and the other does not. Therefore when you have one of each, some of the pigment is produced resulting in an intermediate. • Both letters in uppercase (no lower because no recessive) Codominance: both purebred parental traits are visible in the offspring (both striped and dotted) • Blood types  Ia attaches to a sugar different than the one that Ib attaches to and I does not attach to one at all (polymorphic gene as there are 3 common alleles) • Described by phenotype; a sugar means there is a antigen, b sugar means there is b antigen. I means protein with no function therefore no antigen and no enzyme and no sugars added • A and b are codominant to each other and dominant to i Variations on complete dominance are still okay with Mendel Law  the various relationships are not because of the way they are transmitted, it is because of the way the proteins they encode for are interpreted by the cell. Dominance does not always mean higher frequency Dominant Series: list of all the possible alleles in order from dominant to recessive. Must be determined in reciprocal manner (keep male and female constant) because otherwise sex linked traits cannot be sorted out. Wild type allele is the one with + and is dominant in the population, with all the others present less often than one percent New alleles could arise from mutation in nature, and although they differ in DNA sequence it may not effect the phenotype Wildtype Allele: greater than 1% Mutant Allele: less than 1% Monomorphic: gene with only one wildtype allele (can have lots of alleles) Polymorphic: gene with more than one wildtype allele Pleitrophy: multiple phenotypic effects caused by a single gene (skin colour and height). Some alleles can also affect viability. Assignment of D and R pertains only to the phenotype being regarded. Recessive lethal alleles have a phenotypic ratio is 2:1 (one dies). The wildtype allele in this case is being masked, they are not always dominant. The D and R in this case is not referring to the viability. Allele for yellow coat is D, however it is also recessive for lethality. Similarly, Manx cats also involves spinal development. It is D for taillessness but recessive for lethality. Therefore all Manx cats are heterozygous Multifactorial Inheritance: a phenotype arising from the action of 2 or more genes or from interactions between genes and the environment. The most common traits are determined by more than one gene. (alternative to co or incomplete dominance) Novel phenotypes can emerge from the combined action of the alleles of two genes (chicken combs): 9:3:3:1 indicates 2 genes responsible for comb shape acting independently of each other in simple D/R manner: 1/16 is new!! (same ratio as dihybrid, but only one trait is being affected) Complementary Gene Action: two or more genes can work in tandem, in the same biochemical pathways to produce a particular trait. Their protein products work one after another in a process (metabolic) to produce a trait. Heterogenous Trait: mutation at any one of a number of genes can give rise to the same phenotype  2 wrongs make a right, the wildtype phenotype is rescued by complementation Complementary Gene Action (heterogenous trait): PHENOTYPIC RATIO 9:7 (two genes are involved in the pigmentation) in order to get the right, the two parents have to be mutants in the two different genes, NOT the same gene. Complementation Testing: to determine if two individuals have a mutation in the same gene, as well as to see if two genes are needed to produce a phenotype. Complementation only occurs if the parents have mutations in DIFFERENT genes Epistasis: gene interaction where the effects of an allele at one gene hide the efforts of alleles at another gene. To be epistatic means to have control over another gene. Recessive Epitasis have a 9:3:4 PHENOTYPIC RATIO To determine genotype you would do a test cross with recessive for first trait and the non epistasic version of the second Dominant Epitasis: 12:3:1 PHENOTYPIC RATIO. Hypothesis Testing: deciding between different hypothesis using specific breeding tests • New phenotype would indicate that either more than 1 gene or incomplete dominance of alleles of 1 gene • Cant test cross humans because we don’t have enough offspring and we reproduce slowly. Therefore we do pedigree analysis Genotype does not always mean the same phenotype (enviro influence, chance, modifier genes) Penetrance: percentage of the population with a particular genotype, that demonstrate the expected trait (some people that don’t show it) Expressivity: degree/intensity with which a particular genotype is expressed in a phenotype within a population (grey) Both: not all showing and showing to different degrees Ie. Retinoblastoma, it is the dominant allele however there is only 75% penetrance and within that some only get it in one eye (30% expressivity) Sex Linked Traits: due to genes that are on the x chromosome (y is too small) Eg hemophilia and colour blindness Sex Limited Traits: affect a structure or process that is found in one sex but not the other* Eg. Plumage in birds, milk production, horns and antlers Sex Influenced Traits: show up in both sexes but their expression may differ between the two sexes – not on x or y* Eg. Patterned baldness Factors like temp, light and altitude can affect phenotypic expression of a genotype (affect the enzyme that works on the genes protein product to create a visible trait) Conditional Lethality: particular allele is lethal but only under certain conditions (permissible vs restrictive) eg is malignant hypothermia, triggered by halothane Chance: occurrences in the lives of individuals can influence the expression of some alleles Eg exposure to carcinogens, radiation Key Points • Genes often have multiple alleles and mutant alleles may be D, R ID or CD • If a hybrid that inherits a R mutation from each of its parents had a mutant phenotype, then the R mutations are alleles of the same gene; if the hybrid has a wild phenotype, then the R mutations are alleles of different genes • Most genes encode polypeptides • In homozygous condition, R mutations often abolish or diminish polypeptide activity • Some D mutations produce a polypeptide that interferes with the activity of the polypeptide encoded by the wild-type allele of a gene • Gene action is affected by biological and physical factors in the environment • Two or more genes can influence a trait • Mutant allele is epistatic to a mutant allele of another gene if it has an overriding effect on the phenotype • A gene is pleiotropic if it influences many different phenotypes • Inbreeding increases the freq of homozygotes and decreases the freq of hetero • The effects of inbreeding are proportional to the inbreeding coefficients – the probability that two gene copies in an individual are identical by descent from a common ancestor • The coefficient of relationship is the fraction of genes that two individuals share by virtue of common ancestry Slideshow 3: Pedigree Analysis Most traits do not a simple pattern of Mendelian inheritance because most are multifactorial (influenced by more than one gene) The purpose of pedigrees (family genetic history) is to gain insight into the mutant alleles that are causing abnormalities Monomorphic / Single Gene Traits (only one wild type) gene with mutant phenotypes showing pleitrophy: • Sickle cell disease, tay sachs, phenylketonuria, • Sickle cell anaemia is a syndrome, it varies depending on conditions (altitude) and it helps malaria • Albinism, huntingtons disease, cystic fibrosis, Symbols for pedigree Symbol Meaning Square Male Circle Female Diamond Unspecified Diagonal line Dead Coloured in Have trait Roman numerals Generations Arabic numerals Individuals within a generation Dominant Trait often within a pedigree, at least one affected parent, equal males and female and they can both transmit Recessive Trait  not often within a pedigree, depends on mate being homo or hetero, usually two normal parents, matings between affected people only produced affected children, both males and females can transmit Autosomal vs Sex Linked: one that is conferred on chromo that is not involved in sex determination (44 in humans) vs one that is conferred on chromo that determine sex (2, x and y) Sex Linked D  at least one affected parent, affected females pass on to ½ daughters and sons, affected males pass onto all daughters Sex Linked R more males than females, criss cross (affected males do not pass on to sons), parents of affected males are usually normal Key Points • Pedigrees are used to identify D and R traits in human families • Analysis of pedigrees allows genetic counsellors to assess the risk that an individual will inherit a particular trait • Disorders such as haemophilia and colour blindness, which are X linked R mutations, are more common in males than females • In humans the Y chromo carries fewer genes than the X chromo • In humans pseudoautosomal genes are located both on the X and Y chromo Slideshow 4: Chromosomes 1667: studies that semen contained spermatozoa (something participating in fertilization) 1854: studies with frogs and sea urchins showing union of male and female gametes – equal contribution (gamete union) 1879: observation of threadlike structures in the nuclei of salamander cells during cell division (microscopes were getting better) 1880: advances in microscopes, uses of dyes (see nuclei in the cytoplasm) Early 1900s: linking of chromosome to the Mendelian principles of segregation and independent assortment. It was proposed that each egg/sperm contains only one of each chromo of a pair  Chromosomal Theory of Inheritance: States that hereditary info is on genes and that genes are located on chromosomes; also that egg and sperm contribute equally to the genetic endowment of offspring through their nuclei 1901: x and y chromo determine sex in grasshoppers (they can physically be distinguished) women are homodimetic and males are heterodimetic Not the same in all species! In Drosphilia the presence of y does not determine maleness. Ratio of x to auto determines it. 1:1 is normal female and 1:2 is normal male. No y means the male is sterile Enviro factors can also determine it. Eg temp or incubation, size/dominant behaviour Mitosis and Meiosis are responsible for transmitting genes from one generation to the next and keeping the gene number consistent. Mitosis: nuclear division that results in two daughter cells that are clones/identical to the parent (same number of chromo) Meiosis: nuclear division that results in each egg and sperm containing half the number of chromosomes found in somatic cells. They are unique from parent n: haploid, one complete set of chromo (gametes) 2n: diploid, two complete sets of chromo (somatic) Chromatid is a single DNA molecule Centromere: DNA sequence where packaging doesn’t occur Non homologous chromo have different sequence of genes/different genes. They do not pair at meiosis and are a different size and shape. When there’s homologous chromo then one is maternal and the other is paternal and there have same gene order but different alleles. Sister chromatids have identical alleles (they are the replication result) Packaging (to fit into cell): metaphase chromo (condensed – only way to see in microscope), 30nm chromatin fibre, 11nm nucleosomes, 2nm DNA molecule, Karyotype: visual description of a complete set of chromo in one cell of an organism. Metaphase chromosomes are always used. Each chromosome has two chromatids because they are replicated. 1 to 3 are metacentric and 13 to 15 are acriocentric. There is double the number of chromatids to chromosomes. And there are 23 non homologous chromosomes. The purpose of karyotyping is to reveal abnormalities in chromo number. Trisomy 18 and 13 live till shortly after birth, trisomy 21 is the only viable one as it is so small Key Points • Individual chromo become visible during cell division, between divisions they form a diffuse network of fibres called chromatin • Diploid somatic cells have twice as many chromo as haploid gametes • Sex chromo are different between two sexes, whereas autosomes are the same • In humans sex is determined by a D effect of SRY gene on the Y chromo, the product of this gene, the testis determining factor (TDF) causes a human embryo to develop into a male • In Drosophila sex is determined by the ratio of X Chromo to sets of autosomes; if X:A <= 0.5 the fly is a male (1:2) if X:A >=1 the fly is a female (1:1); and for 0.5
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