BIOL 1P91 - Chapter 17 Student 2024 PDF

Welcome to BIOL 1P91 Part 3  All about me Dr. Lori MacNeil [email protected] MC F229 Office hours by appointment 2  Slido  We will use Slido this term for interactive polls and practic...

Welcome to BIOL 1P91 Part 3  All about me Dr. Lori MacNeil [email protected] MC F229 Office hours by appointment 2  Slido  We will use Slido this term for interactive polls and practice questions  You will be able to access these from any smartphone, tablet or laptop during lecture 3 Mendelian Patterns of Inheritance Chapter 17 4  Chapter 17 Outline Mendel’s Laws of Inheritance The Chromosome Theory of Inheritance  Later Pedigree Analysis of Human Traits Sex Chromosomes and X-Linked Inheritance Patterns Variations in the Inheritance Patterns and their Molecular Basis Gene Interaction Genetics and Probability 5  Explaining Inheritance  Inheritance = the acquisition of traits by their transmission from parent to offspring  Until the late 1800s, many scientists hypothesized that inheritance occurred through pangenesis  “Seeds” produced by all parts of the body  If traits changed over life, the hereditary material would change, and the modified trait would be inherited Theory of acquired characteristics Blending Inheritance  It was also widely accepted that hereditary traits blended together in offspring, with these blending blended traits passed to subsequent generations Until… subsequent generations  Gregor Mendel  “Father of modern genetics”  A priest who studied physics and mathematics  In 1856 began historic studies on pea plants  Published results in 1866 but paper was largely ignored  Work rediscovered ~1900 ClickMendel’s 17.1 to edit Master Laws text of Inheritance 9  Garden Pea, Pisum sativum Several advantageous properties: 1. Genetic variation  Many readily available varieties with different visible characteristics  E.g. Differences in the appearance of seeds, pods, flowers, and stems  Called characters or traits 10  Garden Pea, Pisum sativum 2. Normally self-fertilizing  A modified petal encloses the stamen and stigma, encouraging self-fertilization  Female gamete fertilized by male gamete from same plant  Easy to produce true-breeding lines  Exhibit the same traits from generation to generation 11  Garden Pea, Pisum sativum 3. Ease of making crosses  Flowers are large and easy to manipulate  Can easily manually cross plants = cross-fertilization or hybridization  Offspring = hybrids 12  Single Factor Crosses  Follows the variants of only one character  Generations of a single factor cross are as follows:  P generation: True-breeding parents  F1 generation: First-generation offspring of a P cross  Called monohybrids in single-factor crosses, as true-breeding parents differ in only a single character  F2 generation: F1 monohybrids self- fertilize to produce the F2 generation 13 14  Important Trends in Mendel’s Data  No blending – either a trait was present or it was absent  Traits were “hidden” in the F1 generation but reappeared in the F2 generation  In the F2 generation, ¾ plants showed F1 trait, while ¼ showed hidden trait  = 3:1 ratio 15  Three Important Ideas 1. Traits may exist in two forms, dominant and recessive  Mendel’s data argued strongly against a blending mechanism of heredity  Instead, the F1 generation exhibited a trait like one of the two parents  A dominant trait is seen in a true-breeding parent and its F1 hybrid (heterozygote)  A recessive trait is seen in a true-breeding parent but is masked in the F1 hybrid By convention, an uppercase letter represents the dominant allele, and a lowercase letter represents the recessive allele E.g. Plant height gene: T = tall allele, t = dwarf allele 16  Three Important Ideas 2. An individual carries two genes for a character, and genes have variant forms (now called alleles)  Mendel concluded that:  The genetic determinants of traits are “unit factors” (genes) that are passed intact from generation to generation  Every individual carries two genes for a given character  The gene for each character may exist in two variant forms (alleles) 17  Three Important Ideas 3. The two alleles of a gene separate during the process that gives rise to haploid cells and gametes, so each sperm and egg receives only one allele  In the F2 generation, Mendel observed approximately a 3:1 ratio between the dominant and recessive trait  Mendel realized that this ratio could be explained by alleles of the same gene separating from each other during the process of gamete formation 18 19 Mendel’s Law of Two copies of a gene segregate Segregation of from each other during gamete Alleles formation, so every gamete receives only one allele 20  Genotype and Phenotype  Genotype - Genetic composition of individual  Homozygous: Individuals with two identical copies of a gene  e.g. TT = homozygous dominant; tt = homozygous recessive  Heterozygous: Individuals with two different alleles of the same gene  e.g. Tt = heterozygous  Phenotype - Characteristics (traits) of an organism that are the result of their genotype  e.g. TT and Tt have a tall phenotype, tt plants have a dwarf phenotype 21  Punnett Squares  A common way to predict the outcome of simple genetic crosses  Example: What offspring are expected in a cross between two heterozygous tall plants? Five steps: 1. Write down the genotypes of both parents  Male parent = Tt  Female parent = Tt 2. Write down the possible gametes that each parent can make  Male parent = T and t  Female parent = T and t 22  Punnett Squares 3. Create an empty Punnett square Male gametes T t T Female gametes t 23  Punnett Squares 4. Fill in the possible genotypes of Male gametes the offspring by combining the alleles of the gametes in the T t empty boxes T TT Tt Female gametes t Tt tt 24  Punnett Squares 5. Determine the relative Male gametes proportions of genotypes and phenotypes in the offspring T t T TT Tt Female gametes Genotypes: TT, Tt, and tt in a 1:2:1 ratio Phenotypes: t Tt tt Tall and dwarf in a 3:1 ratio 25  Testcross  To distinguish between the homozygous dominant and heterozygous genotypes, a testcross can be used  An individual with a dominant phenotype is crossed to a homozygous recessive individual  Phenotypes of offspring are examined  Presence of recessive trait indicates parent was heterozygous 26  Two-Factor Crosses  Follows the inheritance of two different characters  We can imagine two hypotheses for what might happen:  Two genes would segregate together and always be inherited together  = Linked assortment  Predicts a 3:1 ratio of phenotypes  Two genes are independent, so their alleles are randomly distributed into the gametes  = Independent assortment  Predicts a 9:3:3:1 ratio of phenotypes 27  Mendel’s Results  F1 plants are dihybrids – hybrids with respect to both traits  Data for F2 are consistent with independent assortment 28 Mendel’s Law of Independent The alleles of different genes assort independently of each Assortment other during the process that gives rise to gametes 29  Pedigree Analysis  Inheritance of human traits is often analyzed using pedigree analysis  Examines the presence of the trait over course of a few generations in one family  Used to understand the inheritance of genetic diseases that follow simple Mendelian patterns  Disease symptoms can occur as a result of a mutant allele  A pedigree helps determine whether the mutant allele is dominant or recessive  Allows for prediction of the likelihood of an individual being affected ClickPedigree 17.3 to edit Master text Analysis of Human Traits 30  Example: Cystic Fibrosis  Approximately 3% of people of European descent are heterozygous carriers of the recessive CFTR (disease-causing) allele  Phenotypically normal  Individuals who are homozygous for the mutant CFTR allele exhibit disease symptoms  CFTR gene normally encodes a transmembrane chloride channel  Normally chloride ions flow out of cells, increasing water flow out of cells & producing a thin, freely-flowing mucus lining tissues  Mutation prevents channel function, disrupting chloride & water movement  Mucus coating lung, pancreas & other organs becomes unusually thick and sticky 31 32  Human Disease  Many human genetic diseases are recessive  Disease alleles persist in heterozygote carriers who are not affected  Unaffected parents have affected children  Huntington Disease is an example of a human genetic disease that is dominant  Symptoms appear later in life, usually after reproduction  Normal allele encodes a protein that functions in nerve cells  Mutation encodes an abnormal form of the protein, which aggregates in cells and results in defective nerve function 33 Pedigree of a dominant trait 34  Autosomes vs. Sex Chromosomes  Autosomes = Pairs of chromosomes found in both sexes  Most genes are found on autosomes  Sex chromosomes = Distinctive pair of chromosomes that differs between males and females  Found in many, but not all, species with two sexes  Some genes are found on sex chromosomes, which causes distinct inheritance patterns ClickSex 17.4 to edit Master textand X-Linked Inheritance Patterns Chromosomes 35  Human Chromosomes  46 Chromosomes total  22 pairs of autosomes = 44  + one pair of sex chromosomes = 46  Sex is determined by an XY system  Males are XY and females XX  Presence of Y chromosome causes maleness  SRY gene on Y chromosome leads to male development  E.g. XXY = male, XO = female 36  X-Linked Traits  Sex linked genes are found on one sex chromosome but not on the other  In humans, the X chromosome is larger and carries more genes than the Y chromosome  Genes found on the X chromosome but not the Y are known as X-linked genes  Females can be homozygous or heterozygous for X-linked genes  However, males are hemizygous for X-linked genes  Have only one a single copy of each X-linked gene  Diseases caused by recessive X-linked mutant alleles occur more frequently in males 37  Example: Hemophilia A  X-linked recessive disease characterized by excessive bleeding  Results from a defective clotting protein If parents are a heterozygous female and an unaffected male, only sons will exhibit the disorder. Chance of having an affected child = 25% Chance of having an affected daughter = 0% Chance of having an affected son = 50% 38  Feature Investigation Morgan’s Drosophila Experiments  Drosophila also have XY sex chromosomes  The first trait (gene) to be localized to a specific chromosome was an X- linked trait in Drosophila in 1910  Thomas Hunt Morgan obtained a white-eyed male from a true-breeding red-eyed line  = New mutation  Conducted fly crosses to study the inheritance of the white-eyed trait and found a connection between eye colour and sex 39  Morgan’s Drosophila crosses  Note Drosophila notation: Xw+ = normal allele; Xw = mutant allele 40  Morgan’s Drosophila Crosses  The P-generation cross Male gametes between a white-eyed male and Xw Y red-eyed female produced all red-eyed offspring Xw+ Xw Xw+ Y Xw+ Red, Red, Female gametes female male Xw+ Xw Xw+ Y Xw+ Red, Red, female male 41  Morgan’s Drosophila crosses  Next, Morgan crossed the F1 offspring with each other to obtain the F2 generation 42  Morgan’s Drosophila Crosses  In the F2 generation, Morgan Male gametes counted: Xw+ Y  1,011 red eyed males Xw+ Xw+ Xw+ Y  782 white-eyed males  2,459 red-eyed females Xw+ Red, Red, Female gametes female male  0 white-eyed females Xw+ Xw Xw Y  These results are consistent Xw Red, White, with the eye colour gene being female male located on the X chromosome 43  Protein Function Explains Dominance  Wild-type allele: Prevalent allele in a population  In most cases, encodes a protein that is made in the proper amount and functions normally  Mutant alleles: Alleles that have been altered by mutation  Tend to be rare in natural populations  Mutations that produce recessive alleles typically decrease or eliminate the synthesis or functional activity of a protein  = loss of function 17.5 Variations in Inheritance Patterns and their Molecular 44 Click to edit Master text  Protein Function Explains Dominance  Recall that in a simple dominant/recessive relationship, the recessive allele does not affect the phenotype of the heterozygote  A single copy of the dominant allele produces the dominant phenotype  How is this explained at the molecular level?  The dominant allele produces functional protein, while the recessive allele has a mutation and does not produce functional protein  ~50% of the normal amount of protein is sufficient for a normal phenotype  Homozygous recessive individuals (aa) have no or very low levels of functional protein, which produces the recessive phenotype or disease 45 Genotype-Phenotype connection in simple Mendelian Inheritance 46 Some single-gene traits do not exhibit a simple dominant/recessive relationship 47  Incomplete Dominance  Heterozygote exhibits a phenotype that is distinct and intermediate between the two homozygous phenotypes  Different phenotypes are observed with 100%, 50%, and 0% protein level  Degree to which we identify incomplete dominance may depend on how closely we examine an individual’s phenotype  E.g. Phenylketonuria (PKU)  Heterozygotes appear phenotypically normal but have double the normal phenylalanine blood level 48  Codominance  Many genes have three or more variants in a population (= Multiple alleles)  Phenotype depends on which two alleles are inherited  Codominance: Single individual expresses both alleles in a way that leads to both traits in the phenotype  e.g. ABO blood types in humans  Gene encodes an enzyme that attaches different sugars to carbohydrates on the surface of red blood cells  IA allele and IB allele encode enzymes with slightly different active sites that attach different sugars  Recessive i allele is non-functional and does not attach sugars 49 ABO Blood Group 50  Role of the Environment  Phenotypes can also be shaped by the environment  Norm of reaction: The phenotypic range seen in individuals with a particular genotype under differing environmental conditions  e.g. Genetically identical plants grow to different heights in different temperatures 51  Complex Traits  Most traits are influenced by many genes and the environment  e.g. Height is affected by genes that encode proteins involved in growth hormones, cell division, nutrient uptake, metabolism & more  A gene interaction occurs when a single character is controlled by the products of two or more genes  In an epistatic gene interaction, the alleles of one gene mask the expression of the alleles of another gene  Often arise because two or more different proteins are involved in a single cellular function  e.g. Both encode enzymes in an enzymatic pathway that leads to the formation of a single product ClickGene 17.6 to edit Master text Interaction 52  Example: Epistasis in Sweet Peas  Mendel crossed true breeding purple x true breeding white flowers and found:  F1 were all purple  F2 were ¾ purple and ¼ white  Consistent with single gene dominant inheritance  However, in early 1900s, Bateson and Punnett crossed two different true-breeding white strains and found:  F1 were all purple!  F2 were purple to white in a 9:7 ratio  Deduced that two different genes are involved in flower colour determination  Plants must have at least one dominant allele for each of these genes to have purple flowers 53  e me y zym En z En C P Enzymatic pathway: Precursor Intermediate Purple Pigment 54  Epistasis in the sweet pea  We can describe the relationship among alleles as follows:  C (one allele for purple) is dominant to c (white)  P (an allele of a different gene for purple) is dominant to p (white)  cc masks P, and pp masks C, resulting in white flowers in either case  A plant that is homozygous for either c or p has white flowers, even if it has a dominant purple-producing allele for the other gene  = Epistasis 55  Polygenic Inheritance  Thus far, we have looked at discrete, or discontinuous traits  Have clearly defined phenotypic variants that don’t overlap  Purple/white flowers, red/white eyes  Traits that show continuous variation over a range or phenotypes are called quantitative traits  E.g. Height, weight, skin colour, metabolic rate, heart size in humans  Most quantitative traits are polygenic (determined by multiple genes) & influenced by environmental factors 56 Example: grain pigmentation in wheat 57 58