Lecture 15: Mendelian Genetics & Discrete Trait Inheritance PDF

Lecture 15: Mendelian Genetics & Discrete Trait Inheritance Readings Ch  Learning Objectives Be familiar with how Mendel’s work led to an understanding of inheritance Be able to preform single and double factor crosses, and report the resulting genotype and phenotype...

Lecture 15: Mendelian Genetics & Discrete Trait Inheritance Readings Ch  Learning Objectives Be familiar with how Mendel’s work led to an understanding of inheritance Be able to preform single and double factor crosses, and report the resulting genotype and phenotype ratios Understand how the laws of independent assortment and segregation relate to the inheritance of traits and alleles Be familiar with terminology (e.g., gene, trait, phenotype, etc…) Know the difference between a discrete and quantitative trait Understand how X-linked inheritance works Know some other types of inheritance patterns (e.g., incomplete dominance, co-dominance, etc….)  Theory of Heredity Storage of information in DNA is common to all living things DNA (packaged in discrete units – chromosomes!) is passed from parent to offspring Trait: Any characteristic of an individual Heredity: The transmission of traits from parents to their offspring Gregor Mendel (1822-1884) Interested in how traits are passed from parent to offspring 1866: Experiments on plant hybridization, Proceedings of the Natural History Society of Brunn. Darwin’s work on natural selection was published around the same time… but Mendel and Darwin were unaware of each other’s work!  Mendel’s work was lost…. But rediscovered in 1900! Mendel was addressing the basic questions: Why offspring resemble their parents? How transmission of traits occurs Common Garden Pea (Pisum sativum) Why was the Common Garden Pea a GREAT model organism for testing inheritance? – It is easy to grow – Its reproductive cycle is short – It produces large numbers of seeds – Its mating is easy to control – Its traits are easily recognizable  1ST Filial- “son or daughter”  What traits did Mendel Study? An individual’s observable features comprise its phenotype Mendel’s pea population had two distinct phenotypes for each of the seven traits What Traits Did Mendel Study? Mendel worked with pure lines (sometimes called true breeding) – They produced identical offspring when self- Pure pollinated lines He used these plants to create hybrids Hybrids – He mated two different pure lines that differed in one or more traits  Mendel noticed something interesting in his crosses! 1. Dominant Phenotype- hide the expression of the recessive in F1 generation 2. Recessive phenotype re-appeared in F2! But always in a ratio of 3:1 Similar patterns of recurring ratios and re- emerging recessive traits were found F1 crosses of the 7 traits he studied! 3.14:1 265.8  Genes, Alleles, and Genotypes During Mendel’s time the term “gene” didn’t exist, and Mendel’s discoveries were some of the first surrounding inheritance of traits Hereditary determinants for a trait are now called genes Mendel also proposed that: – Each individual has two versions of each gene – These different versions of a gene are called alleles Different alleles are responsible for the variation in the traits that Mendel studied! Genes, Alleles, and Genotypes The alleles found in an individual are called its genotype An individual’s genotype affects its phenotype Genotype Phenotype OR Mendel put the puzzle together F1 i ne e L r Pu Hy br i d F2  Mendel’s Conclusions Alternative alleles lead to Phenotype alternative traits: Genotype domina nt yellow YY Dominant allele – UPPER CASE - Y Yy recessive allele – lower case - y recessiv green Phenotype – appearance of e yy individual Genotype – alleles received from parents  A cross between two homozygotes Meiosi s Meiosi s of offspring are heterozygous for Yellow color A cross between a homozygote and a heterozygote Phenotype Ratio: the number of times a specific combination of alleles Y y appears in the predicted phenotypes of any offspring ¾ are yellow Y ¼ are green YY Yy Genotype Ratio: the number of times y a specific combination of alleles Yy yy appears in the predicted genotypes of any offspring ¼ are YY (homozygous dominant) ¼ are yy (homozygous recessive) ½ are Yy (heterozygous) Why does it happen in this way?  Practice Crosses Y y Y y Y y YY Yy Yy yy Y y YY Yy Yy yy Phenotype Ratio: Phenotype Ratio: 100% yellow 50% yellow 50% Green Genotype Ratio: Genotype Ratio: 50% homozygous dominant (YY) 50% homozygous recessive (yy 50% heterozygous (Yy) 50% heterozygous (Yy) Quick Terminology Review: Genotype, Phenotype, Alleles What are the alleles? aa Aa Which coat color is dominant? Which coat color is recessive? aa Aa aa Aa What are the genotypes present in this A a example? a Aa aa a Aa aa What are the phenotypes present in this example?  Pedigrees eminder to self: draw one out on board, too!   Independent Assortment & The Principle of Segregation The Law of Independent Assortment can be explained by the behavior of chromosomes during meiosis Random alignment of chromosome pairs during meiosis I leads to the independent assortment of genes found on different chromosomes How homologous chromosomes align in Meiosis 1 will have downstream effects on which gamete is “given” each chromosome (from mother vs from father, with a lot of crossing over vs with a little crossing over) Independent Assortment Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 23  Independent Assortment & The Principle of Segregation Mendel developed the principle of segregation: Two alleles that code for Maternal Egg the same trait (ex: pea color) must 1/ Donor 2 Y 1/ 2 separate during gamete formation Y y y Happens during Anaphase of Meiosis Paternal Sperm Donor Y 1/2 YY Yy Yy 1/2 y Yy yy  The Principle of Segregation The egg-donor in a heterozygous F1 pea cross has a genotype of Rr. What alleles could be found in the gametes of this individual, and in what proportions? What process determines which gametes receive which allele?  The Principle of Segregati on  Two-factor cross Follows inheritance of two different traits (ex: color and seed shape) Mendel crossed strains of pea plants that bred true for two characteristics Possible patterns: Two genes are linked – variants found together in parents are always inherited as a unit Two genes are independent – variants are randomly distributed into gametes Linked Genes – Located next to each other on the Same Chromosome The physical association of two or more genes found on the same chromosome – Is called linkage Linked genes – Are predicted to always be transmitted together during gamete formation – Should violate the principle of independent assortment  Linked Assortment vs Independent Assortment Seed gene Color gene OR Seed gene Chr 1 Color gene Chr 1 Chr 5  Linked Assortment vs Independent Assortment Seed gene Color gene OR Seed gene Chr 1 Color gene Chr 1 Chr 5 Outcome of a two-factor cross = 315/556 = ~ 9/16 = 9:3:3: ~3/16 = 1 ~3/16 Ratio = ~1/16 556 total seeds Crossing the true-breeding parents produced dihybrid offspring – offspring are hybrids with respect to both traits Data for F2 hybrids is consistent only with the independent assortment hypothesis because of 9:3:3:1 Ratio Independent Assortment in 2 Factor Cross Mendel’s laws are true for all genes: Assortment Segregation BUT!!! Not all traits follow ratios observed by Mendel (ex: 3:1, 9:3:31…)  X-linked inheritance in D. melanogaster Mutation = Heritable change in gene Most common phenotype Meiosis of X & Y Sex Chromosomes  X-linked Inheritance of eye color in fruit flies ene for white eye color is on the X-chromosome, NOT an autosomal chr X-linked Inheritance of eye color in fruit flies Hemophilia: X-linked recessive disease & Reading PedigreesX X X Y H h h Functional dominant copy that “masks” effects of h “Carrier” “Hemophilia”  X-linked Recessive XX X Y XHXH-normal female XhXh-female with H h h hemophilia Functional dominant XHXh-Carrier-carries the copy that allele, but is “masks” effects of h “Carrier” “Hemophilia” phenotypically normal XHY-normal male XhY-male with hemophilia XHXh x X X Y XX H X H h XH XHXH H h Y XHY X hY Incomplete Co- Dominance Dominance Blended “phenotype”  Pleiotropy Gene that influences more than one trait Ex: Gene for Marfan syndrome (mutant FBN1) – Tall – Disproportionately long limbs and fingers – Abnormally shaped chest – Severe heart problems Polygenic Traits Polygenic (or Quantitative) Traits: Multiple genes on different loci control phenotype  Traits: Qualitative vs Quantitative Qualitative Traits Quantitative Traits Discrete phenotypes Distributions of Examples: phenotypes Yellow or purple flowers Influenced by: genotypes Have cystic fibrosis or don’t at different loci and Can roll tongue or can’t environment A.k.a polygenic traits Examples: Height Skin color Weight Environmental Phenotype Variation Warm Cold Differences among individuals due temperat temperat ure ure to exposure to different (>30°C) (