NYA - Lecture 11 Mendelian Inheritance PDF
Document Details
Uploaded by Deleted User
NYA
Tags
Summary
This document covers concepts related to Mendelian inheritance, including genotype, phenotype, alleles, and Mendel's experiments. It details the process of creating Punnett squares to predict phenotypic ratios. This introduction to genetics discusses important terminology and principles of inheritance.
Full Transcript
101-NYA Lecture 11 MENDELIAN INHERITANCE Introduction to Genetics Mendel Mendelian Genetics Introduction to Genetics Important Terminology: Genotype: The genetic makeup, or set of alleles, of an organism Phenotype: The observable physical and physiological traits of an organism, which are...
101-NYA Lecture 11 MENDELIAN INHERITANCE Introduction to Genetics Mendel Mendelian Genetics Introduction to Genetics Important Terminology: Genotype: The genetic makeup, or set of alleles, of an organism Phenotype: The observable physical and physiological traits of an organism, which are determined by its genetic makeup Characters: An observable heritable feature that may vary among individuals (e.g., hair colour, flower colour) Traits: One of two or more detectable variants in a genetic character (e.g., blond hair colour, red hair colour, purple flower colour) Introduction to Genetics Gene Locus: Each gene occurs on a specific chromosome and is located at a specific site Gene locus That location is called the gene locus (plural loci) (same for everyone) Introduction to Genetics Gene Locus: Recall Homologous chromosomes in diploid organisms: Two loci exist for each gene Gene Brown Blue Gene locus locus One locus on each homologous chromosome Homologous chromosomes code for the same genes but may have different alleles for that gene (different versions of that gene) e.g., two chromosomes may have Homologous genes coding for eye color, but one may chromosomes code for brown eyes, the other for blue Introduction to Genetics Alleles and Genotypes: Dominant allele: An allele that masks the presence of another allele Recessive allele: An allele whose presence can be masked by a dominant allele Combinations of alleles: Purple = Dominant allele (P) White = Recessive allele (p) P P p P p p Homozygous for the Heterozygous Homozygous for the dominant allele recessive allele Introduction to Genetics Alleles and Genotypes: Introduction to Genetics Heredity: Genetics deals with the way biological characteristics are inherited Heredity: The transmission of traits from one generation to the next How are traits transmitted? Introduction to Genetics Blended Inheritance: The explanation of heredity most widely in favour during the 1800s was the “blending hypothesis” The idea that genetic material contributed by the two parents mixes in a manner analogous to the way blue and yellow paint blend to make green Ironically, even Darwin believed in blending inheritance It is ironic because this mechanism is incompatible with his theory of evolution by natural selection Introduction to Genetics Blended Inheritance: Hypothesis predicts that, over many generations, a freely mating population will give rise to a uniform population of individuals Is this what we see? The blending hypothesis also fails to explain other phenomena of inheritance, such as traits reappearing after skipping a generation Gregor Mendel Gregor Mendel 1822-1884 Austrian Augustinian Priest “Father of modern genetics” Credited with developing formal explanation of how characteristics were passed from one generation to the next How did he do this? Mainly worked on pea plants Chose easy to observe characteristics like flower colour, etc. Careful mathematical analysis of the results gave rise to Mendel’s Principles (Mendel’s Model) Gregor Mendel Gregor Mendel 1822-1884 Mendel focused on 7 contrasting traits Mendelian Genetics Mendel’s First Experiment: Pea flowers come in two colors: purple and white He took true-breeding or pure-breeding plants: Plants that over the generations of self-pollination, had produced only the same variety as the parent. E.g., plants with purple flowers will always have offspring with purple and those with white will always have white when self- pollinating Mendel asked himself: “What would happen if a plant with purple flowers was artificially crossed with one that had white flowers?” X Mendelian Genetics Mendel’s First Experiment: 1. Removed stamens (male sex organs) from purple flowers 2. Took pollen from plant with white flowers 3. Fertilized purple plant 4. Collected the seeds (peas) and planted them the following year Results: All of the offspring of the first filial (F1) generation had purple Mendelian Genetics Mendel’s First Experiment: “What would happen if the F1 offspring were mated with each other?” “Monohybrid Cross”= a genetic mix between two individuals who have opposite homozygous genotypes When F1 offspring are crossed with each other: 75% or ¾ of F2 offspring had purple flowers 25% or ¼ had white flowers!! What Mendel observed: (1) when you cross a true- breeding purple flower with a true-breeding white flower, you get all purple P: flowers in offspring (aka the F1 generation) F1: What Mendel observed: (2) when you cross two of these F1 generation purple flowers, you get 3:1 ratio of purple: white P: flowered offspring (aka the F2 generation) F1: F2: What Mendel inferred: the only way these ratios work is if 1) every individual has 2 discrete recipes (he calls genes) for all properties. P: F1: F2: What Mendel inferred: the only way these ratios work is if 2) every individual passes on 1 of these copies of a gene (allele) to their P: offspring F1: F2: What Mendel inferred: the only way these ratios work is if 3) which of your two alleles you pass on to your offspring is random every P: time you have an offspring F1: F2: Mendelian Genetics Mendel’s Model: The are four main concepts to this model: 1. Inherited characteristics are controlled by discrete chemical units (genes) 2. Each individual contains a pair of such heritable factors (alleles) for a particular gene (diploid) 3. Law of segregation: During the formation of gametes, this pair is separated (or segregated), so that only one member of the pair appears in any one gamete 4. At fertilization, the single allele in the sperm and the single allele in the egg are combined so that the new individual again has a pair of alleles for that trait Mendelian Genetics Homologous chromosomes Law of Segregation: Mendel had no idea of Original Diploid METAPHASE I meiosis (was all done cell: Tt mathematically) Modern Explanation: Based on meiosis Diploid (2N) organisms produce haploid (n) gametes Result is 4 sex cells each with 1 set of chromosomes The Law of Segregation: Both alleles will always end up in different gametes T t Mendelian Genetics Genotype and Phenotype Revisited: Genotype: short hand notation for the combination of alleles present for the characteristic in a particular organism Phenotype appearance of the characteristic (ex. flower color) Genotype Phenotype PP (Homozygous) Purple pp (Homozygous) White NB. Phenotype does not always reveal genotype! Pp (Heterozygous) Purple Mendelian Genetics Back to Mendel’s First Experiment: Genotypes and phenotypes of the F1 generation: 100% purple phenotype 100% Pp (genotype) – they are all heterozygous Punnett Square: Allows us to predict ratios! Gametes of parent 1: PP P P Pp Pp parent 2: pp Gametes of p -purple- -purple- Pp Pp p -purple- -purple- Mendelian Genetics Back to Mendel’s First Experiment: Phenotypic ratio: Genotypic ratio: 1 PP: homozygous dominant (purple) 2 Pp: heterozygous (purple) 1 pp: homozygous recessive (white) Mendelian Genetics Review: Parental generation (true-breeders) Phenotype: Purple White Genotype: PP pp F1 generation (offspring of original cross) Phenotype: Purple Genotype: Pp F2 generation (grand-children) Phenotype: Purple Purple White 3:1 Genotype: PP Pp pp 1:2:1 Mendelian Genetics A word of caution! Ratios in previous example are what would be expected if the results of a large number of crossings were studied! They are statistical results of large samples They do not necessarily reflect what would be found in the crossing of a single pair of parents Example: coin toss (heads 50%, tails 50%) Mendelian Genetics Procedure for Solving Genetics Problems: 1. Read problem and record question(s) to be answered 2. Decide on notation for gene and alleles and record decision 3. Decide on the genotype for the two phenotypes 4. Decide on genotypes of parents and record decision 5. Decide on phenotypes of parents and record decision Jamie is a biological female who is an albino (a recessive trait) while Pat is a biological male that has normal pigmentation, but is a carrier for albinism. What is the probability that Jamie and Pat will have an albino child? What is the probability that they will have a child that is a carrier for the albino allele? (Note: carrier = does not have disease phenotype, but carries 1 copy of disease allele) Choose a letter for the alleles: Aa aa Let A = allele for normal skin pigmentation a = allele for albinism A a a a A a a a Jamie is a biological female who is an albino (a recessive trait) while Pat is a biological male that has normal pigmentation, but is a carrier for albinism. What is the probability that Jamie and Pat will have an albino child? What is the probability that they will have a child that is a carrier for the albino allele? (Note: carrier = does not have disease phenotype, but carries 1 copy of disease allele) Punnett Square Strategy that allows us to predict genotypic and a a phenotypic ratios of offspring, given alleles of parents A Aa Aa a aa aa ANSWER: 0.5 probability of offspring are albino. 0.5 probability of their offspring are carriers of the albino allele. Mendelian Genetics Sample Genetics Problems: Tay Sachs is a recessive inherited disease affecting the brain. Children who have inherited the disease die in early childhood. Clara and Robert have two children. Their third child is born with Tay Sachs disease. What is the probability that their next child will have Tay Sachs disease. Mendelian Genetics The Testcross: Question: What is the genotype of this flower? It could be PP or Pp – is there a way of finding out? A cross with a recessive individual can be used to test whether an organism with a dominant phenotype is homozygous or Mendelian Genetics Monohybrid Cross: Mendel derived the law of segregation by performing breeding experiments in which he followed only a single character (such as flower color) What happens if you look at more than one character? Mendelian Genetics Dihybrid Cross: Monohybrid crosses: focus on ONE trait Dihybrid crosses: focus on TWO traits 1. Shape of seeds R – smooth r - wrinkled 2. Colour of seeds Y – yellow y – green Mendelian Genetics More Than One Character: Yy Rr Two pairs of alleles on nonhomologous chromosomes. What happens to these Y Y Y Y chromosomes as they go through meiosis? R R R r What if the chromosomes or inherited from your mother (blue) and those inherited from y y y y your father (red) never separated? R r r r Mendelian Genetics Dihybrid Cross Y y Y y Mendel’s hypothesis: Independent assortment R r R r Y y Y y R r R r X Y y Y y r R r R Mendelian Genetics Dihybrid Cross Mendel identified the Law of Independent Assortment by following two characters at the Y y Y y same time. Two pairs of alleles on R r R r nonhomologous chromosomes. Each pair of alleles inherited independently. During gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair. Mendelian Genetics Law of Independent Assortment Ex: Organism with 2 pairs of chromosomes: 2N = 4 - 1 pair of long chromosomes (homologous) - 1 pair of short chromosomes (homologous) Two ways homologous pairs of chromosomes can align at Metaphase I Possibility 1 LONG | LONG SHORT | SHORT Possibility 2 LONG | LONG SHORT | SHORT Mendelian Genetics Independent Assortment and Gamete Production Q: How many gametes will an individual with the following genotype produce? AaBBccDdEE Number of possible gamete combinations = 2number of heterozygote genes Therefore, since AaBBccDdEE is only heterozygote for two genes: Number of gametes = 22 = 4 ABcDE, aBcDE, ABcdE, & aBcdE Mendelian Genetics Independent Assortment and Probabilities Q: What fraction of the offspring of parents each with the genotype Kk Ll Mm will be kk ll mm? Ans. Each of the characters, K, L, and M, will assort independently. The chance of any specific homozygous recessive appearing is 1/4. The chance of all three appearing is therefore (1/4) (1/4) (1/4) or 1/64. K k Example calculation for the Kk * Kk the chance of a child to be K KK Kk kk=1/4 (repeat with all gene pairs, Ll Mm) k Kk kk Mendelian Genetics Dihybrid Cross Dihybrid cross Guinea pigs 2 pairs of alleles, on non homologous BBSS bbss chromosomes: Black color dominant over brown: Black = BB or Bb, brown=bb BS Gametes bs Short hair is dominant over long: Short =SS or Ss, long = ss F1 Q: Homozygous black, homozygous generation short haired guinea pig crossed with a brown, long haired guinea pig All BbSs Parents: BBSS X bbss F1 = BbSs Now cross an F1 X F1 Mendelian Genetics Gametes from F1 female Dihybrid Cross 1 1 1 1 4 4 4 4 BS Bs bS bs Gametes formed by segregation and BBSS BBSs BbSS BbSs independent 1 Black, Black, Black, Black, assortment of alleles. 4 BS short short short short F1 = BbSs X BbSs BBSs BBss BbSs Bbss 1 Black, Black, Black, Black, Each F1 guinea pig = 4 4 Bs short long short long kinds of gametes with BbSS BbSs bbSS bbSs equal probability. 1 Black, Black, Brown, Brown, 4 bS short short short short BbSs Bbss bbSs bbss F2 generation 1 Black, Black, Brown, Brown, bs 4 short long short long Mendelian Genetics Dihybrid Cross 9:3:3:1 ratio: expected ratio in a dihybrid F2 cross if 2 alleles are on nonhomologous chromosomes example: 9 3 3 1 16 16 16 16 Black, Black, Brown, Brown, short-haired long-haired short-haired long-haired F2 phenotypes