Medical Genetics Lecture 2 PDF

MEDICAL GENETICS LECTURE 2 INSTRUCTOR: OTABIL  2 MENDELISM (Mendelian Genetics) VOCABULARY Locus Allele Genotype Phenotype Heterozygous Homozygous Dominant Recessive Law of segregation...

MEDICAL GENETICS LECTURE 2 INSTRUCTOR: OTABIL  2 MENDELISM (Mendelian Genetics) VOCABULARY Locus Allele Genotype Phenotype Heterozygous Homozygous Dominant Recessive Law of segregation P F1 F2 Monohybrid Dihybrid Mendel was an Austrian monk at the St. Thomas Monastryin brünn (Brno, Czech Republic). Studied botany and mathematics. Carried out groundbreaking studies on inheritance https://www.youtube.com/watch?v=mehz7tcxjse  Discovered the basic principles of heredity. Worked with the common garden pea plant, Pisum sativum. Why do you think he choose pea plants?  5 Reasons for choosing the pea plant The pea plant was easy to cultivate with relatively short life cycle (annual plant). Has discontinuous characteristics such as flower color and pea texture. Perfect flowers (i.e., both female and male parts are present on one plant) and can be self- fertilized (i.e. the ovule is fertilized by pollen from the same flower).  6 Features of Mendel’s Experiment Paid attention to a single character at a time. e.g., the shape of the seeds rather than the whole plant. Characters are noticeable features that vary among individuals. Each variant of a character is a trait.  7 Features of Mendel’s Experiment Mendel chose to examine only simple traits: on or off We now know these are single traits. Traits such as yield, and seed weight are multiple gene traits known as quantitative traits.  8 GENE Gene = A length of DNA that codes for a particular protein. Proteins determine an inherited trait. Genes reside on chromosomes. The location of that gene on a chromosome is known as its locus.  9 Allele Allele is a variation of a gene; the different forms of a gene that determine alternative traits. –Each homologous chromosome has one allele –Every different version is an allele of that gene  10 Example of Allele  11 Another important feature of Mendel’s work was his quantitative approach. He counted the number of progenies of each kind to ascertain whether carriers of alternate traits always appeared in the same proportions.  12 Mendel’s Experiment Prevented self-fertilization by removing anthers from “test” flowers. He studied traits that show alternative forms. E.g., tall vs. dwarf. In the parental, or P1 generation, tall plants were pollinated by dwarf plants and vice versa.  13 (a) Flower of a pea plant, cut to show male and female flower parts. (b) Using artificial cross-fertilization, Mendel controlled mating between plants.  14 Mendel’s Experiment Offspring of the cross of P1 individuals are called the first filial generation, or F1. The F1 are also referred to as Hybrids. When tall hybrid plants (1064) were self-fertilized and the F2 classified, some were tall (787) and some dwarf (277) A nearly perfect 3:1 ratio (2.84:1).  15 Other terminologies A hybrid is the offspring resulting from combining the qualities of two organisms of different breeds, varieties, species or genera through sexual reproduction. Monohybrid refers to a heterozygous hybrid concerning a specified gene e.g., the offspring of tall and dwarf peas. They are hybrid for only one characteristic e.g., height. Since all the F1 offspring were tall, they (tall) were referred to as the Dominant trait. The alternative trait (i.e., dwarfness) is called recessive.  16 Dominance applies to the appearance of a trait in the heterozygous condition. Zygotes of individual organisms carrying two units of one allele (DD or dd) are homozygous and those with two different alleles (Dd) are heterozygous. Phenotype refers to the visible expression of a trait or it is any measurable characteristic or distinctive trait possessed by an organism. The trait may be visible, such as colour of a flower or the texture of hair.  17 Genotype vs. Phenotype  18 Principles of Segregation During sexual reproduction, the members of each of the alleles (e.g.,TT, Tt or tt) separate into different reproductive cells or gametes of the male and female parents. The gametes then fuse and give rise to the progeny. Fertilization gives each new individual two factors for each trait.  19 Mendel called this, the principle or rule of segregation The significant inference from his results was that the separation or segregation of pairs of determiners resulted in “purity of gametes”. The concept of segregation can be phrased as the separation of paired genes (allelic pairs) from one another and their distribution to different sex cells  20 Mendel tracked characters for numerous generations P= parental F1= first filial (son) generation F2= Second filial generation Mendel noticed a pattern of inheritance 21 22 Testing the Principle 23 It must be noted that, the separation of the alleles could be detected only in the heterozygous parent (Dd) that produced two different kinds of gametes: (D) and (d) The dwarf (dd) could produce only one kind of gamete (d) When the F1(Dd) were crossed back to the dwarf (dd) variety, half the progeny was tall, and half were dwarf. This demonstrates more conclusively the principle.  24 Recessiveness Recessive alleles are expressed only in homozygous (dd) individuals. Carriers (Dd) are not detectable phenotypically. Recessive alleles can be identified experimentally by crossing potential carriers to homozygous recessive individuals.  Testcross 25 Such a cross of an organism with an unknown genotype to a known homozygous recessive organism is called a testcross. Testcrosses are very useful in genetics in determining the genotypes of individual organisms. Backcross 26 In backcross, the F1 (progeny) is mated or crossed back to one of their parents or with an individual with a parental genotype. The difference between the two crosses is that; In the testcross, a recessive homozygote is always used as one of the testcross parents; this is not necessarily true in a backcross Thank you