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Genetics (patterns of inheritance) MENDELIAN GENETICS branch of biology that studies how genetic characteristics are inherited MENDELIAN GENETICS Gregory Mendel, an Augustinian monk (18221884), was the first who systematically studied how genes work GENE • Portion of DNA with meaningful information determines a characteristic (eyes color, type of hair, height, etc.)’ Transcription Translation Proteins: characteristic • Genes are passed from one generation to another by means of reproduction offspring Mendel published his work in 1866, but his ideas were not accepted until 1900 • In organisms with sexual reproduction Fertilization and Meiosis (crossing-over, independent assortment) ensure genetic variability 1 Genetics (patterns of inheritance) Why was Mendel successful? A) He used pea plants, whose sexual reproduction he could easily control. Pea plants… Are easy to grow Come in many readily distinguishable varieties Are easily manipulated Can self-fertilize B) He studied simple well-defined characteristics (or traits), such as flower color, and he varied one trait at a time. Previous investigators had tried to study many complex traits, such as human height or intelligence C) He repeated his crosses multiple times and applied statistical tests to his results Some genetics terms to know… LOCUS (pl. loci) Specific place where a gene is located in a chromosome Sickle cell hemoglobin Normal hemoglobin Locus Free or Unattached earlobes Attached earlobes Blood type A ALLELES Blood type O Homologous chromosomes: Diploid organism Alleles for blood type •Two or more forms of a gene may be present in the population •Alleles are different “versions” of a gene located at the same locus in homologous chromosomes •In diploid organisms, only two alleles may be present in a given individual 2 Genetics (patterns of inheritance) GENOTYPE Combination of alleles present in an individual from mother and father. The genotype is always diploid Green Yellow (e.g. blood type: AO, AB, etc) pods (GG) pods (gg) Letters in uppercase and lowercase are used to gametes: g name the genes present in one individual gametes: G Alleles… G = Green pods, g = yellow pods Homozygous Genotypes Heterozygous Genotype GG, gg Gg Two identical alleles for a trait or characteristic Breeding GG x gg Two different alleles for a trait Phenotype: all Green pods Genotype: Gg PHENOTYPE The way each combination of alleles expresses in the organism as a result of the genetic interaction. What we “see” in an organism as a result of genes interaction The “outcome” of the genotype, the observable characteristic or trait Carrier: Any individual who is heterozygous opr hybrid for a characteristic Then…what allele expresses (is visible) in the individual? Mendel’s laws of heredity 1) Law of dominance If two different alleles are present in an individual… the dominant allele overshadows or masks the recessive allele. In heterozygous organisms, only the dominant version of the trait is observed Green pods (GG) Yellow pods (gg) gametes: G gametes: g Breeding GG x gg A Punnett square shows the possible combinations of parents’ gametes and the possible offspring genotypes that result from each cross. DOMINANT and RECESSIVE ALLELES Masks the effects of other alleles for the trait (characteristic) when an heterozygous genotype is present. The upper case (E) is designated for the dominant, Will not express unless it is present in a recessive homozygous (gg) genotype. The lower case is used for the recessive alleles (e) Phenotype: all Green pods Genotype: Gg 3 Genetics (patterns of inheritance) During the production of sex cells or gametes in meiosis, the DNA information from mother and father segregate (separate) in order to become haploid (N) 2) Law of segregation What Mendel did •Monohybrid cross: only one trait is involved in the analysis What happens during meiosis Diploid parents (2n) Interphase (S) DNA replication Anaphase I: Different traits end up in different sex cells (gametes): Segregation What Mendel observed Phenotype 3:1 75% Green pod 25% White pod Genotype: 50% Gg 25% GG 25% gg 3) Law of independent assortment What Mendel did •Dihybrid cross: two traits are analyzed together During the production of sex cells or gametes in meiosis, the assortment (left and right) of each pair of chromosomes is random compared to the other pairs F1 x F1 Phenotype 9:3:3:1 9 (G pod / Y seed) 3 (G pod / y seed) 3 (g pod / Y seed) 1 (g pod / y seed) What happens during meiosis Meiosis I Metaphase: G Independent g assortment G Y y Y Option 1 y g Y y Y Meiosis II Option 2 y Y y Y Possible gametes G G GY y Gy g g gY gy 4 Genetics (patterns of inheritance) Mendelian patterns of inheritance in human traits Mendelian genetics allows to explain many inherited traits… Important! Dominant traits are not necessarily “normal” or more common Recessive traits do not always mean “bad” traits Example: blue eyes are recessive! Human Disorders Controlled by a Single Gene Genetic disorders are anyways normally caused by recessive alleles… Why? Most dominant disorders would kill the individual before reproduction or severely reduce chances of mating! 5 Genetics (patterns of inheritance) Variations of Mendel’s Laws A) X-Linked or Sex linked Genes 2 alleles, but located in the X chromosome… X and Y chromosomes are non-homologous: different size, different genes! Because each chromosome has many genes, these genes are inherited as a linkage group. Genes found on Xchromosomes are called X-linked The Y is much smaller than the X chromosome and has less genes. In men, because there is only one X, genes on the Xchromosomes will be expressed. Example: HEMOPHILIA is a X-linked trait in humans. Let N=normal allele and n=hemophilia allele If a woman who is heterozygous (carrier) for the trait has children with a man who is normal Genotype woman=XN Xn and male=XN Y As meiosis separates alleles for the trait, only one allele can be present for the trait in the sex cells, the gametes (sex cells) this couple can produce are… Gametes woman=XN or Xn and male=XN or Y sex cells male sex cells female XN Y XN XN XN XN Y Xn XNXn Xn Y For this couple, every baby girl will be normal, but half of their sons will be hemophilic! 6 Genetics (patterns of inheritance) B) Incomplete Dominance 2 alleles, but 3 phenotypes are possible! 1 gene with 2 alleles, but… Homozygous RR is red Homozygous rr is white With a heterozygous condition, both alleles express themselves The final phenotype in the heterozygous is a “mix” of the two alleles Hypercholesterolemia (dangerously high levels of cholesterol in the blood) is incompletely dominant C) Multiple alleles 3 alleles exist, which leads to multiple phenotypes 1 gene, that has 3 alleles with different genetic interactions Human blood type has 3 alleles: A, B, O A and B are codominant Both are expressed in the heterozygous AB condition A and B are dominant over 0 Genotype A0 will be phenotype A (blood type A) Genotype B0 will be phenotype B (blood type B) As O is recessive to the other alleles, only the genotype OO produces phenotype O (blood type O) 7 Genetics (patterns of inheritance) D) Polygenic Inheritance Some traits are determined by several genes located at several different loci (places) on the same or different chromosomes. Polygenic inheritance is the additive or cumulative effects of many genes on a single phenotype Skin color determination Because there are several loci for skin color, there are many possible variations depending on the number of dominant genes present in the individual Aabbccdd eye color, hair color, and height are examples of polygenic traits AaBBCCDD 8