MENDELIAN GENETICS branch of biology that studies how genetic characteristics are inherited MENDELIAN GENETICS Gregory Mendel, an Augustinian monk (1822-1884), 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 Mendel published his work in 1866, but his ideas were not accepted until 1900 Proteins: characteristic Genes are passed from one generation to another by means of reproduction offspring In organisms with sexual reproduction Fertilization and Meiosis (crossing-over, independent assortment) ensure genetic variability 1
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 Locus Normal hemoglobin Free or Unattached earlobes Attached earlobes Blood type A Blood type O ALLELES 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
GENOTYPE Combination of alleles present in an individual from mother and father. The genotype is always diploid (e.g. blood type: AO, AB, etc) Letters in uppercase and lowercase are used to Green pods (GG) Yellow pods (gg) name the genes present in one individual gametes: G gametes: g Alleles G = Green pods, g = yellow pods Breeding GG x gg Homozygous Genotypes GG, gg Two identical alleles for a trait or characteristic Heterozygous Genotype Gg Two different alleles for a trait 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 Phenotype: all Green pods Genotype: Gg 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) gametes: G Breeding GG x gg Yellow pods (gg) gametes: g 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
2) Law of segregation 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) What happens during meiosis What Mendel did Monohybrid cross: only one trait is involved in the analysis Diploid parents (2n) Anaphase I: Different traits end up in different sex cells (gametes): Segregation Interphase (S) DNA replication What Mendel observed Phenotype 3:1 75% Green pod 25% White pod Genotype: 50% Gg 25% GG 25% gg 3) Law of independent assortment During the production of sex cells What Mendel did Dihybrid cross: two traits are analyzed together What happens during meiosis Meiosis I G G Metaphase: Independent g g assortment Y Y y y Y Y y y 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) Y Option 1 G GY Meiosis II y Possible gametes G g Gy Option 2 Y y g gy gy 4
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
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 X- chromosomes 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 X- chromosomes 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=x N X n and male=x N 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=x N or X n and male=x N or Y sex cells male X N Y sex cells female X N X n X N X N X N X n X N Y X n Y For this couple, every baby girl will be normal, but half of their sons will be hemophilic! 6
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
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 Aabbccdd Because there are several loci for skin color, there are many possible variations depending on the number of dominant genes present in the individual eye color, hair color, and height are examples of polygenic traits AaBBCCDD 8