A Comprehensive Guide to Genetics and Heredity

SBI3U Genetics Study Notes The study of heredity, variation, and the transmission of genetic information from one generation to the next is done in the field of biology known as genetics. Genetics is the study of heredity, the process by which a parent transmits specific genes to their children; children inherit the genes that express particular traits from their biological parents. DNA: molecule inside the nucleus that gives instructions to make proteins etc. DNA Nucleotide includes: ● 1 Phosphate group ● 1 Deoxyribose Sugar ● 1 Nitrogenous Base (Adenine, Thymine, Guanine, Cytosine) Base Pair: AT and CG ● Gene: segment of DNA that encodes information Meiosis: process at which sex cells (gametes) with a haploid number of chromosomes are produced Two Key Outcomes: 1. Genetic Reduction: daughter cells produced have half the amount of genetic information (23 chromosomes) 2. Genetic Recombination: products have different combinations of alleles (forms of a gene), genetically different offspring Meiosis: ● takes place in ovaries and testes ● two nuclear divisions ● human cell with 46 chromosomes (2n) will undergo meiosis, produce gametes with 23 chromosomes (n), known as gametogenesis ● each gamete has 23 chromosomes ● produces 4 haploid daughter cells that are genetically different ● fertilization: haploid (n) sperm cell + haploid (n) egg cell = diploid (2n) zygote ● zygote will produce multicellular organism Vocabulary: Chromosomes- humans have 46, 23 pairs, 1 pair of sex chromosomes, 22 pairs of autosomes, 2 sister chromatids held by a centromere- chromatins Centromere- middle region of chromosome, attracts spindle fibers Somatic cells- body cells (non-sex cells), undergo mitosis Haploid (n)- half the genetic information (23 chromosomes) Diploid (2n)- complete set of genetic information (46 chromosomes) Homologous chromosomes (tetrad)- pair of chromosomes similar in shape, size, gene arrangement and gene information (not identical as they may carry different forms of the same gene) (Example: Chromosome #1) Synapsis- pairing of homologous chromosomes, during prophase 1 in meiosis 1 Crossing over- exchange of chromosomal segments between a pair of homologous chromosomes Cytokinesis- division of cytoplasm (two cells produced) Meiosis 1: first division, chromosome number reduced in half, synapsis and crossing over occur Interphase 1: ● DNA double (46-92) Prophase 1: ● ● ● ● each pair lines up side by side (synapsis) segments of chromosomes exchanged (crossing over) centrioles move to poles spindle apparatus forms Metaphase 1: ● pairs of homologous chromosomes line up along the equator (middle of cell) ● spindle fibers attach to centromere of each homologous chromosome (where centromeres end up is random) Anaphase 1: ● homologous chromosomes separate ● move to opposite poles (segregation) ● one chromosome (two chromatids) move to pole, now haploid ● errors likely to occur here as chromosomes divide into new cells Telophase 1: ● ● ● ● ● chromosomes begin to uncoil spindle fibers disappear cytokinesis takes place nuclear membrane forms around each group of chromosomes diploid cells (46 chromosomes) Meiosis 2: second division, no replication of genetic material, 4 haploid cells produced at the end, all are genetically different Prophase 2: ● ● ● ● nuclear membrane disappears centriole pairs move to opposite ends spindle fibers reform chromosomes condense Metaphase 2: ● sister chromatids attach to spindle fibers in middle of the cell Anaphase 2: ● sister chromatids are pulled apart to opposite sides of the cell ● errors likely to occur here as chromosomes divide into new cells Telophase 2: ● ● ● ● nuclear membrane reforms around sister chromatids centriole pairs disappear DNA uncoils cytokinesis occurs Gamates: ● production of egg and sperm cells (4 egg, 1 sperm) ● haploid= 23 chromosomes ● daughter cells are not identical to parent cell Importance of Meiosis: ● crossing over and independent assortment give rise to genetic variation within individuals ● independent assortment: where chromosomes end up when they split and go to the poles, different genes go both ways (occurs in Anaphase 1 and 2) ● crossing over: genetic material is mixed with that of the other in the homologous pair, creates a mix of genes when new cells are formed (occurs in prophase 1) ● most gamete cells receive mix of paternal and maternal chromosomes Gametogenesis: ● ● ● ● ● ● production that creating a sperm or egg differs greatly between spermatogenesis and oogenesis spermatogenesis: converts the spermatocyte into four spermatids oogenesis: asymmetric cell division produces one large cell and three small ones that degenerate into three polar bodies during birth, eggs are produced until prophase 1, meiosis will continue for one cell each month beginning at puberty, final stage of meiosis 2 is not completed unless fertilized when meiosis 2 is completed, the mature egg (n) has been fused with a sperm cell (n) to create a diploid (2n) zygote Abnormal Meiosis: Nondisjunction: ● occurs when two homologous chromosomes move to the same pole during meiosis 1 ● results in one of the daughter cells having an extra chromosome, while the other is missing one ● cells do not function properly ● can also occur in any cell if chromatids do not separate ● three of one chromosome: trisomy ● one less chromosome: monosomy Chromosomal disorders: Down syndrome: Extra chromosome 21 ● intellectually disabled ● heart defects ● physical abnormalities Klinefelter syndrome: Extra X chromosome ● occurs in males (extra female chromosome) ● tall and slim ● few pubic hair, facial hair and underarm hair Turner Syndrome: missing X chromosome ● ● ● ● ● ● occurs in females short stature sterile lack of ovarian development poor breast development no menstruation *Disorders affecting autosomes have the greatest affect on a person’s health as they impact genetic info and behavior. *XX= Female, XY= Male Vocabulary: Chapter 5: Alleles: two or more variations of a gene, alleles are located at the same position on a pair of homologous chromosomes Dominant: alleles of this type determine the expression of the genetic trait (capital letter) Recessive: alleles of this type are “masked” by dominant alleles (lower case letters) Phenotype: the observable trait of an organism (eg. hair and eye colour) Genotype: the alleles that make-up a gene (specific trait) (eg. AA, Aa, aa) Homozygous: a genotype in which the allele pairs are the same (eg. AA or aa) Heterozygous: a genotype that includes a dominant and a recessive allele (eg. Aa) Mendelian Genetics: Mendel’s Law of Heredity 1. Each parent contributes one allele during cross-fertilization. 2. Dominant allele is always expressed when the recessive allele is present. 3. Each pair of alleles segregates during formation of sex cells. Hybrid: the offsprings from two pure lines, often heterozygous Monohybrid Cross: cross where one trait is observed Dihybrid Cross: a cross where 2 traits are observed Monohybrid Crosses: (2×2 Punnett Square) -Does it with one trait at a time. -Demonstrates how dominance and recessive genes could be passed onto from one generation to another. P1 Generation: BB bb F1 Generation: Bb Bb Bb Bb F2 Generation: BB Bb Bb bb **F2 Generations will always have 3:1 ratio of dominance and recessive Dihybrid Crosses: (4×4 Punnett Square) -Demonstrates Mendel’s law of independent assortment that the inheritance of one trait doesn’t dictated the inheritance of another -In Dihybrid crossing, 2 traits are put to the test Solving: Distributive property both genotypes *9:3:3:1 Ratio for two heterozygous Test Cross: ● use when the genotype of 1 parent is unknown but they express a dominant phenotype ● parent is crossed with a homozygous recessive parent ● determining whether or not the parent is homozygous dominant or heterozygous ● two cases for monohybrid ● four cases for dihybrid Incomplete Dominance: ● not all alleles are fully dominant or recessive ● heterozygous genotype can show an intermediate phenotype (blending of alleles to produce a new phenotype) ● occurs when two alleles are equally dominant, produce a new heterozygous genotype ● example: red dragon + white dragon = pink dragon Codominance: ● occurs when both alleles are fully expressed ● one allele does not mask the other ● example: red bull + white cow = roan calf (mix of white and red hair that appears in patches) Lethal Alleles: ● alleles that have a detrimental effect on the organism ● if lethal gene is dominant and expressed immediately, organism would die, eliminate gene from population ● if organism lives long enough to reproduce, gene is passed on to next generation ABO Blood Group: ● system shows both multiple alleles and codominance, more than two alleles possible for a given name Genotype Phenotype IAIA or IAi -dominant Type A IBIB or IBi –dominant Type B IAIB – dominant Type AB ii –recessive Type O Sex-linked Inheritance: ● traits controlled by genes on either the X or Y chromosome are called seclinked traits ● they are identified by their different rate of appearance between the genders ● most X-linked traits are recessive, affected male passes the allele onto daughter ● females must inherit both alleles to be affected while males only need one ● only females can be carriers ● example: CVD- color vision deficiency, hemophilia