Test Guide Unit 3 Heredity Genetics

Unit 3: Heredity/Genetics Probability ❖ *Remember that probability is only the likelihood that something will happen; it doesn’t guarantee anything. ➢ Ex 1: coin flip: 50% (½) heads, 50% (½) tails ■ Probability of 3 heads in a row: ½ x ½ x ½ = ⅛ (every flip is separate event) ➢ Ex 2: birth: ½ daughter, ½ son ■ Probability of 2 daughters: ½ x ½ = ¼ (every birth is separate event) Gregor Mendel’s Laws ❖ Gregor Mendel analyzed inheritance patterns in plants by using probability ❖ Mendel’s 3 laws were: ➢ Law of Dominance ■ States that a cross between homozygous dominant (TT) and homozygous recessive (tt) will result in hybrid offspring (Tt) with 2 different alleles ● The hybrid’s phenotype (physical appearance) will be the dominant trait. ➢ Law of Segregation ■ States that when gametes form, the 2 alleles from the parent cell separate ■ Can be proven by doing a monohybrid cross (cross between 2 hybrid/ Tt x Tt) ● Monohybrid phenotypic ratio - 3:1 ● Monohybrid genotypic ratio - 1:2:1 ■ *Backcross - can be used to find out unknown genotype (B_) by crossing it with homozygous recessive (bb) ● If unknown genotype is BB, all offspring will exhibit dominant trait (Bb) ● If unknown genotype is hybrid (Bb), ½ of offspring is predicted to exhibit recessive trait (bb) ● *if phenotypic ratio = 1:1, the cross was Bb (hybrid) x bb ➢ Law of Independent Assortment ■ Applies when cross is between 2 hybrid organisms for 2 traits located on separate chromosomes ■ States that inheritance of one gene does not affect inheritance of another ■ Supported by a dihybrid cross (TtYy x TtYy); T = tall, t = dwarf, Y = yellow seed, y = green seed ● Dihybrid phenotypic ratio - 9:3:3:1 (9 tall yellow, 3 tall green, 3 short yellow, 1 short green) Non-Mendelian Inheritance ❖ Incomplete Dominance - blending inheritance (ex. Red x White = Pink) ❖ Codominance - both traits are fully expressed (ex. Person with both A allele and B allele will show both traits as someone with AB blood type) ❖ Multiple alleles - more than 2 alleles of a gene (ex. Blood type has 3 alleles: A, B, O) ❖ Polygenic inheritance - when multiple genes control one trait (ex. hair/skin color) ❖ Sex-linked genes - genes that are located on the sex chromosomes ➢ mostly only considering X-linked since they are more common ➢ Often recessive (color-blindness or hemophilia) ➢ Affected fathers pass to all daughters but never to sons ➢ Carrier mother = 50% chance of passing on gene to son ■ If son has affected X-linked gene, he has the trait ■ If daughter has one copy of affected X-linked gene, she is carrier (uncommon for her to inherit both affected X-linked genes) ❖ *gene expression is sometimes affected by the environment (ex. Fruit flies: vestigial wing gene is affected by temperature) ❖ *an individual’s sex can influence inheritance patterns (different from sex-linked) ➢ Ex. baldness being expressed differently in females and males Karyotype ❖ Method used in the lab to check various qualities of chromosomes (can detect abnormalities in size, shape, or quantity of the chromosomes) ➢ Humans have 44 autosomes and 2 sex chromosomes Pedigree ❖ An organized family tree that shows the phenotypes of each family member for a certain trait ➢ Circles = females, squares = males ➢ Shaded in = expresses trait Mutations ❖ Any random mistakes in the genome ❖ Can happen in both somatic cells (cancer) and germ cells (effects on future offspring) ❖ Can be caused by chemicals, radiation, etc. ❖ 2 types of mutations: ➢ Gene mutations - change in the sequence of nucleotides in DNA ➢ Chromosomes mutations ■ Deletion - when a chromosome fragment without a centromere is lost as the cell divides ■ Translocation - when a chromosome fragment attaches to a different (nonhomologous) chromosome ■ Inversion - when a chromosome fragment attaches to its original chromosome in the reverse direction ■ Polyploidy - when a cell has extra sets of chromosomes ■ Nondisjunction - when homologous chromosomes do not separate properly during meiosis Examples of Human Disorders Caused by Gene/Chromosome Mutations ❖ Genetic Disorders ➢ Cystic fibrosis - autosomal recessive; fluid builds up in digestive tract, lungs, etc. ➢ Tay-Sachs disease - autosomal recessive; abnormal brain function due to shortage of enzymes that help break down lipids ➢ Phenylketonuria - autosomal recessive; body can’t break down phenylalanine, can cause mental disabilities ➢ Huntington’s Disease - autosomal dominant; affects nervous system, causes early death ➢ Color blindness - sex-linked recessive; inability to see certain colors ➢ Hemophilia - sex-linked recessive; blood doesn’t clot normally ❖ Chromosomal Disorders ➢ Klinefelter’s syndrome - male with another X chromosome (XXY); small testes + sterile ➢ Down syndrome - extra chromosome 21; higher risk of Alzheimer’s + leukemia, possible mental disabilities Experiments that proved DNA was inherited material ❖ 1927 - Griffith discovered that bacteria can absorb genetic material from their environment + add it to their own genes (bacterial transformation) ❖ 1944 - Avery, MacLeod, McCarty provided evidence for DNA being the genetic material in Griffith’s experiment ❖ 1952 - Hershey and Chase did an experiment and provided evidence to prove that proteins were not genetic material, but DNA (nucleic acid) was. ❖ 1950 to 1953 - Rosalind Franklin used X-ray crystallography analysis to prove that DNA was a double helix ❖ 1962 - Watson and Crick described DNA as a double helix (and won Nobel Prize) ❖ 1953 - Meselson and Stahl provided evidence supporting that DNA replication is semiconservative DNA Structure ❖ Is double helix + made up of 2 complementary strands that are in opposite orientations ❖ Is a polymer (monomers = nucleotides) ➢ Nucleotides - 5-carbon sugar (or deoxyribose), phosphate group, nitrogenous base ■ 4 nitrogenous bases: adenine, thymine, cytosine, guanine (A, T, C, G) ■ A with T, C with G ❖ 2 DNA strands are held together by hydrogen bonds DNA Replication (Eukaryotes) ❖ Semiconservative replication (each new DNA molecule = 1 old strand, 1 new strand) ❖ Happens during interphase (specifically the S phase) ❖ DNA polymerase starts DNA replication + proofreads new DNA strands for mutations ❖ Hydrogen bonds are broken to separate 2 DNA strands ❖ Each DNA strand is template for new strand ➢ If template strand = AAATCGGAC, new strand = TTTAGCCTG ❖ Telomeres (nonsense DNA sequences) protect genes in chromosomes since DNA replication causes loss of nucleotides at the ends. RNA Structure ❖ Has only one strand ❖ Is a polymer (monomer = nucleotides) ➢ Nucleotides - 5-carbon sugar (or ribose), phosphate, nitrogenous base ■ 4 nitrogenous bases: adenine, uracil (not thymine), cytosine, guanine ❖ RNA types: ➢ messenger RNA (mRNA) - made from template DNA strand during transcription; brings DNA sequence from nucleus to ribosomes to be translated ➢ ribosomal RNA (rRNA) - are a part of ribosomes ➢ transfer RNA (tRNA) - carries amino acids to mRNA (in ribosome) during translation to create a polypeptide (which is folded into a protein) How DNA makes Proteins ❖ Transcription ➢ mRNA sequence (codon sequence) is created from DNA template ■ DNA sequence: AAA TAA CCG GAC, ■ Transcribed mRNA sequence: UUU AUU GGC CUG ➢ First, RNA polymerase binds to DNA strand at promoter region and separates the 2 strands. ➢ Then, it keeps adding nucleotides and stops when a STOP codon is reached ➢ Completed mRNA separates from DNA template strand and double helix reforms ❖ RNA Processing ➢ Happens before mRNA strand moves out of nucleus ➢ snRNPs remove noncoding regions (introns) and only coding regions (exons) remain ➢ exons are then spliced back together and the processed mRNA moves out of nucleus ❖ Translation ➢ mRNA is translated into a sequence of amino acids which is folded into a protein ➢ Occurs at a ribosome ➢ tRNA carries specific amino acids in cytoplasm to ribosomes (the amino acid depends on codons of mRNA) ■ Codon chart will show what amino acid corresponds with what codon ➢ A single amino acid can be encoded by multiple codons Gene Regulation ❖ In humans, it is a complex process where cells turn off/on certain genes ❖ Basic model for gene regulation is the operon in bacteria ➢ Group of functional genes + on/off “switches” ❖ 2 types of operons: ➢ Lac/inducible operon - off until turned on when stimulated by environment ➢ repressible operon - on until turned off when not needed ❖ Operon parts: ➢ promoter - “on” switch; place where RNA polymerase binds and starts transcription ➢ operator - place where the repressor binds and turns off the Lac operon ➢ TATA box - sequence of adenine and thymine; helps RNA polymerase bind to promoter Mutations ❖ Types of Gene Mutations ➢ Point mutation - base-pair substitution; one nucleotide is switched for another (ex. Sickle cell anemia) ■ Can be detrimental, beneficial, or have no effect ➢ Insertion/Deletion - deletion is adding a letter to DNA sequence, insertion is adding a letter; causes a frameshift, where everything after mutation is affected ■ Can cause abnormal/no polypeptide ❖ Chromosome Mutations ➢ Abnormalities in # of chromosomes/chromosome structure ■ Aneuploidy - any abnormal chromosome # (ex. Trisomy-21 or Down syndrome); caused by nondisjunction ● Polyploidy - type of aneuploidy with extra chromosomes sets Genetic Engineering/Recombinant DNA ❖ Recombinant DNA - combination of DNA from two different sources into a single cell ➢ Can be used for genetic engineering Tools/Techniques for Genetic Engineering ❖ Restriction enzymes - tool that cuts DNA at specific places (recognition sequences) ➢ Pieces of DNA cut by restriction enzymes = restriction fragments ❖ Gel electrophoresis - separation of DNA molecules based on size and charge ➢ Smaller molecules = faster ➢ DNA is cut into pieces by restriction enzymes before running a gel ❖ Polymerase chain reaction (PCR) - fast replication of DNA outside of a cell ➢ DNA produced from this is used in labs to make comparisons/study