Genetic variation = variation in DNA sequence in our genomes
Genetic variation is ensured in crossing over and independent assortment
Crossing over: occurs during prophase I, genetic material is exchanged between maternal and paternal chromosomes
Independent assortment: occurs during metaphase I, gametes are created and carry different combinations of maternal and paternal chromosomes
Errors in meiosis caused by non-disjunction or caused by changes in chromosome structure
Non-disjunction: when chromosomes don't separate correctly in meiosis and result in abnormal number of chromosomes
Euploidy (correct number of chromosomes), aneuploidy (incorrect number of chromosomes), monosomy = missing chromosome, polysomy = additional chromosome(s)
Anaphase I: Homologous chromosome pairs don't separate to opposite poles, no normal gametes
Anaphase II: sister chromatids don't separate to opposite poles, half of gametes have correct chromosome number
Extra chromosome 21 (Trisomy 21) = Down Syndrome, 1 sex chromosome = Turner Syndrome (only in females), Extra Y chromosome = Jacob's Syndrome (only in males)
Chromosome structure errors: During crossing over the chemical bonds holding the DNA are broken and reformed
chromosomes don't reform properly or non-homologous pairs cross over
Deletion: piece of chromosome is deleted
Duplication: section of chromosome is repeated (appears 2 or more times in a row)
Inversion: section of chromosome reattaches to chromatid in wrong orientation - DNA sequence is reversed
Translocation: Section of 1 chromosome becomes attached to a different chromosome
Genetic Testing: type of medical test that identifies change in chromosomes, genes or proteins
Genetic screening in newborns: routine tests are done in hospital as a part of a newborn screening program (not mandatory)
Blood test taken from baby's heel and analyzed for various genetic disorders
Prenatal Genetic Testing: procedures and tests performed on the fetus to look for genetic-based abnormalities
Gene therapy: experimental treatment to cure genetic disorders
Insert a healthy normal form of a gene into the tissue cells affected, the gene replaces the effects of the mutated gene and restores the function of the protein
Amniocentesis: Doctor removes a sample of amniotic fluid
Usually done in the 2nd trimester of pregnancy (15-18 weeks)
Recommended to women with risk factors such as age 35 and older, previous child/pregnancy with a birth defect, blood test/ultrasound suggesting a birth defect or a family history of genetic disorders
Chronic Villus Sampling (CVS): The doctor removes a sample of the placenta
Usually done between 10-14 weeks in the pregnancy
Recommended for women with a higher chance of having a baby with a genetic condition/birth defect
Kucha Translucency Screening: A scan that detects chromosomal abnormalities
Genetic counselling
Genetic counsellors help with genetic health, understanding genetic risks and giving information
Human cell has 23 pairs of homologous chromosomes (1 set from paternal, 1 set from maternal)
Diploid organism = homologous pairs (Somatic cells: body cells)
Haploid Organism = 1 set of chromosomes (Reproductive cells: found in reproductive tissue of an organism which produce sperm + egg
Male gamete = sperm, female gamete = ovum
Homologous chromosomes: pair of chromosomes similar in shapes and sizes
Homologous pairs (tetrads) carry genes controlling the same traits
Each locus (position of a gene) = same position on homologues
Autosomes code for majority of offspring's traits, sex chromosomes code for sex of offspring (2 X = female, 1 X + 1 Y = male
Meiosis = process which gamete cells are produced and diploid cells become haploid cells
2 cell divisions (Meiosis I and Meiosis II) and 1 duplication of chromosomes
Meiosis in males = spermatogenesis and produces sperm, meiosis in females = oogenesis and produces ova
Occurs in reproductive tissue, 4 haploid daughter cells are produced from 1 diploid parent cell
Meiosis I
Interphase I = chromosomes replicate, each duplicated chromosome has 2 sister chromatids
Prophase I = 90% of process, chromosomes condense, synapsis occurs, crossing over occurs
Metaphase I = tetrads align on metaphase plate, independent assortment occurs
Anaphase I = Homologous chromosomes separate and move towards poles, sister chromatids remain attached
Telophase I = Each pole has haploid set of chromosomes and cytokinesis occurs and 2 haploid daughter cells form
Meiosis II
Prophase II = chromatin condense to form chromosomes, nuclear membrane breaks down, duplicated centrioles migrate to opposite poles, spindle fibers form
Metaphase II = spindle fibers attach to chromosome centromere and align along equatorial plate, sister chromatids face opposite poles
Anaphase II = centromere splits, sister chromatids are pulled to opposite poles by spindle fibers
Telophase II = nuclear membrane forms around each set of chromatids, spindle fibers appear, cytokinesis occurs and 4 haploid daughter cells are produced
Karyotypes = person's set of chromosomes (humans have 23 pairs of chromosomes, 46 in total)
Autosomal vs Sex chromosomes
22 pairs of autosomal (regular) chromosomes and 1 pair of sex chromosomes (XX → female, XY → male)
DNA determines the structure of proteins which are needed because actions of living thins depend on enzymes (proteins)
DNA Structure: 2 molecules arranged into a ladder-like structure (Double Helix)
Made up of millions of tiny subunits called Nucleotides which consist of phosphate group, pentose sugar and nitrogenous base
4 types of nitrogenous bases: Adenine, guanine, thymine and cytosine
Adenine (A) + Thymine (T) form one base pair while Cytosine (C) + Guanine (G) form one base pair
Due to this pairing, the order of the bases in 1 strand determine the order of the bases in the other strand
The bases are arranged in triplets called codons (AGG-CTC-AAG-TCC-TAG)
Phosphate and sugar form the backbone of the DNA molecule and the bases form the "rungs"
Genes: a segment of DNA that codes for a certain trait found at a specific part on a chromosome
Genes code for specific traits because they code for the production of proteins which give us unique phenotypes
DNA replication: Each cell has a copy of DNA that was in the fertilized egg of the zygote and before the cell divides the chromosomes go through DNA replication
Sequencing of nucleotides contain info that works through proteins
Proteins fold into complex 3-D shapes and become key regulators
DNA doesn't leave the nucleus so it uses a messenger (mRNA) to travel the cell (ribosomes via rough ER) to make a protein that travels throughout the body
mRNA leaves the nucleus with the code for a protein
Codons code for proteins such as Amino acids and have stop/start codons
Nucleotide sequence transcribed from DNA to mRNA is the genetic message
RNA has 3 main differences from DNA: RNA = single stranded vs. double stranded, ribose sugar vs. deoxyribose and uracil (U) instead of thymine (T)
Mutations are caused by errors in replications, transcription, cell division or external agents
Mutations in reproductive cells means the gene is apart of the organism and can cause ew traits, non-working proteins, death of organism or structural/functional problems
Mutations in body cells aren't passed down to offspring but can be harmful and have an impaired function of cell
Point Mutation & Frame-Shift Mutation
Point Mutation: 1 change in a nucleotide changes entire meaning
Frame-Shift Mutation: Single base pair is added/deleted, entire strand shifts and entire protein is changed
Gregor Mendel = botanist in mid 1800s who studied pea plants and noticed that the offspring had different characteristics than their parents
Law of segregation
Inherited traits are determined by 2 alleles of a gene
The alleles segregate into each of the gametes of the parents during meiosis
In fertilization, each offspring contains one allele from each parent
Principles of dominance
Form of trait that's expressed in an individual depends on whether they inherit dominant/recessive alleles
Dominant allele is present, only the dominant for is expressed
In order for the recessive form to be expressed, there needs to be 2 recessive alleles
Vocabulary
Gene: a section of DNA that codes for a particular trait
Alleles: different variations of a gene, rep. using upper (dominant) and lower (recessive) case letter
Dominant: form of trait that always appears when an individual has an allele for it, allele is always expressed
Recessive: form of trait that always appears when an individual has 2 alleles for it, allele is masked in presence of another
Genotype: combination of alleles that code for a trait
Phenotype: appearance of the trait in an organism
Homozygous: genotype made up of the same 2 alleles, 2 upper case alleles = homozygous dominant, 2 lower case alleles = homozygous recessive
Heterozygous: genotype made up of 2 different alleles, uppercase and lower case letter
Punnett Squares
Mendel examined the inheritance one trait at a time (monohybrid cross)
Mendel used a Punnett square to show the multiple combinations of alleles
Complete Dominance: 1 allele is always dominant over the other
Monohybrid cross = single trait crossing, dihybrid cross = inheritance pattern for two traits
Law of Independent Assortment: states that the alleles of one gene sort into gametes independently of the alleles of another gene
Non-Mendelian Genetics: some inherited traits don't follow what Mendel saw
Non-mendelian genetics: incomplete dominance, codominance, polygenic inheritance, modifier genes, multiple alleles and sex-linked traits
Incomplete dominance: neither allele dominates the other, results in heterozygote showing a third phenotype that's in between both traits (blending)
Capital letters carried as superscripts, base letter stays the same
Superscript letter is represented using different letters because alleles = dominant
Codominance: both alleles are fully expressed, results in heterozygote showing a third phenotype with both traits shown (patchy)
Capital letters are carried as superscripts
Polygenic inheritance: the additive effects of 2 or more genes on a single phenotypic characteristic
Multiple genes working together = continuous variation
Controlled by the action of many genes
Modifier genes: a gene can modify the expression of a second gene instead of masking its effects
Environmental factors affect the expression of traits
Temperature can determine whether a gene is turned on or off
Linked genes are on the same chromosome
Homologous chromosomes exchange genetic material during synapsis, changing the combination of alleles possible
Traits close together will occur together with higher frequency
Thomas Morgan created a white-eyed male offspring from two red-eyed parents while studying eye colour in fruit flies
Red-eyed female with a white-eyed male = red-eyed F1 offspring
F2 white-eyed flies = male → eye colour is connected to gender lead Morgan to believe eye colour is on X chromosome which is why white eyes is recessive in females and expressed in males
Most sex-linked traits = X-linked (carried on the X chromosome)
X chromosome is much bigger and contains more genes than the Y chromosome
Some sex-linked traits are associated with disorders
Most are found on the X chromosome, Y-linked = rare
Males have a higher risk because they only have one X chromosome
Y-linked inheritance: Y-linked genes can only be transmitted from father to son
The Punnett squares are used to predict the outcome of the sex-linked inheritance
Most disorders = recessive, some = dominant
Carrier = heterozygous female for the trait
Multiple Alleles: interaction of more than 2 alleles for 1 gene
May show multiple patterns (some alleles show complete dominance, others show codominance)
Methods used in Agriculture
Selective Breeding: process of breeding plants and animals for desirable traits
Artificial Insemination: the transfer of processed semen into a female's reproductive tract
Embryo Transfer: fertilizing an egg artificially and transferring it into a recipient female
Assisted reproductive technologies for humans
Artificial Insemination: sperm is collected and concentrated, then introduced into a woman's vagina
In Vitro Fertilization (IVF): immature eggs are retrieved, joined with sperm in a lab and embryos are inserted into the uterus
Option for women with blocked Fallopian tubes
Cloning: Gene cloning, Therapeutic cloning and Reproductive cloning
Gene cloning: manipulating DNA to produce multiple copies of a gene or another segment of DNA in foreign cells
Therapeutic cloning: producing genetically identical cells that are used to treat various diseases, cloned cells grow new tissues and organs
Somatic cell nuclear transfer (SCNT); an egg cell's nucleus is removed and replaced with the nucleus of a somatic cell of a donor
Reproductive cloning: production of cell clones with the aim of producing a genetically identical organism (rarely sucessful)
Also uses SCNT