2Genetics Genetics is the science of heredity Genetics explains how genes bring about characteristics in living organisms and how those characteristics are transmitted from parents to offspringGenetics is at the center of all biology because gene activity underlies all biological processes!
3GeneticsRemember, genes are discrete units of genetic (hereditary) information consisting of a specific nucleotide sequence in DNA
4Experimental Genetics The modern science of genetics began in the 1860’s when Gregor Mendel, an Austrian monk studied the principles of genetics by breeding garden peasAvailable in a wide variety of shapes and colorsCheap and abundantShort generation times with large amounts of offspring
5Experimental Genetics Mendel studied 7 characters (heritable features) each with its own distinctive trait (variant of that character)He created true-breeding lines; lines of peas that were homologous for each traitA true breeding line had only the genes that coded for that trait, both chromosomes had the same version of the geneFor example, a true breeding purple pea plant had only ‘purple’ genes, not white
7Experimental Genetics Mendel wanted to see what happened when he crossed true-breeding lines for one trait with true-breeding lines for another traitHis results led to the establishment of several principles:Mendel’s Law of DominanceMendel’s Law of SegregationMendel’s Law of Independent Assortment
8What would happen if you cross a purple flower with a white flower? Mendel’s results indicated that 3 of the 4 flowers produced had purple flowers, while 1 had whiteHow?
9Mendel’s Law of Dominance The white and purple flowers of the pea plants are two versions of a gene for flower colorAlternative versions of a gene are called allelesMendel’s law of dominance states that when an organism has 2 different alleles for any given character, 1 allele will dominate
11Mendel’s Law of Dominance For each character, an organism inherits 2 alleles, 1 from each parentThese alleles may be the same or differentAn organism that has 2 identical alleles for a gene is said to be homozygousAn organism that has 2 different alleles for a gene is said to be heterozygous
12Mendel’s Law of Dominance If the 2 alleles of an inherited pair differ, then one allele will determine the organism’s appearance over the other, and is called the dominant alleleThe other allele has no noticeable effect on the organism’s appearance and is called the recessive alleleWe use upper and lower case letters to describe the dominant and recessive alleles, respectively
13Mendel’s Law of Segregation A sperm or egg carries only 1 allele for each inherited characterThis is because allele pairs segregate (separate) during gamete formation (meiosis!)When sperm and egg unite during fertilization, they each contribute their own allele, restoring the paired ‘condition’ to the offspring
14Mendel’s Law of Independent Assortment The alleles of a gene pair separate from one another independently of the other alleles of another gene pair during segregation (meiosis)The origin of any particular allele will be randomly selected from paternal or maternal chromosomes via the process of crossing-over (why, for example, a cat’s color is independent of its tail length)
15Mendel’s Law of Independent Assortment For example, Aa will segregate from Bb, or in other words, the color of the flower is independent from the inheritance of the height of the plant
16In this example, yellow and green are 2 traits for the color character (indicated by Y and y, respectively) and round and wrinkled are 2 traits of another character (indicated by R and r, respectively)RRYYrryyGametesRYryRrYySperm1–4RY14–rY14–Ry14–ry1–4RYRRYYRrYYRRYyRrYy14–rYRrYYrrYYRrYyrrYyYellowround1–4Ry16––9RRYyRrYyRRyyRryyGreenround16––31–4ryYellowwrinkledRrYyrrYyRryyrryy16––3Greenwrinkled116––
17Genetics terminologyThe complete genetic make-up of an organism is called its genotypeThe physical expression of the genotype is its phenotypeHomozygousfor thedominant allelerecessive alleleHeterozygousGenotype:PBaPPaabBb
18Phenotypes can reveal genotypes Chocolate labs are labrador retrievers that are homozygous recessive for coat colorBlack labs have at least 1 copy of the dominant allele; but their genotype can be Bb or BBbbB_
19Phenotypes can reveal genotypes How can you determine your dog’s genotype (without a blood test)?You can testcross your dog; mating your dog with a homozygous recessive dog (bb; a chocolate lab)If the black lab was BB, all of its offspring will be black (Bb)If the black lab was Bb, half would be black (Bb) and half would be brown (bb)
20Testcross:GenotypesB_bbTwo possibilities for the black dog:BBorBbGametesBBbbBbbBbbbOffspringAll black1 black : 1 chocolate
21Geneticists use the testcross to determine unknown genotypes Mendel used testcrosses to verify that he had true-breeding lines of pea plantsMendel performed his experiments nearly 100 years before the discovery of DNA!The testcross continues to be an important tool of geneticists for determining genotypes
22Mendel’s laws reflect the rules of probability Mendel’s strong background in mathematics (and physics and chemistry…) served him well in his studies of inheritanceHe knew he needed large sample sizesThe laws of inheritance reflect the probability of an event occurringThe probability of having a girl: 1 in 2The probability of rolling a 5 on a dice: 1 in 6The probability of drawing a queen from a deck of cards: 4 in 52 (1 in 13)
23Probability An event that is certain to occur has a probability of 1 An event that is certain not to occur has a probability of 0When you flip a coin, the probability of getting heads (or tails) is 1 in 2 every time you toss the coin; independent of previous tosses!
24Segregation and fertilization as chance events Bb maleFormation of spermBb female1–2B1–2bFormation of eggsBBBb1–2B1–41–4bBbb1–2b1–41–4F2 genotypes
25Extra credit opportunity!!! Try it at home…Toss a coin 100 times and record the outcomes; your answer should be close to ½ for heads and ½ for tails (if you are using a fair coin…)Submit your answers (and perhaps some photographic/video proof) for extra credit!!!
26Genetic traits may be tracked Individuals exhibiting a recessive trait would be homozygous recessive (carry 2 copies of the recessive allele)Individuals exhibiting a dominant trait, however, could be homozygous dominant (carry 2 copies of the dominant allele) or be heterozygous (carry 1 copy of the dominant and 1 copy of the recessive allele)
28Genetic disordersGenetic disorders may be inherited as a recessive or dominant traitMost human genetic disorders are recessive; most people who have recessive disorders are born to normal parents who are both heterozygous for the allele controlling the disorderIn this way, the parents are carriers of the recessive allele, but are phenotypically normal
29Offspring produced by parents who are carriers for a recessive trait NormalDdNormalDdParentsDSpermdDdNormal(carrier)DDNormalDoes this mean that deaf parents always have deaf children?DEggsDdNormal(carrier)ddDeafdOffspring
30It is said that everything should be tried once, except square-dancing and inbreeding…. It is relatively unlikely for 2 carriers of a rare, harmful allele will meet and mateHowever, the probability increases greatly if close relatives marry and have childrenA mating of close relatives, called inbreeding, is more likely to produce offspring homozygous for recessive traits
31Genetic disorders Let’s take a non-human example… Dog breeds that have been inbred for appearance frequently exhibit serious genetic disorders, such as weak hip joints, eye problems, etc.Endangered species frequently suffer from inbreeding (reduced numbers increase chances of close matings)
32A case study: The Florida Panther The Florida panther population once numbered in the 30’s in the 1990’sClose matings resulted in reduced sperm counts, heart defects, and low survival rates among kittensIntroduction of the Texas Panther in recent years has yielded hybrids with a higher survival rate (controversial!)
33Genetic disorders Why are most genetic disorders recessive? Dominant alleles that cause lethal diseases are much less common than lethal recessivesThis is because the dominant allele cannot be carried by heterozygotes without it affecting them (and subsequently kill the embryo)In contrast, recessive alleles are continually carried from generation to generation by healthy (unaffected) heterozygous carriers
34Genetic disordersMost dominant genetic disorders can be eliminated when it causes the death of an individual before he/she has a chance to mate (and pass along his/her alleles)A lethal dominant allele, however, can escape elimination when it does not cause death until a relatively advanced ageHuntington’s Disease – degenerative disease of the nervous system does not appear until years of age (50% chance of inheriting it)
36Three’s a crowd…Mendel was fortunate in that he chose characters for which there were only 2 allelesMany genes, however, have more than 2 alleles in the populationMore often than not, the inheritance patterns of a particular trait are more complex
37Incomplete dominanceIn some allele combinations, dominance does not existInstead, 2 traits are blended together to form a 3rd traitIn snapdragons (a plant, not a cool dragon, unfortunately) when a red plant is crossed with a white plant, some offspring are red, some are white and some are pink!
38Red RR White rr P generation Gametes R r F1 generation Pink Rr Gametes –2R1–2rSperm1–2R1–2rF2 generationRRrR1–2REggsRrrr1–2r
39Incomplete dominanceIn this case, heterozygous individuals exhibit a third phenotype, pink.The resulting pink flowers are Rr and can produce red, white or pink offspring of their ownIn the case of incomplete dominance, the phenotype does reveal the genotype for all traits!
40Multiple alleles Multiple alleles exist for most genes For example, the ABO blood group in humans involves 3 alleles of a single gene: A, B, and OAn individual can have type A, B, O, or AB bloodThe A and B alleles are co-dominant; both alleles are expressed in heterozygous individuals
41BloodGroup(Phenotype)GenotypesRed Blood CellsOOOAO or AAACarbohydrate ABO or BBBCarbohydrate BABAB
42Safe a life, give bloodBlood group AB can receive blood from any blood type, but can only donate to ABBlood group A can receive blood from only A or O, but can donate to A or ABBlood group B can receive blood from only B or O, but can donate to B or ABBlood group O can receive blood only from O, but can donate to A, B, O or AB!
43Blood type complications In addition to blood ‘type’, our red blood cells may (or may not) have a protein known as the Rh factorIndividuals without this factor have a “negative” blood type, while those with this factor have a “positive” blood typeProblems can occur when an Rh- mother carries an Rh+ child, especially for children conceived after the birth of an Rh+ child
44Multiple allelesNo matter how many alleles for a given gene are in a population, a diploid individual will only have 2 alleles, one on each homologous chromosomeHomozygousfor thedominant allelerecessive alleleHeterozygousGenotype:PBaPPaabBb
45The chromosome basis of inheritance Mendel established his principles (laws) of inheritance long before mitosis and meiosis were understood, and longer still before chromosomes were ‘discovered’The chromosome theory of inheritance states that genes occupy specific loci, or positions, on chromosomes and it is the chromosomes that undergo segregation and independent assortment during meiosis
46Fertilization among the F1 plants All round yellow seeds(RrYy)F1 generationRyrYRrrRMetaphase Iof meiosis(alternativearrangements)YyYyRrrRAnaphase Iof meiosisYyYyRrrRMetaphase IIof meiosisYyYyGametesyYYyYYyyRRrrrrRR1–4ry1–4rY14–Ry14–RYFertilization among the F1 plantsF2 generation9:3:3:1
47Genes on the same chromosome tend to be inherited together The number of genes in a given cell is far greater than the number of chromosomesEach chromosome contains hundreds or thousands of genesGenes located close together on the same chromosome tend to be inherited together and are called linked genesLinked genes do not follow Mendel’s law of independent assortment
49Not accounted for: purple round and red long Explanation: linked genesParentaldiploid cellPpLlPLplMeiosisMostgametesPLplFertilizationSpermPLplPLPLPLMostoffspringPLplEggsplplplPLpl3 purple long : 1 red roundNot accounted for: purple round and red long
50Sex chromosomes and sex-linked genes The X-Y system of sex chromosomes is only 1 of several sex-determining systemsInsects have an X-O system; females have 2 X chromosomes, while males have only 1 (XO)Some organisms lack sex chromosomes altogether and sex is instead determined by chromosome numberOther organisms have temperature-dependent sex determination!
51Sex chromosomes and sex-linked genes In addition to bearing genes that determine sex, sex chromosomes contain genes for characters unrelated to genderA gene located on a sex chromosome is called a sex-linked geneSex-linked disorders typically affect human males, since they have only one copy of each chromosome
52Sex-linked disorders Hemophilia is a sex-linked recessive trait Hemophiliacs lack 1 or more proteins required for blood clotting, and bleed excessively when injured as a resultHemophilia is a caused by a recessive allele on the X chromosomeWoman can be carriers, but rarely suffer from the condition……Why?