Presentation on theme: "Genetics. Genetics is the science of heredity Genetics explains how genes bring about characteristics in living organisms and how those characteristics."— Presentation transcript:
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 offspring Genetics is at the center of all biology because gene activity underlies all biological processes!
Genetics Remember, genes are discrete units of genetic (hereditary) information consisting of a specific nucleotide sequence in DNA
Experimental Genetics The modern science of genetics began in the 1860s when Gregor Mendel, an Austrian monk studied the principles of genetics by breeding garden peas – Available in a wide variety of shapes and colors – Cheap and abundant – Short generation times with large amounts of offspring
Experimental 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 trait – A true breeding line had only the genes that coded for that trait, both chromosomes had the same version of the gene – For example, a true breeding purple pea plant had only purple genes, not white
Flower color White Axial Purple Flower positionTerminal Yellow Seed color Green Round Seed shapeWrinkled Inflated Pod shape Constricted Green Pod colorYellow Tall Stem lengthDwarf
Experimental Genetics Mendel wanted to see what happened when he crossed true-breeding lines for one trait with true-breeding lines for another trait His results led to the establishment of several principles: – Mendels Law of Dominance – Mendels Law of Segregation – Mendels Law of Independent Assortment
What would happen if you cross a purple flower with a white flower? Mendels results indicated that 3 of the 4 flowers produced had purple flowers, while 1 had white How?
Mendels Law of Dominance The white and purple flowers of the pea plants are two versions of a gene for flower color Alternative versions of a gene are called alleles Mendels law of dominance states that when an organism has 2 different alleles for any given character, 1 allele will dominate
P plants 1–21–2 1–21–2 Genotypic ratio 1 PP : 2 Pp : 1 pp Phenotypic ratio 3 purple : 1 white F 1 plants (hybrids) Gametes Genetic makeup (alleles) All All Pp Sperm PP p ppPp P p P p P P p PP pp All Gametes F 2 plants
Mendels Law of Dominance For each character, an organism inherits 2 alleles, 1 from each parent These alleles may be the same or different An organism that has 2 identical alleles for a gene is said to be homozygous An organism that has 2 different alleles for a gene is said to be heterozygous
Mendels Law of Dominance If the 2 alleles of an inherited pair differ, then one allele will determine the organisms appearance over the other, and is called the dominant allele The other allele has no noticeable effect on the organisms appearance and is called the recessive allele – We use upper and lower case letters to describe the dominant and recessive alleles, respectively
Mendels Law of Segregation A sperm or egg carries only 1 allele for each inherited character This 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
Mendels 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 cats color is independent of its tail length)
Mendels 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
1–41–4 1–41–4 1–41–4 1–41–4 1–41–4 1–41–4 1–41–4 1–41–4 9 –– 16 3 –– 16 3 –– 16 1 –– 16 Sperm Gametes RRYY rryy RrYy ry RY RRYY rryy RrYy ry RY RrYy rrYYRrYY RRYyRrYY RRYy rrYy Rryy RRyy rY Ry ry Yellow round Green round Green wrinkled Yellow wrinkled RY rY Ry In 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)
Homozygous for the dominant allele Homozygous for the recessive allele Heterozygous Genotype: PBa P PP a aa b Bb Genetics terminology The complete genetic make-up of an organism is called its genotype The physical expression of the genotype is its phenotype
Phenotypes can reveal genotypes Chocolate labs are labrador retrievers that are homozygous recessive for coat color Black labs have at least 1 copy of the dominant allele; but their genotype can be Bb or BB B_ bb
Phenotypes can reveal genotypes How can you determine your dogs 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)
B_ or Two possibilities for the black dog: Testcross: Genotypes Gametes Offspring1 black : 1 chocolate All black Bb bb BB Bbbb B b Bb b b B
Geneticists use the testcross to determine unknown genotypes Mendel used testcrosses to verify that he had true-breeding lines of pea plants Mendel performed his experiments nearly 100 years before the discovery of DNA! The testcross continues to be an important tool of geneticists for determining genotypes
Mendels laws reflect the rules of probability Mendels strong background in mathematics (and physics and chemistry…) served him well in his studies of inheritance He knew he needed large sample sizes The laws of inheritance reflect the probability of an event occurring – The probability of having a girl: 1 in 2 – The probability of rolling a 5 on a dice: 1 in 6 – The probability of drawing a queen from a deck of cards: 4 in 52 (1 in 13)
Probability An event that is certain to occur has a probability of 1 An event that is certain not to occur has a probability of 0 When 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!
1–21–2 1–21–2 1–21–2 1–21–2 1–41–4 1–41–4 1–41–4 1–41–4 Formation of eggs Bb female F 2 genotypes Formation of sperm Bb male B B B B B B b b b bb b Segregation and fertilization as chance events
Extra 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!!!
Genetic 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)
Genetic disorders Genetic disorders may be inherited as a recessive or dominant trait Most 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 disorder In this way, the parents are carriers of the recessive allele, but are phenotypically normal
Parents Normal Dd Offspring Sperm Eggs dd Deaf d Dd Normal (carrier) DD Normal D D d Dd Normal (carrier) Normal Dd Offspring produced by parents who are carriers for a recessive trait Does this mean that deaf parents always have deaf children?
It 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 mate However, the probability increases greatly if close relatives marry and have children A mating of close relatives, called inbreeding, is more likely to produce offspring homozygous for recessive traits
Genetic disorders Lets 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)
A case study: The Florida Panther The Florida panther population once numbered in the 30s in the 1990s Close matings resulted in reduced sperm counts, heart defects, and low survival rates among kittens Introduction of the Texas Panther in recent years has yielded hybrids with a higher survival rate (controversial!) www.bigcatrescue.org/catswild/florida_panther.htm
Genetic disorders Why are most genetic disorders recessive? Dominant alleles that cause lethal diseases are much less common than lethal recessives This 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
Genetic disorders Most 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 age – Huntingtons Disease – degenerative disease of the nervous system does not appear until 35-40 years of age (50% chance of inheriting it)
Threes a crowd… Mendel was fortunate in that he chose characters for which there were only 2 alleles Many genes, however, have more than 2 alleles in the population More often than not, the inheritance patterns of a particular trait are more complex www.flickr.com/photos/27887160@N02/2601298345/
Incomplete dominance In some allele combinations, dominance does not exist Instead, 2 traits are blended together to form a 3 rd trait In 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!
P generation 1–21–2 1–21–2 1–21–2 1–21–2 1–21–2 1–21–2 F 1 generation F 2 generation Red RR Gametes Eggs Sperm RR rR Rrrr R r R r R r Pink Rr R r White rr
Incomplete dominance In 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 own In the case of incomplete dominance, the phenotype does reveal the genotype for all traits!
Multiple 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 O An individual can have type A, B, O, or AB blood The A and B alleles are co-dominant; both alleles are expressed in heterozygous individuals
Blood Group (Phenotype) Genotypes O A OO AO or AA Red Blood Cells Carbohydrate A B BO or BB Carbohydrate B AB
Safe a life, give blood Blood group AB can receive blood from any blood type, but can only donate to AB Blood group A can receive blood from only A or O, but can donate to A or AB Blood group B can receive blood from only B or O, but can donate to B or AB Blood group O can receive blood only from O, but can donate to A, B, O or AB!
Blood type complications In addition to blood type, our red blood cells may (or may not) have a protein known as the Rh factor Individuals without this factor have a negative blood type, while those with this factor have a positive blood type Problems can occur when an Rh - mother carries an Rh + child, especially for children conceived after the birth of an Rh + child
Multiple alleles No matter how many alleles for a given gene are in a population, a diploid individual will only have 2 alleles, one on each homologous chromosome Homozygous for the dominant allele Homozygous for the recessive allele Heterozygous Genotype: PBa P PP a aa b Bb
The 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
F 1 generation R Metaphase I of meiosis (alternative arrangements) r Y y R r Y y R r Y y All round yellow seeds (RrYy) Anaphase I of meiosis Metaphase II of meiosis R y r Y r y R Y R r Y y R r Y y 1–41–4 R y Ry R y r Y 1–41–4 rY r Y 1–41–4 ry r y 1–41–4 RY R Y R Y Gametes Fertilization among the F 1 plants :3 9 :1 F 2 generation r y
Genes on the same chromosome tend to be inherited together The number of genes in a given cell is far greater than the number of chromosomes Each chromosome contains hundreds or thousands of genes Genes located close together on the same chromosome tend to be inherited together and are called linked genes Linked genes do not follow Mendels law of independent assortment
Purple long Purple round Red long Red round Experiment Purple flower PpLl Long pollen PpLl Prediction (9:3:3:1) Observed offspring Phenotypes 284 21 55 215 71 24
Explanation: linked genes Parental diploid cell PpLl Most gametes Meiosis PL pl PL pl Fertilization Sperm Most offspring Eggs 3 purple long : 1 red round Not accounted for: purple round and red long PL pl PL pl
Sex chromosomes and sex-linked genes The X-Y system of sex chromosomes is only 1 of several sex-determining systems Insects 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 number Other organisms have temperature- dependent sex determination!
Sex chromosomes and sex-linked genes In addition to bearing genes that determine sex, sex chromosomes contain genes for characters unrelated to gender A gene located on a sex chromosome is called a sex-linked gene Sex-linked disorders typically affect human males, since they have only one copy of each chromosome
Sex-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 result Hemophilia is a caused by a recessive allele on the X chromosome Woman can be carriers, but rarely suffer from the condition……Why?