Mendelian Genetics HOMEWORK YOUR JOB IS TO GO TO THIS WEBSITE and LEARN ABOUT PUNNETT SQUARES http://www.mhhe.com/biosci/genbio/ virtual_labs/BL_05/BL_05.html Your evidence for the ACS will include the finished product of 3 punnett squares that you will need to describe and draw and bring to class tomorrow. Mendel, Pea Plants, and Inheritance Patterns Gregor Mendel was a monk from Austria who spent most of his life on a farm making him aware of agricultural principles. He belonged to an agricultural society and won awards
for developing improved varieties of vegetables and fruit. Currently known as the _____________________________ Mendel, Pea Plants, and Inheritance Patterns After entering the monastery Mendel took courses in mathematics, physics, and botany at the University of Vienna. Few scholars of his time showed interest in both plant breeding and mathematics. Shortly after his university training, Mendel started to study Pisum stivum, the garden pea plant. Mendel, Pea Plants, and Inheritance Patterns Why the pea plant?
The plant is self-fertilizing. Its flowers produce both male and female gametes. One lineage of plants can breed true This means that successive generations will be like parents in one or more traits, as when all offspring grown from seeds of self-fertilized, white flowered parent plants also have white flowers. Pea plants can also cross-fertilize when plant breeders transfer pollen from one plant to the flower of another plant. Breeders open a floral bud of a plant that bred true for white flowers or some other trait and snip out its stamens. Now it cannot produce its own pollen. Now, the buds can be brushed with pollen from a plant that bred true for a different version of the traitsay, purple flowers. Mendel, Pea Plants, and Inheritance Patterns Mendel observed 7 characteristics of pea plants. ex: seed color (green or yellow) - seed texture (smooth or wrinkled) - flower color (purple or white) He used his knowledge of statistics to analyze his
observations. Crossed plants (mated), collected seeds, recorded observations, then planted seeds, then recorded new plant characteristics. Mendel, Pea Plants, and Inheritance Patterns As Mendel hypothesized, such clearly observable differences might help him track a given trait through many generations. If there were patterns to the traits inheritance, then those patterns might tell him something about heredity itself. Terms Used In Modern Genetics In Mendels time, no one knew about genes, meiosis, or chromosomes. However, in order for you to understand genetics, you must know the following terms. Genes
Units of information on heritable traits, which parents transmit to offspring. For example: eye color Each gene has a specific location (locus) in chromosomal DNA. Terms Used In Modern Genetics Diploid Pairs of genes (2n) on homologous (coding for the same characteristicslike eye color) chromosomes. Mutation Alters a genes molecular structure and its message about a trait. May cause a trait to change, as when a gene for flower color specifies yellow and a mutant form of the gene specifies white. Alleles All molecular forms of a the same gene.
For example, eye color can range from blue to green to brown to any number of combinations thereof. Terms Used in Modern Genetics Pure bred or true-breeding When offspring inherit a pair of identical alleles for a trait generation after generation. For example: Pugs are true-breeding. Each generation has the same coat color, body type, personality, etc. Hybrid Offspring of a cross between two individuals that breed true for different forms of a trait. Each one has inherited non-identical alleles for the trait. For example: Puggles are a combination of purebred beagles and pure-bred pugs. They will display a combination of the traits from each breed.
Terms Used in Modern Genetics Homozygous A pair of identical alleles on a pair of homologous chromosomes. For example: A child inherits the alleles for brown eyes from both parents (BB) Heterozyous A pair of non-identical alleles on a pair of homologous chromosomes. For example: A child inherits the alleles for brown eyes from his mother and blue eyes from his father (Bb). Terms Used In Modern Genetics Dominant An allele that masks the effect of any recessive allele paired with it. Capital letters signify dominant alleles. For example: A child inherits the alleles for
brown eyes from his mother and blue eyes from his father (Bb), but will display brown eyes. Recessive An allele that requires homozygous in heritance to show up physically. For example: A child inherits the alleles for blue eyes from both parents (bb) and displays blue eyes. Terms Used in Modern Genetics Homozygous dominant An individual has a pair of dominant alleles (BB) for the trait under study. Homozygous recessive An individual has a pair of recessive alleles (bb). Heterozygous
An individual has a pair of non-identical alleles (Bb). BB BB BB bb BB bb bb bb BB
bb Bb Bb Terms Used In Modern Genetics Genotype bb Refers to the particular alleles that an individual carries. For example: BB, bb, Bb Phenotype Refers to an individuals observable traits. For example: Brown eyes or blue eyes.Blue eyes Mendels Experimental Approach For each of Mendels experiments, he performed the cross in a specific order.
P generation (parental) = true-breeding parents. Example: Mom (PP) and Dad (pp) F1 = first filial generation offspring For example: Child of Pgen (Pp) F2 = second filial generation offspring of self-fertilized or cross-fertilized F1 individuals. For example: Grandchild can either be PP Pp PP P F1
F2 Pp pp Pp pp Mendels Theory of Segregation Mendel used monohybrid experiments to test a hypothesis: Pea plants inherit two units of information (genes) for a trait, one from each parent. Example: Two true-breeding flowers are crossed (one is
dominant purple, the other is recessive white).The result is the F1 generation producing offspring that are all heterozygous and purple. Parental Cross = PP X pp P P p Pp Pp p
Pp Pp Monohybrid Experiment Predictions In the F1 generation he allowed the plants to self cross/fertilize to produce the F2, some offspring had F1 Generation = Pp X Pp white flowers. Mendel was confused, what was going on? Mendel crossed seventy plants, and recorded the number of dominant and recessive forms of traits in thousands of offspring. On average, 3 out 4 F2 plants were dominant, and
one was recessive. The ratio hinted that fertilization is a chance event having a number of possible outcomes. P p P PP Pp p Pp
pp Monohybrid Experiment Predictions Mendel knew that the principles of probability, which applied to chance events, could help him predict possible outcomes of genetic crosses. Probability The chance that each outcome of an event will occur is proportional to the number of ways in which the outcome can be reached. Mendel had to do all his probability problems by hand, but thanks to Reginald C. Punnett, we have the Punnett square to help us figure out genetic probabilities. Yay Punnett! Probability is written as 3:4:5:6:7 Monohybrid Experiment Predictions
Mendels experiments crossing two true breeding parents would look like this: Mom plant has (PP) for purple flowers. Dad plant has (pp) for white flowers. p p P P Pp Pp Pp
Pp All the offspring (F1) are heterozygous. The probability for purple flowers in this generation is 4:0. Monohybrid Experiment Predictions Mendels next experiments (F1) would look like this: Mom plant is Pp. Dad plant is Pp. P p
P PP Pp p Pp pp The resulting offspring have a 3:1 chance (probability) of inheriting a dominant allele (purple). A hint on probability: it will always add up to the number of squares
in the Punnett square, so if you come up with 3:2 (which equals 5), your probability is off! Theory/Law of Segregation In modern terms, the theory of segregation is: Diploid cells have pairs of genes (one from each parent) on homologous chromosomes. The two genes of each pair are separated from each other during meiosis, so they end up in different gametes. In other words, if you have inherited purple flowers (P) from one parent and white flowers (p) from another parent, your eggs/ sperm will contain the allele for EITHER purple (P) or white (p). Not both. Pp
The pollen (plant sperm) will each receive either P or p The ovum(plant eggs) will each receive either P or p Testcrosses Unknown genotype (P?)--will show purple flowers Testcrosses help determine Recessive genotype (pp)will show
whether organisms of an white flowers unknown genotype, but who show dominant If 50% of the P ? characteristics, will have Pp offspring are or PP. white, then the By crossing an unknown p Pp unknown is ?p dominant(P?) with a Pp, if none
of known recessive the offspring individual (pp), p ?p Pp are white, the genotypes can be figured unknown is PP out! Theory/Law of Independent Assortment Mendel was wondering how two genes (like purple flowers and round seeds) are sorted into gametes. Do they sort together, i.e. every time a plant gets purple flowers, it gets round seeds, or
separately? In order to figure this out, he did dihybrid experiments. Dihybrid Experiment Parent 1: PPRR Parent 2: pprr PR PR PR PR pr P p R r
P p R r P p Rr P p Rr In the F1 generation each offspring turns out to be a hybrid for both flower color and pea shape. pr P p
Rr P p Rr P p Rr P p Rr pr P p Rr P p
Rr P p Rr P p Rr Notice that each square has 4 letters and two alleles for each trait! pr P p Rr
P p Rr P p Rr P p Rr Dihybrid Experiments Dihybrid experiments follow Parent 1: Purple flowers and Round seeds (PPRR) two different traits from one Both are dominant traits. generation to the next. Parent 2: white flowers and They start with a cross
wrinkled seeds (pprr) between true-breeding Both are recessive traits. homozygous parents that NOW, you must find all differ in two traits (i.e. the different flower color and seed combinations that the shape) governed by alleles two parents can make. of two genes. Parent 1: PR Parent 2: pr Dihybrid Experiment Now, Mendel crossed two of the F1 generation. Plant 1: PpRr
Plant 2: PpRr Before you can start the dihybrid cross, you must figure out all the possible combinations- use the foil method. PR Pr pR pr P p R r P
p R r Dihybrid Experiment Parent 1: PpRr Parent 1: PpRr The four resulting phenotypes are purple and round, purple and wrinkled, white and round, white and wrinkled. Purple and round = 9 Purple and wrinkled = 3 White and round = 3 White and wrinkled = 1
The ratio will be 9:3:3:1 9-Dominant and Dominant 3-Dominant and Recessive 3-Recessive and Dominant 1-Recessive and Recessive PR Pr pR pr PR PP RR PP Rr
Pp RR Pp Rr Pr PP Rr P P rr Pp Rr Pp r r pR Pp RR Pp Rr
p p RR p p Rr pr Pp Rr pp pp r r Pp rr Rr Theory of Independent Assortment
Mendels dihybrid experiments pointed to the fact that traits are not (usually) linked. Meaning that a plant can have purple flowers with wrinkled seeds or purple flowers and round seeds. OR, you can have brown hair and brown eyes, or brown hair and blue eyes, as the two traits are NOT linked together. Flower color and seed shape are NOT linked. Theory of Independent Assortment In modern terms, the theory of independent assortment is: As meiosis ends, genes on pairs of homologous chromosomes have been sorted out for distribution into one gamete or another, independently of gene
pairs on other chromosomes. In other words, when you make eggs/sperm, your offspring can have many different combinations of traits. They do not HAVE to have brown eyes, simply because they have brown hair, as the two traits are independent of one another. This theory is MOSTLY true, but now we know that some traits ARE linked, like red hair and freckles. Brown hair Blue eyes Blond hair Brown eyes Unexpected Patterns Mendel just happened to focus on traits that have clearly
dominant or recessive forms. However, expression of genes for some traits is not as straightforward. Codominance In codominance, a pair of non-identical alleles affecting two phenotypes are both expressed at the same time in heterozygotes. These are for traits where there is more than one dominant option (like eye color!) For example, if one of your parents has yellow hair, and the other has blue, and you get (YB) for hair color, then you will have both blue and yellow hair! Both traits show up in the heterozygous condition. MOM
DAD YY BB YB YB Multiple Alleles When there are 3 or more versions of ONE trait, it may be caused by multiple alleles. It does not mean that a person will have all of the versions, because only two alleles can act at a time. Codominance in real life! Red blood cells have a type of glycolipid on their plasma
membrane that give them their unique identity. The glycolipid comes in slightly different forms: A, B, or O. An enzyme dictates the glycolipids final structure. Three alleles for this enzyme are present in all populations! Both A and B blood types are dominant. O is recessive. They are written as IA, IB, and i. The occurrence of three or more alleles for a single gene among individuals of a population is called a multiple allele system! Codominance in real life! If you inherit IAIA or IAi, you will have A blood type. 40 % of the US population has Type A blood. They can receive both Type A and Type O blood in an emergency. If you inherit IBIB or IBi, you will have B blood type. 11% of the US population has Type B blood.
They can receive both Type B and Type O blood in an emergency. If you inherit ii, you will have O blood type. 45% of the US population has Type O blood. They can only receive type O blood, but they are universal donors! Because both A and B are dominant alleles, if you inherit IAIB, you will have AB blood type. 4% of the US population has Type AB blood. They can receive both Type AB, Type A, Type B blood and O blood in an emergencyThey are universal recipients! Incomplete Dominance In incomplete dominance, one allele of a pair is not fully dominant over its partner, so the heterozygotes phenotype is somewhere between the two homozygotes. For example, if one of your parents has yellow hair, and the other has blue, and you get (Bb) for hair color, then you will have green hair!
A combination/blending of the two in the heterozygous condition! MOM bb DAD Bb Bb Bb Incomplete Dominance in real life! When true-breeding (pure bred) red and white snapdragons are bred together, their offspring end up being pink.
Red snapdragons have two alleles that left them make a lot of molecules of red pigment. White snapdragons have two mutant alleles and are pigment-free. Pink snapdragons have a red allele and a white allele; and make just enough pigment to color the flowers pink. Polygenic Inheritance When more than one gene affects a single trait. Example is Skin Color Epistasis A type of polygenic inheritance where one gene can interfere with the expression of the other genes. Example: Albinism in mammals- one gene blocks the production of pigment
Genes and the Environment The environment can affect the expression of genes in surprising ways. For example, the himalayan rabbit is homozygous for an enzyme that is heat sensitive. In the winter, this enzyme causes the rabbit to grow white fur, in the summer, the enzyme causes the rabbit to grow black fur. In humans, you may have genes that dictate tallness, but if you dont get enough food to eat during development, you may end up short. All three of these children display the symptoms of Rickets
disease. Rickets is the softening of the long bones due to deficiency in the diet or impaired metabolism. Linkage Thomas Hunt Morgan was an American evolutionary biologist, geneticist and embryologist who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries elucidating the role the chromosome plays in heredity. His tool of choice was the FRUIT FLY! Morgan found that some genes rarely separate during crossing over
of meiosis and are called Linked genes. Fruit Fly Chromosomes Linkage Locus- a region on a chromosome where alleles for genes are located. T. H. Morgan found that the farther apart two genes are on a chromosome, the more likely there is to be crossing over between those two genes. The amount of crossing over that occurs is a fairly constant quantity that can be measured. https://www.youtube.com/ watch?v=qCrulK8PPAg
Karyotype- picture of chromosomes Linkage / Sex-linked genes Sex Chromosomes- X and Y only X: carries sex characteristics (among other genes) Y: carries male sex characteristics Autosomes- chromosomes 1-22 Carry all other body information Genes located on sex chromosomes Females pass only X Males can pass either X or Y Recessive condition WILL be expressed if present only one copy Linkage / Sex-linked genes Sex-influence traits color blindness, hemophilia,
muscular dystrophy, malepattern baldness Take the example of baldness: The reason women are rarely bald is that, even if they have the baldness allele on one X chromosome, they usually have the dominant NON-bald on the other. But it shows up in men because the Y chromosome has no allele for that gene at all. In the absence of a dominant allele, the recessive is expressed. How are Sex-Linked genes passed along? Suppose a normal
woman (XBXB) has children by a bald man (XbY) XB XB Xb XB Xb XB Xb Y XB Y XB Y Next Generation: Suppose one
of the carriers marries a normal man. XB Xb XB XB XB XB Xb Y X Y X Y
B b Nondisjunction Nondisjunction- Occurs during meiosis. It is caused by chromosomes not pulling apart correctly. It can cause gametes to lack a chromosome or to have an extra chromosome. Monosomy a condition in which one chromosome of one pair is missing. ex: Turners syndrome Trisomy a condition in which an individual has an extra chromosome in any of the chromosome pairs. ex: down syndrome (also called trisomy21)
Monosomy of X chromosome Turners syndrome is only example of Monosomy in humans. All other monosomies end in nondevelopment of the zygote Signs/Symptoms- short stature, broad chest, neck webbing, heart failure, sterility and vision problems. Trisomy 21 Extra chromosome number 21
Causes Downs Syndrome- intellectual impairment and physical abnormalities including short stature and broad facial profile. Detecting Human Genetic Disorders Genetic screening is an Blood test look for the examination of a persons presence or absence of certain genetic makeup. Involves proteins. constructing a karyotype (picture of an individuals chromosomes grouped in pairs and arranged in Amniocentesis using a needle inserted into the womb to
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