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Chapter 5 Heredity Gregor Mendel • The father of genetics and heredity • Famous for his pea experiments • Was a monk in a monastery when he did his ground breaking work. • Died in 1884. • The genetic experiments Mendel did with pea plants took him eight years (1856-1863) and he published his results in 1865. During this time, Mendel grew over 10,000 pea plants, keeping track of progeny number and type. Mendel's work and his Laws of Inheritance were not appreciated in his time. It wasn't until 1900, after the rediscovery of his Laws, that his experimental results were understood. • Why did it take so long to recognize his work? Are You Unique? • In the past ten thousand years, there have billions of people who have lived. • Have they all been different? Yes – But Why? In December 1999, the first human chromosome was completely sequenced. Chromosome #22 is one of the smallest human chromosomes and has 33.5 million base pairs of DNA. http://www.dnaftb.org/dnaftb/1/conc ept/ Concept Animation Audio video clip 2 Problem http://www.dnaftb.org/dnaftb/15/con cept/ • animation http://www.dnai.org/a/index.html • Copying the code • Reading the code • Controlling the code Getting back to Mendel • Mendelian Genetics • Mendel was interested in the way traits were passed from parents to offspring. = HEREDITY • Mendel simplified his investigation by studying only one organism (the garden pea plant) Why Pea Plants? • They grow quickly • They are usually self pollinating • They come in many varieties • Self Pollinating – contains both male and female reproductive structures – thus pollen from one flower or plant can fertilize the eggs of the same flower or the eggs of another flower on the same plant. How Did He Do it? • Studied only one characteristic at a time (ie. Plant height, or pea color) • He chose plants with 2 forms of the characteristic to be studied. – Short vs. tall – Smooth peas vs. wrinkled peas – Purple flowers vs. white flowers • He always chose true breeding plants – when self pollinating these always produce the offspring with the same characteristics. (SS or ss) or (TT or tt) as the parent – identical alleles for a trait • He started cross breeding 2 plants with different forms of the same trait via crosspollination. Cross Pollination 1) the anthers on the stamen of one plant are removed (so the plant can not self-pollinate) 2) Pollen from another plant is used to fertilize the plant without anthers. * This allows Mendel to control which pollen fertilizes which plant Know This Nomenclature • True Breed Smooth (SS) x True Breed wrinkled (ss) = parents Cross pollination • The offspring of 2 true breeds = f1 generation (all Ss but all smooth) Self pollination (Ss) X (Ss) • The offspring of an f1 generation due to self pollination = f2 generation How did Mendel follow the scientific method? Ask a Question – How are traits inherited? Form a Hypothesis – If traits are inherited then their patterns can be predicted. Test the Hypothesis – Cross true-breeding plants and offspring. Analyze the Results – Identify patterns in inherited traits. Draw Conclusions – Traits are inherited in predictable patterns. How he deciphered heredity • Mendel chose to study only one characteristic, such as plant height, flower color, or seed shape. True breeding plants – a plant that only reproduces offspring with the same traits as the parent when it self-pollinates. (SS or ss) For example- a tall true breeding plant will always produce offspring that are tall. • He then cross pollinated two true breeding plants with different forms of a single trait. For example- a true breeding tall plant with a true breeding short plant. •Cross Breeding – cut the anthers from the stamens of one plant (so that it can’t self-pollinate) then add pollen to it from another plant. • Crossed plants that had true bred smooth seeds (SS) with those that had true bred wrinkled seeds (ss). • The offspring from this first generation (f1) were all smooth. • This same thing occurred no matter what the trait tested – all the f1 generation possessed only one of the parent’s traits. • dominant trait – the one and only trait that shows itself in f1 generation • recessive trait – the trait that does not show itself in the f1 generation Mendel’s first experiment Mendel: Experiment 1 Remember – in sexual reproduction each parent donates 1 gene each. An SS can only donate “S” But a Ss can donate either an “S” or an “s” Mendel’s second experiment • Mendel allowed the first generation (f1) to self pollinate. • This time the f1 generation which was all smooth seeds (the dominant trait) produced a next generation (f2) which possessed some smooth seeds and some wrinkled seeds. • Somehow the recessive trait showed up again. • Every fourth seed was wrinkled. P parents F1 offspring F1 breeding F2 offspring Mendel’s Actual Results 1) P = smooth seeds crossed with wrinkled seeds F1 = all smooth seeds (so smooth is dominant and wrinkled is recessive) F2 = 5,474 smooth seeds and 1,850 wrinkled seeds is a ratio of 2.96 : 1 2) P = green seeds crossed with yellow seeds F1 = all yellow seeds (so yellow is dominant and green is recessive) F2 = 6,022 yellow seeds and 2,001 green seeds is a ratio of 3.01 : 1 3) P = purple flowers crossed with white flowers F1 = all purple flowers (so purple is dominant and white is recessive) F2 = 705 purple flowers and 224 white flowers is a ratio of 3.15 : 1 4) P = constricted pods crossed with inflated pods F1 = all inflated pods (so inflated is dominant and constricted is recessive) F2 = 882 inflated pods and 299 constricted pods is a ratio of 2.95 : 1 5) P = green pods crossed with yellow pods F1 = all green pods (so green is dominant and yellow is recessive) F2 = 428 green pods and 152 yellow pods is a ratio of 2.82 : 1 6) P = terminal flowers crossed with axial flowers F1 = all axial flowers (so axial is dominant and terminal is recessive) F2 = 651 axial flowers and 207 terminal flowers is a ratio of 3.14 : 1 7) P = dwarf stem crossed with tall stem F1 = all tall (so tall is dominant and dwarf is recessive) F2 = 787 tall stems and 277 dwarf stems is a ratio of 2.84 : 1 Let’s Look at the Ratios Flower Color Seed Color Seed Shape Pod Color Pod Shape Flower Position Plant Height 3.15:1 3.00:1 2.96:1 2.82:1 2.95:1 3.14:1 2.84:1 3:1 (purple:white) (yellow:green) (smooth:wrinkled) (green: yellow) (inflated:constricted) (middle:end) (tall:short) Math Break (probability) If Mendel looked at 7 different traits each with only 2 forms, how many different plants are possible? Pea Shape Flower Color Pea Color purple green Smooth white purple Wrinkled white yellow yellow green yellow green yellow green The Answer Is…. 2x2x2x2x2x2x2 Or 2 = 7 128 different combinations What are the chances of getting a plant that is tall, with green wrinkled peas in a inflated yellow pod with purple flowers located at only the ends of branches (assume equal chance of each individual trait possibility)? Mendel’s Brilliant Conclusion The only way to explain the presence of only one trait in the f1 generation and the 3:1 ratios in the f2 generation was if each plant had 2 sets of instructions for each characteristic. Each parent donates one set of instructions known as genes Therefore, every fertilized egg (offspring) would have 2 forms of the same gene for every characteristic. The 2 forms are individually termed an allele. The combination of the 2 alleles in the offspring is termed the genotype. Since we know smooth seeds are dominant as they show up exclusively in the f1 generation we will label that gene with a capital S. The recessive gene we will label as a small s. Parents Pure Breed Wrinkled Seeds Pure Breed Smooth Seeds ss SS S gene (or allele) Offspring (f1) s gene (or allele) Ss genotype which makes all smooth seeds because smooth is dominant In f1 if there were 4 offspring (or 10,000) their only possible allele combination is Ss. The allele combination is called the genotype. Thus 4 Ss which are all smooth seeds. Second Generation – this generation is allowed to self fertilize. The possible male genotypes are Ss and the possible female genotypes are also Ss. Each can send an S allele or an s allele. Male Contribution Smooth Seeds Female contribution Smooth Seeds f1 Ss Ss or or S gene s gene S gene s gene Offspring (f2) SS Ss Ss ss F2 generation • • • • SS Ss Ss ss Smooth Smooth Smooth What is the Ratio? 3:1 smooth to wrinkled Wrinkled What are the percentages? 75% Smooth and 25% Wrinkled Terminology • Homozygous – possessing 2 of the same alleles for a particular trait (SS or ss or TT or tt) • Heterozygous – possessing different alleles for a particular trait (Ss or Tt) • Homozygous dominant = SS or TT • Homozygous recessive = ss or tt The Punnett Square • The Punnett square is a simple grid with the all the possible sperm/male alleles along one side, all the possible egg/female alleles along another side, and all the possible offspring genotypes filling the grid. • Fusion of those two gametes produces the genotype of the offspring (zygote) in the boxes. For a given trait, dominant traits are symbolized by CAPITAL letters and recessive traits by lower case letter.. Always use the same letter for dominant and recessive. DO NOT USE DIFFERENT LETTERS Therefore all genotypes from true breeding organisms are represented by 2 similar alleles –represented by similar letters (pp or PP, rr or RR, etc.) The cross between 2 true breeding plants one purple flowered and one white flowered would be (where purple is dominant and white is recessive): PP x pp The Punnett Square for PP x pp p Flower Color f1 P Pp purple P p – termed allele Pp – termed genotype Purple – termed phenotype Pp Pp purple purple Defn: phenotype – an organism’s inherited appearance The Punnett Square for Pp x Pp P Flower Color f2 P p PP p Pp purple purple Pp pp purple white Punnett Square Practice 1) What happens in f1 when you cross a Pure Breed Green Pod (GG) with a Pure Breed Yellow pod (gg)? 2) What happens in f2 when you allow a green pod genotype of Gg to self pollinate? 3) What happens when you cross a Gg green pod with a yellow pod gg? 4) The allele for a cleft chin, C, is dominant among humans. What would be the results from a cross between a heterozygous woman and a homozygous dominant man? 5) What about a Cc and a cc combination. 6) What is the ratio of offspring with a cleft chin to offspring without a cleft chin in #’s 4 and 5)? If 2 adults both do not have cleft chins. What are the genotype and phenotype possibilities for their children? If a child was born with a cleft chin. What possible genotype combinations could his parents have had? 7) 8) Mini-Lab Assume: Brown eyes are dominant over blue eyes. Take masking tape and label both sides of 2 coins. Label both coins with a B and a b, to represent a mixed genotype for brown eyes. In this example 2 people heterozygous for eye color produce offspring . 1) 2) Flip each coin 50 times. Record your results for each allele from coin 1 and coin 2 and the subsequent genotype and phenotype of the offspring. Flip each coin an additional 40 times and record your results as above. flip 1 2 3 … Coin 1 Allele Coin 2 Allele Genotype Phenotype • A gene can be defined as a region of DNA that controls a hereditary characteristic. It usually corresponds to a sequence used in the production of a specific protein or RNA. • In humans, Genes can be as short as 1000 base pairs or as long as several hundred thousand base pairs. It can even be carried by more than one chromosome. What are genes? • Not as simple as Mendel’s pea plants The 46 human chromosomes = 23 pairs house almost 3 billion base pairs of DNA that contains about 30,000 40,000 protein-coding genes. The coding regions make up less than 5% of the genome (the function of the remaining DNA is not clear) and some chromosomes have a higher density of genes than others. Sex Chromosomes • Females are XX • Males are XY • What sex alleles can the female donate? •X only • What sex alleles can the male donate? •X or Y • Which parent is responsible for the determination of the gender of the child? Males Only seX- linked Traits • Sex Linked Genes A particularly important category of genetic linkage has to do with the X and Y sex chromosomes. These not only carry the genes that determine male and female traits but also those for some other characteristics as well. Genes that are carried by either sex chromosome are said to be sex linked. • Men normally have an X and a Y combination of sex chromosomes (XY), while women have two X's (XX). Since only men inherit Y chromosomes, they are the only ones to inherit Y-linked traits. Men and women can get the X-linked ones since both inherit X chromosomes. Sex cell inheritance patterns for male and female children X-linked traits that are not related to feminine body characteristics are primarily expressed in the observable characteristics, or phenotype , of men. This is due to the fact that men only have one X chromosome. Subsequently, genes on that chromosome that do not code for gender are usually expressed in the male phenotype even if they are recessive since there are no corresponding genes on the Y chromosome to dominate over them, in most cases. X-linkage in men In women, a recessive allele on one X chromosome is often masked in their phenotype by a dominant normal allele on the other. This explains why women are frequently carriers of X-linked traits but more rarely have them expressed in their own phenotypes. X-linkage in women There are about 1,000 human X-linked genes. Most of them code for something other than female anatomical traits. Some of the non-sex determining X-linked genes are responsible for abnormal conditions such as: hemophilia red-green color blindness congenital night blindness high blood pressure duchene muscular dystrophy fragile-X syndrome . Queen Victoria of England was a carrier of the gene for hemophilia. She passed the harmful allele for this X-linked trait on to one of her four sons and at least two of her five daughters. Her son Leopold had the disease and died at age 30, while her daughters were only carriers. As a result of marrying into other European royal families, the princesses Alice and Beatrice spread hemophilia to Russia, Germany, and Spain. By the early 20th century, ten of Victoria's descendents had hemophilia. All of them were men. Queen Victoria (1819-1901) By comparison to the X chromosome, the much smaller Y chromosome has only about 26 genes and gene families. Most of the Y chromosome genes are involved with essential cell house-keeping activities (16 genes) and sperm production (9 gene families). Only one of the Y chromosome genes, the SRY gene, is responsible for male anatomical traits. Because the Y chromosome only experiences recombination with the X chromosome at the ends (as a result of crossing-over), the Y chromosome essentially is reproduced via cloning from one generation to the next. This prevents mutant Y chromosome genes from being eliminated from male genetic lines. Subsequently, most of the human Y chromosome now contains genetic junk rather than genes. Sex-linked genes are genes on the X- and Ychromosomes. Traits controlled by these genes are called sex-linked traits. Two sexlinked traits include hemophilia and colorblindness. . Hemophilia is a genetic disorder in which a person’s blood clots slowly or not at all. If a person has the dominant allele XH, he or she will have normal blood. If a person only has the recessive allele Xh, he or she will have hemophilia Red-green colorblindness is also a genetic disorder. In this disorder, the person does not see red and green properly. This person will see green as gray and red as yellow. If a person has at least one dominant allele XC, he or she will not have colorblindness. If a person has only the recessive allele Xc, he or she will have colorblindness. • Questions • 1) How are the alleles for sex-linked genes passed from parent to child? • ______________________________________________________ ____________ • ______________________________________________________ ____________ • 2) How many X-and Y-chromosomes do males have? ______________________ • 3) How many of each do females have? __________________________________ • 4) Define the carrier of a trait in terms of alleles. ___________________________ • ______________________________________________________ ____________ Yes, someday you will be just like us. Questions • 1) Why are only females carriers for hemophilia? For red-green colorblindness? • ______________________________________________________________________________ • ______________________________________________________________________________ • ______________________________________________________________________________ • 2) Which of the parents can pass the allele for hemophilia to a son? Explain. • ______________________________________________________________________________ • ______________________________________________________________________________ • ______________________________________________________________________________ • 3) Which of the parents can pass the allele for hemophilia to a daughter? Explain. • ______________________________________________________________________________ • ______________________________________________________________________________ • ______________________________________________________________________________ • 4) Explain why in family #3 there are no colorblind children even though one of the parents is • colorblind? • ______________________________________________________________________________ • ______________________________________________________________________________ • ______________________________________________________________________________ • 5) The brother of a woman’s father has hemophilia. Her father does not have hemophilia, but • she is concerned that her son might. Could she have passed the allele for hemophilia onto her • son? Explain. • ______________________________________________________________________________ • ______________________________________________________________________________ • ______________________________________________________________________________ • 6) A woman’s father is colorblind. She marries a colorblind man. Might their son be • colorblind? What about their daughter? Explain why or why not. • ______________________________________________________________________________ • ______________________________________________________________________________ • ______________________________________________________________________________ What is Crossing Over A process in genetics by which the two chromosomes of a homologous pair exchange equal segments with each other. Crossing over occurs in the first division of meiosis NOVA Movies Watch Chapter 1 in total Watch Chapter 2 until courting Watch all of chapter 3 Messages in the Genes (remind about vocabulary) http://www.pbs.org/wgbh/nova/miracle/program.html# MEIOSIS • The process of making sex cells in sexually reproducing organisms. • It differs from meiosis in that it results in sex cells with half the normal number of chromosomes. • ie. – in humans there are normally 46 chromosomes but in human sex cells (sperm or eggs) there are only 23 chromosomes. • In asexual reproduction – mitosis is used because one organism splits to become two organisms. Thus offspring are identical to parent. • In sexual reproduction two organism will make one new one and therefore they must send half the information each. Thus offspring reflect a combination of parents genes. • The matching chromosomes that come from each parent are called homologous. Chapter 5 Section 2 Incomplete Dominance In many ways Gregor Mendel was quite lucky in discovering his genetic laws. He happened to use pea plants, which happened to have a number of easily observable traits that were determined by just two alleles. And for the traits he studied in his peas, one allele happened to be dominant for the trait & the other was a recessive form. Things aren't always so clearcut & "simple" in the world of genetics, but luckily for Mendel (& the science world) he happened to work with an organism whose genetic make-up was fairly clear-cut & simple. If Mendel were given a mommy black mouse & a daddy white mouse & asked what their offspring would look like, he would've said that a certain percent would be black & the others would be white. He would never have even considered that a white mouse & a black mouse could produce a GREY mouse! For Mendel, the phenotype of the offspring from parents with different phenotypes always resembled the phenotype of at least one of the parents. In other words, Mendel was unaware of the phenomenon of INCOMPLETE DOMINANCE. Incomplete Dominance - a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. I remember Incomplete Dominance in the form of an example like so: RED Flower x WHITE Flower ---> PINK Flower It's like mixing paints, red + white will make pink. Red doesn't totally block (dominate) the white, instead there is incomplete dominance, and we end up with something in-between. • We can still use the Punnett Square to solve problems involving incomplete dominance. The only difference is that instead of using a capital letter for the dominant trait & a lowercase letter for the recessive trait, the letters we use are both going to be capital (because neither trait dominates the other) RR x WW or we could use 1 2 1 2 one letter with a superscript F F x F F . WW x True breed white True breed red pink pink pink pink • Incomplete Dominance Sample Questions • 1. A cross between a blue rocket bird & a white rocket bird produces offspring that are silver. The color of rocket birds is determined by just two alleles. a) What are the genotypes of the parent rocket birds in the original cross? • Since there are only 2 alleles & three phenotypes (blue, white, & silver), we must be dealing with incomplete dominance. So the blue parent is homozygous blue (BB) & the white parent is homozygous white (WW). • b) What is/are the genotype(s) of the silver offspring? • The silver offspring are hybrids (BW), one blue allele & one white allele, neither one dominating the other. Instead, we get a blending of blue & white, i.e. silver. silver x silver = BW x BW blue silver silver white • As you can see, 25% (1/4) of the offspring are homozygous white (WW), 25% (1/4) are homozygous blue (BB), & 50% (2/4) are hybrid & therefore have the silver phenotype. 2. The color of fruit for plant "X" is determined by two alleles. When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit. What are the genotypes of the parent orange-fruited plants? Again, it comes in really handy if you can recognize right off the bat that we have three phenotypes & just 2 alleles. That means we are dealing with either incomplete or codominance. Since orange is a blend of red & yellow, it's incomplete dominance. So the "in-between" phenotype is the hybrid, orange in this example. We'll use RR = red, YY = yellow, & our orange fruits are RY. Codominance • First let me point out that the meaning of the prefix "co-" is "together". Cooperate = work together. Coexist = exist together. Cohabitat = habitat together. The genetic gist to codominance is pretty much the same as incomplete dominance. A hybrid organism shows a third phenotype --- not the usual "dominant" one & not the "recessive" one ... but a third, different phenotype. With incomplete dominance we get a blending of the dominant & recessive traits so that the third phenotype is something in the middle (red x white = pink). • In COdominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms. • I remember codominance in the form of an example like so: • red x white ---> red & white spotted • With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together. • R = allele for red flowers W = allele for white flowers • red x white ---> red & white spotted RR x WW ---> 100% RW A very very common phenotype used in questions about codominance is roan fur in cattle. Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together). A good example of codominance. Another example of codominance is human blood type AB, in which two types of protein ("A" & "B") appear together on the surface of blood cells. • There are three forms of the gene (alleles) that control the ABO blood group, which are designated as iA, i B, and i. You have two alleles (one from your mother and one from your father), which are referred to as your genotype. The inheritance of the alleles is codominant, meaning that if the allele is present, it gets expressed. The following genotypes will yield these blood types: • iAiA or iAi - Both genotypes produce the A protein (type A). • iBiB or iBi - Both genotypes produce the B protein (type B). • iAiB - This genotype produces the A and B protein (type AB). • ii - This genotype produces no protein (type O). So, your blood type does not necessarily tell you exactly which alleles you have. For example, a person with blood type A could have either two iA alleles or one iA allele and one i allele. It is possible for two parents with the same blood type (A or B) to have a child with type O blood. Both parents would have to have a mixed genotype, such as one i allele together with either one iA or one iB allele. Sample Questions 1. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull. 2. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur? 3. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). a) What pattern of inheritence does this illustrate? b) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat? Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull • Step #1 --- recognize that "roan" is a codominance trait. • Homozygous white = WW, & roan = RW (a hybrid cow). So our cross is WW x RW & the punnett square should look something like what you see here. • The results: 2/4 offspring (50%) will be roan (RW), & 50% will be white (WW). 2. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur? Well, the only way to have red fur is to be homozygous red (RR). In order to get that genotype in all the offspring both parents must be "RR". A parent with one or more "W" alleles will cause the inheritence of roan fur in some offspring. Go ahead & work out all the punnett squares if you don't believe me. Only RR x RR gives you 100% RR. RR x RW would produce 50% roan, 50% red, RW x RW produces 25% red, 50% roan & 25% white, WW x RW would produce 50% roan, 50% white, & WW x RR would produce 100% roan (RW). A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). 3. a) What pattern of inheritence does this illustrate? Codominance, two phenotypes together at the same time. b) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat? Tabby cats are the hybrids (because they have both colors) & a black cat must be homozygous black. So the cross for this problem is BB (black) x BT (tabby). The p-square is at the right. The results show that 50% of the offspring will be BB (black) & 50% will be tabby (BT). So to answer the question, 0% of the kittens will be tan. Chapter 5 - Heredity Section 2 - Meiosis • In asexual reproduction only one parent is needed for reproduction – in this process the copying of genetic material occurs through mitosis • Most single celled organisms reproduce in this way. Insert picture here Sexual Reproduction • 2 parent cells join together to form a new individual • This type of reproduction must work differently so that each parent may contribute to the genetic makeup of the offspring. In sexual reproduction The parent cells are known as sex cells. Sex cells are different from all other ordinary body cells in that they have only half the usual number if chromosomes. Human body cells normally possess 46 chromosomes (23 homologous pairs) but human sex chromosomes contain only 23 chromosomes (none of which are paired). Sperm – male sex cells Ova or Eggs – female sex cells Each sperm and egg has only one of the chromosomes from each homologous pair. Science Blooper • In 1918, a prominent scientist miscounted the number of chromosomes in a human cell. He counted 48. For almost 40 years, scientists thought that his was correct. In fact, it wasn’t until 1956 that chromosomes were correctly counted and found to be only 46. OOPS Less is More • Why is it important that sex cells have half ? the usual number of chromosomes • Because when the males sperm and the females egg join to forma new individual, each parent donates one-half of a homologous pair of chromosomes. Meiosis • Sex cells are made through a process termed meiosis. • Meiosis – produces new sex cells with half the usual number of chromosomes. – This process involves the chromosomes dividing once and the nucleus dividing twice. Mitosis Revisited http://www.biology.arizona.edu/c ell_bio/tutorials/cell_cycle/cells3 .html Mitosis Demo • Macaroni Meiosis vs. Macaroni Mitosis Mitosis 1) Take your 4 pieces of yarn and create a single circular cell membrane 2) Start with 4 chromosomes ( 2 pairs) 3) Replicate your chromosomes 4) Attach homologous chromosomes to each other 5) Line them up in the middle 6) Separate the homologous chromosomes to 2 sides 7) Take the 4 pieces of yarn close off to 2 cells Meiosis 1) Take your 2 pieces of yarn and create a single circular cell membrane 2) Start with 4 chromosomes 3) Replicate your chromosomes 4) Attach homologous chromosomes to each other 5) Line them up in the middle 6) Separate the homologous chromosomes to 2 sides 7) Take the 2 pieces of yarn close off to 2 cells 8) Now in each of those 2 cells, line the 4 paired chromosomes up in the middle 9) Separate the homologous pieces in each cell 10) Close off the now 4 cells ASIDE In human males , meiosis and sperm production takes about 9 weeks. This continuous process begins at puberty and ends at death In human females, meiosis and egg production begins before birth. The process stops abruptly prior to birth and then re-begins at puberty and continues until menopause. Between puberty and menopause, one egg per ovary resumes meiosis and finishes its development. Therefore, the meiosis of a single egg may take up to 50 years to complete. • Why does the incidence of birth defects and genetic disorders rise with age of the mother? Why is the age of the father less of a factor? • Answer: age of the sex cell Guess Who? If human sex cells are created by meiosis, how are cat sex cells created? Answer: by meowsis Meiosis explains Mendel Male or Female There are two types of chromsomes: Autosomes – the 22 pairs of chromosomes which do not play a role in sex determination. Each has a match. Sex chromosomes – the chromosomes that carry the genes that determine whether the offspring will be male or female. Male or Female Continued • In humans: • X chromosome carries the genetic information for femaleness. • Y chromosome carries the genetic information for maleness. • Females are XX • Males are XY X Y • If females are XX and males are XY which parent is ultimately responsible for the determination of the sex of the offspring? •Answer – the male because only the male can contribute the different sex chromosome (females have no Y’s to contribute) • Recessive inheritance Albinism • X linked traits ie hemophilia • Hemophilia and brachydactaly – dominant inheritance • Cancer heart disesae – inherited predisposition – the role of ones environment – why could George Burns smoke cigars until he was 99? Then interphase doubling occurs NOVA Movies Watch Chapter 1 in total Watch Chapter 2 until courting Watch all of chapter 3 Messages in the Genes (remind about vocabulary) http://www.pbs.org/wgbh/nova/miracle/program.html#