Gregor Mendel traits Heredity Genetics

Transcription

1 Unit 6 Notes In 1851, Gregor Mendel (a priest from Europe) taught high school and maintained the monastery s garden In the garden, Mendel grew hundreds of pea plants and began noticing that they had different physical characteristics (traits) Some pea plants were short, others tall Some pea plants produced green seeds, others yellow Mendel observed that the pea plant s traits were similar to those of their parents Heredity = the passing of traits from parents to offspring Genetics = the scientific study of heredity Mendel is known as the Father of Genetics

2 Mendel s Peas A new organism begins to form when egg and sperm are joined in the process of fertilization When plants fertilize themselves, it is called self-pollination Mendel developed a method by which he could crosspollinate his pea plants, in order to conduct his experiments

3 Mendel s Experiments Mendel started his experiments with purebred plants Purebred = plant that always produces offspring with the same form of a trait as the parent Mendel s First Experiment Mendel crossed 1 purebred tall plant with 1 purebred short plant (P 1 generation) The offspring of the P1 cross were called the first filial generation (F 1 generation) The offspring of the F1 cross were called the second filial generation (F 2 generation) See results in Figure 2, p. 78 Note: the F 2 offspring are ¼ short, ¾ tall

4 - Mendel s Work

5 Other Traits (see Figure 3, p. 79) Mendel observed seven characteristics in total: Seed shape round or wrinkled Seed color yellow or green Seed coat color gray or white Pod shape smooth or pinched Pod color green or yellow Flower position side or end Stem height tall or short

6 - Mendel s Work

7 Dominant and Recessive Alleles Mendel s experiments taught him that individual genes must control the inheritance of traits in peas Alleles = the different forms of a gene Example: stem height gene has a tall allele and a short allele The female parent gives one allele, the male parent gives one allele Mendel also learned that one allele can mask (hide) the other allele Example: the tall allele masked the short allele in the F 1 generation (Figure 2, p. 78)

8 Individual alleles control the inheritance of traits Dominant allele = one whose trait always shows up in the organism when the allele is present Recessive allele = one whose trait is covered up whenever the dominant allele is present Examples: If we cross two tall P 1 plants, can we have a short F 1 plant? If we cross one tall P 1 plant and one short P 1 plant, can we have a short F 1 plant? Offspring are hybrids (they have two different alleles for the same trait) If we cross two short P 1 plants, can we have a short F 1 plant?

9 Using Symbols in Genetics Scientists use letters to represent alleles in genetics A dominant allele is represented by a capitol letter Example: dominant tall stem height = T A recessive allele is represented by the lowercase version of the dominant trait s letter Example: recessive short stem height = t When two dominant parents produce offspring TT When one dominant and one recessive parent produce offspring Tt (hybrid) When two recessive parents produce offspring tt

10 Chapter 3-2 Probability = the likelihood that a particular event will occur Principles of Probability If you tossed a coin What is the probability that the coin would land heads up? What is the probability that the coin would land tails up? In twenty tosses, how many would you predict would land heads up? The laws of probability predict what is likely to occur not necessarily what will occur

11 Probability and Genetics Mendel was the first scientist to recognize that the principles of probability can be used to predict the results of genetic crosses Mendel counted the offspring of every cross he carried out Example: Mendel crossed two plants hybrid for stem height (Tt x Tt) ¾ of the F 1 offspring had tall stems, ¼ had short stems Therefore, the probability of producing long-stemmed offspring is 3 in 4, and the probability of producing short-stemmed offspring is 1 in 4

12 Punnett Squares Punnett square = chart that shows all the possible combinations of alleles that can result from genetic crosses Punnett squares can also predict the probability of a particular outcome Phenotype = physical appearance (what it looks like) Examples: tall, green Genotype = genetic makeup (what allele combination is present) Examples: TT, Yy

13 - Probability and Heredity

14 - Probability and Heredity

15 Homozygous = an organism that has identical alleles for a trait Examples: TT, tt, GG, gg Heterozygous = an organism that has two different alleles for a trait (hybrid) Examples: Tt, Gg For the following examples, use these abbreviations: Homozygous dominant tall = TT Heterozygous (hybrid) tall = Tt Homozygous recessive short = tt

16 Example: P 1 : Purebred tall x Purebred tall F 1 result all tall 100% chance of being tall T T T T TT TT TT TT

17 Example: P 1 : Purebred tall x hybrid tall F 1 result all tall 100 % chance of being tall T T T TT TT t Tt Tt

18 Example: P 1 : Purebred tall x short F 1 result all tall 100% chance of being tall T T t Tt Tt t Tt Tt

19 Example: P 1 : hybrid tall x hybrid tall F 1 result 3 tall, 1 short 75% chance of being tall 25% chance of being short T t T TT Tt t Tt tt

20 Example: P 1 : hybrid tall x short F 1 result 2 tall, 2 short 50% chance of being tall 50% chance of being short T t t Tt tt t Tt tt

21 Example: P 1 : short x short F 1 result all short 100% chance of being short t t t tt tt t tt tt

22 - Probability and Heredity

23 Codominance Sometimes, a dominant allele and a recessive allele do not exist Codominance = alleles are neither dominant nor recessive Examples: Chickens in Figure 10, p. 89 Labrador retrievers (yellow, black, chocolate)

24 - Probability and Heredity

25 Chapter 3-3 Chromosomes and Inheritance In 1903, Walter Sutton studied sex cells in grasshoppers Chromosome theory of inheritance = genes are carried from parents to their offspring on chromosomes Sex cells (eggs and sperm) contain half the number of chromosomes of body cells Meiosis = the process by which the number of chromosomes is reduced by half to form sex cells (eggs and sperm) See Meiosis on p

26 - The Cell and Inheritance

27 Meiosis and Punnett Squares See Figure 14, p. 95 Also, a Punnett square can be used to determine the probability of the gender of offspring Example: P 1 : XY (male) x XX (female) F 1 results 2 females, 2 males 50% chance of being male 50% chance of being female X X X Y

28 - The Cell and Inheritance

29 - The Cell and Inheritance

30 Chromosomes Organisms can vary greatly in the number of chromosomes in their body cells Humans have 46 chromosomes (23 pairs) per body cell Dogs have 78 chromosomes per body cell Goldfish have 94 chromosomes per body cell Note: larger organisms do not necessarily have more chromosomes! Although your body may only have 23 pairs of chromosomes, your body cells contain between 30,000 and 35,000 genes each controlling a particular trait That is why no two people are exactly alike! See Figure 15, p. 96

31 Chapter 3-4 The Genetic Code The main function of genes is to control the production of proteins in the organism s cells Proteins help determine the size, shape, and many other traits of an organism Chromosomes are composed mostly of DNA DNA is composed of four different nitrogen bases (adenine, thymine, guanine, cytosine) A single rung on the DNA ladder contains hundreds of millions of nitrogen bases The nitrogen bases are arranged in a specific order Example: ATGACGTAC

32 The order of the nitrogen bases along a gene forms a genetic code that specifies what type of protein will be produced Groups of three nitrogen bases result in the production of a specific amino acid Amino acids combine to make proteins Think of the following analogy: Nitrogen bases = letters Amino acids = words Protein = sentence

33 How Cells Make Proteins Protein synthesis = the production of proteins The cell uses information from a gene on a chromosome to produce a specific protein Takes place on the ribosomes in the cytoplasm of the cell The Role of RNA RNA and DNA are similar, but differ in important ways RNA looks like only one side of the ladder RNA contains a different sugar than DNA RNA has the nitrogen bases adenine, guanine, and cytosine, but has uracil (U) instead of thymine

34 Types of RNA Messenger RNA (mrna) = copies the coded message from the DNA in the nucleus and carries the message to the cytoplasm Transfer RNA (trna) = carries amino acids and adds them to the growing protein Translating the Code See Protein Synthesis p Know steps one through four!

35 Mutations Types of Mutations Single-base substitution (Example: A attaches instead of G) Chromosomes do not separate evenly during meiosis (resulting in too many or too few chromosomes) The Effects of Mutations Helpful mutations Example: new, better-tasting potatoes Harmful mutations Example: cancerous tumor Neither helpful nor harmful mutations Example: albino animal in captivity

36 - The DNA Connection

37 - The DNA Connection

38 - The DNA Connection

39 Chapter 4-1 Traits Controlled by Single Genes Many traits are controlled by a single gene with two alleles Often one allele is dominant, and one allele is recessive Example: Figure 2, p. 111 P 1 genotype: Ww x Ww P 1 phenotype: widow s peak x widow s peak F 1 genotype: 1 WW, 2 Ww, 1 ww F 1 phenotype: ¾ widow s peak, ¼ straight hair line

40 Multiple Alleles Some human traits are controlled by a single gene with more than two alleles Multiple alleles = three or more forms of a gene that code for a single trait Example: Human blood types (Figure 3, p. 112) There are four main blood types A, B, AB, O Three alleles control the inheritance of blood types The allele for type A and the allele for type B are codominant The allele for type O is recessive Note: People with type O blood are universal donors People with type AB blood are universal recipients

41 Traits Controlled by Many Genes Some human traits show a large number of phenotypes because the traits are controlled by many genes Examples: Height controlled by at least four genes Skin color controlled by at least three genes The genes act together as a group to produce a single trait

42 Male or Female? The sex of a baby is determined by genes on chromosomes Each human body cell has 23 pairs of chromosomes One pair is made of two sex chromosomes The sex chromosomes determine the baby s gender The sex chromosomes are the only pair of chromosomes that do not always match Remember from Chapter 3: XX (Female), XY (Male) See Figure 5, p. 113

43 Sex-Linked Genes Sex-linked genes = genes on the X and Y chromosome Because males have only one X chromosome, males are more likely than females to have a sex-linked trait that is controlled by a recessive allele Example: red-green colorblindness See Figure 6, p. 114 See Figure 7, p. 115 It takes two recessive alleles to have a colorblind female Carrier = one who has a recessive allele, but does not have the trait But, it takes only one recessive allele to have a colorblind male

44 The Effect of the Environment The effects of genes are often altered by the environment Examples: Diet Due to better eating habits, the average height of adults in the U.S. has increased by almost 10cm in the last one-hundred years Medical care Living conditions

45 Chapter 4-2 Genetic Disorders Genetic disorder = an abnormal condition that a person inherits through genes or chromosomes Genetic disorders are caused by mutations (changes in a person s DNA) Examples: Cystic fibrosis Sickle-cell disease Hemophilia Down syndrome

46 Cystic Fibrosis Cystic fibrosis = genetic disorder in which the body produces abnormally thick mucus in the lungs and intestines Bacteria grow in the mucus and cause infections The mutation that causes cystic fibrosis is carried on a recessive allele Currently, there is no cure for cystic fibrosis

47 Sickle-Cell Disease Sickle-cell disease = genetic disorder that affects the production of hemoglobin in blood Hemoglobin = protein in red blood cells that carries oxygen When oxygen concentrations are low, red blood cells take on an unusual shape Figure 9, p. 118 The sickle-shaped cells cannot carry as much oxygen and block blood vessels, resulting in pain and weakness The mutation that causes sickle-cell disease is codominant with the normal allele Currently, there is no cure for sickle-cell disease

48 Hemophilia Hemophilia = genetic disorder in which a person s blood clots very slowly or not at all A person with hemophilia can bleed to death from a minor cut or scrape The mutation that causes hemophilia is caused by a recessive allele on the X chromosome (sex-linked disorder) Currently, there is no cure for hemophilia People with hemophilia can live normal lives they just have to be careful

49 Down Syndrome Down syndrome = results when a person s cells have an extra copy of chromosome 21 (due to an error in meiosis) People with Down syndrome have a distinctive physical appearance and some degree of developmental delay p. 117 Heart defects are common, but can be treated Despite limitations, people with Down syndrome can lead full, active lives

50 Pedigrees Pedigree = a chart or family tree that tracks which members of a family have a particular trait Geneticists use pedigrees to trace the inheritance of traits in humans See Figure 10, p. 119 Circle = female Square = male Colored shape = person has trait Half-colored shape = person is carrier for trait No color in shape = person does not have trait Horizontal line = connects two married people Vertical line & bracket = connects parents to children

51 Diagnosing Genetic Disorders Scientists began diagnosing genetic disorders with Punnett squares and pedigrees Today, scientists use tools such as amniocentesis and karyotypes to help predict genetic disorders Amniocentesis = procedure done before a baby is born which determines whether the baby will have some genetic disorders Cells are taken from the fluid surrounding the baby Karyotype = picture of all the chromosomes in a cell Made from the cells taken by amniocentesis Genetic counseling = guidance for couples with family histories of genetic disorders

52 Chapter 4-3 People have developed several ways to create organisms with desirable traits Selective breeding Selective breeding = the process of selecting a few organisms with desired traits to serve as parents for the next generation Techniques Inbreeding = involves crossing two genetically similar individuals Hybridization = involves crossing two genetically different individuals

53 Cloning Clone = an organism that is genetically identical to the organism from which it was produced In plants, scientists grow new plants from cuttings (small parts of the original plant) In animals, scientists remove an egg, replace the nucleus, and implant the nucleus to develop This process takes three different animals of the same species This is controversial, since removing the nucleus can be considered killing a life

54 Genetic Engineering Genetic engineering = genes from one organism are transferred into the DNA of another organism Also called gene splicing because DNA is cut open and genes are added Genetic engineering was first successful in bacteria See Genetic Engineering on p. 126 We use genetically engineered bacteria to create insulin (a drug to treat diabetes) We also use bacteria to create human growth hormone (a protein controlling growth in children)

55 Genetic Engineering in Other Organisms Bacteria have been implanted into tomatoes, wheat, and rice to enable them to: Survive in colder temperatures Grow in poor soil conditions Resist insect pests Genes have been inserted into animals, which then create medicines for humans Example: cows can produce a protein that clots blood helping those with hemophilia

56 Gene therapy = process of using genetic engineering to try to correct genetic disorders Working copies of a gene are inserted directly into a person s cells Example: engineered viruses can be inserted into the lung cells of people with cystic fibrosis, helping them breathe DNA Fingerprinting DNA fingerprinting = identifying a person by their own unique DNA Scientists have found ways gather DNA samples from hair, skin, and blood at crime scenes These techniques have put many criminals in jail

57 The Human Genome Project Genome = all the DNA in one cell of one organism Researchers estimate that there are 20,000-25,000 genes in one cell s DNA The main goal of the Human Genome Project is to identify the DNA sequence of every gene in the human genome This would help us understand the following: How humans develop What makes our bodies work What causes things to go wrong in our bodies Potential treatments/cures for genetic disorders and disease