Unit 3.notebook June 03, 2014 Unit 3 Genetics Sep 6 5:24 PM Intro Genetics The branch of biology that deals with variation (differences) and inheritance. Feb 27 1:30 PM
Intro Heredity The passing of genetic traits (characteristics) from one generation to the next. Inherited Traits that are passed on. Heredity results in similarities between relatives. Feb 27 1:23 PM Part I: Heredity p. 526 528 Mendelian Genetics Gregor Mendel was a monk who liked peas. Feb 27 2:22 PM
Mendel was interested in heredity and obtained some purebred pea plants. Tall Vs. Short Part I: Heredity Yellow Vs. Green Etc. Purebred An organism descended from an ancestor of a distinct type or breed (e.g. only tall pea plants). Hybrid An organism descended from ancestors of mixed types or breeds (e.g. a tall and short parent). Purebred means bred for a desired characteristic (trait) (Some people only want green peas!) Feb 27 2:26 PM Part I: Heredity Mendel noticed that many traits came in two versions (e.g. for height: tall or short, no medium). An important observation was that when he bred (or crossed) a tall pea plant with a short pea plant all the offspring were tall. Why no short? Feb 27 2:31 PM
Part I: Heredity Mendel Realized that each plant had factors which controlled each trait. He also noticed that some traits hid (masked) the presence of other traits. Dominant A trait that is always expressed and can hide (mask) the presence of a recessive trait. Recessive A trait that is expressed only when a dominant trait is not present. Feb 27 2:38 PM Unit Theory p. 530 Unit Theory Mendel hypothesized that each factor was inherited as an independent unit. Each parent possessed two units but gave only one unit to their offspring. Mendel looked at traits composed of two factors. We now understand heredity to be controlled by genes composed of two alleles. Gene A sequence of DNA that controls the expression of a specific trait and can be passed on to an offspring (inherited). Eg. Height, flower colour, pod colour Mar 1 1:48 PM
Segregation Law of Segregation Inherited traits are determined by pairs of factors (alleles). These factors separate in the gametes, with one in each gamete. Independent Assortment Segregation of factors (alleles) into gametes is done randomly. This creates variation among gametes. These phenomena help explain the processes we learned about in meiosis. Mar 1 1:50 PM Mendel vs. Modern Genetics Allele An alternate form of a gene. Eg. Tall or short, white or purple flowers Homozygous (Purebred) When an individual has two identical alleles for a gene (TT or tt). Heterozygous (Hybrid) When an individual has two different alleles (Tt). Mar 1 1:49 PM
Mendel vs. Modern Genetics p.529 Principle of dominance When individuals with different alleles are crossed only the dominant allele is expressed. Eg. If tall (T) is dominant to short (t) a cross between a tall (TT) and a short (tt) plant will yield only tall (Tt) plants. Mar 1 1:49 PM Punnett Squares p.532 534 A graphical way of predicting the traits of offspring from a cross. Symbols used: 1. X Represents a cross (i.e. two individuals being bred together). 2. Capital letters Represent dominant alleles Lower case represent recessive alleles. Mar 1 1:50 PM
Punnett Squares 3. P generation Symbolizes the parental generation. 4. F 1 generation Symbolizes the first filial generation (i.e. children of the P generation). 5. F 2 generation Symbolizes the second filial generation (i.e. grandchildren of the P generation). Mar 1 1:50 PM Punnett Squares There are several types of punnett squares. 1. Monohybrid Deals with only one trait of the organism (i.e. height). 2. Dihybrid Deals with two traits of the organism (i.e. height and flower colour). 3. Trihybrid Deals with 3. Mar 1 1:51 PM
Punnett Squares The results of a cross are described in two ways: Phenotype Traits that are visible. The phenotypes of both TT and Tt are the same = tall. Genotype Actual genetic makeup of the organism. The genotypes of TT and Tt are different. Mar 1 1:51 PM Punnett Squares Steps for completing Punnett squares: 1. Read the scenario carefully. 2. Decide what you are being asked to do. 3. Make up symbols to represent the alleles and write down a key. Make sure you can tell both alleles apart: Good allele symbolsbad allele symbols 4. Write out the P generation. 5. Do the Punnett square. 6. Do the genotypic and phenotypic ratios. 7. Answer the question that was asked originally. Mar 1 2:00 PM
Test Cross (Back Cross) p. 533 534 Sometimes it is not possible to determine the genotype of an individual from the phenotype. I.e. is a tall plant TT or Tt? However we can determine phenotype by crossing the individual of unknown genotype with a homozygous recessive individual. This type of cross is called a Test Cross or Back Cross. Mar 11 6:27 PM Test Cross E.g. Is the parent s genotype TT or Tt? 1. Crossing a tall phenotype with a short phenotype results in only tall plants. 2. Crossing a tall phenotype with a short phenotype results in about 50% of the offspring being short. Mar 11 6:28 PM
Practice Green is dominant for the colour of Dragons. Fred, a green dragon, is crossed with a red Dragon. There are twelve offspring and seven are red. What is the genotype of Fred? Show your reasoning. Mar 11 6:29 PM Practice 2 A farmer needs to know if the tall pea plant she has is homozygous dominant or heterozygous. So she crosses that plant with a homozygous recessive individual. All of the 27 offspring are tall. What is the genotype of the original plant? Mar 11 6:30 PM
Dihybrid Cross Dihybrid crosses look at more than one gene at a time. E.g. Height & Flower colour They still follow the same basic principles of the monohybird crosses we have been doing. Mar 11 6:30 PM Question A homozygous tall pea plant that is homozygous for purple flowers is crossed with a plant that is heterozygous for both of these traits. Tall is dominant to short and purple flowers are dominant to white flowers. What are the phenotypic and genotypic ratios of the F1 generation? Mar 11 6:32 PM
Question 2 A woman with freckles who is heterozygous for widow's peak marries a man who is homozygous dominant for no freckles and has a straight hair line. What will be the genotypic and phenotypic ratios of their offspring. Mar 11 6:32 PM Incomplete Dominance p.541 Incomplete Dominance Occurs when neither allele is purely dominant nor recessive. Presence of different alleles results in a characteristic that is different from either of the alleles by themselves. It is often described as a blending of characteristics. Mar 11 6:33 PM
Incomplete Dominance Incomplete dominant alleles are also written in a special way. Most often the two alleles are distinguished with the use of a tick. A and A or A 1 Mar 11 6:35 PM Incomplete Dominance An example is flower colour in snapdragons. R = Red R = White RR = Pink Incomplete dominant alleles do not produce the same phenotypic ratios as dominant/recessive alleles. Mar 11 6:35 PM
Co dominance p. 541 So far all the examples we have looked at had one dominant allele which masked the presence of one recessive allele. For some genes there are no recessive alleles. All alleles are always expressed if they are present. Co dominance Occurs when two different alleles are expressed equally and both are considered dominant. Mar 15 7:58 AM Co dominance The way co dominant alleles are written is different from regular dominant/recessive alleles. Each may be given a different capital letter. A and B One letter may be used and then two different letters are written as superscripts. I A and I B Mar 15 7:58 AM
Co Dominance Examples are black and white feathers in barred plumage chickens (p. 541 fig. 16.16) and red and white fur in Roan horses. Ex. B = black feathers W = white feathers BW = black and white feathers Co dominant alleles do not produce the same phenotypic ratios as dominant/recessive alleles. Mar 15 7:59 AM Multiple Alleles p. 541 542 All of the genes we have looked at so far have had only two alleles. Multiple alleles A gene with more than two alleles. (3 allels determines the phenotype) However, each individual can still carry only two alleles at a time. The more alleles possible for a gene, the greater the variation within a population. Mar 17 12:23 PM
Multiple Alleles ABO blood group gene Example: Human Blood type There are four blood types: type A; type B; type AB; type O There are three alleles: A, B, or O. They are most often symbolized as: I A I B or i. I A and I B are co dominant over i. Mar 17 12:23 PM Phenotype Type A blood Type B blood Type AB blood Type O Blood Multiple Alleles Genotypes Mar 17 12:24 PM
Question A person with AB blood is crossed with a person with homozygous B blood. What are the genotypic and phenotypic ratios of the F1 generation? Mar 17 12:28 PM Polygenetic Inheritance p. 549 All of the examples we have done so far have dealt with only one gene controlling a trait. On one chromosome. Some traits are influenced by two or more genes. Inheritance dealing with these traits is called multiple gene inheritanceor polygenetic inheritancẹ Mar 17 12:54 PM
Polygenetic Inheritance Traits that are controlled by several genes result in a greater variety of phenotypes. This variation is even greater than with multiple allele inheritance. With single gene controlled traits, characteristics can be split in to categories (tall/short, white/red). Polygenetic characteristics can not be as easily divided into groups. This results in a bell curve distribution of variation. Twins~! Mar 17 12:54 PM Mar 17 12:54 PM
Polygenetic Inheritance Examples include eye colour, skin colour, cardiovascular disease and athletic ability. One classic example is corn ear length. Length is controlled by two genes with a total of four alleles. The more dominant alleles that are present the longer the ear is. Mar 17 12:54 PM You don't have to do punnett squares for polygenetic!) Mar 11 10:16 AM
Sex Determination Sex chromosomes Chromosomes which determine the sex of an individual. In humans and some other animals, these are represented as an X and Y. Females have XX, males XY. Autosomes All other chromosomes in an organism. Like the autosomes, the sex chromosomes segregate during meiosis. Each gamete receives only one sex chromosome. Mar 18 9:07 AM Sex linked Inheritance p.546 547, 558 Sex linked inheritance The transmission of genes that are located on the sex chromosomes (X or Y chromosomes). Mar 18 9:13 AM
Sex linked Inheritance p. 545 547 Thomas Morgan experimented with fruit flies in 1910. He noticed that most of the flies with white eyes were male. He concluded that the trait for white eyes must be sex linked (located on a sex chromosome). Mar 18 9:13 AM Sex linked Inheritance Based on Morgan s work we now represent sex linked genes in a special way. X R X r Y We will only look at genes linked to the X chromosome. These are called X linked genes. We will NOT look at genes linked to the Y chromosome (Y linked). Mar 18 9:18 AM
Sex linked Inheritance Morgan s Fruit Fly Cross p. 547 Mar 18 9:23 AM Sex linked Inheritance Fathers pass X linked traits to: All daughters (because they receive the X chromosome). No sons (because they receive the Y chromosome). Fathers pass Y linked traits only to sons. Mothers pass X linked traits to both sons and daughters. Mar 18 9:24 AM
Unit 3.notebook June 03, 2014 Sex linked Inheritance The sex linked (X linked) examples we will encounter are: 1. Red green colour blindness Normal vision is dominant. 2. Pattern baldness Normal hair growth is dominant. 3. Hemophilia Normal clotting is dominant. Mar 18 9:25 AM Question 1 In humans the gene controlling red green colour blindness is carried on the X chromosome. Colour blindness is recessive to colour sight. A woman who is a carrier for the colour blind allele mates with a man who is colour blind. What are the genotypic and phenotypic ratios of the F1? Mar 18 9:30 AM