Presentation on theme: "Genetics Explain the basic rules and processes associated with the transmission of genetic characteristics."— Presentation transcript:
Genetics Explain the basic rules and processes associated with the transmission of genetic characteristics
Describe the evidence for dominance, segregation and the independent assortment of genes on different chromosomes, as investigated by Mendel
Introduction Heredity –is the passing of traits from parents to offspring Genetics –is the study of the patterns of inheritance as hereditary characteristics or traits Gregor Mendel –the "father of modern genetics“ –an Austrian monk ideas of inheritance1866 –published his completely new and thoroughly documented ideas of inheritance in 1866
Introduction His model was so simple that scientists who read it at that time considered it "trivial“ –it received little attention and no recognition until it was rediscovered in 1900 after his death (simultaneously by 3 different people) In the meantime, –chromosomes had been named –their movements during mitosis and meiosis observed and described
Introduction 1902 –scientists realized that chromosomes moved precisely as reported by Mendel Once the connection between chromosomes and heredity was established, the science of genetics was reborn Mendel was the first person to realize that genetic traits are inherited as separate particles
Introduction He did not actually see these particles –but he predicted their existence based on patterns of inheritance factorsHe proposed that organisms have a pair of "factors" for each trait –one from each parent We NOW know that the particles of inheritance are segments of DNA –which we call genes
Mendel's Experiments Mendel worked with garden peas –available in many different varieties E.g. pure breeding tall, pure breeding dwarf Pea flowers contain both male and female parts and normally self-pollinate –but can easily be artificially cross-pollinated with other pea plants Mendel crossed plants of two varieties with contrasting traits (such as tall and dwarf) to see what would happen.
Mendel's Experiments Mendel worked with seven traits, each which occurred in two distinct forms Mendel began by studying crosses involving only one trait at a time Example: Flower color –P1: (parental generation) pure-breeding red flowered plants X pure- breeding white flowered plants –F1: (first filial generation) red-flowered hybrids (genetically mixed offspring) –F2: (second filial generation) 3/4 red-flowered and 1/4 white-flowered
Mendel's Laws 1.Inherited characteristics are controlled by pairs of factors –genes –one from each parent. 2.One gene may “mask” the effect of another. –The gene which is expressed is dominant, while the one which is masked is recessive. 3.Pairs of genes segregate during gamete formation –so each sex cell contains only one member of a pair of genes.
Terms Alleles –Two or more alternate forms of a gene, which produce contrasting effects for a certain trait –e.g. red (R) and white (r) for flower colour of peas Homozygous –having two of the same allele eg. red/red (RR) purebred Heterozygous –having two different alleles eg. red/white (Rr)
Terms Genotype –the genetic makeup of an individual Phenotype –the expression of the genes, or appearance of an individual Purebred –an organism having all homozygous gene pairs Hybrid –an organism having at least one heterozygous gene pair
Terms Monohybrid –an organism having only one heterozygous gene pair Dihybrid –an organism having two heterozygous gene pairs
Monohybrid Punnet Square Rr r r RR Male Female
The phenotypic ratio for a monohybrid cross is always 3:1 –dominant trait : recessive trait
Predicting the Outcome of a Genetic Cross When we know the genotypes of parents used in a genetic cross, we can predict the genotypes of the offspring and their expected ratios –Must use a Punnett square is used named after some guy named Punnett A geneticist –Basically it’s a just a fancy chart
Monohybrid Punnet Square Rr r r RR Male Female
If an organism has the dominant phenotype, how can you determine whether it is homozygous or heterozygous?
Test Cross You conduct a test cross. –Mate it with an organism of a known genotype and see what you get. The only known genotype that is observable is HOMOZYGOUS RECESSIVE (rr) For example: –determine whether a red-flowered pea plant is homozygous or heterozygous
Test Cross P: Red flowered X White-flowered R? rr F1: Suppose the cross produces all red flowers Rr Rr Rr Rr If no offspring showing the recessive phenotype are produced, the unknown parent must be … –homozygous
Test Cross P: Red flowered X White-flowered R? rr F1: Suppose the cross produces 50% red flowers and 50% white flowers The only way a white flower could appear is if it received a recessive allele from the unknown parent. –The unknown parent must be … –HETEROZYGOUS
Dihybrid Cross In addition to his monohybrid crosses, Mendel performed dihybrid crosses of plants with two different pairs of contrasting alleles In one experiment, Mendel crossed plants homozygous for seeds that were both round and yellow with plants homozygous for wrinkled, green seeds
Dihybrid Cross All the F1 offspring had round, yellow seeds Self-fertilization of the F1 plants produced and F2 generation of seeds with the following phenotypes: 315 round yellow 108 round green 101 wrinkled yellow 32 wrinkled green
Dihybrid Cross To find the ratio among the F2 phenotypes, –we take the number of offspring in the smallest category and divide it into the number of offspring in the other categories: –then the quotient is rounded to the nearest whole number
Dihybrid Cross Thus Mendel determined the phenotypic ratio in the F2 generation to be 9:3:3:1 –This is the ratio typical of a dihybrid cross in which both pairs of alleles show a dominant-recessive relationship
Dihybrid Cross Mendel explained these data by assuming that the genes governing seed color and seed shape move independently during gamete formation In the process of independent assortment, each pair of alleles behaves as it would in a monohybrid cross - independently of the other pair A dihybrid can produce four possible gene combinations (with equal probability) If the alleles are: R = roundr = wrinkled Y = yellowy = green The possible gamete combinations are RYRy rY ry
Dihybrid Cross RRYY x rryy RY ry RrYy RY, Ry, rY, ry 9/16 round, yellow(R_Y_) 3/16 round, green (R_yy) 3/16 wrinkled, yellow(rrY_) 1/16 wrinkled, green(rryy) Parent: P Gametes: F1: F1 Gametes: F2:
Multiple Alleles & Incomplete Dominance
Compare ratios and probabilities of genotypes and phenotypes for dominant and recessive, multiple, incomplete dominant, and codominant alleles Explain the relationship between variability and the number of genes controlling a trait
Multiple Alleles The genes which we have studied so far have only two different alleles: Many genes actually exist in more than two allelic forms, Although only two genes control coat colour in rabbits it is controlled by a series of four alleles for the same gene: 1.CFull colour 2.C ch Chinchilla 3.c h Himalayan 4.cAlbino
C + any of the 4 C ch + anything but C c h c h, c h c c Multiple Alleles The dominance hierarchy of these alleles is C > C ch > c h > c Determine the genotypes for the following phenotypes: PhenotypePossible Genotype 1.Full color 2.Chinchilla 3. Himalayan 4. Albino
Incomplete Dominance In some cases, a heterozygous organism shows a blending of genes because neither gene is dominant: –this is termed incomplete dominance –for example, in snapdragons, neither the red nor the white allele is dominant…
Incomplete Dominance Red Flowers x White Flowers Pink Flowers ¼ Red flowers ½ Pink flowers ¼ White flowers Parent: F1: F2: RR x rr Rr, Rr, Rr, Rr RR Rr rr
Codominance If two different alleles each contribute to a phenotype, they are termed codominant One of the best-known example of codominant genes occurs in humans, and determine ABO blood types There are three possible alleles: 1. I A 2. I B 3. i
Codominance Blood Type i O I A I B AB I B I B, I B i B I A I A, I A i A Possible GenotypesPhenotype
Chromosome Mapping Explain the influence of gene linkage and crossing over on variability
Chromosome Mapping Chromosomal Theory of Inheritance 1.Genes are located on chromosomes 2.Chromosomes undergo segregation during meiosis 3.Chromosomes assort independently during meiosis 4.Each chromosome contains many different genes
Chromosome Mapping Each chromosome contains hundreds or thousands of genes Genes located on the same chromosome are inherited together, –they are part of a single chromosome that is passed along as a unit linked –such genes are said to be linked
Chromosome Mapping during meiosis, chromosomes may exchange segments of DNA by crossing-over A A a a B B b b C C c c D D d d E E e e F F f f
Chromosome Mapping during meiosis, chromosomes may exchange segments of DNA by crossing-over A A a aA a A a B B b bB B b b C C c cC C c c D D d dD D d d E E e eE e E e F F f fF f F f
Chromosome Mapping The closer two genes are on a chromosome, the fewer the possible points of crossover are between them, and the less frequently such a cross-over will occur In other words, if two genes are close together on a chromosome it is likely they will stay together and not be exchanged between chromatids during meiosis To determine the location of genes along a chromosome is called MAPPING a chromosome
Chromosome Mapping A chromosome map indicates 1.The order in which specific genes occur on a chromosome 2.The distances between the genes
Chromosome Mapping Example: –In Drosophila, the following data was obtained from genetic crosses: 13% recombination between bar eye and garnet eye – High percentage recombination indicates that these two genes are far apart from each other – High likelihood that crossing over will occur between these two genes. 7% recombination between garnet eye and scalloped wings – These two genes are closer together than bar eye and garnet eye 6% recombination between scalloped wings and bar eye
Chromosome Mapping This data can be used to map the chromosome: Bar eye Scalloped wings Garnet eye 6 units 7 units 13 units
Sex Determination There are two chromosomes involved in the determination of sex of most animals, –the sex chromosomes (X & Y) Any other chromosome not involved in sex determination is called an autosome –for example, humans have 22 pairs of autosomes and 1 pair of sex chromosomes in most mammals, females are homozygous and have 2 X chromosomes, while males are heterozygous and carry an X and a Y chromosome
Sex Determination Female = XX Male = XY Females can produce eggs carrying only an X chromosome, Males produce sperm carrying either an X or a Y chromosome (50% of each) Thus, it is the male who determines the sex of his offspring !
Sex Linkage Compare the pattern of inheritance produced by genes on the sex chromosomes to that produced by genes on autosomes, as investigated by Morgan and others.
A True Story One day a geneticist named Thomas Hunt Morgan discovered a mutant white-eyed male fly among his hundreds of red-eyed flies. He bred the white-eyed male with several red- eyed females and observed the offspring: as expected, all the F1 flies had red eyes, showing red to be the dominant allele. He allowed the F1 generation to interbreed freely and observed the F2 generation. Again, as expected, the ratio of red-eyed flies to white- eyed flies was 3:1, except that … every white-eyed fly was male !
Explanation … Morgan concluded that the gene controlling eye-colour was carried on the… X-chromosome
Sex Linkage Like other chromosomes, the sex chromosomes carry many genes Some of the regions of the X- chromosome have a homologous region on the Y- chromosome –There are also large non-homologous portions: That is, the X chromosome carries some genes that have no counterparts on the Y chromosome
Sex Linkage Genes for sex-linked traits are carried on the X chromosome but not on the Y chromosome –Therefore in a male, the gene on the X chromosome is expressed whether it is dominant or recessive –In a female, she must have two recessive alleles to have the recessive phenotype
Fly Solution R = Red r = white X R X R = Red eyed Female X R X r = Red eyed Female X r X r = White eyed Female X R Y = Red eyed Male X r Y = White eyed Male
Fly Solution X R X R x X r Y X R X r x X R Y X R Y Y X R X r XrXr XRXR XRXR X r YX R YY X R X r X R XRXR XrXr XRXR
Example of hemophilia trait – (h) recessive disease
Sex Linkage Some sex-linked traits in humans are –Colour vision –Hemophilia –Duchenne's Muscular Dystrophy
Sex Influenced Genes The main role of sex hormones is to influence the reproductive system and its related organs, –These hormones, however, also affect many other parts of the body Genes that are expressed to a greater or lesser degree as a result of the level of sex hormones are called sex-influenced genes.
Sex Influenced Genes These genes are usually located on the autosomes Males and females with the same genotype may differ greatly in phenotype because the levels of sex hormones For example: –A bull may have a gene for high milk production, but he will notproduce milk because he has low levels of female hormones.
Sex Influenced Genes In humans, the gene for male pattern baldness is autosomal and sex-influenced A man will become bald even if he has only one allele for baldness, because – the male sex hormones somehow stimulate the expression of the allele In a woman, however, the allele acts as a recessive allele so that she must have two balding genes before she loses her hair
protein essential to lifeIf An organism has a mutation that destroys the genetic code for a protein essential to life, the organism will often die prematurely. This gene that fails to code for a functional protein is called a lethal allele It is possible for lethal alleles to be dominant, but most are rapidly eliminated from a population because they cause death before the individual carrying the allele reproduces.
Lethal Alleles An exception of a lethal dominant allele that remains in a population is the one responsible for Huntington's Disease in humans, because this allele is not expressed until later in life ( years of age) An example of a recessive lethal allele in humans is the one for Brachydactyly: –Heterozygotes have a short middle finger bone that makes the fingers appear to have only two bones instead of three –Homozygous babies lack fingers and have abnormal development of the skeleton that result in death in infancy
Lethal Alleles Some human examples of lethal alleles are: –Sickle cell anemia –Tay-sachs disease –Cystic fibrosis –Huntington's disease
Because geneticists are unable to manipulate the mating patterns of people, they must analyze the results of matings that have already occurred. As much information as possible is collected about a family's history for a particular trait, and this information is assembled into a family tree describing the interrelationships of parents and children across the generations –This is called a –This is called a pedigree
Pedigrees From a pedigree you should be able to determine if a particular trait is dominant or recessive, sex-linked or autosomal A pedigree not only helps us understand the past but also helps us predict the future Geneticists, physicians, and genetic counselors use pedigrees –for analysis of genetic disorders –to advise prospective parents of genetic risks involved Complete the pedigree studies on page 632 and page 611 as a class.