Unit 7 Section 2 and 3

Transcription

1 Unit 7 Section 2 and 3 Evidence 12: Do you think food preferences are passed down from Parents to children, or does the environment play a role? Explain your answer. One of the most important outcomes of sexual reproduction is the variety in traits that result from a shuffling of genes. Heredity is the passing of traits or characters from parents to offspring Traits are distinguishing characteristics that are inherited and passed from one generation to the next Ex. Eye color, leaf shape, tail length Evidence 13: Use figure 2 on page 317 to identify 3 traits that vary among the cats. Genetics is the study of biological inheritance patterns and variations in organisms Gregor Mendel is known as the scientist who laid the understanding for genetics. He studied plant variations (peas) because of his background He discovered that there were patterns of inheritance based on appearance. He was the first to develop rules that accurately predict patterns of heredity Mendel chose to use pea plants in his experiments for several reasons o They reproduce quickly o Produce many offspring o He could easily control the mating of them o The sex organs (both female and male) of the plant are in the flower o It typically self-pollinates (mates with itself) o Pea plant contains several traits displayed plainly Mendel observed seven traits that are easily recognized and apparently only occur in one of two forms: 1. flower color is purple or white 5. seed color is yellow or green 2. flower position is axil or terminal 6. pod shape is inflated or constricted 3. stem length is long or short 7. pod color is yellow or green 4. seed shape is round or wrinkled

2 Pure breed (sometimes called a true breed) a type of organism whose ancestors are genetically uniform Organism is only breed or mated with other organisms in the same breed Ex purple flower with a purple flower or Great Dane with a Great Dane All of Mendel s experiments started with a purebred pea plant. All pea plants were allowed to self-pollinate for several generations to ensure pure bred Cross or crossbred refers to mating or breeding of two individual organisms. Mendel s first experiments He bred a purebred purple flower and a purebred white flower He then allowed the purple and white purebreds to cross pollinate (also known as cross fertilize), these were considered the parental generation. The results were that the flowers had hidden traits because the flowers were purple with no white showing. These were called the Filial 1 generation. The F1 generation was allowed to self-pollinate. The results were that white flowers started to appear. These were called the Filial 2 generation. He continued these experiments with the other traits from the peas that he was studying.

3 Parental (P) --- Filial 1 (F1) --- Filial 2 (F2) Filial means son or daughter Mendel s Results from his experiments He discovered that in F1 plants that only one form of a trait would show. He discovered that after F1 self-pollination that some of the missing traits reappeared in the F2 generation. He discovered that the results were a 3:1 ratio from this monohybrid cross. These results are now known as Law of Segregation and Law of Dominance

4 Evidence 14: Why did Gregor Mendel pollinate the plants himself rather than let the plants self-fertilize? Mendel s hypotheses from his experiments make up the Mendelian Theory of Heredity 1. For each inherited trait, an individual has 2 copies of the gene, one from each parent 2. There are alternative versions of genes that we call alleles 3. When 2 different alleles occur together one of them may be completely expressed while the others may have no observable effect on the organisms appearance a. We call these dominant and recessive traits 4. When gametes are formed, the alleles for each gene in an individual separate independently of one another. Thus gametes carry only one allele for each inherited trait. When gametes unite during fertilization each gamete contributes one allele. Genes influence the development of traits Dominant trait o The trait that is shown or expressed when two different alleles are present o Represented by a capital letter = T Recessive trait o The trait that is only shown or expressed when two copies are present o Represented by a lower case letter = t Homozygous o 2 alleles that are the same in an organism = TT or tt Heterozygous o 2 different alleles in an organism = Tt

5 Genotype A set of alleles that an individual has The genetic makeup of a specific set of genes o Ex BB Phenotype The physical characteristics or traits of an individual that we see o Ex brown eyes Punnett square o A grid system for prediction all possible genotypes resulting from a cross o A diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in the cross.

6 Monohybrid cross is the Punnett square that only looks at one trait Dihybrid cross is the punnett square that looks at two traits Law of independent assortment states that the alleles of different genes separate independently of one another during gamete formation Probability is the likelihood that is a specific event will occur o Expressed as a fraction o Probablility = # of one kind of possible outcome o Ex flipping a coin Total # of all possible outcomes

7 Before Mendel people thought offspring were just a mix or blend of parents o Ex if one parent was tall and one parent was short then the child would be medium height He concluded from experiments that each pea has two separate heritable factors for each trait, one from each parent o When gametes (sperm and egg cells) form, each receives only one of the organisms two factors, or genes, for each trait Heterozygous (Bb) the alleles given are both dominant and recessive Homozygous dominant (BB) the alleles given are both dominant Homozygous recessive (bb) the alleles given are both recessive Genotype genetic makeup Phenotype physical characteristics Monohybrid cross one trait Dihybrid cross two traits Autosomes or autosomal chromosomes or body cells chromosome sets 1-22 Gametes or Sex cells or sperm and egg chromosome set 23 2 copies of each gene is given to the offspring in sexual reproduction

8 Many human genetic disorders are caused by autosomal genes. These can be predicted by using a Punnett square Disorders caused by recessive alleles 2 copies of the recessive allele must be present for a person to have this disorder. This disorder will most often appear in offspring of parents who are both heterozygotes. By being heterozytes, the parent is carrying a normal allele that masks the disease causing recessive allele They are said to be carriers not showing the disorder but can pass it down o Ex. Cystic fibrosis is a severe recessive disorder that mainly affects the sweat glands and the mucus glands Disorders caused by dominant alleles Far less common than recessive disorders o Ex. Huntington s disease a nervous system disorder that develops later in life

9 Disorders caused by sex linked traits These are disorders that are linked either to the X or Y chromosome. XX female XY male Female can only give an X chromosome to the offspring Male can give either an X or a Y to the offspring Y chromosome only has the trait to determine the male gender X chromosome has many traits o Ex. Colorblindness and sickle cell anemia Polygenic trait poly means many genic means genes When several genes influence a trait The genes for this may be on the same chromosome or scattered Ex skin color, eye color, height, weight, hair color Incomplete dominance an individual displays a trait that is intermediate between 2 parents This is a heterozygous phenotype that is somewhere between the 2 homozygous phenotypes Neither allele is completely dominant or recessive 1 trait is shown Ex. Red flower reproduces with a white flower and you get a pink flower

10 Codominance when 2 dominant alleles are expressed at the same time Both of the genes give are shown but neither allele is dominant or recessive Ex. Red flowers reproduce with a white flower and you get a white flower with red spots or stripes Sometimes phenotypes are decided by the environment instead of the sum of gene expression Ex. Turtles lay eggs in the sand. If the temperature is warmer the turtles will become females and if it is cooler they will become males Ex twins aren t always identical Pedigree a chart that can help trace phenotypes and genotypes in a family.

11 A family history that shows how a trait is inherited over several generations Like a family tree that shows genetic disease Pedigree tells us where a genetic disorder is found o Autosomal or sex linked (located on the x) o Dominant or recessive o Heterozygous or homozygous Pedigree rules Circle = female Square = male Vertical lines = offspring Read left to right for order of birth Horizontal line = mating Shading indicates disorder o Trait with whole color means it came from homozygous parents o Trait with ½ color ½ not means came from heterozygous parents

12 Another method used to map out the human genes besides the pedigree is the karyotype