Genetics N. Sassaman 8th grade Science 2011-2012 Chromosomes, Genes, and Alleles Chromosomes are made up of many genes joined together like beads on a string. The chromosomes in a pair may have different alleles for some genes and the same allele for others. DNA – the genetic material that carries information about an organism and is passed from parent to offspring. Dominant vs. Recessive Alleles Gregor Mendel’s experiments In all of Mendel’s crosses, only one form of the trait (dominant) appeared in the F1 generation. However, in the F2 generation, the “lost” form (recessive) of the trait always reappeared in about one fourth of the plants. Trait – a characteristic that an organism can pass on to its offspring through its genes. #1-5 on p. 12 packet Dominant vs. Recessive Alleles Letters are used to represent the alleles for a certain trait. A capital letter is used for dominant trait (allele). A lowercase letter is used for recessive trait (allele). Examples: (see chart above) T = Tall height G = green pod t = short height g = yellow pod R = round seed Y = yellow seed r = wrinkled seed y = green seed #6-13 on p. 12 packet Homozygous vs. Heterozygous REMEMBER: each gene for a trait has TWO possible variations called alleles. (one from mother, one from father) THEREFORE: we assign TWO letters to an individual to represent which two alleles they have. Examples: (see chart above) T = Tall height t = short height Height TT = tall plant Tt = tall plant tt = short plant G = green pod g = yellow pod Pod color GG = green Gg = green gg = yellow R = round seed r = wrinkled seed Seed shape RR = round Rr = round rr = wrinkled Y = yellow seed y = green seed Seed color YY = yellow Yy = yellow yy = green Homozygous vs. Heterozygous Homozygous (purebred) – having TWO OF THE SAME alleles for a trait. (homo = same) Heterozygous (hybrid) – having TWO DIFFERENT alleles for a trait. (hetero = different) Height TT = tall plant Tt = tall plant tt = short plant Pod color GG = green Gg = green gg = yellow TT = homozygous tall (dominant) tt = homozygous short (recessive) Tt = heterozygous tall GG = homozygous green pod (dominant) gg = homozygous yellow pod (recessive) Gg = heterozygous green pod Phenotype vs. Genotype An organism’s phenotype is its physical appearance, or visible traits, and an organism’s genotype is its genetic makeup, or allele combinations. pheno = physical (appearance) geno = genes (allele combo) Ex/ tall or short Ex/ TT, Tt, or tt #1-3 on p. 16 packet; #1-3 on p. 18 packet Codominance and Incomplete Dominance Not all alleles for traits are expressed as a dominant/recessive relationship. Codominance occurs when there are two variation of a trait (two alleles), but neither one is dominant over the other. As a result, BOTH alleles are expressed if present in an individual. FB = black feathers FW = white feathers FBFB = black feathers FWFW = white feathers FBFW = BOTH black and white feathers Codominance and Incomplete Dominance Not all alleles for traits are expressed as a dominant/recessive relationship. Incomplete dominance occurs when one allele for a trait is not completely dominant over the other allele. As a result, there is a ‘blending’ of the two specific traits. R = red flowers R’R’ RR’ R’ = white flowers RR = red flowers R’R’ = white flowers RR’ = pink flowers NOTE: the letters used to represent this type of inheritance pattern vary. Sometimes capital/lowercase are used, sometimes capital/capital prime, sometimes two capital letters. Always read problems carefully. Probability and Punnett Squares In a genetic cross, the allele that each parent will pass on to its offspring is based on probability, the likelihood that a particular event will occur. For example: Mendel was the first scientist to recognize that the principles of probability can be used to predict the results of genetic crosses. coat color in guinea pigs Homozygous black (BB) X homozygous white (bb) Geneticists use Punnett squares to show all the possible outcomes of a genetic cross and to determine the probability of a particular outcome. Probability and Punnett Squares How to set up a Punnett Square: 1. Draw a box with four (4) quadrants. 2. Take one parent and write its alleles ABOVE the two columns. 3. Take the other parent and write its alleles NEXT TO the two rows. 4. Fill in each of the boxes by writing the letters (alleles) that ‘match up’ in that box. How to calculate probability: 1. From a Punnett square, all results will be expressed as ‘…out of 4’ or ‘-/4’. For example: 2 out of 4 OR 2/4 of the offspring above have the genotype Tt. 1 out of 4 OR 1/4 of the offspring above have the genotype tt or TT. 2. A percent can then be calculated by dividing the fraction through and X 100 to get 2/ X 100 = 50% percent (%). ¼ X 100 = 25% 4 # 4-5 on p. 16 packet; #5 on p. 19 packet Homework!! • p. 17 in packet, #6-8 • p. 19 in packet, #6-8 Class assignment: • Page 13 in packet, #1-14 *Assume: B = black, b = white • Define key terms: – Multiple alleles – Sex-linked genes – carrier p. 13 in packet 3. Black - 75%, White - 25% 4. Yes – Punnett square only gives chances of a certain outcome. 5. Black (Bb) and white (bb) 6. White is homozygous b/c two of the same alleles; black is heterozygous b/c two different alleles. 7. Black – 50%, White – 50% 8. f 9. a 10. e 11. g 12. b 13. c 14. d Multiple Alleles Some human traits are controlled by a single gene that has more than two alleles. Such a gene is said to have multiple alleles – three or more forms of a gene that code for a single trait. Blood type is determined by a single gene with three alleles. This chart shows which combinations of alleles result in each blood type. IA and IB are codominant to each other and both dominate over i (recessive). http://www.zerobio.com/videos/blood_type.html Multiple Genes Multiple genes that control a trait act together to produce a single trait with a large number of phenotypes. Examples: hair color, eye color, skin color, height Coat Color Black BBEE BbEE Yellow BBee Bbee bbee Coat color in Labrador retrievers (general) Gene #1 – Color (black or brown) B – Black b – brown bbEE Chocolate Gene #2 – Expression of color (yes or no) E – expressed e – not expressed Possible Genotypes BBEe BbEe bbEe Sex-linked Genes The sex chromosomes carry genes that determine whether a person is male or female. They also carry genes that determine other traits. Females have two X chromosomes - XX Males have one X chromosome and one Y chromosome - XY These are homologous chromosomes and separate during meiosis just like the other 22 pairs of homologous chromosome do!! Sex-linked Genes Some human traits occur more often in one gender than the other. The genes for these traits are often carried on the sex chromosomes, most commonly on the X chromosome. Genes on the X and Y chromosomes are often called sex-linked genes. 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. Females, however, tend to be carriers of a recessive sex-linked trait (they have one recessive allele for a trait and one dominant allele) Red-green colorblindness C – normal vision Female: XCXC XCXc XcXc c – colorblind Male: XCY XcY Today’s assignment - p. 14 in packet!! 1. 2. 3. 4. 5. 6. Dd dd Dd dd Yes 50% 14. 15. 16. 17. multiple alleles Sex-linked traits carrier sex chromosomes 7. XCXc 8. XCY 9. XcXc 10. XcY 11. Yes – the father 12. 50% 13. 50% Pedigree - general A pedigree is a chart or “family tree” that tracks which members of a family have a particular trait. (p. 118 in textbook) Pedigree - example The pedigree shows the inheritance of hemophilia, a sex-linked disorder in a family. http://www.dnalc.org/view/16315Animation-13-Mendelian-laws-apply-tohuman-beings-.html Genetic disorders A genetic disorder is an abnormal condition that a person inherits through genes or chromosomes. Examples: Disorder Description Cause Cystic fibrosis Body produces abnormally thick mucus. Recessive allele due to removal of three DNA bases. Hemophilia Blood clots slowly or not at all. Recessive allele on X chromosome. Down’s syndrome Mental retardation and heart defects. Extra copy of chromosome 21.