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Chapter 5B Gregor Mendel • • • Famous pea plant study Mendelian genetics “Father of Genetics” Father of Genetics Monk and teacher. Experimented with purebred tall and short peas. Discovered some of the basic laws of heredity. Studied seven purebred traits in peas. Called the stronger hereditary factor dominant. Called the weaker hereditary factor recessive. Presentation to the Science Society in1866 went unnoticed. He died in 1884 with his work still unnoticed. His work rediscovered in 1900. Known as the “Father of Genetics”. Mendelian Genetics • • • Began with 34 varieties of pea seeds Chose 7 sets of opposing characteristics Chart, page 113 Mendel’s Peas Self-pollination vs. Cross-pollination Mendel’s Observations He noticed that peas are easy to breed for pure traits and he called the pure strains purebreds. He developed pure strains of peas for seven different traits (i.e. tall or short, round or wrinkled, yellow or green, etc.) He crossed these pure strains to produce hybrids. He crossed thousands of plants and kept careful records for eight years. Mendel’s Peas In peas many traits appear in two forms (i.e. tall or short, round or wrinkled, yellow or green.) The flower is the reproductive organ and the male and female are both in the same flower. He crossed pure strains by putting the pollen (male gamete) from one purebred pea plant on the pistil (female sex organ) of another purebred pea plant to form a hybrid or crossbred. Mendel’s Results Mendel crossed purebred tall plants with purebred short plants and the first generation plants were all tall. When these tall offspring were crossed the result was a ratio of 3 tall to 1 short. Mendel’s Experiments He experimentally crosses different strains to develop hybrids. He then crossed the hybrids and analyzed the results. Dominant Traits RULE • • Strong Hereditary traits cover weak traits. Mendal called stronger traits • • DOMINANT Mendal called weaker traits • recessive • Dominant traits are represented by capital letters (T) while recessive traits are represented by lower case letters (t). try and follow the diagram on the next slide while keeping the DOMINANT and recessive letters in mind. ( TT) (tt ) In the diagram above, the dominant allele is represented by ___and the recessive allele is represented by __ . Mendelian Genetics • • • P1: the original parent generation F1: first filial generation; offspring of the P1 generation F2: second filial generation; offspring of the F1 generation If we consider your parents to be the P1 generation, which generation are you? 1. 2. 3. 4. P1 P2 F1 F2 The concept of unit characteristics Factors occur in pairs. Genes occur in pairs because diploid organisms have 1. daughter chromosomes. 2. homologous pairs of chromosomes. 3. gametes. The concept of dominant & recessive Dominant Traits RULE • • Strong Hereditary traits cover weak traits. Mendal called stronger traits • • DOMINANT Mendal called weaker traits • recessive • Dominant traits are represented by capital letters (T) while recessive traits are represented by lower case letters (t). try and follow the diagram on the next slide while keeping the DOMINANT and recessive letters in mind. ( TT) (tt ) Dominant trait • • A trait that is expressed and masks the expression of the other trait Examples on page 113 Recessive trait • A trait which, when in the presence of a dominant trait, is not expressed Language of Genetics Problems • • • Capital letters (T) = dominant Lowercase letters (t) = recessive A, a; B, b; R, r; etc. The concept of segregation A cell forms gametes during which process? 1. 2. 3. 4. Mitosis Meiosis Cytokinesis Fertilization Monohybrid cross • A genetic cross dealing with only one set of characteristics Punnett square • A diagram used to show the possible gamete combinations from a genetic cross PUNNETT SQUARE CROSS T T X Tt CONT’D T T TT X Tt T t T T T t T T T t Allele Types • • • • Homo - Same Hetero - Opposite Pheno – Physical Geno - Genetic Phenotype • • The physical expression of an organism’s genes Examples: tall, short, black Genotype • • The genetic make-up of an individual; the genes it has Examples: Tt, AA, bb Homozygous • • When both alleles in a cell are the same Examples: tt, TT, BB, bb Heterozygous • • When both alleles in a cell are NOT the same Examples: Bb, Tt Locus • The site on a chromosome where a particular gene is located Allele • • One of a pair of genes that has the same position on homologous chromosomes Examples: T or t Gene Expression Are the following sentences true or false? - Homozygous organisms are true breeding for a particular trait. • False - Plants with the same phenotype always have the same genotype. • False Probability • In Mendel’s model of segregation, what was the ratio of tall plants to short plants in the F2 generation? • The ratio was 3 : 1. Incomplete Dominance Incomplete Dominance • Both alleles are expressed, but neither one is dominant. • KEY: a blending of the traits • Example: • When red snapdragons are crossed with white snapdragons, the resulting offspring are pink. 1st generation w C w C r C r w CC r w CC r C r w CC r w CC 2nd generation r C r C w C Genotypic: r r CC r w CC 1:2:1 _________ Phenotypic: w C 1:2:1 CrCw CwCw _________ Human example • Brachydactyly Codominance • • • Two alleles for a gene are both expressed. KEY: both alleles are expressed with no blending. Example: • In horses, red hair + white hair = roan (red and white hairs). Multiple Alleles • • One of several alleles can be at a given locus Example: human blood types (A, B, AB, O) Human Blood Types Dominant Alleles A I and B I Recessive Allele i A I A I A I i IB IB IB i A B I I ii type A blood type A type B type B type AB type O Suppose a woman who has type AB blood marries a man who is heterozygous for blood type A. What blood types might their children have? type AB x heterozygous type A A I A I i A I B I A I A I i A I B I B I i Dihybrid Crosses • • Genetic crosses dealing with TWO characteristics at the same time Example: green/yellow peas AND tall/short pea plants Mendel’s Concept of Independent Assortment • The segregation of one set of alleles during gamete formation is not affected by another set. Multiple Gene Interaction • • Sometimes two or more genes working together result in a single trait. Examples: many human traits such as hair color and skin color Sex-Linked Traits Two Types of Chromosomes • Autosomes • • • non-sex-determining chromosomes humans = 22 pairs Sex chromosomes • • XX = female XY = male Female = XX Male = XY X X X XX XX Y XY XY Sex-Linked Traits • Some genes are carried on the sex chromosomes. • The X and Y chromosomes are not homologous. Sex-Linked Traits • • Males inherit more sex-linked disorders because they only have one gene for the trait. If there is a defective gene on X, there wouldn’t be a normal gene on Y to counteract it. Sex-Linked Traits • • Indicated with a superscript above the X and Y chromosomes Example: • XHXh • XHY Human Examples • Red-green colorblindness • Hemophilia • Lack a blood chemical that allows for blood clotting H X H X normal female XH Xh carrier female A heterozygous female that does not have the disease, but she does carry the gene for the trait Xh Xh hemophiliac female XH Y normal male Xh Y hemophiliac male carrier female x normal male H h X X H X H H X X H h X X Y H X Y h X Y