- Slides: 22
MENDELIAN GENETICS 5 B - Basic Genetics
Mendelian Genetics � � Gregor Mendel Father of genetics 5. 4
Gregor Mendel � � � � Born in Austria (1822) Monk (1843) Selective breeding on pea plants (1857) Wrote paper outlining his conclusions (1865) Abbot of the monastery (1868) Died of a kidney disorder (1884) Scientist rediscover Mendel’s papers (1900)
Gregor Mendel � � Traits Mendel studied See Table 5 -2 on page 113
Mendel’s Experiments � P 1 generation (parental generation) � Self-pollinating plants that bred true for a particular trait � F 1 generation (1 st filial generation) � Resulting plants from two P 1 plants being cross- pollinated � F 2 generation (2 nd filial generation) � Resulting plants from two F 1 plants being allowed to self-pollinate
Mendel’s Observations � � The F 1 generation only showed one trait. The F 2 generation always showed both traits and always in a 3: 1 ratio.
1. Concept of Unit Characteristics Traits in a pea plant are caused by factors occurring in pairs
2. Concept of Dominant & Recessive � � One factor in the pair is dominant to the other factor. The dominant factor will masked the appearance of the recessive factor when both are present.
3. Concept of Segregation � � Organisms form gametes for reproduction. Each gamete has only one factor from each pair.
Genetic Terms � � Phenotype: the expression of a gene in an organism; represented by words Genotype: the pair of genes an organism has; represented by two letters
Genetic Terms � � � Allele: genes which have equivalent positions on homologous chromosomes Locus: the specific site on a chromosome where a particular gene is located Homozygous: both alleles for a trait are the same Heterozygous: the two alleles for a trait are different Test cross: mating an organism that has a dominant phenotype but an unknown genotype with an organism that has a recessive phenotype Pedigree: a chart that geneticists use to trace the presence or absence of a trait in a number of generations
Genetic Terms � � Monohybrid: a cross between two organisms in which the researcher is only interested in one set of alleles Dihybrid: a cross between two organisms in which the researcher is interested in two sets of alleles
Genetic Terms Punnett squares: depicts genetic crosses and determines the probability of an offspring having a particular trait Named after Reginald Punnett, its designer
Completing Genetic Cross Write genotypes correctly 1. Always have 2 letter per trait 2. Use 1 st letter of dominant trait Capitalize dominant allele 3. 4. 5. 6. Example: green/yellow pods use G or g Green pods use G Yellow pods use g BE CAREFUL (Not Messy) with letters! Lowercase recessive allele Homozygous genotype – 2 capital or 2 lowercase letters Heterozygous genotype – 1 capital and 1 lowercase letter
5 Step Method 1. 2. 3. 4. 5. � Write the genotypes of BOTH parents Do a Punnett Square Write the possible genotypes along with the probability for each one Write the possible phenotypes along with the probability for each one Make sure the question has been answered What about the ratios?
Practice � What type of pea plant would one expect in their fields if they cross a homozygous plant with yellow peas and a heterozygous plant with yellow peas?
Practice (use chart on page 113) Write the genotype for: � Homozygous round pea � Heterozygous green pod � Homozygous white seed coat � Heterozygous green pea � A pea plant with constricted pods � A pea plant homozygous for colored seed coats � A pea plant homozygous for yellow pods � A pea plant heterozygous for round peas
Practice (use chart on page 113) Write the phenotype for: � Aa � Gg � RR � Cc
Practice (use chart on page 113) Show your work as you work the crosses 1. What offspring do you expect from two wrinkled peas of the pea plants? 2. What offspring would you expect from two pea plants that are both heterozygous for inflated pods?
Practice How would you determine if a black cat is homozygous (BB) or heterozygous (Bb) for coat color?
Pedigree Circle = female Square = male Shaded = dominant phenotype Unshaded – recessive phenotype ? = unsure