Mendelian Genetics Or, what’s the story with this “Mendelian” guy?
Outline • Mendel’s laws • Segregation • Dominance and recessivity • Punnett squares • Independent Assortment • Unusual Patterns of Inheritance • Incomplete dominance and Codominance
Mendel’s Laws • Quantitative analysis of F2 plants • “selfed” offspring of outcrossed parents • The law of segregation • Describes dominance and recessivity • The law of independent assortment • We have already learned this…
EXPERIMENT The Law of Segregation P Generation (true-breeding parents) Purple flowers White flowers • P Generation • White flowered X purple flowered pea plants, all of the • F1 Generation (“hybrids”) • All purple flowered • F2 Generation • Mostly purple; some white • 3:1 ratio • Purple is “the dominant heritable factor” • white is recessive • “Heritable factor” is what we now call a gene F1 Generation (hybrids) All plants had purple flowers F2 Generation 224 white-flowered plants 705 purple-flowered plants
Why 3:1? Is it always 3:1? • NO. • Ratio depends on who is breeding with whom • RR x RR • RrxRr • rrxrr • RR x rr • Rr x rr • Punnett Square • Diagrams probabilities of inheritance Sperm F2 Generation p P P PP Pp Eggs p pp Pp 3 1
Useful Genetic Vocabulary • Homozygous: two identical alleles for a trait RR / rr • “homozygous dominant” or “homozygous recessive” • heterozygous : two different alleles for a trait Rr • “heterozygous for blah” “carrier” • An organism’s expressed traits do not always reveal its genetic composition • Phenotype: physical appearance (purple flowers) • Genotype: genetic makeup (Pp) • In the example of flower color in pea plants, PPand Pp plants have the same phenotype (purple) but different genotypes
Phenotype Ratio vs. Genotype Ratio Phenotype Genotype PP Purple 1 (homozygous) Pp 3 Purple (heterozygous) 2 Pp Purple (heterozygous) pp White 1 1 (homozygous) Ratio 3:1 Ratio 1:2:1
Mendel’s Model • 4-part hypothesis to explain the white flower weirdness 1) Two version of genes (different alleles) account for variations in inherited characters. • Different alleles vary in the DNA sequence at the specific locus of a gene. • Purple and white flower alleles are 2 DNA variations at the flower-color locus. Allele for purple flowers Homologous pair of chromosomes Modern take Locus for flower-color gene Allele for white flowers
Mendel’s Model 2) Organism inherits two alleles, one from each parent. • Diploid organism= one set of chrms from each parent. • Pair of homologous chrms = two copies of each locus. (Mendel made this deduction without knowing about the role of chromosomes) • The two alleles at a locus on a chromosome may be identical (Mendel’s P generation)… • …or they may differ (Mendel’s F1 hybrids) 3) If two alleles differ, the dominant alleleis fully expressed in the organism’s appearance. • Recessive allele has no effect on the organisms appearance. • e.g., F1 plants had purple flowers
4) 2 alleles segregate (separate) during gamete production. • Mendel’s Second Law (the Law of segregation) • Corresponds to the distribution of homologous chromosomes to different gametes in meiosis. • Each allele exists as a single copy in all gametes. During meiosis I, tetrads can line up two different ways before the homologs separate. OR Mendel had neither an electron microscope nor a crystal ball, and yet…
How Can One Tell the Genotype of an Individual Expressing the Dominant Phenotype? • Testcross • Breed themystery individual with a homozygous recessive individual • R? x rr • Do any offspring display the recessive phenotype? • the mystery parent must be heterozygous
Fig. 14-7 Testcross Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Predictions If PP If Pp or Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs P p pp Pp pp Pp RESULTS or All offspring purple 1/2 offspring purple and 1/2 offspring white
Mendelian Inheritance Reflects Rules of Probability • Tossing coins or rolling dice • The probability of tossing heads is 1/2 • The probability of rolling a 3 with a six-sided die is 1/6, and the probability of rolling any other number is 1 - 1/6 = 5/6 • Tossing a coin has no impact on the outcome of the next toss • Each toss is an independent event • Gamete from a heterozygous parent has a 50% chance of carrying the dominant allele and a 50% chance of carrying the recessive
Complex Inheritance Patterns • Genotype/phenotype • Commonly not as simple as pea plants • Deviations from simple Mendelian patterns: • alleles are not completely dominant or recessive • a gene is carried on the X chromosome • a gene has more than two alleles • a gene produces multiple phenotypes
Sex-Linked Traits • Traits encoded on Chromosome X • Females have 2 copies (XX) • Males have 1 (XY) • Mothers always donate X • fathers determine offspring sex (donate X or Y) • Females can be carriers • Males CANNOT be silent carriers Y Xc XCXC XcY XCXc XCY XC Normal vision female color-blind male XCXc XCY XC
Degrees of Dominance P Generation Red White CRCR CWCW • Complete dominance • phenotypes of the heterozygote and dominant homozygote are identical • Law of segregation • Incomplete dominance • phenotype of F1 hybrids is intermediate between the P phenotypes CR CW Gametes Pink F1 Generation CRCW 1/2 1/2 CR CW Gametes Sperm 1/2 1/2 CR CW F2 Generation 1/2 CR CRCW CRCR Eggs 1/2 CW CRCW CWCW
Codominance X Two dominant alleles affect the phenotype in separate ways white bull brown cow X Roan calf white horse grey horse X white camellia pink camellia Appaloosa horse hybrid camellia
The Law of Independent Assortment • Law of Segregation • follows a single trait • monohybrid cross (e.g., flower color) • Law of Independent Assortment • Follow two traits • dihybrid cross (e.g., seed color and seed shape) • Known information about pea plants from Law of Segregation: • Yellow seed allele (Y) is dominant to green seed allele (y). • Round seed allele (R) is dominant to wrinkled seed allele (r) • If you cross homozygous plants (YYRR x yyrr), what do you predict would happen?
Law of Independent Assortment • Do the traits always stay together? • YR and yr? • If so… • F1 and F2 offspring would still produce yellow, round seeds • Doesn’t happen! • What is the explanation for this?
2 pairs of alleles segregate independently- • Encoded on two different chromosomes • The presence of one specific allele for one trait has no impact on the presence of a specific allele for the second trait • F1 offspring yellow, round seeds. • F1gametesall possible allelic combinations • YR, Yr, yR, and yr would be produced in equal amounts.
EXPERIMENT YYRR yyrr P Generation Gametes yr YR F1 Generation YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Predictions Sperm or Predicted offspring of F2 generation 1/4 1/4 1/4 yr 1/4 YR yR Yr Sperm YR yr 1/2 1/2 1/4 YR YYRr YYRR YyRR YyRr 1/2 YR YyRr YYRR 1/4 Yr Eggs YYRr YYrr Yyrr YyRr Eggs 1/2 yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 Yyrr yyRr YyRr yyrr 3/16 1/16 9/16 3/16 Phenotypic ratio 9:3:3:1 RESULTS Phenotypic ratio approximately 9:3:3:1 315 108 101 32