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 F 2 plants – “selfed” offspring of outcrossed parents The law of segregation – Describes dominance and recessivity The law of independent assortment – We have already learned this…

The Law of Segregation P Generation White flowered X purple flowered pea plants, all of the F 1 Generation (“hybrids”) All purple flowered F 2 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 EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F 1 Generation (hybrids) All plants had purple flowers F 2 Generation 705 purple-flowered plants 224 white-flowered plants

NO. NO. Ratio depends on who is breeding with whom – RR x RR – Rr x Rr – rr x rr – RR x rr – Rr x rr Punnett Square – Diagrams probabilities of inheritance F 2 Generation Sperm Eggs P P PPPp p p pp 31 Why 3:1? Is it always 3:1?

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” do not always An organism’s expressed traits do not always reveal its genetic composition purple flowers Phenotype: physical appearance (purple flowers) Pp Genotype: genetic makeup (Pp) PPPp phenotypegenotypes In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes Useful Genetic Vocabulary

Phenotype Purple 3 Genotype 1 White Ratio 3:1 (homozygous) (heterozygous) PP Pp pp Ratio 1:2: Phenotype Ratio vs. Genotype Ratio

Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers 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. Modern take

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 F 1 hybrids) dominant allele 3) If two alleles differ, the dominant allele is fully expressed in the organism’s appearance. – Recessive allele has no effect on the organisms appearance. – e.g., F 1 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 the mystery individual with a homozygous recessive individual – R? x rr Do any offspring display the recessive phenotype? – the mystery parent must be heterozygous

Fig RESULTS Dominant phenotype, unknown genotype: PP or Pp? Predictions Recessive phenotype, known genotype: pp  If PPIf Pp or Sperm ppp p P P P p Eggs Pp pp or All offspring purple 1 / 2 offspring purple and 1 / 2 offspring white Testcross

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 XCYXCYXCXcXCXc XCXcXCXc XCYXCY XCXC XcXc XCXC Y XCXCXCXC XcYXcY color-blind maleNormal vision female

Degrees of Dominance Complete dominance – phenotypes of the heterozygote and dominant homozygote are identical – Law of segregation Incomplete dominance – phenotype of F 1 hybrids is intermediate between the P phenotypes Red P Generation Gametes White CRCRCRCR CWCWCWCW CRCR CWCW F 1 Generation Pink CRCWCRCW CRCR CWCW Gametes 1/21/2 1/21/2 F 2 Generation Sperm Eggs CRCR CRCR CWCW CWCW CRCRCRCR CRCWCRCW CRCWCRCW CWCWCWCW 1/21/2 1/21/2 1/21/2 1/21/2

Codominance Two dominant alleles affect the phenotype in separate ways white horse grey horse Appaloosa horse X white bull brown cow Roan calf X X pink camellia white camellia 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… – F 1 and F 2 offspring would still produce yellow, round seeds Doesn’t happen! What is the explanation for this?

2 pairs of alleles segregate independently- two different chromosomes – Encoded on two different chromosomes has no impact – The presence of one specific allele for one trait has no impact on the presence of a specific allele for the second trait F 1 offspring  yellow, round seeds. F 1 gametes  all possible allelic combinations – YRYryRyr – YR, Yr, yR, and yr would be produced in equal amounts.

EXPERIMENT RESULTS P Generation F 1 Generation Predictions Gametes Hypothesis of dependent assortment YYRRyyrr YR yr YyRr  Hypothesis of independent assortment or Predicted offspring of F 2 generation Sperm YR yr Yr YR yR Yr yR yr YR YYRR YyRr YYRr YyRR YYrr Yyrr yyRR yyRr yyrr Phenotypic ratio 3:1 Eggs Phenotypic ratio 9:3:3:1 1/21/2 1/21/2 1/21/2 1/21/2 1/41/4 yr 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 3/43/4 9 / 16 3 / 16 1 / 16 9:3:3:1 Phenotypic ratio approximately 9:3:3: