Ch 14 Gregor Mendel and Inheritance Study Began research late 1850’s St. Thomas (Augustinian) monastery, Brno, Yugoslavia

What genetic principles account for the passing of traits from parents to offspring? Mendel’s pea breeding data supported the particulate mechanism 1. “blending” hypothesis –genetic material from the two parents blends together (like blue and yellow paint blend to make green) 2. “particulate” hypothesis –parents pass on discrete heritable units (genes)

Scientific approach Asked question: how are traits inherited? Used convenient model system: garden peas -controlled breeding -a variety of either/or visible traits -quantitative data Derived conclusions based on the data

For example To examine inheritance of flower color Cross-fertilized (hybridized): true breeding purple x true breeding white Measurable character: phenotype

LE 14-2 Removed stamens from purple flower Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower Carpel Stamens Parental generation (P) Pollinated carpel matured into pod Planted seeds from pod Examined offspring: all purple flowers First generation offspring (F 1 ) Materials & Methods 100% purple phenotype

“True-breeding” plants that produce offspring of the same variety when they self-pollinate

True-breeding parents –P (generation) Hybrid offspring (cross between 2 different true-breeding parents) –F 1 (generation) Progeny of F 1 self-pollination –F 2 (generation) Nomenclature

LE 14-3 P Generation (true-breeding parents) F 1 Generation (hybrids) F 2 Generation Purple flowers White flowers All plants had purple flowers Results from two crosses F 1 x F 1 F2F2 P1 x P2

Only purple flowers in the F 1 hybrids –Purple: dominant trait –White: recessive trait Same pattern of dominant inheritance - six other pea plant phenotypes, each represented by two traits Mendel’s “heritable factors” (= genes) (though he did not know the existence of DNA) Conclusions

Mendel’s Model Inheritance patterns obeyed rules of probability 1.Assumes each trait has two variants Current knowledge: phenotype is controlled by genes (genotype) paternal and maternal alleles (genes)=two variants positioned at chromosomal locus

LE 14-4 Allele for purple flowers Homologous pair of chromosomes Allele for white flowers Locus for flower-color gene

2. Offspring inherit two alleles, one from each parent P= purple (dominant allele) p= white (recessive allele) Homozygous if PP or pp (noun; homozygote) Heterozygous if Pp (noun: heterozygote) PP=_____________________________ pp=___________________________ 3. One of the alleles determines phenotype

4. Law of segregation two alleles for any given phenotype separate during gamete formation. Thus, offspring inherit one allele from each parent. During anaphase I When do alleles first separate from each other?

Convenient quantitative way to predict outcome of crosses Punnett Square 1.Name alleles with single letter 2.Write down genotype of each parent 3.Fill in boxes with all possible combinations of alleles according to parental genotypes On Board

What is the frequency of genotypes of the offspring of the parental cross? What is the frequency of phenotypes of the offspring of a parental cross? What is the frequency of genotypes of the F 1 cross progeny? What is the frequency of phenotypes of the F 1 cross progeny? 50% heterozygous; 25% dominant and recessive homozygous 75% purple, 25% white (3:1) 100% heterozygous 100% purple

LE 14-5_2 Appearance: P Generation Genetic makeup: Gametes F 1 Generation Appearance: Genetic makeup: Gametes: F 2 Generation Purple flowers Pp P p P p F 1 sperm F 1 eggs PPPp pp P p 3: 1 Purple flowers PP White flowers pp P p

LE 14-6 Phenotype Purple 3 Genotype PP (homozygous Pp (heterozygous Pp (heterozygous pp (homozygous Ratio 1:2:1 White Ratio 3:1 1

How can we determine the genotype of an individual with a dominant phenotype? Genotype possibilities (purple flower color)? PP or Pp Test cross Hybridize to individual with homozygous recessive individual (pp) Do Punnett for each. Outcome?

LE 14-7 Dominant phenotype, unknown genotype: PP or Pp? If PP, then all offspring purple: pp P P Pp If Pp, then 1 2 offspring purple and 1 2 offspring white: pp P P pp Pp Recessive phenotype, known genotype: pp

Monohybrid vs dihybrid Heterozygous for 1 characterHeterozygous for 2 characters Set up a cross between heterozygous purple flowered pea plants Pp x Pp Do a Punnett Square: determine genotypic and phenotypic ratios.

What are patterns of inheritance 2 characters? Given: true breeding P for pea color & shape Dominant characters Yellow Round Recessive characters green wrinkled Set up Parental Cross Set up F1 cross What is phenotypic frequency of the offspring?

LE 14-8 P Generation F 1 Generation YYRR Gametes YR yr yyrr YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Sperm Eggs YR Yr yrYR yr Eggs YYRRYyRr yyrr yR yr Phenotypic ratio 3:1 F 2 Generation (predicted offspring) YYRR YYRrYyRRYyRr YYRrYYrrYyRrYyrr YyRRYyRryyRRyyRr YyRrYyrryyRryyrr Phenotypic ratio 9:3:3:1 YRYryRyr Sperm

Observation of a dihybrid cross - Each pair of alleles segregates independently of other pairs of alleles during gamete formation - Mendel proposed the law of independent assortment Note: Applies only to genes on different, nonhomologous chromosomes Genes located near each other on same chromosome tend to be inherited together: linkage (Fast forward) Established at metaphase I & II

Autosomes (22 pr) Sex chromosomes

Inheritance of Sex-Linked Genes Sex chromosomes Carry genes that determine sexual phenotype & other traits Sex-linked genes: Any gene on sex chromosome (usually refers to X-chromosome because of bigger size, more genes)

LE Sperm Ova Sperm Ova Sperm Ova How to show inheritance of sex-linked traits: follow X and Y chromosomes in a cross

Sex-linked recessive disorders –Color blindness –Duchenne muscular dystrophy –Hemophilia Present hemophilia problem Mother carrier/ father normal

We’re celebripeas! Questions?