Genetics: Monohybrid & Dihybrid Crosses

The Science of Genetics

• Heredity is the transmission of traits from one generation to the next
• Genetics began in the 1860s with Gregor Mendel, an Augustinian monk.
• Mendel deduced genetics' fundamental principles by breeding garden peas.
• He lived and worked in an abbey in Brunn, Austria (now Brno, Czech Republic).
• In 1866, Mendel argued that parents pass discrete "heritable factors" to offspring. These are now known as genes.
• Genes retain individuality across generations.
• Mendel's paper appeared seven years after Darwin's "The Origin of Species".
• Mendel chose to study garden peas as they have short generation times and produce many offspring per mating.
• Pea plants come in easily distinguishable varieties like purple or white flowers
• A heritable feature that varies among individuals is a character; its variant is a trait.
• Pea plant mating can be strictly controlled allowing manipulation of breeding experiments

Mendel's Breeding Techniques

• Pea plants typically self-fertilize in nature with pollen from the stamens landing on the carpel of the same flower.
• Mendel ensured self-fertilization by covering flowers with small bags to prevent external pollen.
• For cross-fertilization, he cut off immature stamens and dusted the carpel with pollen from another plant.
• The carpel developed into a pod containing seeds, which produced offspring plants (F1).

Mendel's Experimental Approach

• Mendel's success was due to his approach, organism choice, and character selection.
• He observed seven characters, each with two distinct traits.
• Mendel used true-breeding varieties that consistently produced the same traits over generations of self-pollination.
• Mendel crossed true-breeding varieties to observe hybrid offspring.
• Hybridization or genetic cross involves true-breeding parents (P generation) and their hybrid offspring (F1 generation).
• When F1 plants self-fertilize or fertilize each other, the offspring is the F2 generation.
• Mendel's analysis of thousands of genetic crosses helped him deduce the fundamental principles of heredity.
• P generation is your grandparents, the F₁ your parents, and the F2 is you (and any siblings).

Key Terms in Genetics

• Self-fertilization is when a plant's own pollen fertilizes its eggs, resulting in identical offspring.
• Hybrids are offspring from mating individuals of different species or true-breeding varieties.
• F2 generation is the offspring of the F1 generation. F2 stands for second filial.
• Cross-fertilization is mating two sexually reproducing individuals in a controlled genetic experiment.
• P generation is the parent individuals in studies of inheritance. Parental is what P stands for.
• F1 generation is the offspring of two parental (P generation) individuals. F1 stands for filial.
• Hybridization is the process of crossing two different true-breeding varieties to produce offspring with traits from both.

Mendel's Law of Segregation

• Mendel tracked the inheritance of characters with two forms (e.g., flower color).
• He crossed true-breeding pea plants with purple and white flowers, resulting in only purple flowers in F1 plants.
• To see if the white-flower trait was lost, he mated the F1 plants with each other.
• In the F2 generation, he observed a 3:1 ratio of purple to white flowers, concluding that the white-flower factor was masked in F1 plants.
• In the F2 generation, 705 plants were purple and 224 were white out of 929 plants.
• F1 plants carried two factors for flower color.

Mendel's Hypotheses About Inheritance

• Alternative Versions of Genes: Genes have alternative versions called alleles, accounting for variations in traits.
• Inheritance of Alleles: Organisms inherit two alleles for each gene, one from each parent.
• Homozygous: Two identical alleles.
• Heterozygous: Two different alleles.
• Dominant and Recessive Alleles: When alleles differ, the dominant allele determines appearance, while the recessive allele has no noticeable effect.
• Law of Segregation: During gamete production, allele pairs separate so each sperm or egg carries only one allele for each trait.
• At fertilization, alleles from each parent combine, restoring the paired condition in the offspring.
• Mendel's law of segregation states that allele pairs separate during gamete formation.
• In the F1 generation, plants have one purple-flower allele (P) and one white-flower allele (p).
• During gamete formation in the F1 plants, half of the gametes receive the purple-flower allele (P), and the other half receive the white-flower allele (p).
• During pollination among the F1 plants, the gametes unite randomly.
• Possible Combinations include:
  • PP (purple flowers)
  • Pp (purple flowers)
  • pP (purple flowers)
  • pp (white flowers)
• Three out of four combinations result in purple flowers, and one results in white flowers in F2
• The expected ratio of purple to white flowers in the F2 generation are 3:1.
• The Punnett square visually represents the four possible combinations of gametes and the resulting offspring in the F2 generation.
• A quarter of F2 plants have two alleles for purple flowers and will have purple flowers.
• Half of the plants have one allele for purple flowers and one for white flowers and will also have purple flowers, as purple is the dominant trait.
• A quarter of the plants have two alleles for white flowers and will have white flowers.
• This results in a 3:1 ratio of purple to white flowers in the F2 generation.
• Geneticists distinguish between an organism's observable traits (phenotype) and its genetic makeup (genotype).
• In the F2 generation, the phenotypic ratio of purple to white flowers is 3:1, and the genotypic ratio is 1 PP:2 Pp:1 pp.

Chromosomes and Alleles

• Every diploid cell has pairs of homologous chromosomes.
• Chromosomes carry alleles of the same genes at the same locations where one chromosome comes from each parent.
• The transmission of genetic traits exemplifies the theme of information.
• Each labeled band on the chromosomes represents a gene locus.
• Alleles of a gene reside at the same locus on homologous chromosomes and may be identical (homozygous) or different alleles (heterozygous).
• Connection illustrates Mendel's law of segregation where chromosomal locus will be discussed later.
• An individual is heterozygous (Bb) where each gamete formed has either the B or b allele.
• The B and b alleles are located at the same gene locus separate during meiosis I, and are packaged in separate gametes during meiosis II.
• Heterozygous: having two different alleles for a gene.
• Homozygous: having two identical alleles for a gene.
• Dominant allele: determines phenotype of a gene when individual is heterozygous.
• Punnett square: diagram used in the study of inheritance to show results of random fertilization.
• Principle of segregation: general inheritance rule of having two alleles for each gene that form gametes during meiosis.
• Alleles can also be seperated by monohybrid cross when experimentally matting individuals.
• Genotype refers to the individual's genetic makeup.

Independent Assortment

• Mendel deduced his law of segregation by following one character through two generations.
• A monohybrid cross involves two heterozygous individuals for one character.
• Certain alleles are dominant, like the round allele (R) over wrinkled (r), and yellow (Y) over green (y).
• Mendel wondered what would happen if he crossed plants that differed in both seed shape and seed color.
• Mendel crossed homozygous plants with round yellow seeds (RRYY) and wrinkled green seeds (rryy) to produce only RY and ry gametes.
• The F1 generation would consist of hybrids heterozygous for both characters (RrYy), known as dihybrids that display the round yellow seed phenotype.
• For the F2 generation, Mendel crossed RrYy F1 plants determine if seed color and shape genes would be transmitted (dependent assortment) or separately (independent assortment).
• This dihybrid cross involved two heterozygous organisms.
• The alleles for seed color and shape would be inherited together, as they came from the P generation, or independently.
• The dependent assortment hypothesis predicted that each F2 plant would inherit one of two possible sperm (RY or ry) and one of two possible eggs (RY or ry), resulting in four combinations.
• Mendel’s actual results did not match this prediction, refuting the hypothesis
• The independent assortment hypothesis predicted that F1 plants would produce four different gametes (RY, rY, Ry, ry).
• Each F2 plant would receive one sperm and one egg resulting in 16 combinations.
• This fertilization would lead to four different seed phenotypes (round yellow, round green, wrinkled yellow, wrinkled green) in a 9:3:3:1 ratio.
• The 9:3:3:1 ratio indicates alleles segregate independently of the others.
• This is shown in a 4x4 Punnett square for the dihybrid cross in the F2 generation
• This also applied the the same way in a Labrador with different coat and eye colors.

Gene Mapping

• The inheritance of one character does not affect the inheritance of another. This concept is known as Mendel's law of independent assortment.
• F1 Generation: Focus on a single trait in a monohybrid cross: Mendel observed the dominant and recessive alleles for one characteristic.
• F2 Generation: Expanded focus to at least two traits in a dihybrid cross where he examined how these traits assorted independently.
• This transition helped Mendel uncover the principles of inheritance and independent assortment.
• With from the 9:3:3:1 ratio, it was observed that there are 12 plants with round seeds to 4 with wrinkled seeds, indicating monohybrid crosses result for each character
• Testing various dihybrid combinations showed data consistently close to the predicted 9:3:3:1 ratio for independent assortment

Chromosome Theory of Inheritance

• An individual’s genotype can be found by testcross (mating the individual with an unknown genotype with a homozygous recessive individual).
• Mendel published results in 1866 were that the chromosome theory relates the behavior of chromosomes to Mendel's "heritable factors" (genes).
• Genes are located on chromosomes, and their segregation/independent assortment explains inheritance patterns.
• Observing the chromosomal basis of Mendel's laws can be shown by tracking two genes (seed shape and seed color) during meiosis and fertilization in pea plants
• During metaphase I of meiosis, the alleles segregate as the homologous chromosomes separate in anaphase I: gametes end up with a single long chromosome with one of the alleles
• Fertilization then recombines the alleles.
• Mendel’s laws are based on how genes behave during meiosis demonstrated: pairs of alleles (different versions of a gene) separate from each other during gamete production as homologous chromosomes separate during anaphase I.
• Independent Assortment’s states that the way one pair of alleles segregates is independent of the way another pair of alleles segregates
• This is due to the random orientation of different chromosome pairs during metaphase I of meiosis.
• Chromosome theory is a basic principle in biology that genes are located on the chromosomes and that there activity during meiosis explains the inheritance patterns

Linked Genes

• Genes on the same chromosome tend to be inherited together, meaning they can not follow Mandel's law.
• A heterozygous plant mainly produces PL and pl gametes rather than equal numbers of four types,
• The purple long and red round traits were due to fertilization of those gametes
• Genes in closer proximity on the same chromasome are referred to as being linked together

Genetic Variation

• Crossing over creates genetic variation by producing recombinant gametes.
• Proximity determines how likely genes are to be separated by crossing over, which determines their likelihood to be inherited or independently.
• Fruit Flies (Drosophila melanogaster) were genetically researched as they breed quickly, are inexpensive, and produce new generations every two weeks
• A wild-type (most common traits) fruit fly crossed with a mutant (less common traits)
• A testcross can reveal if the genes are linked based on phenotypic rations.and the testcross reveals mostly parental phenotypes, and a percentage shown via recombinant observation.
• Morgan cross allowed for observations of recombinant chromosomes .

Genetic Maps

• Sturtevant developed a method using crossover data from fruit flies.
• A genetic map shows an ordered list of genetic loci along a chromosome.
• The farther apart two genes are on a chromosome, the more likely crossover events will occur.
• Recombination data can determine the genes' positions on chromosomes.
• Mapping genes demonstrated sequence: Cinnabar is roughly midway between black body is on the short.
• Linkage mapping helps determine the relative positions of genes in various organisms without needing sophisticated equipment
• By designing Drosophila crosses to provide recombination data for a new gene, the likely location can be found

Explore monohybrid crosses, dihybrid crosses, and genetic recombination frequencies. Understand independent assortment and gene mapping. Determine gene location based on recombination frequencies.