Mendel's Test Cross Quiz

Genetic Mapping and Recombination

• Alfred Sturtevant utilized the frequency of recombination between gene pairs to create genetic maps, demonstrating the relative distances between genes on chromosomes.
• The gene pair that showed very low recombination in Morgan's experiments was the white and yellow color genes in fruit flies, indicating they are linked closely on the same chromosome.
• Genetic maps are significant for whole genome sequencing as they provide a reference framework for understanding gene location and function, aiding in the identification of genetic disorders.

Contributions of Morgan and Sturtevant

• Alfred Sturtevant mapped the locations of genes on chromosomes, contributing to the foundation of linkage mapping in genetics.
• In Morgan's experiments, the gene pair that exhibited higher recombination was the body color and wing size genes, showing they are farther apart on the chromosome.

Mendelian Genetics

• Mendel primarily studied traits such as seed shape and color, focusing on discrete phenotypic characteristics in pea plants.
• A plant that is heterozygous for genes controlling one character is called a dihybrid, showcasing different alleles for the different traits.

Test Cross Purpose

• A test cross is performed to determine the genotype of an organism with a dominant phenotype by crossing it with a homozygous recessive parent, revealing the alleles passed to the offspring.
• It is not possible to predict the genotypic composition of a tall plant (TT or Tt) based solely on phenotype because two different genotypes can result in the same tall phenotype.

Allele Separation and Cross Outcomes

• When tall (TT) and dwarf (tt) plants produce gametes, the alleles separate during meiosis, with each gamete receiving one allele from each parent.
• The result of crossing a tall (TT) plant with a dwarf (tt) plant will yield all offspring with the tall phenotype (Tt), but their exact genotype remains uncertain in all cases.

Test Cross Details

• In a test cross, the dominant phenotype organism is typically crossed with a recessive parent to determine its genotype.
• Mendel performed a test cross using true breeding recessive strains to track inheritance patterns in subsequent generations.

Genetic Probabilities and Predictive Tools

• The probability of a gamete from a heterozygous Tt plant containing the 'T' allele is 50%, as each allele has an equal chance of being passed on.
• The Punnett Square aids in visualizing the potential combinations of alleles and the resulting genotypes of offspring from genetic crosses.
• Mendel proposed two general rules (law of segregation and law of independent assortment) based on his observations of monohybrid crosses to explain hereditary patterns.

Historical Figures in Genetics

• The Punnett Square was developed by British geneticist Reginald Punnett and has become a fundamental tool in genetics.
• From the progeny of a test cross, the genotype of the test organism can be inferred, as homozygous recessive offspring will reveal the presence or absence of dominant alleles.