Mendel's Experiments and Genetic Hypotheses

• Linkage analysis forms the basis of genetic mapping.
• Genetic mapping relies on the principles of inheritance.

Mendel's Experiments

• Gregor Mendel, a 19th-century Czech monk, established the fundamental laws of genetic inheritance through breeding pea plants.
• Mendel chose simple, monogenic characteristics for his experiments.

Mendel's First Cross

• The parental (P1) generation involved crossing wrinkled seeds with smooth seeds.
• The first filial (F1) generation produced all smooth seeds upon crossing.
• The second filial (F2) generation showed a 3:1 ratio of smooth to wrinkled seeds.

Mendel's Genetic Hypothesis

• Genes exist in pairs, with each parent contributing two copies of a gene.
• The "A" form leads to smooth seeds, while the "a" form results in wrinkled seeds.
• Parents produce gametes (eggs, pollen) containing one copy of the gene.
• Fertilization yields the F1 generation, which is all smooth due to the dominance of the "A" form over "a."
• F1 plants produce equal numbers of A and a gametes, resulting in F2 plants with a 1/4 AA (smooth), 1/2 Aa (smooth), and 1/4 aa (wrinkled) ratio.

Cross with Two Genes

• Crossing two genes yields a 9/16 yellow/smooth, 3/16 green/smooth, 3/16 yellow/wrinkled, and 1/16 green/wrinkled ratio

Summary of Mendel's Experiments

• Genes in an organism come in pairs.
• Genes have forms called alleles, which can be dominant or recessive.
• One gene from each pair goes into a gamete (segregation).
• Gametes combine randomly during fertilization.
• Offspring types and numbers in a cross follow set laws.
• Separate genes should behave independently (exceptions do occur).

Mendel's Law of Inheritance

• Law of Segregation: gene pairs separate during gamete formation, so each gamete carries only one member of each pair.
• Law of Independent Assortment: during gamete formation, alleles of one gene segregate independently of alleles of other genes.

Bateson and Punnet

• In a dihybrid cross, the P1 is purple flower, long pollen grain (both are dominant)
• the P2 is a red flower, round pollen grain

Thomas Hunt Morgan Experiment

• The experiment was done on fruit flies (Drosophila)
• The traits are eye color and wing shape
• The P1 had red eyes and normal wings.
• The P2 had a mutant with purple eyes and vestigial wings.

Linkage and Crossing Over

• Linkage refers to two connected concepts:
  • Two or more genes can be on the same chromosome.
  • Close genes tend to be inherited as a unit.
• Chromosomes are called linkage groups.
• They includes a group of linked genes.
• The number of linkage groups equals the types of chromosomes in a species.
• Humans have autosomal linkage groups, an X chromosome linkage group, and a Y chromosome linkage group.
• Genes far apart on the same chromosome can independently assort through crossing-over during meiosis.
• It Occurs during prophase-I of meiosis
• Homologous chromosomes exchange DNA segments.

Partial Linkage and Complete Linkage

• Genes are arranged in a linear order on a chromosome.
• The order and distances can be mapped by crossing-over frequency.
• Linked loci that are syntonic do not show independent assortment.
• There is not a 9:3:3:1 ratio in F2 cross- not 1:1:1:1 ratios in test cross
• Closely linked genes are, e.g. 2%
• Partially linked genes are, e.g. 30%

Recombination Fractions

• A recombination event gives 1/2 parental type and 1/2 recombinant type offspring.
• Recombination fraction RF = R/R+P
• RF is between 0 and 0.5 (0 and 50%)
• The closer together the genes are, the smaller the RF
• RF = 0.5 for unlinked genes (very far apart or on different chromosomes)