Mendel's Genetics

Alleles

• An individual's complete set of alleles makes up their genotype, while the products of what's encoded by those alleles make up their phenotype.
• Several alleles may interact with one another in different ways to give a specific phenotype.
• Different alleles arise through mutations, which are changes in the gene's nucleotide sequence.
• Mutations may arise through:
• Errors in replication;
• Environmental factors;
• Chemicals, etc.

Genotype and Phenotype

Genotype: allele combinations that you have

• Two of the same alleles: homozygous Example: allele A and A
• Two different allele: heterozygous Example: allele A and a

Phenotype: physical trait that results due to the allele combination that you have

• If you have two AA, you have brown eyes.
• If you have an A and a (Aa), you have brown eyes.
• If you have aa, you have blue eyes.
• Therefore, two genotypes (AA and Aa) can result in one phenotype (brown eyes).

History of Mendelian Genetics

• Gregor Mendel (1822-1884) was a pioneer in the study of genetics.
• Laws of inheritance derived by hybridization experiments using garden peas.
• Hybridization is the interbreeding between two different varieties of an organism.
• Mendel followed a small number of easily recognizable traits.

• Mendel's strains were true breeding, meaning that the offspring's physical appearance is identical to that of the previous generation. Example: Plants with yellow seeds only produce plants with yellow seeds, plants with purple flowers only produce plants with purple flowers, etc.

Mendel's Objective

1. The use of true breeding strains rather than complicated and poorly characterized ones.
2. A test cross occurs when you breed an organism with one that is homozygous recessive.
3. Focusing on just one or a few traits at a time.
4. The counting of progeny and looking for statistical patterns.

• P1 Generation: refers to the parental generation.
• F1 Generation: refers to the first offspring generation.

Mendel's Law of Segregation

• Mendel's most important discovery was that the F1 progeny from parental strains with different traits were not true-breeders.
• True-breeding organisms are those that always pass down their phenotypic traits to its offspring.
• F2 Generation comes from the F1 generation self-fertilizing.
• Mendel found that the recessive trait reappeared in the F2 generation.
• The recessive trait always consistently appeared at a dominant:recessive ratio close to 3:1.

The Principle of Segregation

• Each cell in a pea plant contains two alleles of each gene (one from mom, one from dad) except for in reproductive cells, which contain only one allele.
• In true-breeding parental strains, the two alleles are identical. These plants are said to be homozygous.
• Each gamete (reproductive cell) contains just one allele of each gene.
• When gametes are formed, the two alleles of a gene segregate – half of the gametes get one allele, while the other half get the other allele. This is the principle of segregation.
• In homozygous plants, all the gametes will have the same allele.

• A zygote is formed after fertilization, and it is comprised of the union of two gametes – one from each parent.
• When the two gametes that formed the zygote carried different alleles for a gene, the resulting zygote is a hybrid containing two different alleles and is said to be heterozygous.
• These progeny then form gametes, and the alleles again segregate.

• The zygote (F1) will all be yellow, since this is the dominant allele.
• Self-fertilizing the F1 generation gives rise to F2. The resulting progeny of the F2 generation can be determined using a Punnett square.
• Note that the yellow seeds in the F2 generation have the same phenotype but different genotypes (can be AA or Aa at a ratio of 1AA:2Aa).
• Plants with the AA or Aa genotypes can be distinguished based on the seeds they produce when self-fertilized.
• Plants with the AA genotype produce only yellow seeds, while those with the Aa genotype produce seeds with a ratio of 3:1 (yellow:green).

• A testcross can also be used to test segregation:
• Cross the F1 progeny with the true-breeding recessive strain instead of allowing them to self-fertilize.
• The recessive trait will only be observed if the F1 strain is heterozygous.

Inheritance Terminology

• B and b
• B and B'
• B1 and B2

• Alleles for a given gene affect the same character but can specify different traits.

• Mendel’s Law of Segregation – two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes.
• Members of an allelic pair segregate during meiosis I and come together at fertilization; each gamete carries one allele of each allelic pair.

Mendel’s Model Quick Summary:

Important Notes on Genetics:

• Alleles usually remain unchanged, although rarely they can be mutated.
• A character can be affected by many genes and also the environment (i.e. one allele pair doesn’t always define the trait of a character, sometimes multiple genes are required to fully determine a characteristic).
• Gene expression of the genes of the genome are what cause the resultant phenotype.
• In a real-world population (mating random) pairs of genotypes mate with frequencies that depend on their proportions in the population