Genetics Fundamentals

• Genetics is a branch of biology focused on studying heredity.
• Heredity is the biological process where parents pass certain genes to their offspring.

Fertilization

• Fertilization is the fusion of sperm and egg cells, leading to the formation of a zygote.
• Sperm and egg cells each contain 23 chromosomes.
• When combined, the zygote contains 46 chromosomes.

Traits and Characteristics

• A trait is a general attribute of an individual.
• A characteristic is a distinctive quality of a certain organism.
• Hair color is an example of a trait.
• Black or brown hair is an example of characteristics.
• Traits can be dominant or recessive.

Dominant and Recessive Genes

• Dominant genes mask other genes.
• Recessive genes are masked or hidden by dominant genes.
• Examples of dominant traits include cleft chin, widow's peak, dimples, brown or black hair, brown eyes, freckles, and free earlobes.
• Examples of recessive traits include having no cleft chin, no widow's peak, no dimples, blonde hair, grey or blue eyes, no freckles, and attached earlobes.

Traits and Genes

• Traits and characteristics are dictated by genes.
• Genes are fundamental units of heredity in living organisms.
• Genes consist of DNA and are responsible for coding instructions for building and maintaining cells and passing genetic information from one generation to the next.

Alleles

• An allele is a variant form of a gene that arises by mutation and is found at a specific place on a chromosome.
• Alleles contribute to the genetic diversity within a population by introducing variations in traits.

Homozygous and Heterozygous Genes

• Genes can be homozygous or heterozygous.
• Paired chromosomes have the same kind of genes but may be different alleles.
• Homozygous dominant individuals have two dominant alleles (AA).
• Homozygous recessive individuals have two recessive alleles (aa).
• Heterozygous individuals have different alleles (Aa).

Genotype vs Phenotype

• Genotype is an organism's genetic information.
• Phenotype is the set of observable physical traits.
• The homozygous dominant genotype (BB) results in a specific observable trait (purple).
• The heterozygous genotype (Bb) also results in a the same observable trait (purple).
• The homozygous recessive genotype (bb) results in a different observable trait (white).

Gregor Johann Mendel

• Gregor Johann Mendel excelled in physics and mathematics and completed his studies in 1843.
• He chose a different solution to his predicament, entering the Altbrünn monastery as a novitiate of the Augustinian order, where he was given the name Gregor.
• In 1850, Mendel failed an exam for teacher certification and was sent to the University of Vienna for two years to benefit from a new program of scientific instruction.
• Mendel conducted his experiments in genetics using the edible pea (Pisum sativum).
• Mendel chose the plant because of the distinct varieties, the ease of culture and control of pollination, and the high proportion of successful seed germinations.
• From 1854 to 1856, he tested 34 varieties for constancy of their traits.

Why Garden Pea Plant?

• The garden pea plant is easy to grow and cultivate.
• The garden pea plant has a short generation time.
• The garden pea plant reproduces by self-fertilization and cross-pollination.
• The garden pea plant has seven distinct contrasting traits.

Seven Distinct Traits of Pea Plant

• The seven distinct traits include seed shape (round or wrinkled), seed color (yellow or green), and flower color (purple or white).
• The seven distinct traits include pod shape (inflated or constricted), pod color (yellow or green), flower position (axial or terminal), and stem height (tall or dwarf).

Gregor Mendel's Pea Plant Experiment

• Mendel chose seven traits expressed in a distinctive manner to study the transmission of characters.
• These include plant height (short or tall) and seed color (green or yellow).
• Mendel crossed varieties that differed in one trait, such as tall crossed with short.
• The first generation of hybrids (F1) displayed the character of one variety but not that of the other.
• One character was dominant and the other recessive.
• In the second generation (F2), the recessive character reappeared.
• The proportion of offspring bearing the dominant trait to offspring bearing the recessive trait was very close to a 3:1 ratio.
• One-third of descendants (F3) of the dominant group were true-breeding, and two-thirds were of hybrid constitution.
• The 3:1 ratio could be rewritten as 1:2:1, such that 50% of the F2 generation were true-breeding, and 50% were still hybrid.

Law of Segregation of Genes

• Every individual organism contains two alleles for each trait, and these alleles segregate (separate) during meiosis, such that each gamete contains only one of the alleles.
• An offspring receives a pair of alleles for a trait by inheriting homologous chromosomes from the parent organisms, meaning one allele for each trait from each parent.

Law of Dominance

• The presence of an allele doesn't guarantee that the trait is expressed in the individual.
• If the two alleles of an inherited pair differ, that means in a heterozygous condition, one allele determines the organism's appearance and is called the dominant allele.
• The other has no noticeable effect on the organism's appearance and is called the recessive allele.
• The dominant allele hides the phenotypic effects of the recessive allele.

Law of Independent Assortment

• Alleles for separate traits are passed independently of one another.
• That indicates that the biological selection of an allele for one trait has nothing to do with the selection of an allele for any other trait.
• Mendel found support for this law in his dihybrid cross experiments.
• In his monohybrid crosses, there was an idealized 3:1 ratio between dominant and recessive phenotypes.
• In dihybrid crosses, however, he found a 9:3:3:1 ratio.

Monohybrid

• Monohybrid is a type of Punnett square that crosses one trait only.

Dihybrid

• Dihybrid is a type of Punnett Square where crosses 2 traits at the same time.

Trihybrid

• Trihybrid is a type of Punnett Square where crosses 3 traits at the same time.