Fundamentals of Genetics

Gregor Mendel He his famous for his work with pea plants It was the center of his research on heredity Heredity: the transmission of characteristics from parents to offspring Austrian monk. Was put in charge of the gardens at the monastery. Studied science and math at a nearby university. Applied what he learned about statistics to the garden.

Gregor Mendel Mendel noted 7 characteristics of pea plants Each occurred in two different traits – genetically determined variations of characteristics We have traits too…cleft chin, attached earlobes, hitchhiker thumb that occur in those 2 variations

Particulate Hypothesis of Inheritance Parents pass on to their offspring through separate and distinct factors that are responsible for inherited traits.

Mendel’s Methods Self-pollination occurs when the pollen is transferred to the stigma of that flower or another on the same plant Cross-pollination occurs between the flowers of 2 different plants Why pea plants? They’re easy to grow and have many varieties. Pollination occurs when pollen grains from the male plant are transferred to the stigma of the female plant. He could easily control how pea plants were pollinated Mendel was able to select parent plants with specific traits and then observed which traits appeared in the offspring plants He interrupted the natural pollination process He carried out cross- pollination for the plants

Mendel’s Experiments Mendel first studied each characteristic and it’s varied traits individually He first grew plants that were true-breeding for each trait Plants that are true-breeding for a trait ALWAYS produce offspring with that trait when self-pollinating. He didn’t’ care if it had white flowers and yellow seeds…only that it had white or purple flowers He did this for several generations until he had 14 true-breeding plants.

Mendel’s Experiments Mendel then cross-pollinated pairs of plants with contrasting traits for a single characteristic. For example, he paired a true-breeding purple plant with a white one. He named the true-breeding parents the P generation.

Mendel’s Experiments When the plants matured, Mendel recorded the number of each type of offspring produced The offspring of the P generation was called the F1 generation He allowed the F1 generation to self-pollinate The offspring of the F1 generation was called the F2 generation Mendel repeated this process hundreds of times, documenting the results

Mendel’s Results After cross pollinating the P generation, he observed that all the flowers of the F1 generation were purple. After F1 self-pollinated, 75% of the F2 flowers were purple, but 25% were white. For every 3 purple flowers there was 1 white flower  a 3:1 ratio. This happened for every trait he studied.

Mendel’s Conclusions Mendel’s experiments were monohybrid crosses: Mating of two organisms that differ in only one character. In each case, one trait disappeared in the F1 generation. That trait would reappear in the F2 generation. When it reappeared, it was always in a 3:1 ratio. Mendel determined that each trait is controlled by a pair (2) of factors.

Alleles Mendel’s “factors” are now known as genes Each of Mendel’s factors had 2 forms The alternative forms (versions) are referred to as alleles We now know that there can be many versions of the same gene Remember, chromosomes occur in pairs, thus genes also occur in pairs Sex cells (gametes – haploid) have one form of a gene on their chromosomes Somatic (body) cells (diploid) have two forms (alleles) You get one from mom and one from dad

Mendel’s Laws Law of Segregation (1st Law): During gamete formation (aka _______________), the alleles for each gene separate from each other so that each gamete carries only ONE allele for each gene. This occurs during Metaphase I. When two gametes combine (sexual reproduction), the offspring receives one allele for a gene from each parent.

Mendel’s Laws Law of Independent Assortment (2nd Law): Genes for different traits can segregate independently during gamete formation. The selection of one allele has NOTHING to do with the selection of another…it is totally random. This occurs during Prophase I. Mendel saw evidence of this when he did dihybrid crosses: mating of two organisms that differ in two factors (i.e. color AND pea shape).

Mendel’s Laws Law of Dominance (3rd Law): Some alleles are “dominant” while others are “recessive”. Dominant refers to an allele that masks the presence of another allele for the same characteristic. Recessive refers to to an allele that is masked by the presence of another allele for the same characteristic. An organism with at least one dominant allele with display the effect of that allele. A COMPLETELY DOMINANT TRAIT WILL ALWAYS HIDE (COVER UP) A RECESSIVE TRAIT. THE ONLY WAY A RECESSIVE TRAIT WILL SHOW IS IF THE DOMINANT ALLELE IS NOT THERE! Letters are used to represent alleles: Dominant alleles are represented by CAPITAL letters  R Recessive alleles are represented by lower case letters  r The actual letters used usually comes from the first letter of the dominant trait. When gametes combine at fertilization, offspring receive 1 allele from each parent. Genes can be RR, Rr, or rr.

Example In Mendel’s peas, purple flowers are dominant and white flowers are recessive. Purple is represented as ‘P’, white is represented as ‘p’. Flowers get one letter from mom, and one from dad. They can combine to give you ‘PP’, ‘Pp’, or ‘pp’. Which combination would give you a Purple flower? PP Pp pp

Example In dogs, black fur is dominant and brown fur is recessive. Black fur is represented as ‘B’, brown fur as ‘b’. Puppies get one letter from mom, and one from dad. They can combine to give you ‘BB’, ‘Bb’, or ‘bb’. Which combination would give you a brown puppy? BB Bb bb

Genotype and Phenotype The letters describe an organism's genetic makeup: its genotype. It consists of the alleles that the organism inherits from its parents. 2 alleles for each trait  its genes. RR, Rr, rr. An organism’s appearance is its phenotype. Blonde, tall, green, wrinkled, smooth. Can be affected by genes AND by environment.

Homozygous and Heterozygous When both alleles of a pair are alike, the organism is homozygous for that trait. Homozygous dominant (BB) OR Homozygous recessive (bb). When both alleles of a pair are different, the organism is heterozygous for that trait. Bb.