FUNDAMENTALS OF GENETICS

WHAT IS GENETICS?§ Genetics: the science that studies how

genes are transmitted from one generation to the next

§ Gene: a segment of DNA that controls a hereditary trait

§ Chromosomes = long chains of genes

§ Heredity: the transmission of characteristics from parents to offspring

§ Traits: the characteristics an organism has (ex: eye color, skin color, hair color, height)

GREGOR MENDEL§ Austrian monk born in 1822§ Known as the “Father of Genetics”§ Discovered three laws of genetics that

would forever change biology1. Law of Dominance & Recessiveness2. Principle of Segregation3. Principle of Independent Assortment

§ Conducted experiments with pea plants§ His greatest contribution was to

demonstrate that inherited characteristics are carried by genes

Mendel’s Garden Peas¡ Observed seven characteristics of pea plants① Plant height (tall vs. short)② Flower position along stem

(axial vs. terminal)③ Pod color (green vs. yellow)④ Pod appearance

(inflated vs. constricted)⑤ Seed texture (round vs. wrinkled)⑥ Seed color (yellow vs. green)⑦ Flower color (purple vs. white)

¡ Garden peas a good choice because:üThey were readily availableüThey were easy to grow üThey grew rapidly

¡ Collected seeds from his pea plants and carefully recorded each plant’s traits and seeds over a span of several years

MENDEL’S EXPERIMENTS§ Pea flowers are normally self-pollinating § Resulting embryos will have the same

characteristics as the parent plant

§ Mendel knew that these pea plants were “true breeding” (pure)

§ Mendel cross-pollinated pairs of plants that were true breeding for contrasting traits of a single characteristic§ He called the true breeding parents the P

generation

¡P generation = Parental generation¡F1 generation = First generation of offspring¡F2 generation = Second generation of offspring

¡Hybrids: the offspring of parents with different traits

§ In one of his experiments, Mendel crossed true-breeding tall plants with true-breeding dwarf plants

§ Tall x dwarf à all tall offspring§ The F1 hybrids were all tall§ All of the offspring had the appearance of only one of the

parents§ The trait of the other parent seemed to have disappeared§ Mendel thought that the dwarf trait had been lost

¡ Next, Mendel allowed the hybrid tall offspring from the first generation to self-pollinate

F1 tall x F1 tall à offspring: ¾ tall and ¼ dwarf¡ He found that ¾ of the offspring were tall and

¼ of the offspring were dwarf¡ Offspring always at a 3:1 ratio

MENDEL’S RESULTS & CONCLUSIONS§ Mendel’s observations & records led him to hypothesize

that biological inheritance is determined by “factors” that are passed from one generation to the next§ Today, we know these factors as genes or alleles§ He hypothesized that each trait was inherited by means of

a separate allele§ Reasoned that a pair of alleles must control each trait

Recessive & Dominant Traits

¡ Mendel realized that some traits are dominant over other traits¡ He hypothesized that the trait appearing in the F1 generation was

controlled by a dominant factor¡ He thought that the trait that did not appear in the F1 generation but

appeared in the F2 generation was controlled by a recessive factor¡ A trait controlled by a recessive factor had no observable effect on

an organism’s appearance

DOMINANT VS. RECESSIVE?1. Pause note taking

2. Clasp your hands together

3. Which thumb did you put on top? ¡Left?¡Right?¡Dominant: Placing left on top of right¡Recessive: Placing right on top of left

§ Mendel’s observations from his experiments can be summarized in two principles:§ The Law of Segregation § The Law of Independent Assortment

The Law of Segregation

§ Mendel concluded that the paired factors separate during the formation of reproductive cells

§ Law of segregation states that a pair of factors is separated during the formation of gametes (egg and sperm cells)

The Law of Independent Assortment

§ Mendel also concluded that the factors for individual characteristics are not connected

§ The law of independent assortment states that factors separate independently of one another during the formation of gametes

HOMOZYGOUS OR HETEROZYGOUS

¡ An offspring will inherit two alleles for a trait,one allele from each parent

¡ The combination of alleles received may either be homozygous or heterozygous

¡ Homozygous = the two alleles are the same ¡ Ex: TT or tt

¡ Heterozygous = the two alleles are different¡ Ex: Tt

GENOTYPES & PHENOTYPES¡Genotype: the genetic makeup of an

organism

¡Phenotype: the physical characteristics of an organism (what the organism looks like)

Ex:Genotype Phenotype

TT TallTt Talltt dwarf