Chapter Three Study Guide
Traits- physical characteristics studied in genetics, they are inherited from parents.
Hair color
Eye color
Shape of nose
Dimples
Heredity- The passing of traits from parents to offspring.
Genetics- The scientific study of heredity.
Gregor Mendel- Father of genetics. Studied cross-pollinated pea plants.
Purebred- always produces offspring with the same form of a trait as the parents
*Purebred short pea plants (tt) *Purebred tall pea pants (TT)
Genes- The factors that control traits.
An offspring receives genes form both parents.
Alleles- Different forms of genes. Some alleles are dominant and some are recessive.
Dominate Allele- a trait that always shows up when present. (Capital Letter)
Recessive Allele- a trait that is masked when a dominant allele is present. It will only show up when it is
paired with another recessive allele. (Lowercase Letter)
Hybrids- Have two different alleles for a given trait.
Example: T (tall allele), t (short allele)
Hybrid plant’s alleles: Tt
Probability- The likelihood that a particular event will occur. (Chance)
*Mendel was the first scientist to recognize that probability can be used to predict genetic crosses.
Punnett Square: *A chart that shows all the possible combinations of alleles.
*It determines the probability of a particular outcome.
Punnett Square
Phenotype- an organism’s physical appearance, it’s visible trait. Genotype- The genetic makeup, or allele
combinations of a specific trait. (Tt, RR, Bb)
(red hair, blue eyes, wrinkled seeds)
Homozygous- an organism with two identical alleles for a trait.
(TT tall, tt short [purebred])
Heterozygous- an organism that has two
different alleles for a trait. (Tt [hybrid])
Codominance: *The alleles are neither dominant nor recessive. * Both alleles are expressed in the offspring.
*Both letters are written as capitals with superscripts.
B W
(F F )
Walter Sutton: *First person to come up with the idea that chromosomes were the key to inheritance.
Chromosome Theory of Inheritance:
Genes are carried from parent to offspring on chromosomes. [Found in the nucleus.]
Sex Cells:
One from each parent…………….
Egg Cell (Female)
Sperm Cell (Male)
*Each sex cell has exactly half the number of chromosomes found in an organism’s body cells.
Ex: Grasshopper
Egg Cell
12 chromosomes
Sperm Cell +12 chromosomes
24 chromosomes in a grasshopper’s body cells
Meiosis- The process by which the number of
chromosomes is reduced by half to form sex cells.
*Punnett squares show how alleles separate when sex cells form during meiosis.
Mutations in sex cells can be passed on to offspring. (Be able to give examples)