CHAPTER 10.2 & 10.3 NOTES
Mendelian Genetics
 The passing of traits to the next generation is called inheritance, or heredity.
 Mendel performed cross-pollination in pea plants.
 Mendel followed various traits in the pea plants he bred.
 The parent generation is also known as the P generation.
 The offspring of this P cross are called the first filial (F1) generation.
 The second filial (F2) generation is the offspring from the F1 cross.
Genes in Pairs
 Allele
 An alternative form of a single gene passed from generation to generation
 Dominant (Purple flower color)
 Expressed form of a trait represented by a capital letter and first letter of the trait (P).
 Recessive (White flower color)
 Only appears when both alleles are recessive. Represented by a lower case letter of the dominant trait (p)
 An organism with two of the same alleles for a particular trait is homozygous.
 PP or pp
 An organism with two different alleles for a particular trait is heterozygous.
 Pp
Genotype and Phenotype
 An organism’s allele pairs are called its genotype.
 Example would be Pp
 The observable characteristic or outward expression of an allele pair is called the phenotype.
 Example would be purple flower color
Mendel’s Law of Segregation
 The law of segregation, states that the two alleles for a trait segregate (separate) when gametes are formed.
 Two alleles for each trait separate during meiosis.
 During fertilization, two alleles for that trait unite.
 Each offspring receives one allele from each parent resulting in two alleles for each trait.
Mendel’s Law of Independent Assortment
 Mendel found that the inheritance of one trait, such as plant height, did not influence the inheritance of any other trait,
such as flower color.
 The law of independent assortment states that the alleles of different genes separate independently of one another
during gamete formation.
 This occurs during meiosis I when the homologous chromosomes line up along the metaphase plate
Monohybrid Cross
 A cross that involves hybrids for a single trait is called a monohybrid cross.
Dihybrid Cross
 The simultaneous inheritance of two or more traits in the same plant is a dihybrid cross.
 For example, if you crossed a yellow round pea (YYRR) with a green wrinkled pea (yyrr)
 What would be the predicted offspring genotypes and phenotypes?
 Four types of alleles from the male gametes and four types of alleles from the female gametes can be produced.
 The resulting phenotypic ratio is 9:3:3:1.
Incomplete Dominance
 In some organisms, however, an individual displays a trait that is intermediate between the two parents, a condition
known as incomplete dominance.
 For example, when a snapdragon with red flowers is crossed with a snapdragon with white flowers, a snapdragon with
pink flowers is produced.
 For some traits, two dominant alleles are expressed at the same time.
 In this case, both forms of the trait are displayed, a phenomenon called codominance.
 Codominance is different from incomplete dominance because both traits are displayed.
 Instead of pink you would get a red and white flower
Polygenic Inheritance
 When several genes influence a trait, the trait is said to be a polygenic trait.
 The genes for a polygenic trait may be scattered along the same chromosome or located on different chromosomes.
 Familiar examples of polygenic traits in humans include eye color, height, weight, and hair and skin color.
Multiple Alleles
 Genes with three or more alleles are said to have multiple alleles.
 Example: Blood types A, B, AB, and O
 Even for traits controlled by genes with multiple alleles, an individual can have only two of the possible alleles for that
 Geneticists often prepare a pedigree, a family history that shows how a trait is inherited over several generations.
 Pedigrees are particularly helpful if the trait is a genetic disorder and the family members want to know if they are
carriers or if their children might get the disorder
 Scientists can determine several pieces of genetic information from a pedigree:
 Autosomal or Sex-Linked? If a trait is autosomal, it will appear in both sexes equally. If a trait is sex-linked, it is
usually seen only in males. A sex-linked trait is a trait whose allele is located on the X chromosome.
 Dominant or Recessive? If the trait is autosomal dominant, every individual with the trait will have a parent with
the trait. If the trait is recessive, an individual with the trait can have one, two, or neither parent exhibit the
 Sex-linked traits occur on the X chromosomes
 Females have 2 Xs so they must have both defective alleles to have the genetic disorder
 Males only have 1 X making sex-linked disorders much more common in males
 Hemophilia is a sex-linked trait a condition that impairs the blood’s ability to clot.