AP Bio Unit 5

Mendel and the Gene Idea

• Mendel’s work: He studied pea plants to understand heredity

• Why peas?": short generation time, many traits with two distinct forms, and controlled breeding

• True-breeding plants: plants that produce offspring identical to themselves when self-pollinated

• Cross-pollination: Mendel controlled which plants were bred together

Genetic Terminology

• Gene: unit of heredity; segment of DNA coding for a trait

• Allele: different versions of a gene

  • Dominant allele (A)

  • recessive allele (a)

• Homozygous: two identical alleles (AA or aa)

• Heterozygous: two different alleles (Aa)

• Genotype: genetic makeup (AA, Aa, or aa)

• Phenotype: physical expression of the gene

A. Mendel’s Law of Inheritance

• A. Law of segregation

  • Two alleles for a gene separate during gamete formation, so each gamete carries only one allele

  • Demonstrated using a monohybrid cross (one trait)

  • Example: crossing purple (P) and white (p) flowers—> F1 generation is all purple (Pp), but F2 generation follows a 3:1 ratio of purple: white

• B. Law of Independent Assortment

  • Genes for different traits segregate independently during gamete formation

  • applies to genes on different chromosomes or far apart on the same chromosome

  • demonstrated using dihybrid cross (two traits at once)

  • Example: a cross between yellow round (YYRR) and green wrinkled (yyrr) peas produces a 9:3:3:1 ratio in the F2 generation

Degrees of Dominance

• Complete Dominance: one allele completely masks the others

• incomplete Dominance: heterozygous phenotype is a blend of both alleles

  • (red x white = pink flowers)

• Codominance: both alleles are fully expressed in heterozygous (ex: blood type AB)

  • Multiple Alleles and Blood Type

    • Some traits have more than two alleles. ex: Human ABO blood type

    • IA, IB (codominant) and i recessive

    • Genotypes: A (IA IA or IA i), B(IB IB or IBi ), AB(IA IB) O(ii)

Pleiotropy

• One gene influences multiple traits

  • example: sickle cell disease affects red blood cell shape, oxygen transport and resistance to malaria

Epistasis

• One gene affects the expression of another gene

  • example, a Labrador retriever coat color (one gene determines pigment, another determines pigment deposition)

Polygenic inheritance

• multiple genes contribute to a single trait

• characters that vary along a continuum or describing a normal distribution

• traits show continuous variation (ex: skin color, height)

• bell curve distribution

Environmental Impact on Phenotypes

• Nature vs Nature: Environment can affect gene expression.

  • Example: Himalayan rabbits have fur that turns black in cold temperatures

Solving Genetics Problems (Probability)

• Multiplication Rule: probability of independent events occurring together = product of their probabilities

  • words like “and”

• Addition Rule: probability of either of two mutually exclusive events occurring = sum of their probabilities

  • words like “or”

Key Ratios to remember:

• Monohybrid: 3:1

• Dihybrid: 9:3:3:1

• Incomplete Dominance: 1:2:1

• Test cross 1:1

Mendelian Genetics in Humans

• Many human traits follow Mendelian patterns of inheritance

• recessive disorders: require two copies of the allele (aa): example, cystic fibrosis, sickle cell

• An autosomal pattern refers to the inheritance of genes found on autosomes (non-sex chromosomes). An autosomal inheritance can be dominant or recessive.

  • Dominant Inheritance

    • Only ONE copy of the dominant allele is needed for the trait or disorder to be expressed

    • affected offspring can have one affected parent

    • DOES NOT skip generations (appears in every generation)

      • Examples: Huntington’s, Marfan Syndrome, Dwarfism

      • Genotypic possibilities:

        • AA—> AFFECTED

        • Aa—→ Affected

        • aa—> not affected

  • Recessive Inheritance

    • Two copies of a recessive allele are needed for a trait or disorder to be expressed

    • carriers (Aa) do not show symptoms but can pass the allele to offspring

    • can skip generations if two carries (Aa) have children

      • Examples: cystic fibrosis, sickle cell anemia

• How to identify an autosomal pattern in pedigrees

  • Autosomal Dominant

    • appears in every generation

    • both males and females are effected equally

    • affected individuals always have at least one affected parent

  • Autosomal Recessive:

    • can skip generations

    • affected individuals may have unaffected carrier parents

    • both males are affected equally

• Sex Linked Inheritance

  • refers to genes located on a sex chromosome

• X-linked recessive

  • more common in males because they only have one X chromosome

  • females need two copies of the recessive allele to be affected

  • affected males inherit the trait from their mother, who may be a carrier

  • can skip generations, as female carriers pass the trait to sons

  • How to identify?

    • more males affected than females

    • affected males come from carriers or affected mothers

    • never passed from father to son

• X-linked Dominant

  • only one copy of the dominant allele is needed to express trait

  • affects both males and females, but females more commonly because they have two X’s

  • affected fathers pass traits to daughter but never to sons

  • appears in every generation