Bio1100 Chapter 7 Chap 6   Genes and Inheritance   Chap 8
  1. Gregor Mendel pioneered the study of inheritance   of observable traits, which we now understand is based on genes  .

    • Gregor Mendel (1822-1884) experimented with pea plants in his monastery in Austria.
      Using true-breeding plants ensured consistent inheritance of a trait (phenotype) such as flower color.
      His experiments involved "crossing" plants with different traits: manually fertilizing eggs of one plant with sperm from another.

    • Alternative versions of a gene that code for different variations of a protein are called alleles.

      Different alleles may result in the production of different variants of a proteins and be observed as different traits (phenotypes) of an organism, such as petal color.

      For each gene, an individual inherits 2 alleles, each inherited from a parent.

      An individual's inherited alleles make up its genotype; which may be observed as a visible phenotype (e.g., flower color).

    • If both alleles inherited from 2 parents are the same, the individual is homozygous.

      If the 2 alleles differ, the individual is heterozygous.

      • In a heterozygote, often the dominant allele determines its phenotype; the other allele is called recessive.

      In this example, the dominant allele expresses purple flowers, while the recessive allele expresses white flowers.

      Possible genotypes are:

      • A homozygous dominant individual shows purple flowers.
      • A homozygous recessive individual shows white flowers.
      • A heterozygous individual shows the dominant phenotype: purple flowers.


      1. Mendel first crossed true-breeding (homozygous) purple with true-breeding (homozygous) white plants.

        • All offspring show purple flowers.

        • Inheritance of the dominant phenotype is explained by separation of alleles during meiosis.

      2. Mendel then crossed the purple offspring.

        • About 1/4 of the next generation reverted to white flowers.


  2. Mendelian inheritance exhibits predictable expression of dominant   and recessive   traits.
    • The offspring between true-breeding (homozygous) purple and white plants must be heterozygous and show the dominant phenotype - purple flowers.

      Meiosis produces gametes that possess either the dominant or the recessive allele.

      Random fertilization among these gametes yields predictable offspring ratio of 1/4 with homozygous recessive genotype.

  3. Many recessive traits are due to an abnormal protein   expressed by the recessive allele.
    • Many phenotypes are the result of expression of an individual's genotype.

      In plants, pigment genes produce pigment proteins which reflect light at different wavelengths.

      Presence of purple pigments result in purple pea flowers (dominant), while a mutant allele yields no purple pigments (recessive).

      Thus homozygous recessive plants show white flowers due to lack of pigments.

    • In animals, pigment genes produce pigments called melanin that color the skin, hair, and eyes (dominant).

      A mutant allele yields no melanin pigments (recessive).

      An animal with a homozygous recessive genotype has a phenotype of albino: it lacks color for those organs.

  4. A Punnett square can be used to calculate the ratio of genotypes   and phenotypes   of offspring based on probability  .
    • Punnett square

      First the possible gametes of the two parents are aligned along two sides of a square.

      Then the gametes are combined to show genotypes of potential zygotes in the cells of the square.

      • All the eggs of homozygous recessive (albino) mother have the recessive allele (m).

      • All the sperm of homozygous dominant (pigmented) father have the dominant allele (M).

      • All the offspring are heterozygous (Mm) in genotype, pigmented in phenotype.



    • If both parents are heterozygous (Mm) in genotype (pigmented in phenotype):

      • 1/2 of both eggs and sperm are dominant (M), 1/2 are recessive (m).

      • Offspring genotype ratio is 1 MM : 2 Mm : 1 mm.

      • Offspring phenotype ratio is 3 pigmented : 1 albino.

      Note these ratios are based on probability, assuming random fertilization of gametes.


      • Assuming brown eyes (B) is dominant over blue eyes (b), can 2 brown-eyed parents have a baby with blue eyes?
        • Yes if the parents are heterozygous; 1/4 of their offspring will be blue-eyed.

  5. A testcross   can be used to determine the genotype of a dominant   individual.
    • Testcross

      How to determine whether an individual exhibiting a dominant phenotype (pigmented) was homozygous (MM) or heterozygous (Mm)?

      Testcross with an individual exhibiting the recessive phenotype (albino), because we know the genotype must be homozygous recessive mm.

      If any offspring is albino, the pigmented parent must be heterozygous (Mm).

  6. Many traits exhibit non-Mendelian inheritance.
    • Incomplete   dominance results when alternative alleles are not fully dominant or fully recessive.
      • Snapdragon flower color exhibits incomplete dominance: the heterozygote phenotype is intermediate between those of the homozygotes.

        In this case the homozygous dominant (CRCR) produces red pigments, while homozygous recessive (CWCW) produces no pigment.

        The heterozygote (CRCW) produces an intermediate mount of pigment, and looks pink.


      • Incomplete dominance yields different inheritance patterns than Mendelian complete dominance.

        A cross between heterozygous parents yield the same genotype and phenotype ratios of 1:2:1.

    • Human blood types exhibit codominance   due to antigen   proteins.
      • Codominance occurs when both alleles contribute expression to the phenotype; an example is human blood types.

        4 phenotypes from the 3 alleles are possible:

        • Type A is either IAIA or IAi.
        • Type B is either IBIB or IBi.
        • Type AB is IAIB.
        • Type O is ii.

        The IA and IB alleles are both dominant over i.

        The IA and IB alleles are codominant.


      • Codominance in human blood types

        The IA and IB alleles add molecules called antigens to the surface of red blood cells.

        The immune system produces antibodies against cells with foreign antigens.

        The IA and IB alleles are codominant because they add different antigens.

        The i allele is recessive because it does not add antigens.

        These traits affect blood transfusions.


      • Donated blood contain only red blood cells (RBC); no antibodies are transfused.

        Type AB individuals are universal recipients, because they do not produce antibodies to either the A or B antigen from any donated RBC.

        Type O individuals are universal donors, because their red blood cells have no antigens to react with antibodies in the blood of the recipient.


    • Some genes reside on the X   chromosome, and exhibit sex-linked inheritance patterns.

    • Sex-linked inheritance

      Some genes are located on the X chromosome (sex-linked).

      Males possess only 1 X chromosome, and exhibit a recessive sex-linked trait (such as color-blindness) if he inherits the allele from his mother.

      Females exhibit the disorder only if she receives the recessive allele from both parents.

      Some examples:

      • color-blind female x normal-sighted male

      • father and son color-blind, mother and daughter normal-sighted


    • A heterozygous female does not exhibit the recessive trait but is a carrier of the recessive allele - she can pass the allele to offspring.

      If a female carrier mates with a normal-sighted male:

      • There is a 50% chance that a daughter will be a carrier like her mother.

      • There is a 50% chance that a son will have the disorder.


    • If a color-blind female mates with a normal-sighted male:

      • All her daughters will be carriers.

      • All her sons will be color-blind.

    • If a father and son are color-blind, but mother and daughter have normal color vision:

      • 50% of the daughters will be color-blind.

      • 50% of the sons will be color-blind.

      Note: the mother must be a carrier to give the disease allele to her son.