Genetics and Inheritance Notes

Mendel's Laws

  • Heredity: The transmission of traits from one generation to the next.
  • Genetics: The scientific study of heredity.
  • Gregor Mendel: Started the field of genetics in the 1860s by breeding garden peas.
    • He deduced the principles of genetics.
    • Relied on mathematics, physics, and chemistry.

9.2 The Science of Genetics Began in an Abbey Garden

  • In 1866, Mendel correctly argued that parents pass on discrete “heritable factors” to their offspring.
  • These heritable factors are today called genes and retain their individuality generation after generation.
  • Character: A heritable feature that varies among individuals (e.g., flower color).
  • Trait: Each variant for a character (e.g., purple or white flowers).
  • Mendel's cross-fertilization technique:
    1. Prevented self-fertilization by cutting off immature stamens (male reproductive organ).
    2. Dusted the carpel (female reproductive organ) with pollen from another plant.
    3. The carpel developed into a pod containing seeds (peas).
    4. Planted the seeds later.
  • True-breeding varieties: Result when self-fertilization produces offspring all identical to the parent.
    • True-breeding parental plants are the P generation.
  • Hybridization/Genetic Cross: Cross-fertilization of two different varieties.
    • The offspring are hybrids, the F1 generation.
    • A cross of F1 plants produces the F2 generation.

9.3 Mendel’s Law of Segregation Describes the Inheritance of a Single Character

  • Monohybrid cross: A cross between two individuals differing in a single character.
  • Mendel performed a monohybrid cross between a plant with purple flowers and a plant with white flowers.

Mendel's experiment with flower color

  • P generation: true-breeding purple and white flower plants.
  • F1 generation: All plants had purple flowers.
  • F2 generation: 3/43/4 of plants had purple flowers, 1/41/4 had white flowers.
  • Mendel’s pea experiment shows dominance with purple color being dominant.

Mendel's conclusions

  • The all-purple F1 generation did not produce light purple flowers, disproving the blending hypothesis.
  • Mendel needed to explain:
    • Why white color seemed to disappear in the F1 generation.
    • Why white color reappeared in one-quarter of the F2 offspring.
    • Dominant character for a trait.
  • Mendel developed four hypotheses (using modern terminology):
    1. Alleles: Alternative versions of genes that account for variations in inherited characters.
    2. For each character, an organism inherits two alleles, one from each parent. The alleles can be the same or different.
      • Homozygous genotype: Has identical alleles.
      • Heterozygous genotype: Has two different alleles.
    3. If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the organism’s appearance and is called the recessive allele.
      • Phenotype: The appearance or expression of a trait.
      • Genotype: The genetic makeup of a trait.
      • The same phenotype may be determined by more than one genotype.
    4. A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation.
      • The fusion of gametes at fertilization creates allele pairs once again.

Explaining the 3:1 ratio in the F2 generation

  • The F1 hybrids all have a Pp genotype.
  • A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine.
  • Phenotypic ratio is 3 purple:1 white
  • Genotypic ratio is 1 PP:2 Pp:1 pp

9.4 Homologous Chromosomes Bear the Alleles for each Character

  • Locus: The specific location of a gene along a chromosome (plural, loci).
  • For a pair of homologous chromosomes (homologs), alleles of a gene reside at the same locus.
    • Homozygous individuals have the same allele on both homologs.
    • Heterozygous individuals have a different allele on each homolog.

9.5 The Law of Independent Assortment is Revealed by Tracking two Characters at Once

  • Dihybrid cross: A mating of parental varieties that differ in two characters.
  • Mendel performed a dihybrid cross with the following results:
    • P generation: round yellow seeds × wrinkled green seeds
    • F1 generation: all plants with round yellow seeds
    • F2 generation:
      • 9/169/16 had round yellow seeds
      • 3/163/16 had wrinkled yellow seeds
      • 3/163/16 had round green seeds
      • 1/161/16 had wrinkled green seeds
  • Mendel needed to explain why the F2 offspring:
    • Had new nonparental combinations of traits
    • Had a 9:3:3:1 phenotypic ratio
  • Mendel:
    • Suggested that the inheritance of one character has no effect on the inheritance of another
    • Suggested that the dihybrid cross is the equivalent to two monohybrid crosses
    • Called this the law of independent assortment

Determining genotype using a test cross

  • A testcross is the mating between an individual of unknown genotype and a homozygous recessive individual.
  • A testcross can show whether the unknown genotype includes a recessive allele.
  • Mendel used testcrosses to verify that he had true-breeding varieties of plants.
  • Example: If a black dog (B_?) is crossed with a chocolate dog (bb), and the offspring are all black, the black dog's genotype is BB. If the offspring are 1 black:1 chocolate, the black dog's genotype is Bb.

9.9 Connection: Many Inherited Traits in Humans are Controlled by a Single Gene

  • Because a trait is dominant does not mean that it is “normal” or more common than a recessive trait
  • Wild-type traits are those most often seen in nature and are not necessarily specified by dominant alleles

Recessive genetic disorders

  • Thousands of human genetic disorders ranging in severity from relatively mild, such as albinism, to invariably fatal, such as cystic fibrosis are inherited as recessive traits.
  • Most people who have recessive disorders are born to normal parents who are both heterozygotes, carriers of the recessive allele for the disorder, and are phenotypically normal

9.11 Incomplete Dominance Results in Intermediate Phenotypes

  • Mendel’s pea crosses always looked like one of the two parental varieties, a situation called complete dominance.
  • For some characters, the appearance of F1 hybrids falls between the phenotypes of the two parental varieties. This is called incomplete dominance.

Codominance and multiple alleles

  • Although each individual carries, at most, two different alleles for a particular gene, in cases of multiple alleles, more than two possible alleles exist in a population
  • Human ABO blood group phenotypes involve three alleles for a single gene.
  • The four human blood groups, A, B, AB, and O, result from combinations of these three alleles.
  • The A and B alleles are both expressed in heterozygous individuals, making both alleles codominant.

9.15 The Environment Affects Many Characters

  • Many characters result from a combination of heredity and the environment. For example:
    • Skin color is affected by exposure to sunlight.
    • Heart disease and cancer are influenced by genes and the environment.
  • Identical twins show that a person’s traits are the results of genetics and environment

Chromosomal Basis of Inheritance

  • The chromosome theory of inheritance states that:
    1. Genes occupy specific loci (positions) on chromosomes
    2. Chromosomes undergo segregation and independent assortment during meiosis

Correlation Between Mendel's Laws and Meiosis

  • Mendel’s laws correlate with chromosome separation in meiosis.
    • The law of segregation states that pairs of alleles separate from each other during gamete formation via meiosis and depends on separation of homologous chromosomes in anaphase I.
    • The law of independent assortment states that each pair of alleles sorts independently of other pairs of alleles during gamete formation and depends on alternative orientations of chromosomes in metaphase 1

9.17 Scientific Thinking: Genes on the same Chromosome Tend to be Inherited Together

  • Bateson and Punnett studied plants that did not show a 9:3:3:1 ratio in the F2 generation. What they found was an example of linked genes, which:
    • Are located close together on the same chromosome
    • Tend to be inherited together

Recombination

  • Crossing over between homologous chromosomes produces new combinations of alleles in gametes.
  • Linked genes can be separated by crossing over, forming recombinant gametes.
  • The percentage of recombinant offspring is the recombination frequency.

Sex Chromosomes and Sex-Linked Genes

  • Among humans and other mammals:
    • XY are males
    • XX are females
  • In addition, human males and females both have 44 autosomes (nonsex chromosomes).
    • 22 somatic chromosomes +
    • a pair of sex chromosomes
    • = 46 total chromosomes
  • In mammals (including humans):
    • the Y chromosome has a crucial gene, SRY, for the development of testes
    • an absence of the SRY gene directs ovaries to develop

9.21 Sex-linked Genes Exhibit a Unique Pattern of Inheritance

  • Sex-linked genes are located on either of the sex chromosomes.
  • The X chromosome carries many genes unrelated to sex.

9.22 Connection: Human Sex-linked Disorders Affect Mostly Males

  • Most sex-linked human disorders are:
    1. Due to recessive alleles
    2. Seen mostly in males
  • A male receiving a single X-linked recessive allele from his mother will have the disorder.
  • A female must receive the allele from both parents to be affected.