Inheritance

Early thought on Inheritance

• Theophrasus (371-287BC) males presence simply ripens the female
• Aristole (384-322) Pangenesis
  • Each body part produces a "seed" that travel to the reproductive organs
  • Darwin modified pangenesis
  • Weismann (1800's) tested pangenesis
    • Proposes germ plasm theory

Breeding

• For thousands of years prior to the identification of DNA as the hereditary material
• Humans breeders knew that crossing specific animals or plants with desirable traits could result in progeny that had different characteristics
• Blending hypothesis
  • Incorrect or at least incomplete

Mendel and the Birth of Genetics

• 1856 Gregor Mendel began work that laid the groundwork that developed into our understanding of genetics
  • Mendel raised several strains of peas for several years
    • Identified consist morphological variations that bred true

Mendels Experiments

• Bred garden pea plants
  • Peas self-fertilize
  • Can be cross fertilized by manipulation

Results of Mendel's Controlled Crosses

• Mendels reported the results from seven trait which he studied
  • Each of the traits existed in two contrasting forms (e.g... red or white flowers; pods yellow or green)
• Selective breeding resulted in all the offspring (F1 or first filial generation) being all alike, they resembled one of the parents
• F2 the traits from the parental (P) reappeared
• Mendels experiments had three unique features that distinguished his work
  • Characteristic had only two possibilities
  • Determined ratios of the characteristics in the progeny
  • Crossed results were followed for at least two generations

Extensions of Mendels Experiments

• Disproved the blending hypothesis
  • Parental generation (P1) crosses yield first filial generation (F1)
    • Seed color could disappear in F1 and reappear in the F2
• Explanation in terms of two forms of the same trait
  • Dominant and Recessive

Dominant and Recessive Traits

• Mendel referred to those traits that appeared in the F1 generation as dominant traits
  • Those latent traits which reappeared in the F2 generation are known as recessive traits
• The breeding of the F1 generation to itself (selfing) resulted in the approximately 1/4 of the offspring displaying the recessive flower color
• Same was true for the other characteristics which he quantified

Genetic Alleles

• 3:1 ratio is possible if there are two hereditary units for each trait, one from each parent
  • Units are called alleles
    • Alternate forms of genes
  • Individuals can have different combinations of these alleles
    • Homozygous and Heterozygous

Genotype versus Phenotype

It is not possible to determine by visual examination whether an individual is homozygous or heterozygous for a given dominant trait

• If allele Z is dominant to allele z the possible combinations are ZZ, Zz or zZ
  • All these trait look the same in the phenotype
    • Phenotype is the outward expression or what you see
    • Test cross required to determine genotype
• If and individual receives zz alleles then both the genotype can be deduced from the phenotypes

Partial Dominance

• Partial dominant traits, have heterozygote phenotypes that are intermediate between the homozygotes
  • Blending hypothesis revisited
  • Mendel studied these traits but did not fully understand the phenomena
• Partially dominant alleles both contribute to the phenotype (neither is dominantto the other)

Principle of Segregation

• Mendels conclusion from his experiments was that there had to be two discrete hereditary units for a given characteristic separated (segregate) during gamete formation
• Segregation occurs during meiosis

• Mendels 1st Law: The Law of Segregation

Multigenic Traits

• Most characteristics are controlled by several genes at once
  • Multigenic inheritance and crosses are referred to as multihybrid crosses
• Inheriting two genes together
  • Mendel's dihybrid crosses of plants with round yellow seeds with plant having wrinkled green seeds produced:
    • F1 all had round yellow seeds
    • F2 had round yellow, wrinkled yellow, round green and wrinkled green seeds (9:3:3:1 ratio)

Mendels Second Law

• The Law of Independent Assortment states that genes can assort to gametes independent of one and other.
  • Important caveat the genes which are assorting independent of each other must be located on non-homologous chromosomes

Product Law

The chance that two independent events will occur together is equal to the product of the chances that each event will occur separately.

• Mendels pea seed experiment: yellow is dominant there for it will be seen 3/4 of the time, while wrinkled is recessive (1/4)
  • Therefore the probability of getting yellow wrinkled seeds is 3/4 x 1/4 or 3/16

How Genes Interact

• We have seen that two genes can interact to modify a phenotype (e.g... partial dominance)
• Most traits (phenotypic) are the result of several genes interacting
  • Biochemical systems such as glycolysis are the result of 12 genes which produce the enzymes

Types of Gene Interactions

• Complementation-can show if a trait in two individuals is produced by the same gene or by two separate genes
• Epistatis- One gene has the effect of suppressing the phenotypic expression of another
• Collaboration-Two gene which interact to produce a novel phenotype
• Modifiers- expression of one gene can be influenced by the expression of others (e.g.. human eye color)
• Multiple-gene inheritance- Many traits have a gradation of phenotypes rather than a limited set
  • Skin color, Human height, IQ etc...
  • Quantitative traits which are polygenic

When Genes have Several Alleles

• Within a population many forms of the same gene (alleles) can be present
  • Blood types in Humans
    • A-B-O blood series
      • Four blood types are generated (A,B, AB, O)
      • Blood groups represent the presence or absence of surface antigens on the red blood corpuscles
        • A individuals do not produce antibodies to the A antigen but do produce antibodies to B antigens
        • B type :no anti-B but do have anti-A
        • AB type no antibodies to the surface antigen
        • O type produces anti-A and anti-B but no antigens

Deleterious Alleles that Arise and Survive in Populations

• The vast number of alterations to genes (mutations) are deleterious
  • Evolution has a billions of years to fine tune the functioning of genetic structures
  • Few mutations provide a selective advantage to the individual
    • Mutations are one of the major source of genetic variability

Heterozygous Maintenance of Deleterious Alleles

• Selection can act against mutations only if it alters the phenotype
  • Dominant deleterious mutations can be selected against and removed from the population quickly
  • Recessive mutations are not as rapidly selected against due the protection of that gene in the heterozygous condition
    • Sickle cell anemia is pleiotrophic

How Sex Alters Inheritance

• Diploid individuals have generally have two copies of each gene, one on each of the homologous chromosomes
  • Species with sexually distinct phenotypes have modified arrangement
    • One homologous pair determines the sexual phenotype (Sex Chromosomes)
    • The chromosomes are referred to as the Autosomal chromosomes

Sex Chromosomes

• In humans and many other species the sexual phenotype is controlled by the sex chromosomes
  • XX condition results in female expression
  • XY produces the male phenotype
• X-chromosome carries several genes while the Y has only a few genes

Sex Linked Characteristics

• Male which receive only one X chromosome will phenotypically express the alleles for the genes which are carried on that chromosome
• No true homologous chromosome is present
• Recessive deleteerious alleles can be directly selected against
  • Color Blindness and hemophilia

Barr Bodies

• Female humans and many other mammalian species receive two X chromosomes.
  • Each X chromosomes has a homologous allele
  • In somatic cells one of these chromosome undergoes condensation and produces a structure called a Barr body
    • In heterozygous women this produces a sexual mosaic of genes
      • Calico cats are an example of this mosaic pattern being expressed in the phenotype

• This page is maintained by James C. Pushnik