Genetics

 

1862	1868	1880

 

Genetic Terminology:

• Trait – any characteristic that can be passed from parent to offspring
• Heredity – passing of traits from parent to offspring
• Genetics – study of heredity
• Alleles – two forms of a gene (dominant & recessive)
• Dominant – stronger of two genes expressed in the hybrid; represented by a capital letter (R)
• Recessive – gene that shows up less often in a cross; represented by a lowercase letter (r)
• Genotype – gene combination for a trait (e.g. RR, Rr, rr)
• Phenotype – the physical feature resulting from a genotype (e.g. tall, short)
• Homozygous genotype – gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr); also called pure 
• Heterozygous genotype – gene combination of one dominant & one recessive allele    (e.g. Rr); also called hybrid
• Monohybrid cross – cross involving a single trait
• Dihybrid cross – cross involving two traits
• Punnett Square – used to solve genetics problems

Blending Concept of Inheritance:

• Accepted before Mendel’s experiments
• Theory stated that offspring would have traits intermediate between those of its parents such as red & white flowers producing pink
• The appearance of red or white flowers again was consider instability in genetic material
• Blending theory was of no help to Charles Darwin’s theory of evolution 
• Blending theory did not account for variation and could not explain species diversity
• Particulate theory of Inheritance, proposed by Mendel, accounted for variation in a population generation after generation
• Mendel’s work was unrecognized until 1900

Gregor Mendel:

• Austrian monk
• Studied science & math at the University of Vienna
• Formulated the laws of heredity in the early 1860’s
• Did a statistical study of  traits in garden peas over an eight year period

 

 

Why peas, Pisum sativum?

• Can be grown in a small area
• Produce lots of offspring
• Produce pure plants when allowed to self-pollinate several generations
• Can be artificially cross-pollinate

GARDEN PEA

Mendel’s Experiments:

• Mendel studied simple traits from 22 varieties of  pea plants (seed color & shape, pod color & shape, etc.)
• Mendel traced the inheritance of individual traits & kept careful records of numbers of offspring
• He used his math principles of probability to interpret results
• Mendel studied pea traits, each of which had a dominant & a recessive form (alleles)
• The dominant (shows up most often) gene or allele is represented with a capital letter, & the recessive gene with a lower case of that same letter (e.g. B, b)
• Mendel’s traits included:

         a. Seed shape —  Round (R) or Wrinkled (r)
            b. Seed Color —- Yellow (Y) or  Green (y)
            c. Pod Shape — Smooth (S) or wrinkled (s)
            d. Pod Color —  Green (G) or Yellow (g)
            e. Seed Coat Color —  Gray (G) or White (g)
            f. Flower position — Axial (A) or Terminal (a)
            g. Plant Height — Tall (T) or Short (t)
            h. Flower color — Purple (P) or white (p)

•  Mendel produced pure strains by allowing the plants to self-pollinate for several generations
• These strains were called the Parental generation or P1 strain
• Mendel cross-pollinated two strains and tracked each trait through two
  generations (e.g. TT  x  tt )

   

  Trait – plant height
  Alleles – T tall, t short
  P1 cross    TT  x  tt				genotype      —    Tt
  	t	t		phenotype    —    Tall
  T	Tt	Tt		genotypic ratio –all alike
  T	Tt	Tt		phenotypic ratio- all alike

   

• The offspring of this cross were all hybrids showing only the dominant trait & were called the First Filial or F1 generation
• Mendel then crossed two of his F1 plants and tracked their traits; known as an F1 cross

• When 2 hybrids were crossed, 75% (3/4) of the offspring showed the dominant trait & 25% (1/4) showed the recessive trait; always a 3:1 ratio
• The offspring of this cross were called the F2 generation
• Mendel then crossed a pure & a hybrid from his F2 generation; known as an F2 or test cross

• 50% (1/2) of the offspring in a test cross showed the same genotype of one parent & the other 50% showed the genotype of the other parent; always a 1:1 ratio

Problems: Work the P1, F1, and both F2 crosses for all of the other pea plant traits & be sure to include genotypes, phenotypes, genotypic & phenotypic ratios.

• Mendel also crossed plants that differed in two characteristics (Dihybrid Crosses)
  such as seed shape & seed color
• In the P1 cross, RRYY  x  rryy, all of the F1 offspring showed only the dominant form for both traits; all hybrids, RrYy

• When Mendel crossed 2 hybrid plants (F1 cross), he got the following results

Problems: Choose two other pea plant traits and work the P1 and F1 dihybrid crosses. Be sure to show the trait, alleles, genotypes, phenotypes, and all ratios. 

Results of Mendel’s Experiments:

• Inheritable factors or genes are responsible for all heritable characteristics
• Phenotype is based on Genotype
• Each trait is based on two genes, one from the mother and the other from the father
• True-breeding individuals are homozygous ( both alleles) are the same
• Law of Dominance states that when different alleles for a characteristic are inherited (heterozygous), the trait of only one (the dominant one) will be expressed. The recessive trait’s phenotype only appears in true-breeding (homozygous) individuals

 

Trait: Pod Color
Genotypes:	Phenotype:
GG	Green Pod
Gg	Green Pod
gg	Yellow Pod

 

• Law of Segregation states that each genetic trait is produced by a pair of alleles which separate (segregate) during reproduction

• Law of Independent Assortment states that each factor (gene) is distributed (assorted) randomly and independently of one another in the formation of gametes

Other Patterns of Inheritance:

• Incomplete dominance occurs in the heterozygous or hybrid genotype where the 2 alleles blend to give a different phenotype
• Flower color in snapdragons shows incomplete dominance whenever a red flower is crossed with a white flower to produce pink flowers

• In some populations, multiple alleles (3 or more) may determine a trait such as in ABO Blood type
• Alleles A & B are dominant, while O is recessive

• Polygenic inheritance occurs whenever many variations in the resulting phenotypes such as in hair, skin, & eye color
• The expression of a gene is also influenced by environmental factors (example: seasonal change in fur color)