Passing genes to the next generation

1
Heredity

• Passing genes to the next generation

2
Father of Modern Genetics

• Gregor Mendel (1822-1884)
• Austrian monk
• Bred pea plants to effectively apply quantitative
  methods to study inheritance
• Unappreciated during his lifetime
• 1900 work was rediscovered and accepted
• Led to modern day genetic research

3
Mendels Experiments

• Some things were already known
• All hybrid plants that are offspring of
  genetically pure parents are similar in
  appearance.
• When these hybrids mate with each other, they do
  not breed true their offspring have a mixture of
  traits.
• Mendel recognized the pattern in the way these
  traits reappeared in offspring.

4
Mendels Experiments, cont.

• Choose Pisum sativum, garden pea, as his organism
  for experiment
• Easy to grow
• Many varieties were available
• Controlled pollination was relatively easy
• Removed anthers to prevent self-pollination
• Obtained plants from commercial sources
• Worked for 2 years to ensure he had true-breeding
  plants
• True-breeding produces same phenotype (physical
  appearance) in every offspring

5
Mendels Experiments, cont.

• Chose easily distinguishable phenotypes to study
• Seed color, height, etc.
• Began experiments by crossing plants from 2
  different true-breeding plants
• These were known as the parental generation or P
  generation
• All offspring looked like one of the parents
• Ex. Tall x short plants all tall offspring
• Offspring known as first filial generation of F1
  offspring
• F2 generation resulted from crossing 2 F1
  individuals
• Mendels F2 generation had 787 tall plants 277
  short plants

6
Mendels Experiments, cont.

• Most breeders thought inheritance involved
  blending of traits
• Mendel did not accept that
• How can something that has been blended become
  separated again?
• Did not explain how traits showed back up in F2
  generation
• Mendel said some hereditary factors were masked
  by others
• Dominant factor that is expressed (ex. Tall)
• Recessive factor that is hidden (ex. Short)

7
The Principle of Segregation

• Allele alternative form of a gene
• Ex. Dominant allele (T) and recessive allele (t)
• Principle of segregation before sexual
  reproduction, 2 alleles carried by an individual
  parent must become separated (segregated)
• Meiosis homologous chromosomes separate
• Each gamete contains only 1 allele of each pair
• Random fertilization leads to 3 possible
  combinations
• TT
• Tt
• tt

8
Predicting Ratio of Traits in Offspring

• Monohybrid cross used to show inheritance
  ratios of different alleles of a single locus
  (location on chromosome)
• Homozygous carries 2 identical alleles on the
  locus
• Heterozygous carries different alleles on the
  locus

9
Predicting Ratio of Traits in Offspring, cont.

• Punnett square is used to show possible egg and
  sperm combinations
• Perform the following cross
• Homozygous Tall (mother) x
• homozygous short (father)
• TT x tt

10
Predicting Ratios, cont.

• B/c of meiosis, the alleles will go in different
  gametes
• Mother will make eggs with T
• Father will make sperm with t

11
Predicting Ratios, cont.

• Each square represents a possible zygote
• 4/4 squares Tt
• 100 Tt ? genotype
• 100 Tall ? phenotype
• Notice the fathers
• phenotype is hidden
• No short plants

Tt
Tt
Tt
Tt
12
Predicting Ratios, cont.

• Now lets cross two offspring, like Mendel did
• TT ¼
• Tt ½
• tt ¼
• All 3 genotypes
• are present

TT
Tt
tt
Tt
13
Test Cross

• Used to determine heterozygosity
• Ex. Guinea pigs with black and brown coats
• Black is dominant to brown
• BB and Bb appear the same phenotypically
• If you had a black guinea pig how could you
  figure out its genotype?
• Perform a test cross
• Mate the black guinea pig with a brown guinea pig
• Brown guinea pig has bb phenotype
• Can only give b

14
Test Cross, cont.

• If the offspring were black and brown, what could
  you infer about the black parent guinea pigs
  genotype?
• Had to be Bb
• If the offspring were all black, what could you
  infer about the black parent guinea pigs
  genotype?
• Had to be BB

15
Predicting Ratios of Two Traits

• Worked same as 1 trait examples
• Ex. AaBb x AaBb
• To find gametes you must FOIL
• First, Outside, Inside, Last
• First AB
• Outside Ab
• Inside aB
• Last ab

16
Patterns of Inheritance

• Complete Dominance
• Dominant allele completely hides recessive allele
• Incomplete Dominance
• Dominant allele and recessive allele mixed in
  heterozygous individual
• Codominance
• Dominant and recessive allele are equally
  expressed
• Sex-Linked
• Trait is carried only on X chromosome
• Multiple Alleles
• More than 2 forms of the gene are present