Mendel and the Gene Idea

1
Mendel and the Gene Idea

• AP Biology Chapter 14

2
Early Ideas about Heredity

• Blending
• Traits from parents act like paints being mixed
• Does NOT fit our observations
• Nonetheless, this was an accepted idea during
  Mendels time.

3
Mendel?

• Came up with a particulate mechanism for
  inheritance
• Said an organisms collection of genes was more
  like a bucket of marbles than a pail of paint.
• Marbles do not get diluted as they are passed
  from person to person
• They are DISCRETE units that retain their
  separate identities in offspring.

4
About Mendel

• Born in Austria
• Became a monk
• While studying at a university, became interested
  in plant breeding and in inheritance
• This is an image of Mendels garden.

5
Why Pea Plants?

• Easily available and in many varieties
• Character inheritable feature
• Example Flower color is a character with two
  possible variations / traits purple or white
• Mendel chose to observe characters that varied in
  a discrete way
• on or off
• No in between
• A very wise choice

6
(No Transcript)
7
(No Transcript)
8
Why Pea Plants?

• Self-fertilizing
• Normally, the reproductive parts are hidden
  within flower petals.
• Mendel could strictly control which plants mated
  with which.

9
Another wise choice by Mendel

• He always started his experiments with plants
  that bred true for the particular character to be
  studied
• True-breeding
• Means a self-fertilizing plant ALWAYS has
  offspring of the same variety.
• Example Purple flowered parent ALWAYS has
  purple flowered offspring.

10
A Typical Mendelian Breeding Experiment

• Cross two PARENTS that are TRUE-BREEDING, yet
  contrasting varieties
• Called P generation
• Crossing two different varieties is called
  hybridization
• Their offspring are called the F1 generation
• Allow the F1 generation to self-fertilize
• The F1s produce a generation called the F2
  generation

11
What Mendel Did that was DIFFERENT

• The CRITICAL thing that Mendel did in his
  experiments that NO ONE else had done was to
  COUNT the different varieties of offspring that
  resulted from the different crosses.
• In particular, it was his counting and analysis
  of the F2 generation plants that allowed him to
  reveal two of the most fundamental principles of
  genetics
• Law of segregation and law of independent
  assortment

12
Another thing that Mendel did

• He grew A LOT of peas
• Thus he had VERY large sample sizes
• This allowed the ratios to really become apparent
  to him
• He also kept really good records of his work and
  data.

13
What Mendels Numbers Told Him

• Cross a pure purple with a pure white
• Get F1s that are ALL purple
• Did the white trait disappear?
• Allow F1s to self fertilize
• Get F2s that show up in numbers like these
• 705 purple 224 white
• This is a ratio of approximately 31
• Told him the white trait did NOT disappear, nor
  was it diluted.

14
Mendels Hypothesis Part 1

• Alternative versions of genes account for
  variations in inherited characters.
• Example the gene for flower color in pea plants
  exists in two versions purple and white.
• The alternate versions are called alleles.

15
Mendels Hypothesis Part 1

• Relating Genes to Chromosomes
• Each gene resides at a specific locus on a
  specific chromosome
• The DNA at that locus can vary, but only a little
  bit in its information content.
• The purple flower and white flower alleles are
  two variations possible on the flower color locus
  on one of a pea plants chromosomes.

16
Mendels Hypothesis Part 2

• For each character an organism has TWO alleles
• Two sets of chromosomes in each body cell
• One from each parent
• Thus, one allele for every character from each
  parent two alleles per character per organism.
• Amazingly, Mendel was able to figure this out
  while possessing NO knowledge of chromosomes.

17
Mendels Hypothesis Part 3

• If the two alleles at a locus differ, then the
  dominant allele determines the organisms
  appearance
• The recessive allele has no noticeable effect

18
Mendels Hypothesis Part 4

• Law of Segregation
• Two alleles for a character separate during
  gamete formation and end up in different gametes
• This means that any given sperm or egg gets only
  ONE of the TWO alleles that are present in the
  body cells of the organism.
• This describes MEIOSIS
• Homologous pairs separate at meiosis
• Each member of a pair carries its own allele for
  a given character.
• Of course, if an organism is true-breeding for a
  trait, then both chromosomes will have the same
  allele, so all gametes will be the same with
  regard to that allele. If they differ than
  gametes will differ.

19
Punnett Squares

• Punnett Squares show how Mendels model accounts
  for the 31 ratios he observed in F2 generations.

20
Vocabulary

• Homozygous
• Heterozygous
• Phenotype
• Genotype

21
Test Cross
22
Monohybrid vs. Dihybrid Crosses

• Monohybrid
• Parents differ in one trait
• Dihybrid
• Parents differ in two traits

23
Dihybrids and the Law of Independent Assortment

• When following TWO traits and not just one
  (example flower color and plant height) each
  PAIR of alleles segregates independently of other
  pairs of alleles when gametes are forming in
  meiosis
• In other words, the homologous pairs of
  chromosomes order themselves randomly along
  either side of the metaphase plate.

24
(No Transcript)
25
Dihybrid Cross
26
Laws of Probability

• The same laws of probability govern Mendels Laws
  as govern coin tosses and rolls of dice.

27
Laws of Probability

• Multiplication Rule
• Used to determine the probability that two or
  more independent events will occur to gether in
  some specific combination
• What is the chance that two coins tossed together
  will both be heads?
• Multiply the probability of one event by the
  probability of the other event
• ½ X ½ ¼

28
Multiplication Rule in a Cross

• A plant heterozygous for seed shape (Rr) is
  crossed with another heterozygote.
• Each gamete produced has a ½ chance of carrying R
  and a ½ chance of carrying r.
• The same applies to each sperm cell.
• Thus, the probability of getting a rr offspring
  is ¼.

29
Rule of Addition

• The probability that any one of two or more
  mutually exclusive events will occur is
  calculated by adding together their individual
  probabilities.
• The probability for one possible way of obtaining
  an F2 heterozygote (Rr) is ¼. The probability
  for the other possible way is ¼.
• The rule of addition says ¼ ¼ ½

30
(No Transcript)
31
Complex Problems Can Be Solved Using the Laws of
Probability

• Tracking 3 different characters in a cross
  (trihybrid)
• Plant 1 purple flowers, yellow and round seeds
  (heterozygous for all)
• PpYyRr
• Plant 2 purple flowers (heterozygous), green
  and wrinkled seeds
• Ppyyrr

32
Complex Problems Can Be Solved Using the Laws of
Probability

• PpYyRr X Ppyyrr
• What fraction from this cross would be predicted
  to have the recessive phenotypes for at least two
  of the three characters?
• First, list all genotypes possible (given these
  two parents) that would fulfill this condition
• PPyyrr, Ppyyrr, ppyyRr, ppYyrr, ppyyrr

33
Complex Problems Can Be Solved Using the Laws of
Probability

• Next, use the rule of multiplication to calculate
  the probability of each of these genotypes
  occurring from this cross
• Take each pair in the cross as though it were a
  monohybrid
• What is the liklihood of getting ppyyRr from
  PpYyRr X Ppyyrr
• Likelihood of Pp x Pp giving pp is ¼
• Likelihood of Yy x yy giving yy is ½
• Likelihood of Rr x rr giving Rr is ½
• ¼ x ½ x ½ 1/16

34
Complex Problems Can Be Solved Using the Laws of
Probability

• Likelihood of getting ppYrr ¼ x ½ x ½ 1/16
• Likelihood of getting Ppyyrr ½ x ½ x ½ 1/8
• Likelihood of getting PPyyrr ¼ x ½ x ½ 1/16
• Likelihood of getting ppyyrr ¼ x ½ x ½ 1/16

35
Complex Problems Can Be Solved Using the Laws of
Probability

• Finally, the law of addition is used to add
  together the probabilities for all of the
  different genotypes that fulfill the condition of
  at least two recessive traits.
• 1/16 1/16 1/8 1/16 1/16 6/16 or 3/8

36
Other Inheritance Patterns

• Mendels original observations are just the tip
  of the iceberg with regard to the complexity of
  genetics
• There are many other inheritance patterns that
  exist beyond Mendels Complete Dominance
• Complete Dominance
• What weve been studying typical Mendel
• One allele completely masks another
• Phenotypes of the heterozygote and homozygote
  dominant are indistinguishable

37
Codominance

• Two alleles affect a phenotype in separate and
  distinguishable ways.
• Blood types

38
Incomplete Dominance

• When heterozygote phenotypes fall in the middle
  between either homozygote
• Appears blended

39
Relation between Dominance and Phenotype

• Dominant and recessive alleles do not actually
  interact at all. One does not truly dominate
  the other.
• Genes code for proteins.
• Consider the case of a recessive genetic disorder
• recessive alleles are very often broken genes
• The gene does not work to make the protein it
  should perhaps it does not make any protein at
  all.
• The dominant gene in such a relationship is
  just the working gene. It is making proteins.
• In organisms that are homozygous dominant, both
  alleles are working and making the protein the
  organism needs
• In organisms that are heterozygous, one copy of
  the gene is working and making protein while the
  other copy is not however in this condition the
  organism still appears normal. Apparently
  because making only ½ the amount of protein is
  still adequate.
• In the homozygous recessive organism, there are
  NO copies of the working gene, thus NO protein is
  being made. In this case, normally the organism
  has a problem or at least some phenotype that is
  distinguishable.

40
Classic Example Tay Sachs

• Recessive disorder
• Protein required breaks down lipids in the brain
• Homozygous dominant people have two working
  copies of this gene and make plenty of the
  protein
• Heterozygotes have one working copy and one
  broken copy. They make half the protein of a
  homozygous dominant person but this must be
  enough because they show no symptoms
• Homozygous recessive people do not make ANY of
  this protein. This results in catastrophic build
  up of lipid in the brain which essentially
  crushes brain tissue. Death typically occurs by
  age 5.

41
Frequency of Dominant Alleles

• Dominant alleles are more common, right?
• NOT necessarily for every trait
• Polydactyly is very rare, but is DOMINANT to
  having the normal numbers of fingers.

42
Multiple Alleles

• Though any one individual is only ALLOWED to
  possess TWO alleles per trait (or locus), there
  are OFTEN many MORE than two alleles that EXIST.
• Example ABO blood group
• 3 alleles exist A, B and O
• Nonetheless, any one individual can only possess
  two of these.

43
Pleiotropy

• We often treat genes as though the affect only
  phenotypic trait, but in fact, they often have
  many phenotypic effects.
• Pleiotropy when genes have mulitple phenotypic
  effects
• Sickle cell anemia has MANY effects on the people
  who have it.

44
Epistasis

• Sometimes the gene at one locus can alter the
  phenotypic expression of a gene at a second
  locus. This is epistasis.

45
Example of Epistasis

• Mice
• One gene determines whether a mouse will have
  black fur (dominant) or brown fur (recessive)
• A second gene determines whether or not pigment
  (whether black or brown) will be deposited in the
  hair at all.

46
(No Transcript)
47
Polygenic Inheritance

• Many characters do not appear to have an either
  / or classification
• Many characters differ along a continuum
• Human skin color and height
• Termed quantitative characters
• Quantitative characters often indicate polygenic
  inheritance
• Additive effect of two or more genes on single
  trait
• Skin color is probably a product of at least 3
  separate genes

48
Nature vs. Nurture

• The environment affects the ability of genes to
  be expressed
• A person may have genes that program them to be 6
  feet tall, but if he does not have proper
  nutrition, he will not attain that height.
• It is important to remember that MANY factors can
  be involved in certain aspects of phenotype.

49
Pedigree Analysis
50
(No Transcript)
51
Recessive Disorders

• Thousands of disorders fall into this group
• Carriers (heterozygotes)
• Carry an allele, but have not symptoms
• 2 famous recessive disorders
• Cystic fibrosis
• Sickle Cell Anemia

52
Cystic Fibrosis

• Most common lethal genetic disease in the US
• 1/2500 people of European descent
• 1/25 people of European descent are carriers
• Chloride transport channels in membranes are
  dysfunctional
• Results in too much chloride outside of cells and
  very viscous mucus. Multiple organs are affected.
• Leads to many (pleiotropic) effects
• Poor absorption of nutrients
• Chronic bronchitis
• Recurrent bacterial infections

53
Sickle-Cell Disease

• Most common genetic disorder of people of African
  descent.
• 1/400 African-Americans
• ONE amino acid in the hemoglobin protein is
  wrong.
• Blood cells become misshapen clump and clog
  blood vessesls
• Multiple symptoms throughout the body
  (pleiotropy)

54
Inheritance of Sickle Cell

• Not a straight-forward recessive disorder
• 2 recessive alleles are needed for full blown
  sickle cell disease
• The presence of 1 allele can affect phenotype
• Such people have sickle-cell trait
• They have a few symptoms occasionally, but are
  generally normal
• Thus, this trait in the organism appears
  incompletely dominant
• In fact, in the cells, some cells have deformed
  hemoglobin while others do not. So the trait at
  the molecular level is codominant.

55
Close Relatives and Recessive Disorders
56
Dominant Disorders

• Achondroplasia
• Polydactyly
• Huntintons Disease
• Lethal dominant alleles are far less common than
  lethal recessive alleles
• They cannot be masked.
• Can be passed on only if death is caused at a
  relatively advanced age

57
Multifactorial Disorders

• Heart Disease
• Genetic component environmental factors can
  influence disease
• Very complex

58
Counseling and Testing

• Counseling using family history
• Tests of expectant parents becoming routine
• Fetal Testing
• Amniocentesis
• Chorionic villus sampling
• Newborn screening
• PKU

59
(No Transcript)
60
(No Transcript)
61
(No Transcript)