MENDELIAN GENETICS

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MENDELIAN GENETICS
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ANNOUNCEMENTS

• The first set of genetics problems will be posted
  on the course web site by end of today!
• Exam II will be held in 1.5 weeks
• No class on July 4th

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OBJECTIVES

• Understand Mendels principles/laws governing
  genetics
• Understand the meaning of relevant vocabulary
  discussed in class
• Be able to predict results of a mono dihybrid
  cross using a Punnett square
• Know exceptions to the rules

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INTRODUCTION TO GENETICS TERMINOLOGY

• Diploid
• Cell (or organism) with pairs of homologous
  chromosomes
• Haploid
• Cell/org. with only one chromosome of each kind

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GENETICS TERMINOLOGY

• Gene (Character)
• A feature that is heritable
• Locus
• Specific area on chromosome where a gene is found
• Allele (Trait)
• Variation of a character.
• Diff. forms of the SAME gene.
• Genotype
• The genetic makeup of an organism (combination of
  genes in its nucleus)
• Phenotype
• The physical appearance of an organism

Brown Eyes
Blue Eyes
Black Hair
Red Hair
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GENETICS

• Genetics
• The scientific study of heredity
• Heredity
• The transmission of traits from parents to
  offspring

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MENDEL

• Gregor Mendel
• Interested in basic patterns governing the
  transmission of traits from parent to offspring

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MENDEL

• Mendels work precedes Darwin and an
  understanding of meiosis/genes/chromosomes

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HISTORICAL HYPOTHESES REGARDING INHERITANCE

• The Blending Inheritance Hypothesis
• Offspring represent a blend of characteristics
  from two parents
• The Inheritance of Acquired Characteristics
  Hypothesis
• Traits present in parents are modified, through
  use, and passed on to offspring in modified form

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MENDEL

• Mendel performed crosses between true breeding
  lines of garden peas
• Began studying inheritance of ONE trait
• Monohybrid cross
• Total of 7 traits were studied
• Later, multiple traits studied together

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MENDELS CROSSES

• Mendel's plants had two observable forms
  (alleles) of each character (gene)
• Green or Yellow seeds

• Round or Wrinkled seeds

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Forms of Trait (Phenotype)
Trait
or
Seed shape
Round
Wrinkled
Round
Wrinkled
or
Seed color
Green
Yellow
Yellow
Green
Pod shape
or
Constructed
Inflated
Smooth
Constricted
or
Pod color
Green
Yellow
Green
Yellow
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Trait
Forms of Trait (Phenotype)
or
Flower color
Purple
White
White
Purple
or
Flower and pod position
Axial (on stem)
Terminal (at tip)
Axial (on stem)
Terminal (at tip)
or
Stem length
Dwarf
Tall
Tall
Dwarf
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MENDEL

• Looked at traits exhibited by progeny of crosses
• He studied phenotype and INFERRED GENOTYPE!
• Mendel found observable evidence of how parents
  transmit genes to offspring

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MENDELS PEA PLANTS

• Pisum sativum
• Can self fertilize
• Male female parts on same flower

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• Male parts anthers
• Pollen grain contain sperm
• Female parts pistil
• Ovule eggs

SELF FERTILIZATION
Stigma (receives pollen)
Anthers (produce pollen grains, which contain
male gametes)
Ovules (produce female gametes)
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MENDELS PEA PLANTS

• Controlled Mating
• Cut off reproductive organ to stop self
  fertilization
• Cross fertilize plants in a controlled fashion
• Plant that contributes pollen is considered
  male
• Plant that receives the pollen is considered
  female

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CROSS-POLLINATION
1. Remove anthers from one plant.
2. Collect pollen from a different plant.
3. Transfer pollen to stigma of the 1st
plant (plant without anthers)
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FIRST HALF OF RECIPROCAL CROSS
to stigma of wrinkled-seeded parent
Pollen from round- seeded parent
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SECOND HALF OF RECIPROCAL CROSS
Polen from wrinkled- seeded parent
To stigma of round-seeded parent
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MENDELS CROSSES
Round Parent
x
Wrinkled Parent
All Round Offspring (F1)
(Wrinkled trait disappeared!)
Reciprocal cross (control for sex) produced
same results
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MENDELS CROSSES

• Next, Mendel crossed F1 (bro-sis mating)

Round (F1)
Round
x
Get both types of offsrping (F2)
Wrinkled trait re-emerges in F2
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MENDELS CROSSES

• Mendel Determines (Phenotypic) Ratio
• F2 5474 Round 1850 Wrinkled
• Nearly 3 1
• Similar pattern observed in other 6 traits

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MENDELS CONCLUSIONS
Genes do not blend together. The hereditary
determinants, or genes, maintain their integrity
from generation to generation. They do not
blend together, and they do not acquire
characteristics in response to actions by an
individual
1.
Peas have two versions, or alleles, of each gene.
This is also true for many other organisms.
2.
Each gamete contains one allele of each gene.
Pairs of alleles segregate during the formation
of gametes
3.
Males and females contribute equally to the
genotype of their offspring. When gametes
fuse, offspring acquire a total of two
allelesone from each parent.
4.
Some alleles are dominant to others. When a
dominant and recessive allele for the same gene
are found in the same individual, that individual
exhibits the dominant phenotype.
5.
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MENDELS SYMBOLS

• Mendel developed system of assigning symbols to
  hereditary characters
• Letters represent versions of a gene (alleles)
• Ex If Letter r represents a gene then
• R symbolizes dominant allele
• r symbolizes recessive allele

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PUNNETT SQUARE

• R.C. Punnett, a leading geneticist in the early
  1900s, invented a technique called the Punnett
  Square
• Place gamete possibilities for 2 parents along
  axes
• Internal boxes represent union of gametes
• Used to predict genotype and phenotype of
  potential zygote

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MENDELS CROSS OF PURE LINE PEA PLANTS
PARENT R R

R is round r is wrinkled
PARENT r r
hint gametes along top, whole people on
inside
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MENDELS CROSS OF PURE LINE PEA PLANTS
R Dominant allele for seed shape (round)
r Recessive allele for seed shape (wrinkled)
Round-seeded Father
Wrinkled-seeded Mother
Parental generation (homozygous)
Meiosis
Gametes
Fertilization
F1 generation
Rr
Rr
Rr
Rr
All have Rr genotype (heterozygous) and round
seed phenotype
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MENDELS CROSS OF F1 PLANTS
Crossing the F1s Rr x Rr
R round r wrinkled
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PROBABILITY THEORY

• In Diploid Organisms,
• Each allele has 50 chance of being found in a
  particular gamete
• To calculate likelihood of two alleles combining
  (i.e. fertilization)
• Multiply probabilities together

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MENDELS CROSS OF F1 PLANTS
Round- phenotype Wrinkled phenotype
Mother
R Dominant allele (round) r Recessive allele
(wrinkled)
Rr
Female gametes
r
R
R
R
Father
Rr
RR
Male gametes
Rr
r
Rr
rr
Resulting genotypes 1/4 RR 1/2 Rr
1/4 rr
Resulting phenotypes 3/4
1/4
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POSSIBLE GENOTYPES

• Three possible genotypes for each gene in a
  diploid cell
• Homozygous
• Cell (diploid) with two doses of same allele
• Homozygous Dominant
• Both alleles of a gene are of the Dominant
  variety (RR)
• Homozygous Recessive
• Both alleles of a gene are of the Recessive
  variety (rr)
• Heterozygous
• Cell (diploid) with two differing alleles for a
  gene
• One dominant and one recessive version of gene

Mendel DID NOT use these terms, only inferred
them!
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MENDELS CROSSES

• Dihybrid Crosses
• Mendel used pea plants to follow inheritance of
  TWO traits
• Another pattern began to emerge

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MENDELS SYMBOLS

• Seed Shape
• R is dominant allele (Round)
• r is recessive allele (Wrinkled)
• Seed Color
• Y is dominant allele (Yellow)
• y is recessive allele (Green)

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IN CLASS EXERCISE

• Make a Punnett Square to predict offspring of a
  homozygous dominant individual crossed with a
  homozygous recessive individual!
• What are the genotypes of the parents?
• What are the genotypes of the gametes?
• What about the offspring?

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IN CLASS EXERCISE

• Outcome of Dihybrid Cross With Pure Bred Parents

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MENDELS CROSSES WITH PEAS THAT DIFFER IN TWO
TRAITS
R Dominant allele for seed shape (round)
r Recessive allele for seed shape (wrinkled)
Y Dominant allele for seed color (yellow)
y Recessive allele for seed color (green)
Father
Mother
Parental generation
Meiosis
Gametes
Fertilization
F1 generation
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IN CLASS EXERCISE

• Now, cross the F1s
• What is the phenotypic ratio of the offspring?
• (What Mendel Observed)
• What about the genotypic ratio?

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PUNNETT SQUARE FOR TWO TRAITS
R Dominant allele for seed shape (round) r
Recessive allele for seed shape (wrinkled) Y
Dominant allele for seed color (yellow) y
Recessive allele for seed color (green)
Parental generation
rryy
RRYY
RrYy
F1 generation
ALL
Female gametes
RrYy
1/4 RY
1/4 Ry
1/4 rY
1/4 ry
1/4 RY
RRYY
RrYy
RrYY
RRYy
1/4 Ry
RrYy
Rryy
RRyy
RRYy
RrYy
Male gametes
1/4 rY
rrYy
RrYY
RrYy
rrYY
1/4 ry
rryy
rrYy
Rryy
RrYy
Resulting genotypes
9/16R-Y-
3/16R-yy
3/16rrY-
1/16rryy
9/16
3/16
3/16
1/16
Resulting phenotypes
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F2 OFFSPRING FROM DIHYBRID CROSS
Yellow Green Wrinkled Round
F2 generation phenotype
Mendels Numbers
315
101
108
32 556
Fraction of progeny
9/16
3/16
3/16
1/16 1
Note New mixtures of traits are observed in
F2!!
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How does the 9 3 3 1 ratio for two
traits relate to the 3 1 ratio for one trait?
Round seeds Wrinkled seeds
315 108 101 32
423 133
3 1
Yellow seeds Green seeds
315 101 108 32
416 140
3 1
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MENDELS PRINCIPLE OF SEGREGATION

• Principle of Segregation
• Holds true for one or two traits
• For a single trait, 31 ratio (dominant
  recessive)
• For two traits, 9331 ratio
• Each trait, considered individually, showed 31
  ratio

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PRINCIPLE OF SEGGREGATION
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MENDELS PRINCIPLE OF INDEPENDENT ASSORTMENT

• Law of Independent Assortment
• Genes residing on different chromosomes separate
  without regard for one another

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Hypothetical Example of Independent Assortment
During meiosis I, tetrads can line up two
different waysbefore homologs separate.
OR
Brown eyesBlack hair
Blue eyesRed hair
Blue eyesBlack hair
Brown eyesRed hair
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THE TEST CROSS

• To determine genotype of an individual, cross
  with a homozygous recessive individual

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THE TEST CROSS
Homozygous recessive parent
YOUR HOMEWORK!
?
rryy
ry
All
1/4
1/4
1/4
1/4
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MENDEL

• Why Mendel Was Successful
• Unbiased
• Selected good model organism
• Used pure breeds as parents
• Large sample size
• Quantitative analysis
• Controlled experiments
• Studied obvious traits

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EXTENSION OF MENDELS RULES

• Phenotypic variation is not always due to simple
  dominance/recessive-ness of genes

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EXTENSION OF MENDELS RULES

• Some Traits Deviate from Mendelian Patterns
• Incomplete Dominance
• No allele is completely dominant over another
• Co Dominance
• Two alleles are dominant expressed
• Environmental Effects
• Environmental conditions help determine trait
• Quantitative Traits
• Trait due to interaction between more than one
  gene

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INCOMPLETE DOMINANCE

• No Allele Is Completely Dominant
• Neither red nor white allele is completely
  dominant

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Incomplete dominance in flower color
Parental generation
X
RR
rr
F1 generation All Heterozygotes
Rr
Self-fertilization
F2 generation
1/4 RR
1/4 Rr
1/4 Rr
1/4 rr
Red
Pink
White
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INCOMPLETE DOMINANCE

• Red allele red pigment
• Two copies needed for
• flower to be red
• White allele no pigment
• Need two copies for flower to be white
• In heterozygous form, neither allele dominates
• Red allele specifies enough pigment to make
  flowers pink but not red
• NOT blending because red and white flowers
  appear in F2

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CODOMINANCE

• Two alleles specify different phenotypes, but one
  does not mask expression of the other
• Type AB Blood

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CODOMINANCE

• Blood type
• Determined by type of glycoprotein on RBC
  membrane
• 3 possible alleles
• A
• B
• O
• A B are dominant to O, but co-dominant to each
  other
• The existence of more than two alleles at a gene
  locus is called multiple allelism (polymorphism)

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CODOMINANCE

• Blood Type
• With alleles AA or AO, you have type A blood
• With alleles BB or BO, you have type B blood
• With alleles OO, you have type O blood
• With alleles AB, you have type AB blood
• Both alleles expressed Codominance

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CODOMINANCE

• ABO Blood System
• Three alleles
• Six genotypes
• Four phenotypes

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ENVIRONMENTAL EFFECTS

• Environmental conditions help
• determine expression of genotype
• Ex Siamese cat
• Himalayan rabbit
• Both carry allele specifying heat-sensitive
  enzyme for melanin production

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ENVIRONMENTAL EFFECTS

• Temp. sensitive enzyme acts at surface of body
• In cold areas, enzyme is more active
• ?fur is darker

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QUANTITIVE TRAITS
A result of more than one gene contributing to
phenotype

• Some traits show continuous variation
• Eye color
• Human skin color
• Height
• The greater the number of genes influencing
  trait, the greater the variation in traits
  expressed