Chapter 11 Introduction to Genetics

1
Chapter 11 Introduction to Genetics
CPI BIOLOGY Hollison High School
2
11-1 The Work of Gregor Mendel

• What is Genetics? the study of heredity
• Gregor Mendels Peas
• Pollen plants sperm
• Egg Cells plants reproductive cells
• Fertilization joining of pollen egg cells ?
  develops intos embryo in a seed

Born in 1822. His work with pea plants laid the
foundation for Genetics.
3

• Working with pea plants
• Self-pollinating pollen fertilizes egg cells in
  the SAME flower (single parent reproduction)
• True-breeding offspring genetically identical to
  parents due to self-pollination
• Cross-pollination combining reproductive cells
  from 2 DIFFERENT parent plants

Mendel could cross- breed a purple flower with a
white flower What do you think is the color of
the offspring?
4
Genes and Dominance

• TRAIT specific characteristic (seed color, plant
  height, etc)
• What did Mendel do in his pea plant experiments?
• Studied 7 different traits each with contrasting
  characters (ex) Height, short or tall
• He crossed the plants (with contrasting
  characters) and looked at their offspring

5
P parental generation original pair of
plants F1 first filial generation first
generation Hybrids offspring from parents with
different traits
6
Tracking Generations

• Parental generation P
• mates to produce
• First-generation offspring F1
• mate to produce
• Second-generation offspring F2

7
CROSS-POLLINATION Mendel cut the male parts of
one flower (ouch!) and dusted the female parts
with pollen from another flower.
P GENERATION purple x white flowers F1
GENERATION all purple flowers
HYBRID PLANTS
8

• What happened in Mendels crosses?
• All the offspring only had one of the parents
  charactersthe other parents character
  disappeared!!
• Mendels Conclusions
• Inheritance is determined by factors that are
  passed down
• GENES the factors that determine traits
• Contrasting characters are different forms of a
  gene called ALLELES

9

• Mendels Principle of Dominance

• some alleles are dominant, some are recessive
• DOMINANT ALLELE form of trait that will always
  be exhibited usually expressed in capitals
• RECESSIVE ALLELE form of trait is only exhibited
  when the dominant allele is NOT present
• (ex) Allele for tall is dominant for and the
  allele for short is recessive

10
What happened to the recessive allele?

• Mendel wanted to know if the recessive allele
  disappeared from the F1 plants.
• F1 CROSS He self-crossed the F1 generations to
  make F2 offspring

11
THE F2 GENERATION

• The recessive traits reappeared!!
• ¼ plants had white flowers, the recessive trait
• Summary of Crosses
• tall plants X short plants ? tall plants
• P P F1
• tall plants self-pollinating ? ¼ short, ¾ tall
• F1 Cross F2

12
Explaining the F1 Cross

• Why did the recessive allele reappear? At some
  point, the recessive allele had to separate from
  the dominante allele. This is called
• SEGREGATION separation of alleles ? occurs
  during formation of gametes (eggs sperm) in
  anaphase II of meiosis
• F1 plants inherited 1 tall allele 1 short
  allele from parents
• When gametes are formed, the two alleles
  segregate from each other ? each gamete has 1
  copy of each gene
• So, 2 different types of gametes are formed (one
  w/ tall allele, one w/ short allele)

13
SEGREGATION
14
11-2 Probability Punnett Squares

• Mendel realized that the principles of
  probability can explain the results of genetic
  crosses.
• PROBABILITY likelihood an event will occur (ex)
  Flip coin 3x in a row, 1/8 chance it will be
  heads all 3 times ( ½ x ½ x ½ )
• The pattern in which alleles segregate is
  randomjust like a coin flip! So which ever
  allele gametes receive is also random.

15
Punnett Squares

• What is a Punnett Square?
• A diagram showing the possible genetic
  combinations from a particular cross
• Can be used to predict and compare the genetic
  variations that will result from a cross
• What do the letters represent in a punnett
  square?
• Letters represent alleles capital dominant
  lowercase recessive
• Homozygous has two identical alleles for a trait
  (ex) TT or tt
• Heterozygous has two different alleles for the
  same trait (ex) Tt

16
Punnett Squarefor TT x Tt
Punnett Squarefor YY x yy
17
Genotype vs Phenotype

• GENOTYPE the genetic makeup of an organism (ex)
  TT
• PHENOTYPE the physical characteristics
  exhibitied (ex) tall plant
• In the Punnett Square shown
• What is the genotype of the offspring?
• What is the phenotype?

18
Probability and Segregation

• F2 generation from Tall F1 plants ?
• ¾ tall, ¼ short
• 31 ratio of tall to short plants
• Punnet squares work to predict outcomes, so
  Mendels ideas about segregation are accurate!

19
11-3 Exploring Mendelian Genetics

• Does the gene that determines flower color have
  anything to do with the gene for height?
• (ex) Do all tall plants have purple flowers?
• Mendel performed TWO-FACTOR CROSSES crossing 2
  different genes and following traits as they pass
  from one generation to the next

20
Two-Factor Cross F1

• Two Genes shape of pea color of pea
• The Cross Round yellow peas x wrinkled green
  peas
• RRYY x rryy
• What are the possible alleles parent 1 can pass?
  RY
• What are the possible alleles parent 2 can pass?
  ry
• Draw a Punnett Square for this cross.
• All F1 were RrYy (round and yellow) or HYBRIDS
• This cross does not answer question, but
  provides hybrids for next cross

21
Two-Factor Cross F2

• F1 Generation RrYy
• How would these alleles segregate when F1
  self-pollinated?
• RrYy x RrYy
• Do the two dominant alleles stay together?
• What are the possible alleles each parent can
  pass on?
• There are 4 possible combinations Ry, RY, rY, ry
• Draw a Punnett Square for this cross.

22
INDEPENDENT ASSORTMENT

• The F1 Hybrid cross produces a 9331 phenotype
  ratio
• Mendel found that the 2 alleles (seed shape
  seed color) dont influence each others
  inheritance
• This is called the principle of Independent
  Assortment genes for different traits can
  segregate independently during the formation of
  gametes

23
Independent Assortment
OR
Metaphase I
A
A
A
A
a
a
a
a
B
B
B
B
b
b
b
b
Metaphase II
A
A
A
A
a
a
a
a
B
B
B
B
b
b
b
b
Gametes
B
B
B
B
b
b
b
b
A
A
A
A
a
a
a
a
1/4 AB
1/4 ab
1/4 Ab
1/4 aB
24
Summary of Mendels Principles

• Inheritance of characteristics is determined by
  genes which are passed to offspring
• If 2 alleles of a trait exist, some alleles may
  be dominant, others may be recessive
• Sexually reproducing organisms have 2 copies of
  each gene which segregate during gamete formation
• Alleles for different genes segregate
  independently

25
Beyond Dominant and Recessive Alleles

• Genetics is more complicated
• Some alleles are neither dominant nor recessive
• Many traits are controlled by multiple alleles or
  multiple genes

26
Other Inheritance Patterns

• Incomplete Dominance
• Codominance
• Multiple Alleles
• Polygenic Traits

27
Incomplete Dominance
Homozygous parent (RR)
Homozygous Parent (rr)
X

• When one allele is not completely dominant
  recessive allele is not totally masked
• Heterozygous phenotype is in between the two
  homozygous phenotypes
• (ex) Red snapdragon flowers (RR) X snapdragon
  white (rr) flowers ? pink hybrid flowers (Rr)

All F1 are heterozygous
X
F2 shows three phenotypes in 121 ratio
28
Incomplete Dominance
homozygous parent
X
homozygous parent
All F1 offspring heterozygous for flower color
Cross two of the F1 plants and the F2 offspring
will show three phenotypes in a 121 ratio
29
Codominance

• Both alleles contribute to the phenotype
• Heterozygous genotype expresses both phenotypes
• (ex) Feather colors in chickens white feathers X
  black feathers ? speckled chicken
• (ex) Horse coats red X white? roan coat

30
Codominance ABO Blood Types

• Alleles that controls blood type are codominant
• Two alleles A B are both exhibited when paired,
  a third allele (i) is recessive to others
• AA or Ai Type A Blood
• BB or Bi Type B Blood
• AB Type AB Blood
• ii Type O Blood

31
Multiple Alleles

• gt 2 possible alleles for a gene
• Individuals can still only have 2 alleles each
  but more than 2 alleles exist in a population
• (ex) coat color in rabbits ? lots of options due
  to 4 different alleles
• (ex) blood type is determined by multiple alleles

32
Polygenic Traits

• Traits controlled by the interaction of 2 genes
• (ex) Fruit fly eye color (3 different genes)
• (ex) Skin color in humans (4 different genes),
  eye color, height, weight

33
Applying Mendels Principles

• Early 1900s Morgan used Mendels principles to
  study fruit flies ? advanced study of genetics
• Mendels Principles also apply to study the
  inheritance of human traits and to calculate the
  probability of traits appearing in the next
  generation.

34
11-1 11-2 Mini-Quiz (10pts)

• Who worked with pea plants and came up with the
  fundamental principles of genetics?
• After the P (parent) generation, what are the
  next two generations called?
• Factors that determine traits are called _____
  and contrasting characters of traits are called
  ______.
• When two hybrid plants are crossed, what
  percentage exhibits the dominant phenotype? What
  percentage exhibits the recessive phenotype?
• Draw the Punnett Square for a heterozygous tall
  plant and a short plant. What is the genotype
  ratio for the offspring? What is the phenotype
  ratio for the offspring?

35
11-1 11-2 Mini-Quiz Answer Key (10pts)

• (1) MENDEL
• (2) F1 F2
• (2) GENES ALLELES
• (2) 75 DOMINANT 25 RECESSIVE
• (3) Genotype ratio 50 Tt, 50 tt
• Phenotype ratio 50Tall, 50short