Mendel, genes, alleles and inheritance

1

• Mendel, genes, alleles and inheritance
• Gregor Johann Mendel (1822-1884)
• was an monk who studied segregation of traits
• (differences) in the garden pea (Pisum sativum),
  
• beginning in 1854
• Published his theory of inheritance in 1865
• which was largely ignored until Mendel was
• rediscovered in 1902

2

• Mendel, genes, alleles and inheritance
• Gregor Johann Mendel (1822-1884)
• was an monk who studied segregation of traits
• (differences) in the garden pea (Pisum sativum),
  
• beginning in 1854
• Published his theory of inheritance in 1865
• which was largely ignored until Mendel was
• rediscovered in 1902

3

• Mendel, genes, alleles and inheritance
• Gregor Johann Mendel (1822-1884)
• was an monk who studied segregation of traits
• (differences) in the garden pea (Pisum sativum),
  
• beginning in 1854
• Published his theory of inheritance in 1865
• which was largely ignored until Mendel was
• rediscovered in 1902

4

• Mendel focused on 7 well-defined garden pea
  traits
• Flower/seed coat colour
• Seed colour
• Seed shape
• Pod colour
• Pod shape
• Stem length
• Flower position
• He began by self-fertilizing 34 different pea
  strains (phenotypes) so that they bred true
  (selfing, the opposite of cross-fertilization)
• He then crossed the different phenotypes one at a
  time, counted offspring of each phenotype and
  analyzed the results mathematically

5

• Mendel focused on 7 well-defined garden pea
  traits
• Flower/seed coat colour
• Seed colour
• Seed shape
• Pod colour
• Pod shape
• Stem length
• Flower position
• He began by self-fertilizing 34 different pea
  strains (phenotypes) so that they bred true
  (selfing, the opposite of cross-fertilization)
• He then crossed the different phenotypes one at a
  time, counted offspring of each phenotype and
  analyzed the results mathematically

6
(No Transcript)
7

• Mendel focused on 7 well-defined garden pea
  traits
• Flower/seed coat colour
• Seed colour
• Seed shape
• Pod colour
• Pod shape
• Stem length
• Flower position
• He began by self-fertilizing 34 different pea
  strains (phenotypes) so that they bred true
  (selfing, the opposite of cross-fertilization)
• He then crossed the different phenotypes one at a
  time, counted offspring of each phenotype and
  analyzed the results mathematically

8

• Mendel focused on 7 well-defined garden pea
  traits
• Flower/seed coat colour
• Seed colour
• Seed shape
• Pod colour
• Pod shape
• Stem length
• Flower position
• He began by self-fertilizing 34 different pea
  strains (phenotypes) so that they bred true
  (selfing, the opposite of cross-fertilization)
• He then crossed the different phenotypes one at a
  time, counted offspring of each phenotype and
  analyzed the results mathematically

9
Some basic terminology Generations P
parental generation F1 1st filial generation,
progeny of the P generation F2 2nd filial
generation, progeny of the F1 generation F3
and so on
10
Seed shape comes in two forms smooth and
wrinkled, and the expression of this phenotype in
regulated by a gene We will use the following
symbols S - smooth seed s wrinkled seed The
different variants of the same gene are called
alleles Peas contain two copies of the seed
shape gene, one from each parent, therefore, a
pea could have one of the following
genotypes SS Ss ss SS / ss homozygous Ss
heterozygous
11
Seed shape comes in two forms smooth and
wrinkled, and the expression of this phenotype in
regulated by a gene We will use the following
symbols S - smooth seed s wrinkled seed The
different variants of the same gene are called
alleles Peas contain two copies of the seed
shape gene, one from each parent, therefore, a
pea could have one of the following
genotypes SS Ss ss SS / ss homozygous Ss
heterozygous
12
Seed shape comes in two forms smooth and
wrinkled, and the expression of this phenotype in
regulated by a gene We will use the following
symbols S - smooth seed gene s wrinkled seed
gene The different variants of the same gene are
called alleles Peas contain two copies of the
seed shape gene, one from each parent, therefore,
a pea could have one of the following
genotypes SS Ss ss SS / ss homozygous Ss
heterozygous
13
Seed shape comes in two forms smooth and
wrinkled, and the expression of this phenotype in
regulated by a gene We will use the following
symbols S - smooth seed gene s wrinkled seed
gene The different variants of the same gene are
called alleles Peas contain two copies of the
seed shape gene, one from each parent, therefore,
a pea could have one of the following
genotypes SS Ss ss SS / ss homozygous Ss
heterozygous
14
Seed shape comes in two forms smooth and
wrinkled, and the expression of this phenotype in
regulated by a gene We will use the following
symbols S - smooth seed gene s wrinkled seed
gene The different variants of the same gene are
called alleles Peas contain two copies of the
seed shape gene, one from each parent, therefore,
a pea could have one of the following
genotypes SS Ss ss SS / ss homozygous Ss
heterozygous
15
Seed shape comes in two forms smooth and
wrinkled, and the expression of this phenotype in
regulated by a gene We will use the following
symbols S - smooth seed gene s wrinkled seed
gene The different variants of the same gene are
called alleles Peas contain two copies of the
seed shape gene, one from each parent, therefore,
a pea could have one of the following
genotypes SS Ss ss SS / ss homozygous Ss
heterozygous
16
Seed shape comes in two forms smooth and
wrinkled, and the expression of this phenotype in
regulated by a gene We will use the following
symbols S - smooth seed gene s wrinkled seed
gene The different variants of the same gene are
called alleles Peas contain two copies of the
seed shape gene, one from each parent, therefore,
a pea could have one of the following
genotypes SS Ss ss SS / ss homozygous Ss
heterozygous
17
Seed shape comes in two forms smooth and
wrinkled, and the expression of this phenotype in
regulated by a gene We will use the following
symbols S - smooth seed gene s wrinkled seed
gene The different variants of the same gene are
called alleles Peas contain two copies of the
seed shape gene, one from each parent, therefore,
a pea could have one of the following
genotypes SS Ss ss SS / ss homozygous Ss
heterozygous
18
(No Transcript)
19
(No Transcript)
20
(No Transcript)
21
(No Transcript)
22
?
23
(No Transcript)
24
Note the ratio for dominant recessive
phenotypes in the F2 generation 31
25
(No Transcript)
26
(No Transcript)
27
This type of cross, for one phenotype, is called
a monohybrid cross F1 4/4 offspring express
the dominant phenotype all heterozygous F2 3/4
offspring offspring express the dominant
phenotype one homozygous, two heterozygous 1/4
offspring express the recessive
phenotype homozygous
28
This type of cross, for one phenotype, is called
a monohybrid cross F1 4/4 offspring express
the dominant phenotype all heterozygous F2 3/4
offspring offspring express the dominant
phenotype one homozygous, two heterozygous 1/4
offspring express the recessive
phenotype homozygous
29
This type of cross, for one phenotype, is called
a monohybrid cross F1 4/4 offspring express
the dominant phenotype all heterozygous F2 3/4
offspring offspring express the dominant
phenotype one homozygous, two heterozygous 1/4
offspring express the recessive
phenotype homozygous
30
This type of cross, for one phenotype, is called
a monohybrid cross F1 4/4 offspring express
the dominant phenotype all heterozygous F2 3/4
offspring offspring express the dominant
phenotype one homozygous, two heterozygous 1/4
offspring express the recessive
phenotype homozygous
31
(No Transcript)
32
A wrinkled seed has the recessive phenotype and
is homozygous recessive (ss) A smooth seed
expresses the dominant phenotype but may be
homozygous dominant (SS) or heterozygous
(Ss) How can you tell the smooth seeds
genotype? A test cross involves crossing the
smooth seed in question with a homozygous
recessive (wrinkled) seed
33
A wrinkled seed has the recessive phenotype and
is homozygous recessive (ss) A smooth seed
expresses the dominant phenotype but may be
homozygous dominant (SS) or heterozygous
(Ss) How can you tell the smooth seeds
genotype? A test cross involves crossing the
smooth seed in question with a homozygous
recessive (wrinkled) seed
34
A wrinkled seed has the recessive phenotype and
is homozygous recessive (ss) A smooth seed
expresses the dominant phenotype but may be
homozygous dominant (SS) or heterozygous
(Ss) How can you tell the smooth seeds
genotype? A test cross involves crossing the
smooth seed in question with a homozygous
recessive (wrinkled) seed
35
A wrinkled seed has the recessive phenotype and
is homozygous recessive (ss) A smooth seed
expresses the dominant phenotype but may be
homozygous dominant (SS) or heterozygous
(Ss) How can you tell the smooth seeds
genotype? A test cross involves crossing the
smooth seed in question with a homozygous
recessive (wrinkled) seed
36
(No Transcript)
37
(No Transcript)
38

• Lethal Genes
• Some genes are lethal when both alleles are
  present. Lethality can occur before or after
  birth
• An example is the "creeper" allele in chickens,
  which causes the legs to be short and stunted
• Creeper is a dominant phenotype, heterozygous
  chickens display the creeper phenotype
• If two creeper chickens are crossed, one would
  expect to have (from Mendelian genetics) 3/4 of
  the offspring to be creeper and 1/4 to be normal
• Instead the ratio obtained is 2/3 creeper and 1/3
  normal, this occurs because homozygous creeper
  chickens die.
•                                         

39

• Mendels dihybrid crosses
• Mendel also performed crosses involving two pairs
  of traits,
• eg seed shape and colour
• wrinkled yellow seed x smooth green seed
• If alleles sort independently, four possible
  phenotypes appear in the F2 generation in a
  9331 ratio.
• Mendels Principle of Independent Assortment
• Alleles for different traits assort independently
  of one another.
• Modern formulation of independent assortment
• Genes on different chromosomes behave
  independently in gamete production.

40
(No Transcript)
41
Fig. 10.12a Dihybrid cross F1 generation
42
Fig. 10.12b Dihybrid cross F2
generationRatio9331
43

• Trihybrid crosses
• Involve three independently assorting character
  pairs.
• Results in
• 64 combinations of 8 different gametes
• 27 different genotypes
• 8 different phenotypes (2 x 2 x 2)
• Predicted ratio of phenotypes 279993331

44

• Summary of Mendels Principles
• Mendels Principle of Uniformity in F1
• F1 offspring of a monohybrid cross of
  true-breeding strains resemble only one of the
  parents.
• Mendels Principle of Segregation
• Recessive characters masked in the F1 progeny of
  two true-breeding strains, reappear in a specific
  proportion of the F2 progeny.
• Two members of a gene pair segregate (separate)
  from each other during the formation of gametes.
• Mendels Principle of Independent Assortment
• Alleles for different traits assort
  independently of one another.
• Genes on different chromosomes behave
• independently in gamete production