Genes
• Units of information about specific traits
• Passed from parents to offspring
• Each has a specific location (locus) on a chromosome
Alleles
• Different molecular forms of a gene found on homologous chromosomes
• Arise by mutation
• Dominant allele masks a recessive allele that is paired with it
Allele Combinations
• Homozygous – having two identical alleles – Homozygous dominant, AA – Homozygous recessive, aa
• Heterozygous – having two different alleles– Aa
fertilization produces heterozygous offspring
meiosis II
meiosis I
(chromosomes duplicated
before meiosis)
homozygous dominant parent
homozygous recessive parent
(gametes) (gametes)
Fig. 8-4, p.114
Average F2 dominant-to-recessive ratio for all of the traits studied:
Dominant Form Recessive Form
FLOWER COLOR
FLOWER POSITION
STEM LENGTH
3.15:1
3:1
2.84:1787 tall
705 purple 224 white
227 dwarf
651 along stem 207 at tip 3.14:1
Fig. 8-5, p.115
female gametes
mal
e g
amet
es
aA
a aa
A
aA
aaAa
A
a
aA
aaAa
AaA
a
aA
aaAa
AaAAA
a
Fig. 8-6a, p.115
Probability and the Punnett Square
Monohybrid Cross
Experimental intercross between
two F1 heterozygotes
AA X aa Aa (F1 monohybrids)
Aa X Aa ?
AMonohybrid
Cross
True-breedinghomozygous recessiveparent plant
True-breedinghomozygous dominantparent plant
An F1 plantself-fertilizesand producesgametes:
F1 PHENOTYPES
F2 PHENOTYPES
aa
Aa
AA
aaAa
Aa
Aa Aa
Aa Aa
Aa Aa
Aa Aa
Aa
Aa
AA
aa
A
A
A
A
a a
a
a
AA
Mendel’s Theory of Segregation
• Individual inherits a unit of information (allele) for a trait from each parent
• During gamete formation, the alleles segregate from each other
Dihybrid Cross
AB X ab
Experimental cross between individuals that are heterozygous for different versions of two traits
Dihybrid Cross: F1 Results
AABB aabbx
AaBb
AB AB ab ab
TRUE-BREEDING PARENTS:
GAMETES:
F1 HYBRID OFFSPRING:
purple flowers, tall
white flowers,dwarf
all purple-flowered, tall
AB ab
1AABBpurple-
flowered,tall parent
(homozygousdominant)
2aabb
white-flowered,
dwarf parent(homozygous
recessive)
3 F1 OUTCOME: All of the F1 plants are purple-flowered, tall(AaBb heterozygotes)
X
Fig. 8-7, p.116
AaBb AaBb
meiosis,gamete formation
meiosis,gamete formation
Fig. 8-7, p.116
1/16aaBB
1/16aaBb
1/16aaBb
1/16Aabb
1/16Aabb
1/16AAbb
1/16AABB
1/16AABb
1/16AaBB
1/16AaBb
1/16AABb
1/16AaBb
1/16AaBB
1/16AaBb
1/16AaBb
1/4 AB 1/4 Ab 1/4 aB 1/4 ab
1/16aabb
1/4 AB
1/4 Ab
1/4 aB
1/4 ab
AaBb AaBbX
1/16 white-flowered, dwarf
3/16 white-flowered, tall
3/16 purple-flowered, dwarf
9/16 purple-flowered, tall
Dihybrid Cross: F2 Results
Independent Assortment
• “Units” for one trait were assorted into gametes independently of the “units” for the other trait
• Members of each pair of homologous chromosomes are randomly sorted into gametes during meiosis
Independent Assortment
Metaphase I:
Metaphase II:
Gametes:
1/4 AB 1/4 ab 1/4 Ab 1/4 aB
A A A A
A A A A
AAAA
B B
B B
BB
B B
BBBB
a a a a
aa aa
aaaa
bb b b
bb b b
b b b b
OR
Alleles
• Different molecular forms of a gene
• Arise through mutation
• Diploid cell has a pair of alleles at each
locus
• Alleles on homologous chromosomes
may be same or different
The Y Chromosome
• Small, with few genes
• Master gene for male sex determination – SRY gene (sex-determining region of Y)
• SRY present, testes form
• SRY absent, ovaries form
The X Chromosome
• Carries more than 2,000 genes
• Most genes deal with nonsexual traits
• Genes on X chromosome can be expressed in both males and females
Dominance Relations
Complete dominance
Incomplete dominance
Codominance
Recessive traits
• Recessive inheritance–Two recessive alleles are
needed to show disease
–Heterozygous parents are carriers of the disease-causing allele
–Probability of inheritance increases with inbreeding, mating between close relatives
Dominant Traits
• Dominant inheritance–One dominant allele is needed to
show disease–Dominant lethal alleles are
usually eliminated from the population
Codominance
Codominance• Neither allele is dominant over the
other
• Expression of both alleles is observed as a distinct phenotype in the heterozygous individual
• Observed for type AB blood
Incomplete dominance
Incomplete dominance• Neither allele is dominant over the
other• Expression of both alleles is
observed as an intermediate phenotype in the heterozygous individual
Autosomal Recessive Inheritance Patterns
• If parents are
both
heterozygous,
child will have a
25% chance of
being affected
X-Linked Recessive Inheritance
• Males show disorder more than females
• Son cannot inherit disorder from his father