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