observing patterns in inherited...

Observing Patterns in

Inherited Traits

Chapter 11

Updated Reading 11.1-11.3

Not 11.5-11.7

What you absolutely need to

know

• Punnett Square with monohybrid and

dihybrid cross

• Heterozygous, homozygous, alleles,

locus, gene

• Test cross, P, F1, F2

• Mendel and his work

Early Ideas about Heredity

• People knew that sperm and eggs

transmitted information about traits

• Blending theory

• Problem:

– Would expect variation to disappear

– Variation in traits persists

No Blending Involved

• We cannot say that a red flower crossed

with a what flower produces a pink

flower.

Name this man

Gregor Mendel

• Strong background in plant breeding and

mathematics

• Using pea plants, found indirect but

observable evidence of how parents

transmit genes to offspring

Genetic Terms

A pair of homologous

chromosomes

A gene locus

A pair of alleles

Three pairs of genes

The Garden Pea Plant

• Self-pollinating

• True breeding (different alleles

not normally introduced)

• Can be experimentally cross-

pollinated

Impact of Mendel’s Work

• Mendel presented his results in 1865

• Paper received little notice

• Mendel discontinued his experiments in

1871

• Paper rediscovered in 1900 and finally

appreciated

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

• Arise by mutation

• Dominant allele masks a recessive

allele that is paired with it

Allele Combinations

• Homozygous

– having two identical alleles at a locus

– AA or aa

• Heterozygous

– having two different alleles at a locus

– Aa

Genotype & Phenotype

• Genotype refers to particular genes an individual carries

• Phenotype refers to an individual’s observable traits

• Cannot always determine genotype by observing phenotype. Ex. A blond haired person can produce a red-haired offspring

Tracking Generations

• Parental generation P

mates to produce

• First-generation offspring F1

mate to produce

• Second-generation offspring F2

Monohybrid Crosses

• Use F1 offspring of parents that breed

true for different forms of a trait:

(AA x aa = Aa)

• The experiment itself is a cross between

two identical F1 heterozygotes, which

are the “monohybrids” (Aa x Aa)

F1 Results of One

Monohybrid Cross

Punnett Square of a

Monohybrid Cross

Female gametes

Male

gametes

Dominant

phenotype

can arise 3

ways,

recessive

only 1

aA

aaAa

AaAAA

a

F2 Results of

Monohybrid Cross

Testcross

• Individual that shows dominant

phenotype is crossed with individual

with recessive phenotype (AA x aa)

• Examining offspring allows you to

determine the genotype of the dominant

individual

Dominance Relations

• Complete dominance

• Incomplete dominance

– Heterozygote phenotype is somewhere

between that of two homozyotes

• Codominance

– Non-identical alleles specify two

phenotypes that are both expressed in

heterozygotes

F1 Results of Mendel’s

Dihybrid Crosses

• All plants displayed the dominant form

of both traits: AaBb

• We now know:

– All plants inherited one allele for each trait

from each parent

– All plants were heterozygous (AaBb)

F1 Results of Mendel’s

Dihybrid Crosses

• All plants displayed the dominant form

of both traits: AaBb

• We now know:

– All plants inherited one allele for each trait

from each parent

– All plants were heterozygous (AaBb)

Dihybrid Cross

Experimental cross between individuals

that are homozygous for different

versions of two traits

Now we want to try two traits

Purple AA

Tall BB

• Parent with purple flowers and and long

stems = AABB

• Purple AA

Tall BB

• Parent with white flowers and short

stems

• White aa and short stems bb

• Dihybrid cross is AABB x aabb

Phenotypic Ratios in F2

Four Phenotypes:

– Tall, purple-flowered (9/16)

– Tall, white-flowered (3/16)

– Dwarf, purple-flowered (3/16)

– Dwarf, white-flowered (1/16)

AaBb X AaBb

AaBb x AaBb produces the

following gametes

If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible