Human Genetics

Earlobe Variation

• Whether a person is born with attached or detached earlobes depends on a single gene

• Gene has two molecular forms (alleles)

Earlobe Variation

• You inherited one allele for this gene from each parent

• Dominant allele specifies detached earlobes

• Recessive allele specifies attached lobes

Dominant & Recessive Alleles

• If you have attached earlobes, you inherited two copies of the recessive allele

• If you have detached earlobes, you may have either one or two copies of the dominant allele

Alleles denoted as…

• shorthand notation to show genes and the two copies of each in a cell

• Dominant called D- for detached earlobes

• Recessive called d- for attached earlobes

• So person can be- DD, Dd or dd

Gregor Mendel

• Strong background in plant breeding and mathematics

• Using pea plants, found how parents transmit genes (units of information) to offspring

The Garden Pea Plant

• Self-pollinating

• True breeding (different alleles not introduced)

• Can be experimentally cross-pollinated

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

A pair of homologous chromosomes

A gene locus

A pair of alleles

Three pairs of genes

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

Tracking Generations

• Parental generation P

mates to produce F1

• First-generation offspring F1

mate to produce F2

• Second-generation offspring F2

F1 Results of True-breeding lineages Crossed

F2 Results of Monohybrid Cross

Mendel’s Monohybrid Cross Results

787 tall 277 dwarf

651 long stem

207 at tip

705 purple 224 white

152 yellow428 green

299 wrinkled882 inflated

6,022 yellow 2,001 green

5,474 round 1,850 wrinkled

F2 plants showed dominant-to-recessive ratio that averaged 3:1

Mendel’s Theory of Segregation

• An individual inherits a unit of information (allele) about a trait from each parent

• During gamete formation, the alleles segregate from each other (meiosis!!)

Punnett Square of a Monohybrid Cross

Female gametes

Male gametes

A a

A

a Aa

AA Aa

aa

Dominant phenotype canarise 3 ways,recessive only one

Test Cross

• Individual that shows dominant phenotype is crossed with individual with recessive phenotype

• Examining offspring allows you to determine the genotype of the dominant individual

Dihybrid Cross

Experimental cross between individuals that are homozygous for different

versions of two traits

A 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

F1 Results of Mendel’s Dihybrid Crosses

• All plants displayed the dominant form

of both traits

– All plants inherited one allele for each trait

from each parent

– All plants were heterozygous (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

Explanation of Mendel’s Dihybrid Results

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

aB

AB

AB

abAb

Ab

aB

ab

1/4

1/4

1/4

1/4

1/4 1/4 1/4 1/4

AaBb aabbAabb aaBb

AABB AABb AaBB AaBb

AABb AAbb AaBb Aabb

AaBb aaBB aaBbAaBB

1/161/161/161/16

1/161/161/161/16

1/161/161/161/16

1/16 1/16 1/16 1/16

Independent Assortment

• Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait

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

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

Flower Color in Snapdragons: Incomplete Dominance

Red-flowered plant X White-flowered plant

Pink-flowered F1 plants

(homozygote) (homozygote)

(heterozygotes)

Flower Color in Snapdragons: Incomplete Dominance

Pink-flowered plant X Pink-flowered plant

White-, pink-, and red-flowered plants in a 1:2:1 ratio

(heterozygote) (heterozygote)

Flower Color in Snapdragons: Incomplete Dominance

• Red flowers - two alleles allow them to make a red pigment

• White flowers - two mutant alleles; can’t make red pigment

• Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment

Co-dominance - Multiple alleles

• Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid (called A or B) on blood cells

• 3 alleles total = A gene makes A glycolipid, B gene makes B glycolipid and O gene means neither A nor B genes are present

ABO Blood Type:Allele Combinations

• Type A – AA or Ai

• Type B – BB or Bi

• Type AB - AB

• Type O - ii

ABO Blood Type: Glycolipids on Red Cells

• Type A - Glycolipid A on cell surface

• Type B - Glycolipid B on cell surface

• Type AB - Both glyocolipids A & B

• Type O - Neither glyocolipid A nor B

Environmental Effects on phenotype

• Siamese cats- heat sensitive enzyme works only in cooler areas of the body

• Height of a person- can have gene for more height but nutrition can also dictate height