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
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