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 Heterozygous 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 5,474 round 1,850 wrinkled 6,022 yellow 2,001 green 882 inflated 299 wrinkled 428 green 152 yellow F2 plants showed dominant-to-recessive ratio that averaged 3:1 705 purple 224 white 651 long stem 207 at tip 787 tall 277 dwarf

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 Dominant phenotype can arise 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 purple flowers, tall white flowers, dwarf TRUE- BREEDING PARENTS: AABB x aabb GAMETES: AB AB ab ab AaBb F1 HYBRID OFFSPRING: 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) AaBb X AaBb Four Phenotypes: Tall, purple-flowered (9/16) Tall, white-flowered (3/16) Dwarf, purple-flowered (3/16) Dwarf, white-flowered (1/16)

Explanation of Mendel’s Dihybrid Results If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible 1/4 1/4 1/4 1/4 AB Ab aB ab 1/4 1/16 1/16 1/16 1/16 AB AABB AABb AaBB AaBb 1/4 1/16 1/16 1/16 1/16 Ab AABb AAbb AaBb Aabb 1/4 1/16 1/16 1/16 1/16 aB AaBB AaBb aaBB aaBb 1/4 1/16 1/16 1/16 1/16 ab AaBb Aabb aaBb aabb

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