Introduction to Genetics
Remember DNA RNA Protein Traits DNA contains the code for proteins (protein synthesis remember?) Proteins determine our traits
Remember
Gregor Mendel 1822-1884 Father of Genetics Studied characteristics garden plants Curious about inheritance patterns
Mendel s pea plants Pea plants can: Self fertilize Cross fertilize Have easily observed contrasting traits (ex. tall or short)
Pea traits
Mendel s experiments Parent generation (P generation starting point) Pure Green Pea X Pure Yellow Pea Hypothesis: yellow-green offpsring
Genetic Crossing Parent (green) X Parent (yellow) P P F1 F1 F1 F1 First Generation (F1 - offpsring) **ALL YELLOW**
Mendel s Second Generation F2 Mendel self-fertilized the F1: F1 F1 F2 F2 F2 F2 Green trait reappeared in F2 3:1 ratio (second) generation
Mendel s First Law: The Law of Segregation All individuals have two copies of each allele (i.e. Ss); each gamete receives one copy of every allele during gamete formation.
Hybrid Cross - mating between individuals who have different alleles at one locus of interest Possible gametes passed down Parent (dad)plant: Ss S s Parent(mom) plant: Ss S SS Ss s Ss ss
Principle of Dominance DOMINANT TRAIT Traits that is expressed; what you see (ex. T) RECESSIVE TRAIT Trait that is hidden; (ex. t) Each parent will either pass down an allele that is dominant or an allele that is recessive
Each parent passes down only 1 of 2 possible alleles
Offspring will inherit two alleles, one from mom and one from dad These alleles will either be dominant or recessive
Offspring inherit 2 alleles, one S from each parent Possibilities: SS Ss or ss ss
Common Vocabulary Genotype Phenotype Homozygous Heterozygous
Common Vocabulary cont. Genotypic description: Homozygous dominant: SS Homozygous recessive: ss Heterozygous: Ss Remember: the phenotype is what you see SS and Ss - same phenotype, different genotype
Test cross to determine what parent genotype was Cross between a dominant phenotype and a recessive individual S? X ss Parent genotype can be determined by looking at the offspring
Monohybrid Cross Cross involving only one trait
Dihybrid Cross Cross involving two traits Ex. Shape and Color Round (R) X Yellow (Y) (for peas) RrYy x RrYy
Each allele sorts independently from other alleles in gamete formation
Mendel s Second Law: The Law of Independent Assortment The two alleles for one gene segregate (assort) independently of the alleles for other genes during gamete formation (meiosis). 2 nd Law only refers for dihybrid crosses involving 2 traits the alleles for EACH trait will separate independent of each other.
Summer squash dihybrid ex. W (white) is dominant to w (yellow) D (disk shape) is dominant to d (sphere shape) Genotype WWDD (white, disk-shaped fruit) X wwdd (yellow, sphere-shaped fruit) Phenotype
P (parent) Generation white, disk-shaped yellow, sphere-shaped P F 1 All offspring for F 1 : WwDd X WwDd
Cross all the possibilities for the offspring to determine F 2 F 1 F 1 possibilities F 2 possibilities F 1 possibilities
What would the genotype and phenotype ratios be?
You try it monohybrid test cross In humans, brown eyes (B) are dominant over blue (b)*. A brown-eyed man marries a blue-eyed woman and they have three children, two of whom are brown-eyed and one of whom is blue-eyed. Draw the Punnett square that illustrates this marriage. What is the man s genotype? What are the genotypes of the children?
Try again. In dogs, there is an hereditary deafness caused by a recessive gene, d. A kennel owner has a male dog that she wants to use for breeding purposes. The dog can hear, so the owner knows his genotype is either DD or Dd. If the dog s genotype is Dd, the owner does not wish to use him for breeding so that the deafness gene will not be passed on. This can be tested by breeding the dog to a deaf female (dd). Draw the Punnett squares to illustrate these two possible crosses. In each case, what percentage/how many of the offspring would be expected to be hearing? deaf? How could you tell the genotype of this male dog?
Dihybrid Cross In humans, there is a gene that controls formation (or lack thereof) of muscles in the tongue that allow people with those muscles to roll their tongues, while people who lack those muscles cannot roll their tongues. The ability to roll one s tongue is dominant over non-rolling. The ability to taste certain substances is also genetically controlled. For example, there is a substance called phenylthiocarbamate (PTC for short), which some people can taste (the dominant trait), while others cannot (the recessive trait). To people who are tasters, the paper tastes very bitter, but to nontasters, it just tastes like paper. Let s let R represent tongue-rolling, r represent a non-roller, T represent ability to taste PTC, and t represent non-tasting.
Suppose a woman who is both a homozygous tongue-roller and a non-ptc-taster marries a man who is a heterozygous tongueroller and is a PTC taster, and they have three children: a homozygous tongue-roller who is also a PTC taster, a heterozygous tongue-roller who is also a taster, and a heterozygous tongue-roller who is a non-taster. If these parents would have a bunch more children so that they had 12 in all, how many of those 12 would you expect to be non-tasters who are homozygous for tongue-rolling? If the first child (the homozygous tongue-roller who is also a PTC taster) marries someone who is heterozygous for both traits, draw the Punnett square that predicts what their children will be.
Sex linked Traits Found only on the X chromosome Women - 2 X chromosomes (XX) Men - 1 X chromosome (XY) Any genes found on the X chromosomes are referred to as sex-linked genes.
Designating Sex linked Traits X r - Recessive trait X R - Dominant trait What is the genotype for a female that is homozygous dominant? What is the genotype for a woman that is Homozygous recessive? What is the genotype for a woman that is Heterozygous? What is the genotype for a male that is has the dominant trait? What is the genotype for a man with the recessive trait?
Fruit Flies gene for eye color Eye color gene is carried on the X chromosome (sex-linked) Red eyes dominant - X R White eyes recessive - X r Try it If a white-eyed female fruit fly is mated with a red-eyed male, predict the possible offspring.
Incomplete Dominance - Blending two different phenotypes produce a third phenotype that is a blending of the parental traits. 4 o clock flowers, Red is not completely dominant over white RR Red flowers RW PINK flowers WW White flowers
Incomplete Dominance Cross a Red flower with a White flower. Predict the ratio of offspring?????
Incomplete Dominance Cross a Red flower with a Pink flower, what is the ratio of offspring?????
Codominance - Together two different phenotypes produce a third phenotype where both parental traits appear together.
For Example: Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together).
Codominance Cross a Red cow with a white cow. Predict the ratio of offspring.????
Multiple Alleles trait controlled by more than two alleles Blood Type controlled by 3 alleles, not 2 A, B, O alleles 4 blood types A, B, AB, O
Polygenetic Inheritance many genes affecting one trait
Environmental Factors Can affect whether or not a trait is expressed (turn on the trait or turn it off) Chemicals Radiation Diet Why twins look different due to their environment!