Introduction to Genetics
Remember DNA RNA Protein Traits DNA contains the code for proteins (protein synthesis remember?) Proteins determine our traits
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 (P) generation (generation starting point) Pure Green Pea X Pure Yellow Pea Hypothesis: yellow-green offpsring
Genetic Crossing Parent (green) X Parent (yellow) P P F 1 F 1 F 1 F 1 First Generation (F 1 - offpsring) **ALL YELLOW**
Mendel s Second Generation F 2 Mendel self-fertilized the F 1 : F 1 F 1 F 2 F 2 3:1 ratio F 2 F2 F 2 Green trait reappeared in F 2 (second) generation
Before we can figure out what happened. We need to learn some genetics!
Mendel s First Law: The Law of Segregation 1. All individuals have two copies of each allele (i.e. Ss) 2. Each gamete receives one copy of every allele during gamete formation (S or s) In other words, they SEGREGATE
Hybrid Cross - mating between individuals who have different alleles at one locus of interest Possible allele from gamete passed down to offspring Parent (dad)plant: Ss Parent(mom) plant: Ss S s S SS Ss s Ss ss Possible combinations for alleles of offspring after fertilization
Vocabulary 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, either dominant or recessive
Which allele is dominant? Which is recessive? Parent (dad)plant: Ss In each square (offspring trait), which parent is passing down the dominant trait? The recessive trait? S s Parent(mom) plant: Ss S SS Ss s Ss ss Possible combinations for alleles of offspring after fertilization
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 From Dad (only 1 gamete) From Mom (only 1 gamete)
Common Vocabulary Phenotype Genotype
Phenotype The observable traits of an organism (what you see!)
Genotype the genetic make up combination of alleles located on homologous chromosomes that determines a specific characteristic or trait.
Offspring can have genotypes that are: Homozygous dominant: SS Homozygous recessive: ss Heterozygous: Ss **Remember on allele from mom and one from dad**
You Try It Answer #7 on your PowerPoint Notes
Monohybrid Cross Cross involving only one trait Used to find traits of offspring Can be used to find the traits of parents
You Try It In humans, brown eyes (B) are dominant over blue (b). What are the chances that a blue eyed man and a homozygous brown eyed woman will have children with blue eyes?
You try it monohybrid cross 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?
You Try It If a heterozygous purple flower is mated with another heterozygous purple flower, what ratio of offspring will be white in color? (Use P for purple and p for white)
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 (Dd x dd and DD x dd). 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?
You try it Genetics Protocol Monohybrid practice problems
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, so, color and shape sort independently. i.e. round doesn t necessarily go to the same gamete as yellow
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 more than one trait the alleles for EACH trait will separate independent of each other.
Summer squash dihybrid example W (white) is dominant to w (yellow) D (disk shape) is dominant to d (sphere shape) Cross a white, disk shape with a yellow, sphere shape Genotype WwDd (white, disk-shaped fruit) X wwdd (yellow, sphere-shaped fruit) Phenotype
Let s try it together: Cross WwDd x wwdd What are all the possible allele combinations for WwDd? wwdd?
What would the genotype and phenotype ratios be for this cross? (WwDd x wwdd)
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.
Part A: Suppose a woman who is both a homozygous tongueroller and a non-ptc-taster marries a man who is a heterozygous tongue-roller 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? Part B: 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.
You try it
Non- Mendelian Genetics
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 For male, x r y, x R y For female, x r x r, x R x r, x R x R
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.
Male Pattern Baldness Male-pattern baldness is a sex-linked trait in which affected people become bald. X B = No baldness (dominant) X b = Male-pattern baldness (recessive) If a Male with X b Y genotype mated with a Female with XB X b genotype. What percent of male children will have male-pattern baldness?
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.????
Blood Type a type of codominance There are three alleles of this gene: A, B, and O. A person s blood type is determined by which allele he/she inherits from each parent.
AB, A, B, O type blood A and B genes are co-dominant if both an A and B allele are inherited, both are expressed O is a recessive allele If an A or B gene is inherited along with the O gene, the A or B gene determines the person s blood type. A person is type O only if he/she inherits two O genes.
Polygenic Inheritance many genes affecting one trait 3 genes affect skin color
Epistasis Occurs when two or more genes are involved in determining traits (i.e. skin color) When gene at one locus affects the expression of a gene at another locus
Epistasis example - Mice Black coat (B) is dominant to brown (b) Must have bb to have brown coat A color gene (C) must be present for the color to be deposited on the hair Must have Cc or CC for color, if cc, no color and mouse is white
Linked Genes Genes that are inherited together Located near each other on the same chromosome Chances of them crossing over to another chromosome during meiosis is small Ex. Eye color and wing color in fruit flies are inherited together, not independently sorted during meiosis
Environmental Factors Can affect whether or not a trait is expressed (turn on the trait or turn it off) Chemicals Radiation Diet Stress Why twins look different due to their environment!