Gregor Mendel • Genetics is the field of Biology that studies how characteristics (traits) are passed from parent to child • Gregor Mendel, the Father of Genetics, was an Austrian Monk…gardened and grew peas in the Mid 1800’s • Looked at 7 different traits in pea plants • Height Flower color • Pod appearance pod color • Seed texture seed color • Position of flowers
Gregor Mendel • Mendel noticed that pea plants with purple flowers always had seeds that grew plants with purple flowers, when they self pollinate. He called these plants true breeding. • Self pollinate means the egg and pollen are from the same plant. • The opposite is cross pollinate where egg and pollen come from different plants. Usually we just shorten this term and say “cross” and we use it refer to other living things, not just plants. • When Mendel cross-pollinated purple flowered plants with white flowered plants, he got plants that all had purple flowers! The white trait had disappeared! • We call the ones he started with the P generation (parental) • We call the ones he got from those crosses the F1 generation (filial 1)
Gregor Mendel • Mendel then let that F1 generation self pollinate to make an F2 generation • He found 3/4 of the plants had purple flowers, ¼ of the plants had flowers. This had never happened before. Never before had purple flowered plants made babies with white flowers! • For the other traits, he always came up with the same ratio inthe F2:3:1 • Mendel explained his results by saying • Something controlled traits (and called them factors) • There must be two factors • One factor prevents the other from happening • Mendel concluded that the pair of factors separate during the formation of reproductive cells. This is called the Law of Segregation. • Mendel also crossed different traits, (seed color flower color) and concluded that the factors are not connected. He called this the Law of Independent Assortment
Today… • We know that there are segments of DNA on chromosomes called genes that occur in pairs (because chromosomes do). Each has 1 or more forms called alleles. • We use capital letters for dominant alleles and lowercase letters for recessive alleles. • Dominant means that it does not let the other allele be expressed. Recessive means it is not expressed. • We also know they separate during a process called meiosis (at anaphase I)
Other terms… • Genotype: the alleles present or geneticmakeup; uses letters like PP, Pp or pp • If some thing has the same two alleles, we call it homozygous or pure bred. e.g. PP, pp, BB, bb (note that this doesn’t tell whether something is dominant or recessive) • If some thing has two different alleles, we call it heterozygous or a hybrid. e.g. Pp, Bb • Phenotype: the appearance; uses words like purple or white
Punnett Squares • One way to show the possibilities of a genetic cross is to use a Punnett square • Laws of probability state that there is a 50/50 chance of either allele being passed on, so a Punnett square doesn’t show what will happen. It shows you what can happen and how often. • A punnett square that show one trait is called a monohybrid cross.
Punnett Squares • When giving the results, we use ratios or percentages for both the genotype and phenotype • In this example, the genotypic ratio is 2BB: 2Bb or 50% Bb and 50% Bb • The phenotpyic ratio is all (100%) black fur (note that all 4 will look exactly the same…the Bb are not a little “less black” than the BB)
Complete (simple) dominance • Complete or simple dominance is when the phenotype is one thing or the other, like all of GregorMendel’s … yellow or green seeds, purple or white flowers • You can tell because there is a 3:1 phenotypic ratio in F2. • It’s not always that simple.
Other cases… • In a flower called 4 o’clocks, red and white flowers make pink in the F1. • This is called incomplete dominance … you get a new phenotype, a mixture of dominant and recessive • You see a 1:2:1 phenotypic ratio in the F2
Other cases… • What if both alleles are expressed? For the flowers above they wouldn’t be pink, they’d be red and white at the same time • That’s called Codominance …you get both dominant and recessive at the same time • What would the what the rabbits to the left look like in the F2. • They’d have black and brown spotted fur
Dihybrid Cross • When two traits are looked at, it is called a Dihybrid cross. • Notice there are 2 letters by each row/column outside the Punnett Square, one of each allele (e.g. R and Y, not R and r) • To figure out the ocmbinaton, use FOIL…first, outer, inner last. • Dihybrid crosses have a 9:3:3:1 phenotypic ratio in the F2: • 9: Round, yellow (both dom.) • 3: Round, green (1 dom.,1 rec.) • 3: Wrinkled, yellow (1 rec.,1 dom.) • 1: wrinkled, green (both rec.)
Sex Linked Traits • Recall that the sex chromosomes (X and Y) determines the sex of organism. XX is female and XY is male (in humans) • All other chromosomes are called autososmes • In genetics, genes on the X or Y chromosome are written on an X or Y, still using capital letters for dominant and lowercase for recessive, e.g. XRXR or XrY • Phenotypes mention both sex and trait, e.g. red eyed female. • Males tend to show the recessive traits more, since they only need 1 allele, which they get from their Mom.
But most of the time… • Traits we see are very complex. They are controlled by more than one allele (polygenic traits), like blood type • Or are controlled by more than one gene, like eye color or skin color
Inheritance of traits • A pedigreeshows traits inherited through generations • You can USUALLY tell a trait is: • Sex linked trait if only males have it (or majority males) • Autosomal if roughly equal number males and females • Dominantif every child with trait has at least 1 parent with trait • Recessiveif child with trait may have parents without • Not everyone shows the trait. Some have it and pass it on without showing it. They are called carriers.
Reading a pedigree • squares are males, circles are females • colored in show the trait, blank do not, half colored carry it • trait is sex linked or autosomal? • trait is dominant or recessive? • Kay’s genotype is…. • Tom’s genotype is… • the chances of Jenpassing the disease on to their child is… • the chances of Tim passing the disease on to their child is… • sex linked …more males than females • recessive … Kay does not have it, but her children do • Kay’s genotype is XAXa(carrier) • Tom genotype isXA Y (Sara does not have it) • Jen … is a carrier, so 50/50 • Tim … will pass it to his daughters, who will become carriers, but not to his sons, who get a Y chromosome