Mendel, Probability & Heredity

GREGOR MENDEL “The Father of Genetics” Genetics: The study of heredity. Heredity: The passing of genes/characteristics from parents to offspring.

Mendel was an Austrian monk from the 1800’s. Studied over 30,000 specimens of pea plants during an 8 year period. He studied 7 characteristics of pea plants: 1. Plant height (dwarf vs. tall) 2. Flower & pod position (on side or top of stem) 3. Seed shape (round or wrinkled) 4. Seed color (green or yellow) 5. Pod shape (inflated or constricted) 6. Pod color (green or yellow) 7. Flower color (white or purple)

Mendel observed that pea plants’ traits were often similar to those of their parents. Sometimes, however, he noted that plants had different traits than their parents. He wanted to know why…

Before we discuss Mendel’s question… You do not inherit traits themselves, but the genes that code for them. Most eukaryotic cells contain pairs of chromosomes w/ one chromosome of each pair coming from each of the two parents. Inherited trait = Received from parents. (Hair color, eye color, etc.) Acquired trait = Developed during your life. (Ability to read, write, ride a bike, etc.) Some traits can be influenced by the environment. (Ex. = Skin color can darken due to sun.)

Terms to Know… Purebred: Will always produce offspring w/ a particular trait when allowed to breed naturally. Hybrid: Will produce offspring that are not necessarily identical to itself.

Generations are given different labels when discussing heredity. Mendel’s 1 st experiment crossed a purebred tall plant of w/ a purebred short plant. → P Generation All of the offspring plants were of regular height. → F 1 Generation. He noticed the short trait seemed to have disappeared.

Next, Mendel let the 1 st generation of offspring self- pollinate w/ each other → F 2 Generation. 75% of the plants were regular height, 25% were short. He repeated this experiment for multiple traits – flower color, pod color, etc. He found that similar results were obtained in each experiment.

He realized that… 1.Organisms must have 2 “factors” for each possible trait, one from each parent, that control the inheritance of traits in offspring. 2.Some traits can be masked by others.

Gene: The factor that controls a trait - what is actually passed down from parent to offspring. Alleles: Various forms of the same gene. Ex. = Height. One parent might have the allele for regular height while the other has the allele for short height. Ex. = Eye color. One parent might have the allele for blue eyes & the other has the allele for green eyes.

Genes are located at specific spots on a chromosome. Alleles of a gene will be in the same spot on the chromosome.

Alleles can be dominant or recessive… Dominant: When present, the allele that will always shows up or “wins” in an organism.Dominant: When present, the allele that will always shows up or “wins” in an organism. Recessive: Allele that is not expressed when combined w/ a dominant form of the gene.Recessive: Allele that is not expressed when combined w/ a dominant form of the gene. Example = Blue eyes are recessive, brown eyes are dominant. Two parents w/ brown eyes are more likely to have a child w/ brown eyes, but they have a 25% chance of having a child w/ blue eyes.Example = Blue eyes are recessive, brown eyes are dominant. Two parents w/ brown eyes are more likely to have a child w/ brown eyes, but they have a 25% chance of having a child w/ blue eyes.

Terms to Know… Revisited. We use capital & lowercase letters to indicate whether alleles (traits) are dominant or recessive. For instance, tall pea plants (T) are dominant, and short pea plants (t) are recessive. So, we can have… Purebred: Will always produce offspring w/ a particular trait when allowed to breed naturally. Have 2 dominant alleles (TT) or 2 recessive alleles (tt). Hybrid: Will produce offspring that are not necessarily identical to itself. Organism carries both a dominant & recessive allele (Tt).

Section Probability & Heredity

Heredity is all about probability. You have two alleles for every trait, & you inherit one allele from each parent. Your parents each have two alleles for that trait as well, so you had a 50% chance of inheriting each of their individual alleles. So, there is more than one way for parent alleles to combine! This is why Mendel saw the results he did when he crossed pea plants.

We use punnett squares to figure out the probability of allele combinations from a genetic cross & show possible outcomes.

Using a PUNNETT SQUARE Letters stand for dominant and recessive alleles. A capital letter represents a dominant allele. Lowercase letters represent recessive alleles.

Using a PUNNETT SQUARE The first step in completing a punnett square is figuring out what alleles can be passed down from the parent organisms. Organisms are either heterozygous or homozygous for a trait. Homozygous: Organism has 2 identical alleles for a trait – purebred. (TT, tt) Heterozygous: Organism has 2 different alleles for a trait – hybrid. (Tt) Genotype: An organism’s genetic makeup or allele combination.

Using a PUNNETT SQUARE To set up a Punnett square, draw a large square, and then divide it into 4 equal sections (also squares). It should look something like this:

Using a PUNNETT SQUARE Let’s say you’re asked to cross a red flower (genotype Rr) & a white flower (genotype rr). Rr x rr Place the alleles of one parent on top, and the alleles of the other on the left. You should get something similar to this:

Using a PUNNETT SQUARE Finally, take each letter in each column & combine it with each letter from each row in the corresponding square. You should now have a picture close to this:

Using a PUNNETT SQUARE The two-letter combinations are the possible genotypes of the offspring. They are: Rr, Rr, rr, and rr genotypes. But what does that mean?! What will you see when you look at the offspring – red or white flowers?

A phenotype is the physical appearance or visible traits of an organism. Phenotype is determined by the genotype (actual genetic makeup). Let’s go back to our red & white flowers… Red (R) is dominant to white (r), so red always wins if it’s present. → Offspring with either one or two red alleles will look red! Genotypes of RR or Rr = Red flowers In order to see white (recessive trait), offspring have to be homozygous or purebred for the white allele. (rr)

The probability (chance) that a flower will have a red phenotype is 2/4 or 50%, and the probability of a white phenotype is 2/4 or 50%. So here’s our cross again… What is the probability that the offspring will be red flowers? What is the probability that the offspring will be white flowers?

Time to Practice… 1. Cross a homozygous dominant tall plant with a homozygous recessive short plant. What letters would you use to represent this cross? Dominant trait = tall Recessive trait = short T = tall t = short

 What is the genotype of a homozygous dominant tall plant? Genotype % TT % Tt % tt  TT TT x tt What is the genotype of a homozygous recessive short plant?  tt t t T TT T Phenotype % tall % short Tt

2. Cross a heterozygous brown haired rabbit with a homozygous recessive white haired rabbit. What letters would you use to represent this cross? Dominant trait = brown hair Recessive trait = white hair B = brown b = white

What is the genotype of a heterozygous brown haired rabbit? Genotype % BB % Bb % bb  Bb Bb x bb What is the genotype of a homozygous recessive white haired rabbit?  bb b b BbBb Phenotype % brown % white bb Bb 0 50

3. Cross a homozygous dominant long haired guinea pig with a heterozygous long haired guinea pig. What letters would you use to represent this cross? Dominant trait = long hair Recessive trait = short hair L = long l = short

 What is the genotype of a homozygous dominant long haired guinea pig? Genotype % LL % Ll % ll  LL LL x Ll What is the genotype of a heterozygous long haired guinea pig?  Ll L l LL LL Phenotype % long % short LLLl LLLl

4. Cross a heterozygous wrinkly elephant with another heterozygous wrinkly elephant.  What letters would you use to represent this cross? Dominant trait = wrinkly skin Recessive trait = smooth skin W = wrinkly w = smooth

What is the genotype of a heterozygous wrinkly- skinned elephant? Genotype % WW % Ww % ww  Ww Ww x Ww What is the genotype of a heterozygous wrinkly-skinned elephant?  Ww W w WwWw Phenotype % wrinkly % smooth Wwww WWWw

Phenotype % round % wrinkled 5.A plant that is homozygous dominant for round seeds is crossed with a heterozygous plant. What is the probability that the offspring will have wrinkled seeds? RR x Rr R r RRRR RRRr RRRr R = round Genotype % RR % Rr % rr r = wrinkled