1. Biology Unit 5 Notes: Mendelian Genetics

2. (1) Gregor Mendel Monk Studied genetics of pea plants.
He noticed that:
There were some traits which showed up all of the time (dominant).
There were some traits which showed up some times (recessive).
These traits are passed down from parents.
You can’t tell just by looking at an organism, which genes it has.

5. (2) Dominant + Recessive Traits
Dominant Trait:
These will ALWAYS SHOW UP if they are present.
Representation  Capital Letters
Recessive Trait:
These will ONLY SHOW UP if the organism inherited 2 together.
Representation  Lower Case Letters

7. (3) Inheriting Traits You inherit 2 alleles for every gene.
Gene = Chunk of DNA.
Allele = Variation that chunk can come in.
“Heterozygous” = You inherited 1-Dominant and 1-Recessive allele.
“Homozygous” = You inherited 2 of the same alleles.
“Homozygous Dominant” = You inherited 2-Dominant alleles.
“Homozygous Recessive” = You inherited 2-Recessive alleles

9. (4) Ways to Describe Inheritance
Phenotype:
Physical Description.
Examples:
“White”
“Brown”
“White and Brown Spotted”
Genotype:
Genetic Description.
AA or Homozygous Dominant
Aa or Heterozygous
aa or Homozygous Recessive

11. (5) What’s With These Letters?
The letters represent the alleles an organism can inherit.
You should choose letters with a different Capital + Lower Case appearance:
A and a
B and b
Bad Life Choices: S and s, X and x, C and c
Actual Letters DO NOT MATTER:
Heterozygous needs to be: Capital + Lower
Homozygous Dominant needs to be: 2 Capital
Homozygous Recessive needs to be: 2 Lower

13. (6) What Are Punnett Squares?
Punnett Square = Tool used to make genetic predictions.
They do NOT tell you with certainty how many offspring will have which traits!!!
Tells you probability.
“There is a ¼ or 25% chance that the offspring will….”
SET-UP:
1 Parent’s genotype on the top
Other Parent’s genotype on the side

14. (7) Setting Up Punnett Squares
1 Parent’s Genotype on the Top (doesn’t matter which one).
1 Parent’s Genotype on the Side (doesn’t matter which one).
1 Letter per Box!
For Example:
If one parent is AA and the other parent is aa… A A
Draw your Punnett Square! a a

15. (8) Filling In Punnett Squares
They get filled in like a matrix-box OR like a grid on a map.
The TOP letters copy down into each box.
The SIDE letters copy across into each box.
The dominant trait is always written in front.
For Example:
If one parent is AA and the other parent is aa… A A
Draw your Punnett Square! Aa a a

16. (9) Interpreting Punnett Squares
Each Box = ¼ or 25% chance
For Example:
4/4 Boxes have Aa, so…
There is a 4/4 or 100% chance that the offspring will be Heterozygous.
I could also say that there is a 100% chance that the offspring will look dominant. A A
Draw your Punnett Square! Aa a a