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Genetics, Heredity, and Inheritance Cheat Sheet by

Gregor Mendel

Who Was Gregor Mendel?
Gregor Mendel was born in 1822. He worked in a monastery garden, using pea plants to study heredity.
What led to Mendel's experi­ments?
Through his experience breeding plants, Mendel knew that patterns of inheri­tance sometimes seemed simple, and sometimes didn't. Mendel noticed that a trait would sometimes skip a genera­tion, and then show up again. He wanted to know why this happened.
Why did Mendel choose pea plants for his experi­ments?
Garden peas were a good choice for multiple reasons. They grew quickly, exist in many varieties, and can self-p­oll­inate. Self-p­oll­inating plants contain both male and female reprod­uctive struct­ures. This means pollen from one flower on a plant can pollinate itself or another flower on that plant.
What did Mendel study?
During his experi­ments, Mendel studied one charac­ter­istic at a time. He studied charac­ter­istics that had two forms. He was careful to use truebr­eeding plants. When these plants self-p­oll­inate, the offspring has the same trait as the parent. Mendel cross-­pol­linated true-b­reeding plants to carry out his experi­ment.
What were Mendel's two experi­ments?
In his first experi­ment, Mendel studied 7 charac­ter­istics. He performed crosses between the two traits of each charac­ter­istic. The offspring of these crosses is called the first genera­tion. One trait (dominant) always appeared, and one trait (reces­sive) seemed to disappear. Mendel then performed another experi­ment. He allowed the first generation to self-p­oll­inate. The recessive trait appeared at a 3:1 ratio (25%).
What did Mendel realize as a result of his two experi­ments?
Mendel realized that his results could only be explained if each parent had two sets of instru­ctions per charac­ter­istic (genes). Each parent gives the offspring one set, so it has two. The different forms of a gene are called alleles. This can be shown with a Punnett square.

Punnett Square

Genotype: Inherited combin­ation of alleles
Phenotype: An organism's appearance


What are the two types of reprod­uction?
Asexual reprod­uction: One parent is needed for reprod­uction. Internal structures of the cell are copied by mitosis. The parent cell divides creating two identical daughter cells. Most one-celled organisms reproduce this way.
Sexual reprod­uction: Two parent cells, known as sex cells, join together to form a new indivi­dual. One half of the chromo­somes an organism needs is given to it by each parent cell. Male sex cells are called sperm. Female sex cells are called eggs, or ova. Each sex cell has one of the chromo­somes from each homologous pair.
What is meiosis?
A process which produces new cells with half the usual number of chromo­somes (sex cells). The chromo­somes are then copied and the nucleus divides twice. The result is sperm and eggs with half the number of chromo­somes found in a normal body cell.
Who is Walter Sutton?
Walter Sutton was a young graduate student who made an important observ­ation. He realized that GENES ARE LOCATED ON CHROMO­SOMES!
What are sex chromo­somes?
Sex chromo­somes carry genes that determine whether the offspring is male or female. Female: XX Male: XY Females always pass on X. Males pass X or Y. If an X sperm fertilizes an egg, a female is born. If a Y sperm fertilizes an egg, a male is born.




What are the subunits of DNA?
DNA is made of four subunits, known as nucleo­tides. Each consists of a phosphate, a sugar, and a base. The three bases are:
(A) Adenine - Pairs with T
(T) Thymine - Pairs with A
(G) Guanine - Pairs with C
(C) Cytosine - Pairs with G
Because A always bonds with T and G always bonds with C, one side of a DNA sequence is comple­mentary to the other.
How does DNA replicate?
DNA replicates by splitting down the middle. Each original molecule is like a template. A comple­mentary molecule forms along it.
What are Chargaff's rules?
Erwin Chargaff found that the amount of adenine in DNA is equal to that of thymine, and the amount of guanine is equal to that of cytosine.
Who was Rosalind Franklin?
In the lab of Maurice Wilkins, Rosalind Franklin created X-Ray images of DNA. These suggested that DNA had a spiral shape. James Watson and Francis Crick later used this research.
Who are James Watson and Francis Crick?
Watson and Crick built on the research of others and concluded that DNA resembles a double helix shape. Upon discovery of this, Crick is said to have exclaimed, "We have discovered the secret of life! "

Patterns of Inheri­tance

Law of Dominance
One allele is dominant. It always shows unless it isn't present (Homoz­ygous Recessive)
Law of Segreg­ation
During production of gametes, 2 copies of each allele separate. Therefore, one allele is needed from each parent.
Law of Indepe­ndent Assortment
Individual alleles assort indepe­nde­ntly. This gives different traits equal chance of occurring together.
Incomplete dominance
Dominant allele partially expressed, resulting in an interm­ediate phenotype (Example: Rr If Red is Dominant and White is recessive, the phenotype is pink).
2 dominant alleles, Phenotype is neither dominant nor recessive.
Example: In blood, types A and B are codomi­nant, O is recessive. AB blood is therefore possible.
Sex-Linked Inheri­tance
A trait is associated with the X or Y chromo­some. (Males can never be carrier).
Polygenic Inheri­tance
Multiple genes affect one trait (Examples: Hair color, skin color, eye color).


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