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Biology Genetic Inheritance - Heredity Cheat Sheet by

This will include terms, test crossings, blood types, and whatever I was taught in class.

Terms - Alphab­etical

Allele: Different form of the same gene
Continuous Variation: Occurs when a phenotype shows multiple variations in a population through multiple genes
Gene: Basic unit of DNA - hereditary material.
It occupies a specific location on a chromosome and determines a particular charac­ter­istic in an organism
Genotype: An organi­sim's genetic inform­ation
Heredity: Transfer of traits from parent to offspring
Phenotype: Set of observable traits
Polygenic: Multiple genes contri­buting to one trait eg. height or weight.

Gregor Mendel

Studied the phenotypes of genera­tions of peas.

Some of the pea plant phenotypes are:
- Purple flowers or white flowers
- Yellow seeds or green seeds
- Wrinkled seeds or round seeds
- Tall or short plants


Humans have 2 alleles for each gene

Dominant allele: Will mask a recessive gene – TT or Tt

Recessive allele: Is only expressed when a gene has two of this type of allele – tt


Homozygous dominant genotype: 2 dominant alleles (TT or AA)
Homozygous recessive genotype: 2 recessive alleles (tt or aa)
Hetero­zygous genotype: one dominant allele and one recessive allele (Tt or Aa)

One-Trait Inheri­tance

The types of gametes that are produced depends on the genotype of the parent cell.

Examples of one-trait inheri­tance
 Cheek dimples
 Free or attached earlobes
 Freckles

One-trait crosses consider only one set of alleles -
e.g. HH x Hh

Remember that a gamete (egg or sperm) has only
one allele
 e.g. a ‘H’ or a ‘h’

Punnet Squares

Can be used to predict genotypes and phenotypes of offspring from genetic crosses.

Steps for a Punnet Square (based on the picture below)

Step 1: Figure our the genotypes of the parents - Rr x RR

Step 2: Place one Parent on the top and one on the outside

Step 3: Cross them. Always make sure to put the capital letter first

Punnet Squares

Monohybrid Cross

Both parents are hetero­zygous (or a hybrid) for a single (mono) trait.
Based on the picture below the trait is freckles.

Monohybrid Cross


Genotypic ratio: the number of offspring with the same genotype

Phenotypic ratio: the number of offspring with the same outward appearance

 What is the genotypic ratio for the Monohybrid cross above?
 1: 2: 1 (1 FF: 2 Ff: 1 ff)

 What is the phenotypic ratio?
 3: 1 (3 with freckles and 1 with no freckles)

For a monohybrid cross, 3:1 phenotypic ratio is always expeccted when one allele is completely dominant over the other.

Test Crossing

Determine whether an organism with a dominant phenotype is homozygous or hetero­zygous.
The genotype is unknown (Hh or Hh)

Rules of a test cross

1. If the cross yeilds 100% dominant phenotype offspring, the parent is homozygous dominant.

2. If the cross yeilds 50% dominant phenotype and 50% recessive phenotype offspring, the parent is hetero­zygous.

Example shown in picture below.

Test Crossing

Mendelian Laws

Law of Segreg­ation
During formation of gametes, the 2 traits carried by each parent will separate
Law of Indepe­ndent Assortment
The homologous pairs separate into gametes is completely random, so any possible combin­ation can occur.
If the genes for two different traits are found on different chromo­somes, we can use a dihybrid cross to look at the possible genotype and phenotype outcomes.

Dihybrid Cross

A cross between two indivi­duals that carry two different traits that determines the probab­ility of the traits being passed onto the offspring.

A dihybrid is hetero­zygous for 2 traits.

A dihybrid cross always gives a 9:3:3:1 phenotypic ratio

Dihybrid Cross


Non-Me­ndelian Inheri­tance

Polygenic Inheri­tance
Multiple genes affect one trait (eg. hair colour, eye color, skin colour)
Incomplete Dominance
Dominant allele is partially expressed, resulting in an interm­ediate phenotype
(Eg. Red X White = Pink hetero­zyg­ous).
both alleles are expressed in the hetero‐ zygote neither is dominant or recessive, but there is no blending
Sex-linked Inheri­tance
Traits that are carried on by the sex chromo­somes

Codomi­nance - Blood Types

Different combin­ations of the three blood-type alleles produce four different possible phenot­ypes, or blood types (A, B, AB, and O)

IA and IB are completely dominant over i, and IA is codominant with IB

IA -> dominant allele for blood type A

IB -> dominant allele for blood type B

i -> recessive allele for blood type O

Blood Types Cont'

Type AB blood has no antibo­dies, any blood can be donated to them - universal acceptor

Type O - universal donor

X-linked Influences

Some inheri­tance patterns depend on which chromo­somes the gene is located on.

The X chromosome contains many genes that are not related to sex charac­ter­istics, and those genes are called X-linked (genes on the X chromosome unrelated to sex charac­ter­ist­ics).

Human X-linked, recessive traits include hemoph­ilia, which is a blood clotting disorder and red-green colour blindness.

Each of these traits are much more common in males than females because males have only one X chromosome and females need to inherit two mutated alleles to have the disease

Genetic Disorders

Sickle­-cell Anemia
Autosomal Recessive. The hemoglobin gene is mutated, causing abnormal red blood cell formation and reduced capacity to carry oxygen.
Tay Sachs
Autosomal Recessive. Lysosomal enzyme is mutated, causing brain deteri­oration leading to death
Cystic Fibrosis
Autosomal Recessive. the chloride channel gene is mutated, causing altered water balance inside of cells, and this leads to excessive mucus produc­tion, which impacts the lungs, liver, pancreas, and sweat glands.
Huntin­gton's Disease
Autosomal Dominant. CAG nucleotide repeats are inserted into a gene that affects a protein in the brain.
Recessive X-linked. Affects a gene involved in blood clotting.


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