Biology Genetic Inheritance - Heredity Cheat Sheet

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

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’

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

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

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.

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.

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

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

I^A and I^B are completely dominant over i, and I^A is codominant with I^B

I^A -> dominant allele for blood type A

I^B -> dominant allele for blood type B

i -> recessive allele for blood type O
.

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