Lesson 2: Applying Mendel’s Principles

Question 1

Mendel and
Autosomal Traits

Answer 1

Autosomal traits are
traits expressed from a
gene on an autosomal
chromosome.

Example: Flower colour

Question 2

Applying Mendel’s Principles

Answer 2

Each organism has two factors for each trait.
These factors we now know are genes located on
chromosomes. The different forms are called
alleles. An organism receives one copy of each
gene from each parent.

The two alleles for each trait must separate
when gametes are formed through meiosis. A
parent passes on only one allele for each trait to
any offspring.

The alleles for each trait behave independently
of alleles for other traits during gamete
production.

Question 3

Predicting Offspring: The
Punnett Square

Answer 3

R.C. Punnett devised a system to predict
the probability of traits of offspring.

Question 4

Using the Punnett Square Examples 1

Answer 4

One common Mendelian ratio is a result of the cross between a purebred dominant and a
purebred recessive.

All offspring will be heterozygous and display the dominant trait.

Ex. Big ears are dominant (E) to small ears (e). A homozygous dominant woman has a
child with a man who has small ears.

Question 5

Using the Punnett Square Examples 2

Answer 5

Another common Mendelian ratio occurs when two
heterozygous individuals have offspring. This is called a
monohybrid cross.

The genotypic ratio that results is 1AA: 2Aa: 1aa

The phenotypic ratio that results is 3 dominant: 1
recessive.

Ex. In garden gnomes beards are dominant to no beard.
Two heterozygous individuals have offspring.

Question 6

Using the Punnett Square Examples 3

Answer 6

Another common Mendelian ratio results when more than one trait is considered. This is when
organisms that are heterozygous for two traits have offspring. This is called a dihybrid cross.

The gamete possibilities are as follows:

AaBb 🡪AB, Ab, aB or ab gametes

Example: Pea seed color and shape

yellow = dominant (A), green = recessive (a)

smooth = dominant (B), wrinkled = recessive (b)

Question 7

Test Cross

Answer 7

Used to determine if a parent is homozygous or heterozygous for a dominant trait.

This is done by crossing it with a homozygous recessive individual and observing
the results in the offspring.