Key Concept B: Monohybrid Crosses & Mendel’s 1st Law of Segregation

How he discovered the law

● In his first set of experiments, Mendel investigated the pattern of inheritance of one set of traits. His one-trait experiments have been categorized as monohybrid crosses.

● The observed results of Mendel’s monohybrid crosses with pea plants resulted in Mendel’s First Law: the Law of Segregation

Mendel’s Law of Segregation:

Every individual has 2 copies of each gene (1 on each homologous chromosome) When any individual produces gametes, the copies of the gene separate (segregate) so that each gamete only receives one copy. If an individual has 2 different alleles, the gamete will receive one or the other.

B2. Test Cross

Test Cross: testing an unknown plant or animal by crossing it with a known homozygous recessive (ex. aa)

• Used to find the unknown genotype of an individual. The unknown is always Crossed with a homozygous recessive individual.

• If it ends up Dominant is is homozygous and if it’s mixed it’s heterozygous.

B3. Multiple Alleles

Multiple Alleles: A gene with more than two alleles.

• As there are more than two alleles, upper and lower case letters are not used to signify dominance and recessiveness.

B4. Incomplete Dominance

Incomplete Dominance: Describes a condition where there is partial expression of both alleles: neither of two alleles for the same gene can completely conceal the presence of the other.

• Three phenotypes exist for incomplete dominant traits with heterozygous individuals being an intermediate condition of the two alleles.

• Because there is no completely dominant allele, superscript notation is used in place of upper and lower case letters.

B4. Co-Dominance:

Describes a condition in which both alleles are fully expressed.

B5. Sex-Linked Traits

Thomas Morgan (1866-1945) is considered to be one of the modern father of genetics and was made famous for his extensive fruit fly experiments.

• He concluded that sex-linked traits are expressed in different ratios by male and female offspring, because they are governed by the segregation of X and Y chromosomes.

• There are much fewer genes located on the Y chromosome than on the X chromosome.