Different alleles may result in the production of different variants of a proteins and be observed as different traits (phenotypes) of an organism, such as petal color.

For each gene, an individual inherits 2 alleles, each inherited from a parent.

An individual's inherited alleles make up its genotype; which may be observed as a visible phenotype (e.g., flower color).

If the 2 alleles differ, the individual is heterozygous.

• In a heterozygote, often the dominant allele determines its phenotype; the other allele is called recessive.

In this example, the dominant allele expresses purple flowers, while the recessive allele expresses white flowers.

Possible genotypes are:

• A homozygous dominant individual shows purple flowers.
• A homozygous recessive individual shows white flowers.
• A heterozygous individual shows the dominant phenotype: purple flowers.

1. Mendel first crossed true-breeding (homozygous) purple with true-breeding (homozygous) white plants.
   • All offspring show purple flowers.

   • Inheritance of the dominant phenotype is explained by separation of alleles during meiosis.

2. Mendel then crossed the purple offspring.
   • About 1/4 of the next generation reverted to white flowers.

In plants, pigment genes produce pigment proteins which reflect light at different wavelengths.

Presence of purple pigments result in purple pea flowers (dominant), while a mutant allele yields no purple pigments (recessive).

Thus homozygous recessive plants show white flowers due to lack of pigments.

Meiosis in the heterozygous offspring produces gametes that possess either the dominant or the recessive allele.

First the possible gametes of the two parents are aligned along two sides of a square.

Then the gametes are combined to show genotypes of potential zygotes in the cells of the square.

• All the eggs of homozygous recessive (albino) mother have the recessive allele (m).

• All the sperm of homozygous dominant (pigmented) father have the dominant allele (M).

• All the offspring are heterozygous (Mm) in genotype, pigmented in phenotype.

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A mutant allele yields no melanin pigments (recessive).

An animal with a homozygous recessive genotype has a phenotype of albino: it lacks color for those organs.

• Due to meiosis, 1/2 of both eggs and sperm are dominant (M), 1/2 are recessive (m).

• Offspring genotype ratio is 1 MM : 2 Mm : 1 mm.

• Offspring phenotype ratio is 3 pigmented : 1 albino.

Note these ratios are based on probability, assuming random fertilization of gametes.

• Assuming brown eyes (B) is dominant over blue eyes (b), can 2 brown-eyed parents have a baby with blue eyes?
  • Yes if the parents are heterozygous; 1/4 of their offspring will be blue-eyed.

How to determine whether an individual exhibiting a dominant phenotype (pigmented) was homozygous (MM) or heterozygous (Mm)?

Testcross with an individual exhibiting the recessive phenotype (albino), because we know the genotype must be homozygous recessive mm.

If any offspring is albino, the pigmented parent must be heterozygous (Mm).

Some genes are located on the X chromosome (sex-linked).

Males possess only 1 X chromosome, and exhibit a recessive sex-linked trait (such as color-blindness) if he inherits the allele from his mother.

Females exhibit the disorder only if she receives the recessive allele from both parents.

Some examples:

• color-blind female marries normal-sighted male

• father and son color-blind, mother and daughter normal-sighted

If a female carrier mates with a normal-sighted male:

• There is a 50% chance that a daughter will be a carrier like her mother.

• There is a 50% chance that a son will have the disorder.

• All her daughters will be carriers.

• All her sons will be color-blind.

• 50% of the daughters will be color-blind.

• 50% of the sons will be color-blind.

Note: the mother must be a carrier to give the disease allele to her son.