Question

Albinism in humans is inherited as a simple recessive trait. Determine the genotypes of the parents and offspring for the following families. When two alternative genotypes are possible, list both. (a) Two parents without albinism have five children, four without albinism and one with albinism. (b) A male without albinism and a female with albinism have six children, all without albinism.

Short answer

Answer: In the first scenario, both parents have a genotype of Aa and their children can have genotypes of AA, Aa, or aa. In the second scenario, the male parent has a genotype of AA and the female parent has a genotype of aa. All their children have a genotype of Aa.

Step-by-step solution

Identify the genotype of the child with albinism

The child with albinism must have the genotype "aa", as it requires two copies of the recessive allele to express the trait.

Determine the parents' genotypes from their children

Since the parents have a child with albinism, both parents must carry the recessive allele "a". This means that the parents are both heterozygous (Aa) for the trait, as they both do not express albinism.

Determine the genotypes of the other children

These children could have either of the two genotypes without albinism: "AA" or "Aa". To determine the probabilities for each genotype, we can use a Punnett square or calculate probabilities directly. Since both parents are heterozygous (Aa), each child has a 25% chance of being "AA", a 50% chance of being "Aa", and a 25% chance of being "aa" (expressing albinism). In this case, four children without albinism could have either the "AA" or "Aa" genotype. (b) A male without albinism and a female with albinism have six children, all without albinism.

Identify the genotype of the female with albinism

Given that the female expresses albinism, her genotype must be "aa".

Determine the genotype of the male without albinism from their children

Since all six children are without albinism and have received one allele from each parent, the male without albinism must have the genotype "AA", so his children inherit the "A" allele and do not express albinism.

Determine the genotypes of the children

Since the father has the genotype "AA" and the mother has the genotype "aa", each child must inherit one "A" allele from the father and one "a" allele from the mother. Therefore, all six children have the genotype "Aa" without expressing albinism.

Key concepts

Recessive Traits

Recessive traits refer to genetic characteristics that are expressed only when an individual has two copies of a recessive allele, represented as "aa." In genetic inheritance, alleles can be either dominant or recessive. Dominant alleles mask the effects of recessive alleles in heterozygous individuals (carriers), whose genotype is "Aa." Such carriers will not express the recessive trait but can pass the recessive allele to their offspring.

Recessive traits typically require genetic analysis to understand their inheritance patterns. This is because a person with a recessive trait must have inherited the recessive allele from both parents. Each parent would thus be a carrier or have the trait themselves.

• **Example**: Albinism in humans.
• **Probability**: 25% chance for two heterozygous (Aa) parents to have a child with a recessive trait (aa).

This means understanding recessive traits is crucial for predicting the likelihood of certain genetic traits appearing in offspring.

Genotype

The term "genotype" describes the genetic makeup of an individual with respect to a particular trait. It refers to the specific alleles that an organism carries in its cells. Unlike the phenotype, which is the physical expression of a trait, the genotype provides the genetic blueprint.

Genotypes are denoted by letters, with a common convention being:

• **'AA'** - Homozygous dominant
• **'Aa'** - Heterozygous
• **'aa'** - Homozygous recessive

This notation helps in predicting inheritance patterns, as seen in the study of genetic traits like albinism. By analyzing parents' genotypes, one can predict possible offspring characteristics using tools like the Punnett square. Genotype analysis is vital in genetic counseling to assess and address potential hereditary conditions.

Albinism

Albinism is a condition caused by a lack of melanin pigment in the skin, hair, and eyes. It is inherited as a recessive trait, which means an individual must inherit two recessive alleles ("aa") to express it physically. People with albinism often have lighter skin and hair, along with vision problems, due to the lack of pigmentation.

The disorder results from a mutation that affects melanin production, vital for coloration in skin, hair, and eyes. Since parents without albinism can still have children with albinism if both carry the recessive allele (Aa), understanding albinism requires comprehending recessive inheritance patterns. With both parents carrying one recessive allele, there is a 25% chance with each pregnancy that the child will have albinism if the alleles combine as "aa."

• Prevention: Genetic screenings can help identify carriers.
• Management: Sun protection and vision support are vital for affected individuals.

By studying albinism, researchers can uncover insights into genetic mutations and inheritance.

Punnett Square

The Punnett square is a simple graphical tool used to predict the possible genotypes of offspring from parental genetic combinations. It helps determine the probability of an offspring inheriting particular alleles. To create a Punnett square, you list one parent's alleles on the top and the other parent's alleles on the side, then fill in the squares inside.

In genetic problems, the Punnett square is particularly useful for visualizing how traits like albinism are inherited, especially when considering heterozygous parents ("Aa"). For example:

• Top Row: Possibly "A" or "a" alleles from one parent
• Side Column: Possibly "A" or "a" alleles from the other parent

The resulting boxes reflect all potential allele combinations for the offspring. This visualization illustrates not only the probability of different genotypes but also enhances understanding of genetic inheritance. Such tools are essential for those studying genetics to quickly and easily calculate genetic outcomes.