AP Biology: Genetics & Heredity

What are Mendel's two fundamental laws of inheritance?

Mendel's Law of Segregation states that two alleles for a heritable character separate during gamete formation. The Law of Independent Assortment states that each pair of alleles segregates independently of other pairs of alleles during gamete formation.

Differentiate between genotype and phenotype.

Genotype refers to the genetic makeup, or set of alleles, of an organism. Phenotype refers to the observable physical and physiological traits of an organism, which are determined by its genotype and environmental influences.

Define an allele and explain the terms dominant and recessive in relation to alleles.

An allele is an alternative form of a gene. A dominant allele expresses its phenotypic effect even when heterozygous, while a recessive allele only expresses its phenotypic effect when an individual is homozygous for that allele.

What is the difference between an individual who is homozygous and one who is heterozygous for a particular gene?

A homozygous individual has two identical alleles for a given gene (e.g., AA or aa). A heterozygous individual has two different alleles for a given gene (e.g., Aa).

What is the purpose of a Punnett Square in genetics?

A Punnett Square is a diagram used to predict the allele combinations and resulting genotypes and phenotypes of offspring from a genetic cross. It illustrates the probability of each outcome.

When would a geneticist perform a test cross, and what information does it reveal?

A test cross is performed to determine the genotype of an individual showing a dominant phenotype but with an unknown genotype. The unknown individual is crossed with a homozygous recessive individual; if any offspring are recessive, the unknown parent must be heterozygous.

Describe incomplete dominance and give an example.

In incomplete dominance, the phenotype of heterozygotes is intermediate between the phenotypes of the two homozygotes. For example, a red flower crossed with a white flower may produce pink offspring.

Explain codominance and provide a classic human example.

Codominance occurs when both alleles for a gene are fully expressed in the heterozygote phenotype without blending. A classic example is human ABO blood type, where A and B alleles are codominant, resulting in AB blood type.

What is polygenic inheritance, and how does it differ from Mendelian single-gene traits?

Polygenic inheritance involves multiple genes affecting a single phenotypic character, resulting in a continuous variation of phenotypes. Unlike single-gene traits with distinct categories, polygenic traits like human height or skin color show a range of expressions.

What is a sex-linked trait, and why are X-linked recessive disorders more common in males?

A sex-linked trait is a characteristic whose gene is located on a sex chromosome, typically the X chromosome in humans. X-linked recessive disorders are more common in males because they have only one X chromosome, so if they inherit a recessive allele on it, they will express the trait.

How can geneticists use a pedigree chart to determine the inheritance pattern of a trait?

A pedigree chart uses symbols to represent family relationships and phenotypes across generations. By analyzing patterns like affected individuals, sex ratios, and occurrence in parents/offspring, geneticists can infer if a trait is dominant, recessive, autosomal, or X-linked.

Define linked genes and explain how recombination frequency relates to gene mapping.

Linked genes are genes located close together on the same chromosome that tend to be inherited together. Recombination frequency, the percentage of recombinant offspring, is used to estimate the distance between linked genes on a chromosome; a higher frequency indicates a greater distance.

What is nondisjunction, and what chromosomal abnormality can it lead to?

Nondisjunction is an error during meiosis or mitosis in which homologous chromosomes or sister chromatids fail to separate properly. This can lead to aneuploidy, a condition where an organism has an abnormal number of chromosomes, such as trisomy (e.g., Down Syndrome).

Explain mitochondrial inheritance.

Mitochondrial inheritance is a non-Mendelian inheritance pattern where traits are passed down exclusively from the mother to all her offspring, regardless of sex. This is because mitochondria, containing their own DNA, are inherited from the egg cell during fertilization.

List the five conditions required for a population to be in Hardy-Weinberg equilibrium.

For a population to be in Hardy-Weinberg equilibrium, five conditions must be met: no gene flow, no mutation, random mating, no natural selection, and an extremely large population size.