Genetics

The One Thing

Genetics explains heredity and variation through genes, which occur as allele pairs and segregate during gamete formation. Mendel’s laws allow the inheritance of contrasting characteristics to be predicted, while sex chromosomes determine chromosomal sex and influence sex-linked inheritance.

Reactions, Processes and Experiments

Key Terms

• Heredity: The transmission of characteristics from parents to their offspring.
• Variation: Differences in characteristics among individuals of the same species.
• Gene: A unit of heredity that controls a particular characteristic.
• Allele: An alternative form of a gene, such as the allele for tallness or dwarfness.
• Chromosome: A thread-like structure in the nucleus that carries genes.
• Dominant allele: An allele that expresses its effect even when only one copy is present.
• Recessive allele: An allele whose effect is expressed only when two copies are present and no dominant allele masks it.
• Genotype: The genetic constitution of an organism, such as TT, Tt or tt.
• Phenotype: The observable appearance or characteristic of an organism.
• Homozygous: Having two identical alleles for a gene, such as TT or tt.
• Heterozygous: Having two different alleles for a gene, such as Tt.
• Gamete: A reproductive cell, such as a sperm or egg, containing one allele of each gene pair.
• Mendel: Gregor Johann Mendel, an Austrian monk whose pea-plant experiments established the basic laws of inheritance.
• Monohybrid cross: A genetic cross involving one pair of contrasting characteristics.
• Dihybrid cross: A genetic cross involving two pairs of contrasting characteristics.
• Punnett square: A diagram used to predict the possible genotypes and phenotypes of offspring.
• Test cross: A cross between an organism showing a dominant phenotype and a homozygous recessive organism to determine the unknown genotype.
• Law of Dominance: When two contrasting alleles occur together, only the dominant allele is expressed in the phenotype, while the recessive allele remains masked.
• Law of Segregation: The two alleles of a gene pair separate during gamete formation, so each gamete receives only one allele.
• Law of Independent Assortment: Alleles of different gene pairs separate independently during gamete formation, provided the genes are not linked.
• Sex chromosomes: Chromosomes involved in determining sex; in humans, females are XX and males are XY.
• Autosomes: Chromosomes other than the sex chromosomes; humans have 22 pairs of autosomes.
• Sex-linked inheritance: Inheritance of a characteristic controlled by a gene located on a sex chromosome, usually the X chromosome.
• X-linked recessive trait: A recessive characteristic controlled by a gene on the X chromosome; it is more common in males because males have only one X chromosome.
• Carrier: A person who possesses a recessive allele but does not show the trait, commonly used for heterozygous females carrying an X-linked recessive allele.
• Pedigree: A diagram that traces the inheritance of a characteristic through several generations of a family.

Easily Confused

• Genotype and phenotype: Genotype is the genetic constitution, such as Tt; phenotype is the observable characteristic, such as tallness.
• Homozygous and heterozygous: Homozygous organisms have identical alleles, such as TT or tt; heterozygous organisms have different alleles, such as Tt.
• Dominant and recessive alleles: A dominant allele is expressed when one copy is present; a recessive allele is expressed only when unmasked, usually by the absence of a dominant allele.
• Monohybrid and dihybrid crosses: A monohybrid cross involves one pair of contrasting characteristics; a dihybrid cross involves two pairs.
• Law of Dominance and Law of Segregation: Dominance concerns which allele is expressed in a heterozygote; segregation concerns the separation of allele pairs during gamete formation.
• Sex chromosomes and autosomes: Sex chromosomes determine chromosomal sex and may carry sex-linked genes; autosomes are the other 22 chromosome pairs in humans.
• Carrier female and affected female: X^A X^a represents a carrier female who does not show the X-linked recessive trait; X^a X^a represents an affected female.
• Autosomal recessive and X-linked recessive inheritance: An autosomal recessive phenotype generally requires two recessive alleles; a male can express an X-linked recessive phenotype with one recessive allele on his only X chromosome.
• Independent assortment and linkage: Independent assortment applies most clearly to genes on different chromosomes or sufficiently far apart on the same chromosome; it does not apply in the same way to linked genes.

What Gets Asked

• Questions may require a monohybrid cross such as TT × tt, identifying that all F1 offspring are Tt and tall. A common error is to describe the F1 plants as homozygous.
• Questions may require the F2 results of Tt × Tt, including both the genotypic ratio 1 TT : 2 Tt : 1 tt and the phenotypic ratio 3 tall : 1 dwarf. The genotypic and phenotypic ratios must not be interchanged.
• A test-cross question may use Tt × tt and require the 1 tall : 1 dwarf ratio. The recessive parent must be homozygous, not merely phenotypically recessive without the stated genotype.
• Dihybrid-cross questions may require the gametes RY, Ry, rY and ry, the phenotypic ratio 9:3:3:1, or the full genotypic ratio. The four gamete types and their allele combinations must be written accurately.
• Human sex-determination questions may require the combinations XX and XY and an explanation that the father’s sperm determines chromosomal sex. The mother’s ova carry only X chromosomes.
• X-linked inheritance questions may require the genotypes X^A X^A, X^A X^a, X^a X^a, X^A Y and X^a Y, or the cross X^A X^a × X^A Y. A frequent error is stating that a father passes an X-linked trait directly to his sons; fathers pass their Y chromosome to sons.
• Questions may ask for the distinction between expected genetic ratios and actual offspring numbers. The ratios are probabilities, so chance may cause observed numbers to differ.