TEAS 7 · Science · 6 min read
Genetics and Heredity
- Distinguish between key genetic concepts including genes, alleles, loci, genotypes, and phenotypes.
- Apply Mendelian laws and Punnett squares to monohybrid crosses, dihybrid crosses, and testcrosses.
- Analyze variations from classic Mendelian inheritance, including incomplete dominance, codominance, sex-linked traits, and linked genes.
Molecular Foundations: Genes, Chromosomes, and Alleles
Genetics is the scientific study of heredity and the variation of inherited characteristics, whereas heredity is the biological process by which parents pass genes onto their offspring. At the molecular level, a gene is a specific sequence of DNA that encodes instructions for synthesizing a functional product, usually a protein. These synthesized proteins carry out vital biological activities that define living systems.
In human cell nuclei, chromosomes consist of DNA packaged with histone proteins, and they condense tightly during cell division. Human somatic cells are diploid somatic cells that contain two sets of chromosomes, organized into 23 pairs of chromosomes that total 46 chromosomes. Among these are homologous chromosomes, which are pairs of chromosomes of the same size and shape that carry the same genes at the same loci. A locus is the specific physical location of a gene on a chromosome.
Genes exist in different variations known as alleles, which are alternative forms or versions of a specific gene. An individual's genotype refers to their specific combination of alleles for a given gene. In contrast, an individual's phenotype consists of observable physical or physiological traits influenced by genotype and the environment. Everyday environmental conditions interact with an organism's genetic makeup to help shape these observable characteristics.
The specific combination of alleles at a locus defines whether an individual is homozygous or heterozygous. A homozygous individual carries two identical alleles for a specific gene, while a heterozygous individual carries two different alleles for a specific gene. In genetic notation, dominant alleles are conventionally represented by uppercase letters, and recessive alleles are conventionally represented by lowercase letters. A dominant allele masks the phenotypic expression of a recessive allele in a heterozygous organism. For an autosomal trait with complete dominance, the recessive phenotype appears when an individual has two recessive alleles.
Common mistakes
- Assuming that codominance produces an intermediate blended phenotype, when in fact codominance involves full and simultaneous expression of both alleles without blending, while incomplete dominance results in an intermediate blend.
- Concluding that a testcross resulting entirely in dominant offspring definitely proves the unknown parent is homozygous dominant, when in reality a finite number of offspring cannot rule out a heterozygous parent.
- Believing that all genes assort independently, overlooking that linked genes are located on the same chromosome, tend to be inherited together, and are less likely to be separated by crossing over the closer they are.
- Labeling biological males as carriers for X-linked recessive conditions, when males are hemizygous (carrying only one X chromosome) and therefore either express the trait or do not.
Key points
- Genetics is the scientific study of heredity and inherited variation, while heredity is the process whereby parents pass genes, sequences of DNA that encode functional products, usually proteins, to offspring.
- Human somatic cells are diploid cells containing 23 pairs of chromosomes (46 chromosomes total), organized as 22 pairs of autosomes and one pair of sex chromosomes, packaged with histone proteins.
- Homologous chromosomes carry the same genes at the same loci, and alleles represent alternative versions of these genes that establish an individual's genotype and influence their phenotype.
- Mendel established the principle of dominance, the Law of Segregation, and the Law of Independent Assortment through experimental crosses using garden pea plants.
- A monohybrid cross of two heterozygotes (Aa x Aa) yields a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio under complete dominance, with a 25 percent probability of homozygous recessive offspring and a 50 percent probability of heterozygous offspring.
- Incomplete dominance produces an intermediate blend (such as pink snapdragons), while codominance allows both alleles to be fully and simultaneously expressed (such as genotype I^A I^B producing AB blood with both A and B antigens).
- Sex-linked traits primarily reside on the X chromosome; biological males are hemizygous, causing X-linked recessive disorders to appear more frequently in males, while linked genes reside on the same chromosome and tend to be inherited together.
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