Key Terms

Mendelian genetics: genetic analysis based on observation of a phenotype that’s controlled by a single gene

Genotype: the gene sequence that determines the characteristics of an organism

Phenotype: the detectable characteristic of an organism

Dominant traits: traits that are inherited unchanged in a hybridization.

Recessive traits: traits that become latent, or disappear, in the offspring of a hybridization, but reappears in the progeny of the hybrid offspring

Homozygous: a gene with two identical alleles on their homologous chromosomes

Heterozygous: a gene with two different alleles on their homologous chromosomes

Mendelian Principles

Law of segregation: during gamete production, the two copies of alleles of a parent’s gene separate, and only one gene is passed on from each parent to the offspring

Law of independent assortment: alleles in a pair separate and are inherited by chance

Law of Dominance: some alleles are dominant while others are recessive

Punnett Square

Punnett square: a diagram to predict the outcomes of a cross between two individuals and their traits

To prepare a Punnett square, all possible combinations of the parental alleles are listed along the top of a grid for one parent and the side of a grid for the other parent. This represents their meiotic segregation into haploid gametes. Then the combinations of egg and sperm are made in the boxes in the table to show which alleles are combining. Each box represents the diploid genotype of a zygote that could result from this mating. Since each possibility is equally likely, genotypic ratios can be determined from a Punnett square. If the pattern of inheritance (dominant or recessive) is known, the phenotypic ratios can be also be determined.

Monohybrid cross: a cross between individuals that differ in only one trait

Dihybrid cross: cross between individuals that differ in two traits

When crossing two heterozygotes in a monohybrid cross, the genotypic ratio is 1:2:1 and the phenotypic ratio is 3:1.

When crossing two heterozygotes in a dihybrid cross, the genotypic ratio is 4:2:2:2:2:1:1:1:1 and the phenotypic ratio is 9:3:3:1.

Sex-linked Traits

Sex-linked genes: genes located on either sex chromosome

In humans, sex-linked usually refers to genes located on the larger X chromosome.

When a gene being examined is present on the X chromosome, but not on the Y chromosome, it is said to be X-linked. When a female parent is homozygous for a recessive X- linked trait, she will pass the trait on to 100 percent of her offspring. Her male offspring will express the trait, as they will inherit their father's Y chromosome.

In humans, the alleles for certain conditions (some forms of color blindness, hemophilia, and muscular dystrophy) are X-linked. Females who are heterozygous for these diseases are said to be carriers and may not exhibit any phenotypic effects. These females will pass the disease to half of their sons and will pass carrier status to half of their daughters; therefore, recessive X-linked traits appear more frequently in males than females.

Sex-linked traits: traits located by genes located on the sex chromosome. (ie red-green color blindness is on a recessive gene found on the X chromosome)

The Product Rule and Sum Rule

The product rule: the probability of two independent events occurring together can be calculated by multiplying the individual probabilities of each event occurring alone.

The sum rule: the probability of the occurrence of one event or the other event, of two mutually exclusive events, is the sum of their individual probabilities (applied when  considering two mutually exclusive outcomes that can come about by more than one pathway).

To use probability laws, you must work with large sample sizes because small sample sizes are prone to deviations caused by chance.