13.2 Mendelian Inheritance and Variation

13.2 Mendelian Inheritance and Variation

Praxis 5442 focus (ETS III.C): Candidates should construct and interpret Punnett squares for simple dominant/recessive traits, explain how mutations contribute to variation (harmful, beneficial, or neutral), and contrast causes of conditions such as cystic fibrosis (gene-level) versus Down syndrome (chromosomal).

Once students know that genes have alleles, they need a tool to predict inheritance. Gregor Mendel’s work with pea plants supplies that tool for middle school: track one trait at a time, assign letter symbols, and use a Punnett square to show possible offspring genotypes.

Core Mendelian vocabulary

• Dominant allele: expressed in the phenotype when at least one copy is present (often written as a capital letter, e.g., P).
• Recessive allele: expressed in the phenotype only when two copies are present (e.g., p).
• Homozygous: two identical alleles (PP or pp).
• Heterozygous: two different alleles (Pp).
• Genotype: the allele pair itself.
• Phenotype: the observable trait (purple flowers, white flowers).

These definitions are operational: Praxis items often give phenotypes and ask which genotypes are possible, or give a cross and ask for expected ratios.

Worked example: purple and white flowers

Classic teaching model (aligned with Mendel’s flower-color style problems):

• Allele P = purple flowers (dominant)
• Allele p = white flowers (recessive)

A heterozygous purple plant has genotype Pp. Crossing two heterozygotes models a common classroom lab:

Parents: Pp × Pp

Each parent can pass P or p with equal probability (for a simple monohybrid model). The Punnett square:

Genotype ratio: 1 PP : 2 Pp : 1 pp
Phenotype ratio: 3 purple : 1 white

Why? PP and Pp both show purple because P is dominant; only pp shows white.

Worked probability check: Probability of white offspring = probability of pp = 1/4 = 25%. Probability of purple = 3/4 = 75%. If a class grows 80 offspring plants from this cross (large-sample idealization), about 60 purple and 20 white are expected—real counts vary by chance.

Quick variants teachers should recognize

Exam tip: If a white-flowered plant appears, its genotype must be pp for this trait model. A purple-flowered plant could be PP or Pp—a test cross with pp distinguishes them.

Mutations and variation

A mutation is a change in DNA sequence. Mutations are a source of new alleles and therefore of genetic variation. For Praxis middle school depth, classify effects by outcome—not by assuming “mutation = bad.”

Context matters: a mutation helpful in one environment can be harmful in another. Mutations can occur spontaneously during replication or be increased by mutagens (certain radiation or chemicals)—keep explanations age-appropriate and evidence-based.

Mutations in body (somatic) cells are not typically passed to offspring; mutations in cells that form gametes can be inherited. Middle school items may not always stress this distinction, but teaching scenarios about “will the child inherit it?” reward careful wording.

Contrasting causes: cystic fibrosis vs Down syndrome

Teachers and Praxis stems often juxtapose genetic conditions that look similar to students (“both are genetic”) but differ in mechanism.

Cystic fibrosis (CF) — typically a single-gene disorder

Cystic fibrosis is commonly taught as an autosomal recessive condition caused by mutations in the CFTR gene, which affects a protein that helps regulate salt and water movement in certain tissues. In the simple Mendelian classroom model:

• Let F = typical-function allele, f = disease-related allele.
• Someone with cystic fibrosis has genotype ff.
• Heterozygous Ff individuals are carriers (usually without the full disease phenotype in basic models).

Two carrier parents (Ff × Ff) yield the same Punnett pattern as the flower example: about 1/4 chance of ff offspring. Emphasize: this is a gene-level change (allele mutation), not an extra chromosome.

Down syndrome — chromosomal (trisomy 21)

Down syndrome is typically caused by an extra copy of chromosome 21 (trisomy 21), often from nondisjunction during meiosis (chromosomes fail to separate properly). The result is 47 chromosomes in many cases rather than 46. This is not explained by a single dominant/recessive allele pair in a Punnett square for one gene.

Common wrong answer to watch for: Claiming Down syndrome is “a recessive gene like cystic fibrosis.” Correct the level: gene mutation vs chromosome number error.

Variation beyond one gene

Real traits can involve incomplete dominance, codominance, multiple alleles, polygenic inheritance, and environmental effects (nutrition affecting height, sunlight affecting some plant pigments). Praxis 5442 still leans on clean Mendelian examples for calculation items, but teaching-scenario stems may ask you to recognize when a simple dominant/recessive model is insufficient.

Classroom checkpoint ideas

• Have students complete blank Punnett squares for PP × pp, Pp × pp, and Pp × Pp, then state genotype and phenotype ratios.
• Use colored beads for alleles and build “offspring” to show chance variation around 3:1.
• Case-sort cards: “gene mutation,” “chromosomal nondisjunction,” “environmental influence.”
• Discuss respectful, accurate language for genetic conditions while keeping mechanisms clear for the exam.

Mendelian tools predict how existing alleles combine. The next section explains how variation and selection, over many generations, connect to evolution and the evidence that supports it.