12.2 Genetics: Genes and Alleles

Genetics explains how traits are inherited. For USTET Science, you do not need advanced molecular pathways; you need clean definitions, dominance logic, and the ability to complete a one-gene Punnett square quickly and accurately.

DNA, Genes, and Chromosomes

DNA (deoxyribonucleic acid) stores hereditary information as a sequence of nucleotide bases: adenine (A), thymine (T), cytosine (C), and guanine (G). A pairs with T; C pairs with G.

A gene is a specific segment of DNA that influences a trait—often by coding for a polypeptide. Genes occupy fixed positions (loci) on chromosomes. Humans are diploid: chromosomes come in homologous pairs, so you typically inherit two copies of each autosomal gene—one from each parent.

Key distinction: "Gene" names the trait slot (for example, flower color). "Allele" names which version occupies that slot (purple vs white).

Genotype and Phenotype

• Genotype — the organism's allele combination (AA, Aa, or aa).
• Phenotype — the observable characteristic (purple flowers, attached earlobes, blood type display, and so on).

Identical phenotypes can hide different genotypes when dominance is involved. For a completely dominant trait, both AA and Aa may show the same dominant phenotype, while only aa shows the recessive phenotype.

Homozygous and Heterozygous

Only the homozygous recessive genotype expresses a recessive phenotype under complete dominance. That is why recessive traits can "skip" generations: heterozygote carriers look dominant but can pass the recessive allele to offspring.

Dominance Patterns You Should Know

Complete Dominance (Classical Mendelian)

In complete dominance, the dominant allele fully masks the recessive allele in heterozygotes. Mendel's pea experiments popularized this pattern: a single dominant allele for purple flowers was enough for purple phenotype.

Incomplete Dominance

In incomplete dominance, heterozygotes show a blended intermediate phenotype. Classic example: red flower (RR) × white flower (rr) → pink heterozygotes (Rr). Here, phenotype ratios among heterozygote self-crosses often appear as 1:2:1 for both genotype and phenotype.

Codominance

In codominance, both alleles are fully expressed together. Human ABO blood groups involve codominance of IA and IB (with i recessive). A person with genotype IAIB has AB blood type—both A and B antigens present.

USTET items most often assume complete dominance unless the question clearly describes blending or dual expression.

Mendel's Core Ideas (Working Version)

Gregor Mendel proposed principles that still organize introductory genetics:

1. Segregation — the two alleles of a gene separate during gamete formation so each gamete carries one allele.
2. Independent assortment — alleles of different genes segregate independently if genes are on different chromosomes (or far apart).
3. Traits are determined by discrete hereditary factors (genes), not by blending of parental fluids.

Meiosis provides the physical basis for segregation and independent assortment: homologous chromosomes separate in meiosis I, and different pairs orient independently.

Punnett Squares: The Exam Skill

A Punnett square is a grid that combines possible parental gametes to predict offspring genotypes.

Monohybrid Cross Steps

1. Assign allele symbols (dominant capital, recessive lowercase).
2. Determine parental genotypes.
3. List gametes each parent can produce.
4. Fill the square with all gamete combinations.
5. Translate genotypes into phenotypes and compute ratios.

Worked Example: Heterozygote × Heterozygote

Cross: Aa × Aa (complete dominance; A = dominant trait).

Genotype ratio: 1 AA : 2 Aa : 1 aa (1:2:1). Phenotype ratio: 3 dominant : 1 recessive (3:1).

This 3:1 phenotypic ratio is one of the most frequently tested results in entrance-exam genetics.

Worked Example: Test Cross

A test cross mates an individual showing the dominant phenotype (genotype unknown: AA or Aa) with a homozygous recessive (aa).

• If offspring are all dominant phenotype → unknown parent was likely AA.
• If offspring are about 1:1 dominant:recessive → unknown parent was Aa.

Test crosses are designed to reveal hidden heterozygosity.

Another Common Cross: AA × aa

All offspring are Aa. Phenotype: 100% dominant. Genotype: 100% heterozygous. Questions sometimes ask for genotype rather than phenotype—read carefully.

Pedigree and Carrier Logic (Brief)

Even without drawing full pedigrees, remember:

• Two unaffected parents can have an affected child if both are heterozygous carriers of a recessive allele.
• An affected child with recessive disorder has genotype aa; each parent must have contributed an a allele.
• Dominant traits do not skip as easily: an affected child usually has at least one affected parent (barring new mutation).

Sex-Linked Traits (High-Yield Basics)

Genes on the X chromosome show distinctive inheritance. In XY systems:

• Males (XY) express whatever allele is on their single X (hemizygous).
• Females (XX) need two recessive alleles to express an X-linked recessive trait.

That is why X-linked recessive conditions (such as classical red-green color blindness patterns in textbooks) appear more often in males. Fathers pass their X to all daughters and their Y to all sons; they do not pass their X to sons.

Connecting Genetics Back to Cells

Alleles are DNA sequences in chromosomes inside the nucleus. Mitosis preserves allele combinations in body cells; meiosis reshuffles alleles into gametes. Punnett squares are simply a probability model of which gametes meet at fertilization.

Strategy for USTET Genetics Items

1. Identify whether the question asks for genotype, phenotype, or probability.
2. Confirm the dominance rule (complete, incomplete, or codominance).
3. Write parental genotypes before guessing.
4. Build a quick Punnett square for any non-obvious cross.
5. Convert counts to ratios or percentages only after the square is filled.

If you keep definitions sharp—gene vs allele, genotype vs phenotype, homozygous vs heterozygous—and practice the Aa × Aa and test-cross patterns, you can answer most USTET genetics questions in under a minute.