Genetics Basics: Heredity, Traits and DNA

• DNA: deoxyribonucleic acid, the molecule that carries genetic information.
• Gene: a stretch of DNA whose information contributes to a functional product, such as a protein or ribonucleic acid (RNA).
• Chromosome: a structure containing a long DNA molecule packaged with proteins in a eukaryotic cell.
• Allele: a version of a DNA sequence at a particular genetic location.

Genes contribute to how cells work and how traits develop. Some genes provide instructions for proteins. Others produce functional RNA rather than a protein. One gene does not necessarily control exactly one trait.

Most DNA in a human cell is in nuclear chromosomes. Mitochondria also contain DNA. Saying “all DNA is inside the nucleus” therefore misses an important exception.

Does a gene have only two possible alleles?

No. A population can contain several versions of the same gene. For a typical autosomal gene, a person usually has two copies. One comes from each biological parent. Autosomal means located on a chromosome other than a sex chromosome.

The number of copies in one person is not the number of possible alleles in the population. Simple pea crosses use two allele symbols because the example follows two chosen variants.

Genotype describes an organism's genetic makeup. In a single-gene example, AA, Aa and aa identify different allele combinations. Phenotype means an observable characteristic, such as flower colour or blood group.

Phenotype is not limited to visible appearance. Blood group is a phenotype even though you cannot identify it by looking. Genetic makeup and environmental conditions can both influence a phenotype.

Can different genotypes give the same phenotype?

Yes. Suppose A gives purple flowers and is completely dominant over a, which gives white flowers. AA and Aa plants both have purple flowers. Their genotypes differ, even though their flower-colour phenotype matches. An aa plant has white flowers in this model.

In complete dominance, one allele determines the studied phenotype when paired with the other allele. The expressed allele is called dominant. The other allele is called recessive for that trait.

Homozygous means the two alleles match. Heterozygous means they differ. Dominant does not mean stronger, more useful or more common. A recessive allele remains present in Aa and can pass to offspring.

Do all traits follow complete dominance?

No. In incomplete dominance, a heterozygote has an intermediate phenotype. In codominance, both allele-associated features are expressed. Many traits also involve several genes. Do not force every human characteristic into an AA/Aa/aa flower-colour model.

Gregor Mendel studied inheritance through crosses between pea plants with contrasting characteristics. His experiments helped distinguish inherited factors from a permanent blending of parental traits.

The law of segregation states that paired alleles separate during gamete formation. Gametes are reproductive cells, such as eggs and sperm. In the simple diploid model, each gamete receives one allele from the pair.

Diploid means having two chromosome sets. Meiosis is a division process that halves the chromosome-set number. Reproductive cells with one set can combine at fertilisation without doubling the number each generation.

An Aa parent can therefore make A gametes or a gametes. Each type has probability 1/2 in this model. Fertilisation combines one gamete from each parent. The offspring receives a pair again.

Segregation explains how a recessive phenotype can reappear after being hidden in heterozygous parents. The recessive allele was not destroyed or converted into the dominant allele.

A Punnett square predicts possible offspring allele combinations from specified parental genotypes. Put one parent's possible gametes across the top. Put the other parent's gametes down the side. Combine their alleles in each box.

Worked example: Aa × Aa

Assume complete dominance, equal gamete probabilities and equal survival of the resulting genotypes.

The four equally likely outcomes are AA, Aa, Aa and aa. The expected genotype ratio is 1:2:1. With complete dominance, AA and Aa share the dominant phenotype. The expected phenotype ratio is 3:1.

For one offspring, P(aa) = 1/2 × 1/2 = 1/4, or 25%. The dominant phenotype has probability 3/4, or 75%.

Does this guarantee three dominant offspring in every group of four?

No. These ratios describe probabilities and expected proportions across many offspring. A small group can differ by chance. Previous offspring do not change the next independent offspring's probability in this model.

For AA × Aa, the AA parent contributes only A gametes. The Aa parent contributes A or a with equal probability.

The expected genotype ratio is 1 AA : 1 Aa. All offspring have the dominant phenotype under complete dominance. There is no aa outcome because the AA parent cannot contribute a.

For Aa × aa, the expected outcomes are half Aa and half aa. This gives a 1:1 phenotype ratio under the same assumptions. A cross with a homozygous recessive organism is called a test cross when testing a dominant-phenotype organism's genotype.

For AA × Aa, the probability of an aa offspring is 0 in this model.

Independent assortment means inheritance at one gene does not determine inheritance at another in the stated model. Genes on different chromosomes generally assort independently because chromosome pairs orient independently during meiosis.

Genes close together on the same chromosome are linked. They tend to travel together rather than assort independently. Recombination can separate linked alleles. Genes far apart on one chromosome can behave approximately as independently assorting genes.

For two independently assorting genes, AaBb can produce AB, Ab, aB and ab gametes. Each has probability 1/4 in the simple model. Check the independence assumption before applying a two-trait probability calculation.

Not in the usual medical sense. A carrier commonly has a recessive-condition allele without showing the associated condition. The carrier can still pass that allele to offspring.

In a simplified autosomal recessive model, Aa is an unaffected carrier and aa is affected. A is the functioning allele in this example. Calling every Aa organism a disease carrier confuses a general genotype with a condition-specific label.

Two Aa carriers have a 1/4 probability of an affected aa offspring in each independent conception. That probability does not mean every fourth child must be affected. Real genetic conditions may need more detailed models.

Most human body cells with nuclei usually contain 46 chromosomes, arranged in 23 pairs. A typical egg or sperm contains 23 chromosomes, not 23 pairs. Fertilisation restores the usual paired number.

The 23 pairs ordinarily include 22 autosomal pairs and one sex-chromosome pair. These numbers describe the usual human chromosome arrangement, not every person or every cell. Mature red blood cells lack a nucleus. Chromosomal variations also occur.

The chromosomes carry many genes, not one chromosome for each trait. Nuclear chromosomes explain much inheritance, but mitochondrial DNA is a separate part of the genetic material.

Inherited DNA can influence hair texture, eye colour, pigmentation and blood group. Playing chess, speaking a particular language and riding a bicycle require learning. A parent's mastered skill is not passed directly through the parent's DNA.

However, “genetic” and “environmental” are not always opposite boxes. Several genes can influence one characteristic. Nutrition, exposure and experience can also influence how characteristics develop.

Why are attached earlobes a poor single-gene example?

Earlobe attachment has genetic influences, but it is not a simple dominant-versus-recessive classroom trait. Multiple genetic regions contribute, and intermediate forms occur. Curly hair also has complex genetic influences. Freckling has genetic influences alongside environmental exposure.

Inherited influence does not guarantee that a child will show an identical parental characteristic. Use a specified pea-flower model for simple Mendelian calculations. Do not infer a human genotype from appearance alone.

Natural selection occurs when heritable differences affect reproductive success in a particular environment. Charles Darwin helped explain evolution through this process. Alfred Russel Wallace independently developed the same central idea.

Imagine beetles with inherited differences in colour. On a dark background, birds may catch more conspicuous beetles. Better-camouflaged beetles may leave more offspring. Their colour-associated variants can become more common over generations.

The population changes across generations. Individual beetles do not develop useful inherited colours simply because they need camouflage. Survival matters because it can affect reproduction; survival alone is not the complete definition of selection.

Homologous structures share an evolutionary origin, even when their present functions differ. A bird's forelimb and a human arm share the basic vertebrate forelimb plan.

Analogous structures perform similar functions but evolved independently in the comparison being made. Bird and insect wings are analogous as flight structures. Similar function alone does not establish close common ancestry.

Fossils preserve remains, impressions or traces of past organisms. They document past life and help reconstruct change through time. The fossil record is incomplete because preservation is uncommon.

Speciation is the formation of new species. Geographic separation can reduce gene flow between populations. Different changes may accumulate until reproductive barriers arise.

For sexually reproducing organisms, reproductive isolation limits successful interbreeding between populations. Separation does not automatically create a new species after a fixed time. Speciation can also occur without geographic separation. The details depend on the organisms and the species definition being used.

The Combined Defence Services Examination I, 2026, General Knowledge Question 6 compares physical characteristics with chess skill. The champion has curly hair, attached earlobes and freckles. The learned chess skill is the intended non-inherited skill among those choices.

The physical characteristics have genetic influences, but their transmission is not guaranteed. They should not be taught as three simple one-gene traits. The exam distinction concerns a learned achievement versus inherited biological influences.