Inheritance, cell division and variation

Inheritance, cell division and variation

Inheritance concerns the transmission of genetic information from parents to offspring. Genes are regions of DNA that contribute to biological characteristics, often by coding for products that influence cell processes. Different versions of a gene are called alleles. Many visible characteristics also depend on environmental conditions and interactions among many genes, so not every trait follows one simple dominant–recessive pattern.

In diploid organisms, chromosomes occur in homologous pairs and a cell usually contains two alleles of a particular gene, one inherited from each parent. Homozygous means having identical alleles at the locus; heterozygous means having different alleles. Genotype describes the genetic combination, while phenotype describes the expressed characteristic. A phenotype may reflect both genotype and environment, rather than genetic information alone.

In a simplified complete-dominance example, let A produce a dominant phenotype and a be recessive. Both AA and Aa show the dominant phenotype, while aa shows the recessive phenotype. Dominant does not mean stronger, better or more common. It describes the expression of an allele in a heterozygote under the stated inheritance model. Other patterns include incomplete dominance and codominance.

A genetic cross begins with parental genotypes and the possible gametes each parent can form. Crossing Aa with Aa gives possible offspring AA, Aa, Aa and aa. Under the simple model, the expected genotype ratio is 1:2:1 and the phenotype ratio is 3:1. These are probabilities over repeated offspring, not a promise that every small family will contain exactly that ratio.

Meiosis produces haploid reproductive cells and contributes to variation through processes including segregation and recombination. Fertilisation combines gametes and restores the diploid chromosome number. Mitosis, in contrast, normally supports growth, repair and some forms of asexual reproduction by producing cells with the same chromosome number as the parent cell, apart from mutations or abnormal events.

Variation provides the differences on which natural selection can act. If heritable features affect survival or reproductive success in an environment, allele frequencies can change over generations. Individuals do not deliberately develop needed inherited features because they want to adapt. In exam answers, separate the source of variation, the environmental selection pressure and the difference in reproductive contribution, and avoid treating a short-term personal change as automatic genetic evolution.

Following chromosome behaviour

A chromosome contains a long DNA molecule associated with proteins. Before cell division, DNA is copied so each replicated chromosome consists of two sister chromatids. During mitosis, the chromatids separate and are distributed to daughter cells. In a normal diploid body cell, this preserves chromosome number and usually produces genetically very similar cells. Mitosis supports growth, replacement and some forms of asexual reproduction. A mutation can introduce a difference, so “genetically identical” is an idealised description of ordinary mitosis rather than a guarantee that no change is ever possible.

Meiosis involves two successive divisions following one round of DNA replication. Homologous chromosomes separate in the first division; sister chromatids separate in the second. The resulting cells have a haploid chromosome set. Crossing over and independent assortment create new combinations of alleles, while random fertilisation combines gametes from two parents. These mechanisms explain much genetic variation in sexually reproducing populations. They do not create every new allele: mutation is the ultimate source of new DNA variants.

Using genetic symbols carefully

At a simple locus with complete dominance, use one capital and one corresponding lowercase letter, such as A and a. AA is homozygous dominant, Aa heterozygous and aa homozygous recessive. The genotype is the allele combination; the phenotype is the expressed characteristic under particular conditions. Dominant does not mean stronger, healthier, more common or better. A recessive allele may be common in a population, and a dominant allele may be rare. Environmental conditions and many genes influence numerous traits, so not every human characteristic can be predicted with a single-locus school model.

For Aa × Aa, each parent can produce A or a gametes. Combining these gives AA, Aa, Aa and aa. The expected genotype ratio is 1:2:1 and the phenotype ratio is 3:1 if A is completely dominant. These are probabilities across many offspring, not a guarantee that every four offspring contain exactly three dominant phenotypes. Each fertilisation is a new event under the assumptions of the model. State those assumptions when interpreting a numerical ratio.

Beyond one straightforward dominant allele

In codominance, both alleles contribute distinctly to a heterozygous phenotype. The human ABO blood-group system provides an example: Iᴬ and Iᴮ are codominant, and both are dominant to i. A parent with genotype Iᴬi can produce Iᴬ or i gametes. A parent with Iᴮi can produce Iᴮ or i, giving possible offspring groups AB, A, B and O in this simplified genetic cross. Blood transfusion decisions involve additional compatibility testing and are not determined safely by a classroom cross alone.

Sex-linked inheritance often concerns genes on the X chromosome. In a simple X-linked recessive model, an XY individual has one copy of the relevant X-linked gene and can express the recessive allele without a corresponding dominant allele on a second X. A heterozygous XX individual may be described as a carrier in that model. Use the correct symbols and trace which parent contributes which chromosome. Do not claim that a father passes his X chromosome to a son in the usual XX/XY model; the son receives his X from his mother and Y from his father.

Dihybrid reasoning and variation

A dihybrid cross follows two loci. For AaBb, possible gametes are AB, Ab, aB and ab if the loci assort independently. Crossing two such heterozygotes gives an expected 9:3:3:1 phenotype ratio when each locus has complete dominance and the stated independence assumptions hold. Linked genes or gene interactions can change the ratio. A useful shortcut is to multiply independent probabilities: the chance of aa from Aa × Aa is 1/4, and bb from Bb × Bb is 1/4, giving 1/16 for aabb if independent. Explain independence rather than applying the product automatically to any two events.

Continuous variation produces a range of values, such as height in a defined population. It commonly reflects many genes and environmental influences. Discontinuous variation has distinct categories, such as ABO groups, although the biology may still be complex. Environmental variation is not automatically inherited: improved muscle size through training does not rewrite the corresponding inherited alleles in a person's gametes. Conversely, inherited differences can affect responses to the same environment. Avoid treating genetic and environmental influences as mutually exclusive explanations.

Self-check and clear exam answers

If a diploid cell has 20 chromosomes, ordinary mitotic daughter cells have 20 and the gametes produced by meiosis have 10. If asked about chromosome count, do not substitute DNA content or count each replicated chromatid as a separate chromosome before separation. For a genetic cross, state parental genotypes, list gametes, combine them systematically, and then give genotype and phenotype probabilities separately. A correct final ratio without a correct cross may conceal a misunderstanding. Finally, distinguish selection from inheritance: a favourable allele can become more common across generations, but organisms do not choose the alleles their offspring receive.