Genes, inheritance and selection Study Guide

Predicting the results of single gene crosses involves understanding how characteristics are passed from parents to offspring. Each characteristic, like eye colour or height, is controlled by a gene, and each gene can have different versions called alleles. For example, a gene for pea plant height might have one allele for tall plants (T) and another for short plants (t). Alleles can be dominant or recessive. A dominant allele always expresses its trait if present, while a recessive allele only expresses its trait if two copies are inherited (one from each parent).

We represent an individual's genetic makeup, or genotype, using letters to show the alleles they have (e.g., TT, Tt, tt). The observable characteristic, or phenotype, is what we see (e.g., tall, short). A Punnett square is a tool used to predict the genotypes and phenotypes of offspring from a genetic cross. It graphically shows all possible combinations of alleles the offspring could inherit from their parents. Each box in the Punnett square represents a possible genotype for the offspring, and by counting these, we can determine the probability of different phenotypes and genotypes occurring in the next generation.

For example, if we cross two tall pea plants that are heterozygous (Tt), meaning they carry one dominant tall allele and one recessive short allele, the Punnett square would show that there's a 25% chance of homozygous dominant (TT - tall), 50% chance of heterozygous (Tt - tall), and 25% chance of homozygous recessive (tt - short) offspring. This gives a phenotypic ratio of 3 tall: 1 short. Understanding these principles allows us to predict the likely outcomes of genetic crosses based on the parental genotypes.

In biology, the terms haploid and diploid refer to the number of sets of chromosomes found within a cell's nucleus. Most cells in a multicellular organism are diploid, meaning they contain two complete sets of chromosomes. One set comes from the organism's mother and the other set comes from the father, resulting in pairs of homologous chromosomes. For instance, in humans, a diploid cell has 46 chromosomes (23 pairs). These diploid cells are formed through mitosis, a type of cell division that produces two identical daughter cells, each with the full complement of chromosomes. Conversely, haploid cells contain only one complete set of chromosomes. These are typically the gametes, or sex cells (sperm and egg), which are involved in sexual reproduction. In humans, a haploid cell has 23 single chromosomes. Haploid cells are formed through meiosis, a specialized type of cell division that reduces the chromosome number by half. When a haploid sperm cell fertilises a haploid egg cell, their nuclei fuse, restoring the diploid number of chromosomes in the resulting zygote, which then develops into a new organism. This alternation between haploid and diploid states is fundamental to sexual reproduction and maintains the species' characteristic chromosome number across generations.