Genetics revision notes

Chromosomes and Sex

Human body cells contain 23 pairs of chromosomes, totalling 46. One pair consists of the sex chromosomes, which determine an individual's biological sex. Females have two X chromosomes (XX genotype), while males have one X and one Y chromosome (XY genotype). The other 22 pairs are called autosomes and carry genes for other characteristics.

Gamete Formation and Fertilisation

During gamete formation (meiosis), the pairs of chromosomes separate. A female produces egg cells, and each egg contains a single X chromosome. A male produces sperm cells; approximately half of these sperm will carry an X chromosome, and the other half will carry a Y chromosome. Sex is determined at fertilisation. If an X sperm fertilises the egg (X), the zygote will be XX (female). If a Y sperm fertilises the egg (X), the zygote will be XY (male). This gives a 50% probability for either sex.

Genetic Diagrams for Sex Determination

Genetic diagrams, such as Punnett squares, can illustrate sex determination. Parental genotypes are XX (female) and XY (male). Gametes for female are X and X. Gametes for male are X and Y. The potential offspring genotypes are XX and XY, each with a 50% probability, demonstrating equal chances for male or female offspring.

Benefits of Sexual Reproduction

Sexual reproduction creates genetic variation within a population. This variation arises from the combination of genetic material from two different parents, and the independent assortment and crossing over during meiosis. Increased variation enhances the adaptability of a species to environmental changes, such as new diseases or shifts in climate, by ensuring that at least some individuals have advantageous traits for survival.

Limitations of Sexual Reproduction

A significant limitation is the need to find a mate. This process can be energy-intensive, time-consuming, and risky due to predator exposure. Competition for mates can also be intense, and successful reproduction is not always guaranteed. In addition, sexual reproduction is generally a slower process compared to asexual reproduction, often resulting in fewer offspring per reproductive event.

Basic Genetic Units

Chromosomes are thread-like structures containing DNA, located in the nucleus. Genes are specific segments of DNA on a chromosome that code for particular traits, like eye colour. Alleles are alternative forms of a gene, for instance, the gene for eye colour might have an allele for blue eyes and an allele for brown eyes. Dominant alleles are expressed even if only one copy is present, while recessive alleles are only expressed when two copies are present.

Genotype vs. Phenotype and Zygote Formation

Genotype refers to an organism's genetic makeup (e.g., TT, Tt, tt). Phenotype is the observable physical characteristic resulting from the genotype (e.g., tall, short). An individual is homozygous if they have two identical alleles (e.g., TT or tt) or heterozygous if they have two different alleles (e.g., Tt). Gametes are haploid reproductive cells (sperm or egg). The fusion of two gametes during fertilisation results in a diploid cell called a zygote, which develops into a new organism.

Monohybrid Crosses and Punnett Squares

A monohybrid cross focuses on one genetic trait determined by two alleles. Punnett squares are diagrams used to predict the genotypes and phenotypes of offspring resulting from a genetic cross. Each box in the square represents a possible combination of alleles from the parents. Dominant alleles are typically represented by an uppercase letter (e.g., 'A'), and recessive alleles by a lowercase letter (e.g., 'a'). Genotypes can be homozygous dominant (AA), heterozygous (Aa), or homozygous recessive (aa). Phenotypes are the observable characteristics, which are determined by the genotype. Outcomes are typically expressed as ratios (e.g., 3:1 phenotypic ratio) or percentages.

Dominant and Recessive Traits

A dominant trait is expressed (phenotypically observed) when an individual has at least one copy of the dominant allele. For example, if 'A' is dominant for blue flowers and 'a' is recessive for white flowers, both AA and Aa genotypes would result in blue flowers. A recessive trait is only expressed when an individual inherits two copies of the recessive allele (e.g., 'aa' for white flowers). Understanding dominance and recessiveness is crucial for interpreting genetic crosses and pedigree charts. If a recessive trait appears in offspring from parents who do not show the trait, it indicates that both parents must be heterozygous carriers.

Pedigree Analysis

Pedigree charts are visual tools that map the inheritance of a specific trait through several generations of a family. Standard symbols are used: squares for males, circles for females, and shaded shapes for individuals expressing the trait. A horizontal line between a male and female indicates a mating pair, and vertical lines connect parents to offspring. Pedigrees help determine if a trait is dominant or recessive, and to deduce the genotypes of family members. For instance, if an affected child has unaffected parents, the trait must be recessive, and the parents must be carriers. If every affected individual has at least one affected parent, it suggests a dominant trait.