Genes, inheritance and selection Revision Notes

Diploid Cells: Two Sets of Chromosomes

Diploid cells contain a full set of chromosomes, meaning two copies of each chromosome. These are represented as '2n', where 'n' is the number of unique chromosomes in a single set. One set comes from the maternal parent and the other from the paternal parent. Most body cells (somatic cells) in multicellular organisms are diploid. For humans, diploid cells have 46 chromosomes, organised into 23 pairs. This ensures genetic diversity and redundancy in case of gene mutations.

Haploid Cells: One Set of Chromosomes

Haploid cells contain only one complete set of chromosomes, represented as 'n'. These cells are characteristic of gametes (sex cells) like sperm and egg cells. In humans, haploid cells have 23 individual chromosomes. The purpose of haploid cells is to combine during fertilisation to form a new diploid organism, preventing the chromosome number from doubling with each generation. Meiosis is the cell division process that produces haploid cells.

Role in Sexual Reproduction

The distinction between haploid and diploid is fundamental to sexual reproduction. Gametes (sperm and egg), being haploid, each contribute one set of chromosomes. When fertilisation occurs, the haploid sperm nucleus fuses with the haploid egg nucleus, resulting in a diploid zygote. This zygote then undergoes mitosis to develop into a multicellular organism, with all its somatic cells maintaining the diploid chromosome number.

Alleles and Genes

Inheritance is the passing of characteristics from parents to offspring. A gene is a section of DNA that codes for a specific characteristic. Each gene can have different forms called alleles. For example, a gene for eye colour might have an allele for brown eyes and an allele for blue eyes. We inherit two alleles for each gene, one from each parent. These alleles determine our genetic traits.

Dominant and Recessive Alleles

Alleles interact to determine the observable characteristic, or phenotype. A dominant allele always expresses its characteristic, even if only one copy is present (e.g., brown eye allele 'B'). It masks the effect of a recessive allele. A recessive allele only expresses its characteristic if two copies are present in the genotype (e.g., blue eye allele 'b') and no dominant allele is present. Geneticists use uppercase letters for dominant alleles and lowercase letters for recessive alleles.

Genotype and Phenotype

Your genotype is the combination of alleles you have for a particular trait. For instance, 'BB', 'Bb', or 'bb' for eye colour. Your phenotype is the physical expression of your genotype – the observable characteristic, such as 'brown eyes' or 'blue eyes'. An individual is homozygous if they have two identical alleles (e.g., 'BB' or 'bb'). An individual is heterozygous if they have two different alleles (e.g., 'Bb').

Predicting Inheritance with Punnett Squares

Punnett squares are grids used to predict the possible genotypes and phenotypes of offspring resulting from a genetic cross. To construct one, write the alleles from one parent along the top, and the alleles from the other parent down the side. Then, fill in the squares by combining the alleles from each row and column. This allows you to calculate the probability of offspring inheriting specific traits.

What is Genetic Variation?

Genetic variation describes the natural differences in the genetic material (DNA) among individuals within the same species. This leads to observable differences in their characteristics, such as different fur colours in a fox population or varied heights in humans. It's the reason why offspring are not exact copies of their parents, nor are siblings identical (unless they are identical twins). These differences arise from various genetic factors that are passed down through generations.

Importance of Genetic Variation for Survival

Genetic variation is essential for a species to adapt and survive environmental changes. If a population lacks genetic diversity, all individuals might be equally vulnerable to new diseases, predators, or significant climate shifts. However, with extensive genetic variation, some individuals will have traits that make them more resilient or better suited to new conditions. These individuals are more likely to survive, reproduce, and pass on their advantageous genes, allowing the species to continue.

Sources of Genetic Variation

The primary sources of genetic variation are mutations and sexual reproduction. Mutations are random, permanent changes in the DNA sequence. While many mutations are harmful or neutral, some can be beneficial, introducing new traits into a population. Sexual reproduction also contributes significantly by combining genetic material from two different parents, creating unique combinations of traits in the offspring. This constant shuffling and occasional introduction of new genes ensure a diverse gene pool.

Basic Principles of Inheritance

Inheritance describes how characteristics are passed from parents to offspring. Each characteristic is controlled by one or more genes. For single gene crosses, we focus on traits determined by a single gene. Organisms typically inherit two alleles for each gene, one from each biological parent. These alleles can be homozygous (two identical alleles, e.g., AA or aa) or heterozygous (two different alleles, e.g., Aa). Dominant alleles (capital letters) express their trait if present, while recessive alleles (lowercase letters) only express their trait when two copies are present.

Genotype and Phenotype

The genotype refers to the specific combination of alleles an individual possesses for a particular gene. For example, a plant could have the genotype 'TT' (homozygous dominant), 'Tt' (heterozygous), or 'tt' (homozygous recessive) for stem length. The phenotype is the observable characteristic or trait that results from the genotype and environmental interactions. For instance, 'TT' and 'Tt' genotypes might both result in a 'tall' phenotype if 'T' is dominant, while 'tt' would result in a 'short' phenotype.

Using Punnett Squares for Predictions

Punnett squares are a visual tool used to predict the probabilities of offspring genotypes and phenotypes in a genetic cross. To use one, first identify the genotypes of the parents. Then, separate the alleles for each parent to represent the gametes they can produce. Place one parent's gametes along the top of the square and the other parent's gametes along the side. Fill in the grid by combining the alleles from the top and side of each box. Each box represents a possible offspring genotype. Finally, count the resulting genotypes and phenotypes to determine their ratios or probabilities.

Interpreting Results of Monohybrid Crosses

A monohybrid cross involves tracking the inheritance of a single characteristic. After completing a Punnett square, interpret the results to understand the proportion of different genotypes and phenotypes among potential offspring. For example, a cross between two heterozygous parents (Aa x Aa) typically yields a genotypic ratio of 1 AA : 2 Aa : 1 aa. If 'A' is dominant, this translates to a phenotypic ratio of 3 dominant trait : 1 recessive trait. Understanding these ratios is crucial for predicting the outcomes of genetic crosses.