Mendel and Genetic Crosses (Leaving Cert Biology): Revision Notes

Mendel and Genetic Crosses

Understanding genetic terminology

Before diving into genetic crosses, it's essential to understand the specialised language that geneticists use. This terminology forms the foundation for understanding how traits are passed from parents to offspring.

Genes and chromosomes

A gene is a section of DNA that controls the production of a protein. Every gene has a specific location on a chromosome called its locus (plural: loci). Think of the locus as the gene's address on the chromosome.

Alleles - different versions of genes

Alleles are different forms of the same gene. For example, a gene controlling coat colour might have one allele for black coat and another for white coat. Most organisms have two alleles for each trait because they inherit one from each parent.

There are two main types of alleles:

• Dominant alleles prevent recessive alleles from being expressed and are represented by capital letters (e.g., B)
• Recessive alleles are prevented from being expressed when a dominant allele is present and are shown with lowercase letters (e.g., b)

Genotype vs phenotype

Understanding the difference between genotype and phenotype is crucial:

• Genotype: The genetic make-up of an organism (the actual alleles present)
• Phenotype: The physical expression or appearance that results from the genotype

The relationship between these can be summarised as:

$\text{Genotype} + \text{Environment} = \text{Phenotype}$

This means that whilst genes provide the blueprint, environmental factors can also influence how traits are expressed.

Homozygous and heterozygous

Organisms can be classified based on their allele combinations:

• Homozygous: Having two identical alleles (e.g., BB or bb)
• Heterozygous: Having two different alleles (e.g., Bb)

Gregor Mendel - the father of genetics

Gregor Mendel is known as the 'father of genetics'. He was born in 1822 in Austria and became a monk at the age of 21. Mendel's success came from his careful, methodical approach to studying inheritance.

Mendel's experimental approach

Mendel carried out numerous experiments on garden pea plants, investigating the inheritance of seven characteristics including stem height, flower colour, and seed shape. His experimental design was revolutionary for several reasons:

• He studied features that displayed only two forms (e.g., stems were either tall or short, seeds were either green or yellow)
• He counted the number of plants with each type of trait, allowing him to detect mathematical ratios
• He collected seeds and grew them in carefully labelled containers
• He studied the appearances (phenotypes) of the resulting plants

Mendel's work involved removing the pollen-producing structures (anthers) from some flowers and transferring pollen to these flowers using a small brush. The treated flowers were then covered with bags to prevent contamination from other pollen sources.

Monohybrid crosses

A monohybrid cross involves studying the inheritance of a single characteristic. This is the simplest type of genetic cross and forms the basis for understanding more complex inheritance patterns.

Working out genetic crosses

When working out genetic crosses, follow these essential rules:

Sample genetic cross - coat colour in rats

Let's examine a practical example. In rats, black coat (B) is dominant over white coat (b). When we cross two rats with genotypes BB (black) and bb (white):

This demonstrates that when a homozygous dominant individual is crossed with a homozygous recessive individual, all offspring are heterozygous and display the dominant trait.

Punnett squares - a useful tool

A Punnett square is a table used to show all the possible genotypes of the progeny of a genetic cross. It's named after English geneticist Reginald Punnett, who also discovered important concepts about genetic linkage and sex determination.

More complex crosses - heterozygous parents

When both parents are heterozygous, we see more interesting ratios. Consider sheep where white wool (W) is dominant over black wool (w):

Pedigree studies

A pedigree is a diagram showing the genetic history of a group of related individuals. Pedigrees are essential tools for tracking the inheritance of traits through families.

Reading pedigrees

In pedigree charts:

• Males are represented by squares
• Females are represented by circles
• Individuals displaying the trait are shown in one colour
• Those not displaying the trait are shown in a different colour
• Lines connect parents to their offspring

Analysing inheritance patterns

Pedigrees help us determine whether a trait is controlled by dominant or recessive alleles. For example, if two parents without a trait have children who display the trait, this suggests the trait is recessive.

Expected ratios in genetic crosses

Understanding the mathematical patterns that emerge from different types of crosses is crucial for predicting inheritance outcomes.

Common genetic ratios

• 1:0 (100%): When crossing homozygous dominant with any genotype
• 1:1 (50%:50%): When crossing heterozygous with homozygous recessive
• 3:1 (75%:25%): When crossing two heterozygous individuals

Key terms for genetic crosses

• Progeny: The offspring that are produced (F₁ is short for the first filial generation)
• F₁ generation: The offspring of the parents in any cross
• Homozygous: The two alleles are identical
• Heterozygous: The alleles are different