Biology unit 2 - area of study 1 outcome 1

What is a gene?

A segment of DNA that codes for a specific protein or trait.

What is an allele?

An alternative version of a gene found at the same locus on homologous chromosomes.

What is a genome?

The complete set of genetic material (DNA) in an organism, including all genes and non-coding regions.

How are genes, alleles, and the genome related?

The genome contains all the DNA of an organism; within it are genes, and each gene may exist in multiple forms called alleles.

What is the difference between dominant and recessive alleles?

A dominant allele is expressed even if only one copy is present (Bb), while a recessive allele is expressed only when two copies are present (bb).

How is genetic information passed from one generation to the next?

Through meiosis and fertilisation, where gametes carry half the genetic information from each parent.

What are homologous chromosomes?

A pair of chromosomes (one from each parent) that have the same size, shape, and genes at the same loci, but may contain different alleles.

Why are homologous chromosomes important?

They allow for genetic variation through crossing over and independent assortment during meiosis.

What are autosomes?

Chromosomes that carry genes for body traits (non-sex chromosomes). Humans have 22 pairs.

What are sex chromosomes?

Chromosomes that determine biological sex — XX for females and XY for males.

How do chromosomes vary among organisms?

Different species have different numbers and sizes of chromosomes.
Example: Humans have 46, fruit flies have 8, dogs have 78.

What is chromosomal variation and why is it significant?

Differences in chromosome number or structure create genetic diversity and can lead to evolution or genetic disorders (e.g., trisomy 21 causes Down syndrome).

What is a karyotype used for?

To visualise and analyse the number, size, and shape of chromosomes in a cell, helping identify abnormalities.

What is meiosis?

A type of cell division that reduces the chromosome number by half, producing four genetically unique haploid gametes from one diploid cell.

Why is meiosis important?

It ensures that offspring have the correct number of chromosomes and introduces genetic variation, which is essential for evolution and adaptability.

What happens to the chromosome number during meiosis?

The chromosome number is reduced from diploid (2n) to haploid (n), so gametes carry only one set of chromosomes.

What is crossing over?

An exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis.

Why is crossing over important?

It produces new combinations of alleles on chromosomes, increasing genetic diversity in offspring.

What is independent assortment?

The random distribution of maternal and paternal homologous chromosomes into gametes during metaphase I of meiosis.

How does independent assortment contribute to genetic diversity?

Each gamete receives a random mix of chromosomes, creating countless possible genetic combinations in offspring.

How does meiosis increase genetic variation in populations?

Through crossing over and independent assortment, meiosis shuffles genetic material, creating unique gametes and promoting adaptability within a species.

How are genotype symbols written?

Dominant alleles are written with uppercase letters (e.g., A), and recessive alleles with lowercase letters (e.g., a).

What is the difference between homozygous and heterozygous genotypes?

Homozygous means having two identical alleles (AA or aa), while heterozygous means having two different alleles (Aa).

What is meant by a gene locus?

The gene locus is the fixed position of a gene on a chromosome.

How can genotype symbols be used in genetic problems?

They are used to predict inheritance patterns and offspring genotypes using Punnett squares.

What is a dominant phenotype?

A trait that is expressed when at least one dominant allele is present (e.g., Aa or AA).

What is a recessive phenotype?

A trait that appears only when two recessive alleles are present (aa).

What is codominance?

When both alleles are fully expressed in a heterozygous individual.
Example: AB blood type, where both A and B antigens are expressed equally.

What is incomplete dominance?

When neither allele is completely dominant, resulting in an intermediate phenotype.
Example: Red (RR) × White (rr) → Pink (Rr).

How do genetic and environmental factors influence phenotype?

Phenotype results from the interaction between an organism’s genotype and environmental conditions like temperature, nutrition, or light exposure.

What are epigenetic factors?

Chemical modifications (like DNA methylation) that affect gene expression without changing the DNA sequence.

How can epigenetic changes influence phenotype?

They can switch genes “on” or “off,” affecting how traits are expressed and sometimes passing these changes to offspring.

Why is it important to understand gene–environment interactions?

Because both genetics and environment work together to shape an organism’s observable traits, influencing health, adaptation, and survival.

What is a pedigree chart?

A diagram showing how a trait is inherited across generations within a family.

What is autosomal inheritance?

Inheritance of traits controlled by genes located on non-sex (autosomal) chromosomes.

What is sex-linked inheritance?

Inheritance of traits controlled by genes located on sex chromosomes, usually the X chromosome.

How can you identify a carrier in a pedigree?

Carriers (heterozygous individuals) are often represented with a half-shaded symbol, meaning they carry one recessive allele but do not express the trait.

How can pedigrees be used to predict inheritance?

By analysing family patterns to determine the probability that offspring will inherit specific traits.

What is a monohybrid cross?

A genetic cross between two individuals focusing on one trait or gene.

How are Punnett squares used in a monohybrid cross?

They predict genotype and phenotype ratios of offspring from two parents with known genotypes.

What is a test cross?

A cross between an organism with an unknown genotype and one that is homozygous recessive, used to determine the unknown genotype.

What is a dihybrid cross?

A genetic cross involving two traits or genes at once.

What is Mendel’s law of independent assortment?

Genes for different traits are inherited independently if they are located on different chromosomes.

What are linked genes?

Genes located close together on the same chromosome that tend to be inherited together.

How does gene linkage affect inheritance?

It reduces the variety of offspring genotypes because linked genes do not assort independently.

How can recombination occur between linked genes?

Through crossing over during meiosis, where sections of homologous chromosomes exchange DNA segments.

How do you recognise gene linkage from test cross results?

If offspring ratios deviate significantly from the expected Mendelian ratio (9:3:3:1), linkage is likely.

Why is analysing genetic crosses important?

It helps predict inheritance patterns, identify carriers, and understand how genetic variation arises through independent assortment or linkage.