Biology OCR gateway Chapter 5

Asexual reproduction

Asexual reproduction involves one parent and produces genetically identical offspring, known as clones.

Sexual reproduction

Sexual reproduction involves the fusion of male and female gametes, resulting in offspring with genetic variation.

Meiosis and its importance

Meiosis is a type of cell division that produces four non-identical haploid cells (gametes). It is crucial for genetic variation and sexual reproduction.

How meiosis create genetic variation

Meiosis shuffles chromosomes during cell division, leading to genetically unique gametes.

A gene

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

An Allele

An allele is a variant of a gene. For example, one allele might code for blue eyes, and another for brown eyes.

The difference between genotype and phenotype

Genotype refers to the genetic makeup of an organism, while phenotype refers to the observable characteristics.

Homozygous

Homozygous means having the same alleles for a specific gene (e.g., AA).

Heterozygous

Heterozygous means having two different alleles for a specific gene (e.g., Aa).

The purpose of a Punnett square

A Punnett square predicts the probabilities of offspring inheriting certain genotypes and phenotypes.

How sex chromosomes are represented in humans

Females have XX chromosomes, while males have XY chromosomes.

Gregor Mendel

Gregor Mendel was a scientist who studied inheritance patterns in pea plants, establishing the principles of dominant and recessive traits.

The 2 types of traits

Dominant and recessive traits

What is meant by 'dominant' and 'recessive' traits?

A dominant trait is expressed if at least one dominant allele is present, while a recessive trait is only expressed if two recessive alleles are present. What is natural selection? Natural selection is the process by which organisms with advantageous traits survive, reproduce, and pass those traits to their offspring.

How mutations drive evolution

Mutations introduce genetic variation, and beneficial mutations may increase an organism's chances of survival and reproduction.

Speciation

• The process by which a new species forms

• It occurs when groups in a species become reproductively isolated and diverge

evidence for evolution

The fossil record, antibiotic resistance

The fossil record

The collection of all known fossils and their relative ages that contains preserved remains or impressions of organisms, therefore providing evidence for evolution

The difference between artificial and natural classification

Artificial classification groups organisms based on observable traits, while natural classification is based on evolutionary relationships.

Artificial classification

Grouping organisms based on observable characteristics

Natural classification

Grouping organisms based on similarities first and then identifying shared characteristics

A genetic disorder

A disease caused by changes or mutations in an individual’s DNA e.g. cystic fibrosis & sickle cell anaemia

How Punnett squares are used to predict genetic disorders

Punnett squares can show the likelihood of offspring inheriting genetic disorders of being carriers of a recessive condition

A carrier

• A person who has or “carries” one copy of a recessive allele for a genetic disorder but doesn't have any symptoms

• A carrier can pass a genetic disorder to their offspring

Continuous variation

• A variation of a that can be measured in a value between a minimum and a maximum e.g. height & weight

• It is usually influenced by multiple genes and the environment

Discontinuous variation

• Characteristics that fall into distinct groups e.g. eye colour and blood group

• It is usually controlled by a single gene

How environmental factors affect continuous variation

Environmental factors such as diet and exercise can influence traits such as height and weight (which are examples of continuous variation)

Antibiotic resistance

• When bacteria evolve to survive antibiotics, making treatments less effective

• It is evidence for evolution

How antibiotic resistance occurs

1. Random mutations in bacterial DNA can provide resistance to antibiotics

2. These bacteria survive and reproduce, passing the resistance to the offspring

Danger of antibiotic resistance

Antibiotic resistance means that antibiotics are less effective, meaning that infections and other illnesses will be harder to treat