Scientific ideas, concepts and processes
Inheritance is the transmission of genetic information from parent to offspring.
Cells
All living things are made of cells.
A cell is the smallest unit of structure and function, and all existing cells come from previously existing cells.
Cells have organelles, each with a specific function. (including the cell nucleus, chromosomes, cytoplasm, cell membrane, ribosomes, endoplasmic reticulum (ER), and mitochondria)
Chromosomes and Genes
The genome is the complete set of inherited information of a living organism.
A chromosome is a strand of DNA, made up of a chain of genes.
Humans have 46 chromosomes in most somatic cells. Different genes are switched on or off within different cells depending on the location and function of the cell.
Human chromosomes exist in homologous pairs. Each pair contains one chromosome from each parent.
A gene is a length of DNA. It is the unit of heredity. A gene can be copied and passed on to the next generation.
An allele is one of two or more alternative forms of a gene.
Sex chromosomes determine the sex of a person and inherited sexual characteristics. A person with two X chromosomes is female and with one X and one Y is male.
In sexual reproduction, a zygote is the result of fertilization when the male and female gametes (sperm and egg cell) fuse.
DNA and Proteins
A gene is a part of a chromosome with instructions coded as four different nucleotide bases (A, T, G and C).
These bases form pairs (A with T, G with C)
Replication (copying) of DNA occurs during mitosis.
Cells make proteins according to the recipe coded in the DNA. The type of protein that is made determines the particular phenotypic traits.
When cells make proteins, the DNA first forms mRNA from the chromosomes in the cell nucleus.
Proteins are made from mRNA in the ribosomes of cells.
Monohybrid inheritance
The genotype is the genetic makeup of an organism in terms of the alleles present (e.g. Bb or cc).
The phenotype is the physical and behavioural traits of an organism due to the way the genotype is expressed within a particular environment. Phenotype depends on genotype and environment.
Homozygous is having two identical alleles of a particular gene (e.g. BB or bb). Two identical homozygous individuals that breed together will be pure-breeding.
Heterozygous is having two different alleles of a particular gene (e.g. Bb or Cc).
A dominant allele is always expressed (represented by a capital letter such as B or C).
A recessive allele is only expressed when there is no dominant allele of the gene present (represented by a lower case letter such as b or c)
A monohybrid cross of dominant and recessive alleles gives phenotypes in the ratios 1:1 and 3:1.
Genetic disease
A mutation is a change in a gene, cause by mutagens such as radiation or certain chemicals. Mutations can be beneficial, or have no effect, but are often problematic.
Sickle cell anemia is caused by a recessive allele. A person with two sickle cell alleles is affected by the disease. A carrier is a person with a single copy of the sickle cell allele and is not affected by the disease, but can pass it on to their offspring.
Genetic testing
A genetic test examines chromosomes, the DNA molecule itself or the proteins it produces to diagnose genetic disorders, or test paternity or ancestry.
Genetic testing could show predisposition to a number of diseases such as cancer and heart disease.
Genetic modification
Genetic engineering transfers the genes of one living organism to a different organism, to produce a protein which gives the organism a particular trait.
Gene therapy on humans is still at an experimental stage and involves using a vector (such as a virus) to transfer a new gene to replace a damaged gene in human cells.
Cloning produces a genetically identical individual by transferring the chromosomes of one organism into the nucleus of an egg cell.
Extra
Mitosis and Meiosis
Mitosis is cell division that produces genetically identical cells (clones). Exact duplication of chromosomes keeps the number of chromosomes constant. Mitosis is involved in growth, repair of damaged tissues, replacement of worn out cells and asexual reproduction.
The phases of mitosis include prophase, metaphase, anaphase and telophase.
A diploid nucleus is a nucleus containing two sets of chromosomes, found in somatic cells.
Meiosis produces gametes (sperm and egg cells). The chromosome number is halved from diploid to haploid. A haploid nucleus contains a single set of unpaired chromosomes.
Crossing over and independent assortment occur during meiosis. They result in genetic variation, so the cells produced are not all genetically identical.
Genetic disease
Huntington’s chorea is a genetic disease caused by a dominant gene which affects people in middle age. People with a single copy of the gene will develop the disease, and genetic tests will show if the person will develop the disease later in life.
Mutations in either of the BRCA genes on chromosomes 13 and 17 increase the risk of breast cancer and ovarian cancer, although a person with the affected allele is not certain to develop the disease.
Genetic technology
Recombinant DNA technology uses an enzyme to cut DNA from one chromosome and a vector to transfer it to another organism. This technology has been used to engineer bacteria to make insulin, human growth hormone and factor VIII.
Genetic testing can be carried out on parents to see if they are carriers of a genetic disorder, and on IVF embryos before they are implanted, on the fetus in the womb, or on a newborn baby to check for inherited diseases.
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