Slides in this set
1. Genes, chromosomes and DNA
· Nucleus: The nucleus is the `brain' of the cell and contains all the genetic information.
· Chromosomes: Chromosomes are long coils of DNA arms. A normal body cell will have 23
pairs of them but a gamete will have 23 individuals.
· DNA: DNA is a double helix and is made up of the four bases: A, T,C and G.
· Genes: Genes control the development of characteristics and are instructions for cells.
· Alleles: Different versions of the same genes.
· Protein: Proteins are the building blocks of life and because we have different versions of
proteins we end up with different characteristics.
· Genotype: An organisms genotype describes the genes it's got.
· Phenotype: The characteristics an organism displays is it's phenotype.
Most cells in the body have a nucleus except a few such as red blood cells. Genes control the
development of characteristics such as hair colour and how an organism functions. There are
two types of proteins: structural and functional. Structural proteins are part of an organisms
structure so they are part of things like skin, hair and blood. An example of a structural protein is
collagen which is found in bones and cartilage. Functional proteins have a role in the organisms
functioning so they are part of things like enzymes. An example of a functional protein is
amylase which is an enzyme that breaks down starch to aid digestion. Some characteristics are
controlled by genes such as dimples but some characteristics are controlled only by
environmental factors such as scars. Some characteristics are controlled by both such as weight.
The two chromatids of a chromosome are held together in the middle which is called the
2. Genes and Variation
· Gametes: Gametes are sex cells which are either sperm cells or ovum (egg) cells.
· Sperm: The male sex cell which has a pointed head to burrow into the egg cell in order to
· Ovum: The female sex cell which contains nutrients to nourish the offspring.
· Sexual reproduction: This is reproduction that involves two parents.
Normal body cells have 23 pairs of chromosomes, so 46 individual chromosomes. Gametes
have 23 individual chromosomes because when they come together during sexual reproduction
there are 46 individual chromosomes (23). 23 + 23 = 46. Which is equivalent to a body cell.
Two chromosomes in a pair always carry the same gene which is found in the same place. Due
to the fact that they come from different parents they will have different alleles of these genes.
Children look like both parents but are not identical to either. Half a child's chromosomes are
from each parent. Every child has a unique combination of chromosomes (except identical
Genes are shuffled randomly into sperm and egg cells. The chromosome pairs separate and go
into different cells. This means that two chromosomes in a pair are never identical due to
different alleles. All sex cells produced by an individual are genetically different.
Ovum (egg) cells can be fertilised by any one of the 50 million sperm that enter the vagina.
Because of this there is a miniscule chance of siblings being identical.…read more
3. Inheritance and Genetic Diagrams
· Homozygous: This means you have two of the same alleles for a certain gene.
· Heterozygous: This means you have to different alleles for that particular gene.
· Dominant: This is the allele that is shown by the organism as a phenotype. It is usually
written with a capital letter.
· Recessive: This is the allele that can only be shown if there are two recessive alleles. It is
usually written with a lower case letter.
· Genetic Diagrams: These show the possible genotypes and phenotypes of the offspring of
On the next slide there will be a genetic diagram which will be fully identified. You need to be
able to do this (define it in the same way). The interpretation tells you: the parents phenotypes,
the parents genotypes, sex cell's genotypes, possible genotypes of offspring and possible
phenotypes of offspring.…read more
The parents are carriers of the `crazy' gene which is recessive. This means that both the alleles for
that gene need to be recessive (which also means they are homozygous).
There are many different types of genetic disorders that are caused by recessive alleles. The most
commonly known one is cystic fibrosis but here is an example of another: phenylketonuria (PKU)
which was mentioned ages ago in a six mark question. Remember? In PKU certain enzymes don't
work as they should and so there is accumulation of the substrate (which it is meant to break
down) in the brain. This causes severe mental retardation and a lowered IQ by the age of 1. It is
found in 1 out of 20,000 new-borns.…read more
4. Genetic diagrams and sex chromosomes
· Punnett squares: This is another type of genetic diagram.
· Sex organs: These are different for males and females. Males have testes and females have
ovaries. Sex organs are part of the reproductive system.
If we use the hamster example again only this time with a Punnett square we can see that it is just
another way of working out the genetic cross:
This tells us the same information as the genetic
diagram but it is much clearer.
1. In percentage what chance is there for the offspring
to be a carrier of the crazy gene?
2. In percentage what is the chance for the offspring to
be a sufferer of the crazy gene?
3. In percentage what is the chance of the offspring
being completely normal?
4. What is the phenotype of the parents?
5. What is the genotype of the parents?
Reginald Crundell Punnett devised the 6. Is the genotype of the parents dominant or
Punnett square. He was also one of the recessive?
first English geneticists. 7. Is the allele for this genetic disease dominant?…read more