genetics and inheritance

• some characteristics are controlled by...

  a single gene
• what do genes control?

  the development of different characteristics
• most characteristics are controlled by...

  several genes interacting
• alleles


  different versions of a gene (which are represented by letters in genetic diagrams)
• how many alleles do humans have?

  humans have two copies (alleles) of every gene in the body - one on each chromosome in a pair
• homozygous


  an organism that has two identical alleles for a trait
• heterozygous


  an organism that has two different alleles for a trait
• how is a characteristic determined if two alleles are different?

  only one allele will determine the present characteristic, and this is the dominant allele
• dominant allele


  an allele whose trait always shows up in the organism when the allele is present
• recessive allele


  an allele that is masked when a dominant allele is present
• how can an organism display a recessive characteristic?

  both alleles must be recessive
• genotype


  an organism's genetic makeup, or allele combinations
• phenotype


  an organism's physical appearance, or visible traits
• what can show how characterises are inherited?

  family trees
• how is cystic fibrosis inherited?

  autosomal recessive trait meaning both parents need to carry a recessive trait for the offspring to inherit the disease
• autosomal recessive


  two copies of an abnormal gene must be present in order for the disease or trait to develop
• How is polydactyly inherited?

  autosomal dominant trait meaning only one parent needs to carry a dominant gene for the offspring to inherit the disease
• autosomal dominant


  only one copy of an abnormal gene must be present in order for the disease or trait to develop
• embryos can be screened for...

  genetic disorders
• how can embryos be screened for genetic disorders?

  - in in vitro fertilisation embryos are fertilised in the lab and then implanted in womb

  - pre-implantation a cell can be removed from each embryo and its genes analysed

  - many genetic orders can be detected in this way

  - DNA can also be obtained from an embryo in the womb that tests for disorders

  - if a genetic disorder will be 'bad' they can be destroyed or terminated
• arguments for embryonic screening

  - it helps to stop people from suffering

  - during IVF, most of the embryos are destroyed anyway, screening just allows the selected one to be healthy

  - treating disorders costs the government (and tax payers) a lot of money

  - there are laws put in place to stop it going to far, as parents cannot select the sex of their baby (unless its for health reasons)
• arguments against embryonic screening

  - ethical issues surrounding the disposal of unused embryos

  - implies that people with genetic problems are undesirable which may increase prejudice

  - screening is expensive

  - there may come a point where everyone wants to screen their embryos and they can pick and choose traits that their baby will have
• Gregor Mendel


  - Austrian monk

  - discovered that there is a pattern to the way that certain traits are inherited by breeding peas to study patterns of inheritance

  - his work began the science of genetics

  - father of modern genetics
• conclusions that Mendel reached

  - characteristics in plants are determined by "hereditary units"

  - hereditary units are passed on to offspring unchanged from both parents, one unit from each parent

  - hereditary units can be dominant or recessive: if an individual has both the dominant and the recessive unit for a characteristic, the dominant characteristic will be expressed
• why were Mendel's ideas not immediately accepted?

  - Mendel's work was new to scientists at the time

  - the background knowledge on genes, DNA and chromosomes required to understand his work had not yet been discovered
• why did people begin to understand Mendel's work?

  - in the late 1800s, scientists became familiar with chromosomes and they were able to observe how they behaved during cell division

  - in the early 20th century, scientists observed similarities in the way that chromosomes and Mendel's "units" acted, which are now known as genes that are found on chromosomes

  - in 1953, the structure of DNA was determined, allowing scientists to go on and find out how genes worked