- The nucleus of a cell contains thread-like structures called chromosomes.
- The chromosome threads carry the gene.
- In the nuclei of sex cells (gametes) there is only a single set of chromosomes.
- Therefore, nuclei of male and female sex cells contain one set of genes.
One chromosome --> many genes --> lots of DNA
- The genetic information from the parents is passed on to the offspring during reproduction. So the offspring cells contain two sets of genes, one set inherited from each parent.
- Different genes control the development of different characteristics of the offspring.
- In most body cels the chromosomes are in pairs. One set came from the female gamete (from the mother) and one set from the male gamete (from the father).
- Differences in the characteristics of individuals, may be due to genetic causes, environmental causes or both.
Types of reproduction
- Asexual reproduction does not involve the fusion of gametes (sex cells). All the genetic information comes from one parent. All the offspring are genetically identical to the parent, so there is little variety.
- Identical copies produced by asexual reproduction are called clones.
- Sexual reproduction invoves the fusion of sex cells (gametes). There is a mixing of genetic information, so the offspring show variation.
- In animals the sex cells are egg and sperm.
- Offspring produced by sexual reproduction are similar to both parents, but cannot be identical. That is because they have a combination of two sets of genes.
- The different forms of a gene are known as alleles.
- Random mixing of genes leads to variation in the offspring. This is important in survival. Some characteristics may give offspring a better chance of surviving difficult conditions.
Genetic and environmental differences
- Differences in the characteristics of individuals of the same kind (same species) may be due to:
- differences in the genes they have inherited
- the conditions in which they have developed
- a combination of both genetic and environmental causes.
- Identical twins are clones who share the same genetic material, but even they will never be exactly identical due to differences in their environmental influences.
- Genes are the most important factor in controlling the appearance of an individual.
- Plants may be affected by lack of light, nutrients or space to grow. The weaker plants may have the same genes as the healthier plants but cannot grow well if deprived of nutrients.
- Human development may be affected during pregnancy. If the mother smokes or drinks a lot of alcohol, the baby may have a low birth weight.
- Once animals are born, too much of too little food can alter their characteristics. For example, genes may determine if someone has the potential to be a good athelete. However, training to develop muscles and eating the correct diet can alter the athelete's body.
From Mendel to modern genetics
- Humans have 23 pairs of chromosomes.
- Genes controlling the same characteristics are called alleles.
- If an allele 'masks the effect of another it is said to be 'dominant'. The allele where the effect is 'masked' is said to be 'recessive'
- Genetic diagrams, including family trees, illustrate how alleles and characteristics are inherited.
- Phenotype - physical appearance of the characteristic, e.g. dimples or no dimples.
- Genotype - the genetic make up - which alleles does the individual inherit? DD, Dd or dd
- Homozygous - both alleles are the same DD (homozygous dominant) or dd (homozygous recessive).
- Heterozygous - the two alleles are different Dd.
- Monohybrid inheritance - the inheritance of a single characteristic controlled by a pair of alleles.
Gregor Mendel observed clear patterns of inheritance in pea plants before anyone had discovered chromosomes. He counted the number of plants with a particular characteristic and recorded his findings carefully. Mendel realised that there are separate units of inheritance.
Inheritance in action
- Chromosomes are made of DNA (deoxyribonucleic acid).
- DNA isa very long molecule with strands which are twisted together into a double helix structure.
- DNA contains four compounds called bases. A-T C-G
- Genes are short sections of DNA with a large number of bases, grouped into threes.
- Each gene has a code to make a specific protein.
- A sequence of three bases is the code for a particular amino acid.
- The order of the bases controls the order of the amino acids in the protein.
- Family trees and Punnett squares are clear ways to illustrate inheritance. When you trace a family tree, look for evidence of which allele (form of gene) is dominant and which reccessive. If a characteristic is 'hidden' in a generation and appears later it is recessive.
Inherited conditions in humans
- Polydactyl can be passed on by one parent who has the allele.
- If an allele is recessive, the person must inherit two recessive alleles to have the disorder.
- Cystic fibrosis is an inherited disorder caused by a recessive allele. The allele affects cell membranes and causes the production of thick sticky mucus. The mucus can affect several organs, including the lungs and pancreas.
- A child must inherit a recessive allele from both parents to develop cystic fibrosis. The disorder can be passed on from two parents who don't have cystic fibrosis themselves. The parents are described as carriers of the allele.
- We can use genetic diagrams to predict how genetic disorders might be inherited and predict the inheritance of genetic disease.
More inherited conditions in humans
Sickle cell anaemia
- This is a genetic disorder caused by a recessive allele.
- The disorder affects the shape of the red bloods which cannot carry oxygen efficiently and may block small blood vessels, causing pain.
- People who are homozygous usually do not survive.
- People who are heterozygous for the sickle-cell anaemia gene, have some sickle blood cells. This gives them protection from the malaria parasite.
- The majority of the population are homozygous for the dominant gene so their red blood cells have the normal shape. Homozygous dominant people catch malaria, which kills many people.
Whole chromosome disorders
- Some inherited conditions are caused by inheritance of abnormal numbers of chromosomes.
- A mistake during meiosis could lead to a cell with too many or too few chromosomes.
- Down's syndrome is caused by the presence of an extra chromosome, 47 instead of 46.
- A child with Down's syndrome could have a range of developmental problems due to the extra chromosome 21.
Screening for genetic disease
- Embryo screening involves tests to diagnose genetic disorders before the baby is born.
- The results of the test may give the parents choices. Sometimes the parents decide to terminate the pregnancy. Other parents decide this is not ethical but can prepare for the delivery of an affected baby.
- In IVF the embryos are screened and only healthy embryos are implanted into the mother. Embryos carrying faulty genes are destroyed and some people think this is unethical.
- DNA is isolated fro the embryo cells.
- A gene probe that will bind on to the gene for a specific disorder is produced.
- The gene probe has a flourescent chemical attached to it.
- The probe is added to a mixture containing the DNA sample from the embryo.
- UV light is used to detect the probe.
- If the probes have attached to the faulty allele and flouresce, the embryo or fetus is affected by the condition.
Concerns about embryo screening include:
- the risk of miscarriage
- the reliability of the information of the screening process as sometimes there can be a false positive or false negative results
- decisions about terminating pregnancy.