inheritance

• A gamete is a sex cell. In humans, gametes are sperm and eggs (ovums). DNA is a large and complex polymer, which is made up of two strands forming a double helix. DNA determines the characteristics of a living organism. With the exception of identical twins, each person's DNA is unique
• • Chromosomes are contained inside the cell's nucleus. These are long threads of DNA, which are made up of many genes.
• • A gene is a small section of DNA on a chromosome, that code for a particular sequence of amino acids, to make a specific protein. It is the unit of heredity, and may be copied and passed on to the next generation.
• Some characteristics are controlled by a single gene, such as fur in animals and red-green colour blindness in humans. Each gene might have different forms, and these are called alleles. 
• Chromosomes are found in the nucleus of a body cell in pairs. One chromosome is inherited from the mother and one is inherited from the father.
• The chromosome in each pair carries the same gene in the same location. These genes could be the same, or different versions.
• • Alleles are different versions of the same gene. For example, the gene for eye colour has an allele for blue eye colour and an allele for brown eye colour. For any gene, a person may have the same two alleles, known as homozygous or two different ones, known as heterozygous.
• • The genotype is the collection of alleles that determine characteristics and can be expressed as a phenotype.
• Alleles may be either dominant or recessive
• • A dominantallele is always expressed, even if one copy is present. Dominant alleles are represented by a capital letter, for example, A. The allele for brown eyes is dominant. You only need one copy of this allele to have brown eyes. Two copies will still give you brown eyes.
• • A recessiveallele is only expressed if the individual has two copies and does not have the dominant allele of that gene. Recessive alleles are represented by a lower case letter, for example, a. The allele for blue eyes is recessive. You need two copies of this allele to have blue eyes.
• • Homozygous alleles are both identical for the same characteristic, for example AA or aa.
• • Heterozygous alleles are both different for the same characteristic, for example Aa.
• Most characteristics are a result of multiple genes interacting, rather than a single gene.
• Genetic crosses of single gene combinations (monohybrid inheritance) can be shown and examined using Punnett squares. This shows the possible offspring combinations could be produced, and the probability of these combinations can be calculated.
• You should always write the dominant allele first.
• How to construct Punnett squares - Higher

  1. Determine the parental genotypes - the allele combinations for the male and female. You can use any letter you like but select one that has a clearly different lower case,
  2. Split the alleles for each parental type and add them into your Punnett square around the edges.
  3. Work out the new possible genetic combinations inside the Punnett square.
  4. Use this to answer the question. You may be asked to comment on the proportion of different allele combinations in the offspring, calculate a probability ratio or just determine the phenotypes of the offspring.
• Method:

  • Draw your Punnett square
  • Add the alleles into your Punnett square around the edges
  • Work out the new possible genetic combinations inside the Punnett square
• Human body cells have 23 pairs of chromosomes in the nucleus.
• Twenty two pairs are known as autosomes, and control characteristics, but one pair carries genes that determine sex - whether offspring are male or female:

  • males have two different sex chromosomes, X Y
  • females have two X chromosomes, XX
• Chromosomes from a male

  These diagrams are known as human karyotypes, and show all the chromosomes aligned in pairs
• A genetic diagram, like a Punnett square, shows how chromosomes may combine in zygotes
• The two possible combinations are:

  • an X chromosome from the mother and an X chromosome from the father - producing a girl (female phenotype from the XX genotype)
  • an X chromosome from the mother and a Y chromosome from the father - producing a boy (male phenotype from the XY phenotype)
• The ratio of female to male offspring is 1:1 - on average, half of the offspring will be girls and half will be boys. This can also be converted into a probability of 50% (XX) and 50% (XY).
• Cystic fibrosis is an inherited disorder of cell membranes that mainly affects the lungs and digestive system
• Cystic fibrosis is an inherited disorder of cell membranes that mainly affects the lungs and digestive system. They can become clogged with lots of thick, sticky mucus as too much is produced. Over many years, the lungs become increasingly damaged and may eventually stop working properly. A number of treatments are available to help reduce the problems caused by the condition, but unfortunately average life expectancy is reduced for people who have it.
• cystic fibrosis is caused by a faulty recessive allele on chromosome 7. 
• To be born with cystic fibrosis, a child has to inherit two copies of this faulty gene - one from each of their parents. Their parents will not usually have the condition themselves, because they will only carry one faulty gene and one that works normally.
• An individual who is homozygous (ff) with the recessive allele will develop cystic fibrosis. Someone who is heterozygous (Ff) will be a carrier of the recessive allele, but will not develop cystic fibrosis and have no symptoms. Someone who is homozygous with the dominant allele (FF) will not develop cystic fibrosis, as you need two faulty alleles (ff) for the condition. In this combination, no faulty alleles are present.
• Polydactyly is an inherited condition in which a person has extra fingers or toes. It is caused by a dominant allele of a gene. This means it can be passed on by just one allele from one parent if they have the disorder.
• Someone who is homozygous (PP) or heterozygous (Pp) for the dominant allele will develop polydactyly.
• Offspring need to carry just one dominant allele from their parents to inherit the polydactyl condition.
• The probability of the offspring having polydactyly is 50% (2 of the 4) and 50% not having it (normal). This can be expressed as a ratio, 2:2 which can be simplified to 1:1.
• Genetic testing involves analysis of a person's DNA to see if they carry alleles that cause genetic disorders. It can be done at any stage in a person's life.
• • Antenatal testing is used to analyse an individual's DNA or chromosomes before they are born. This testing is offered to couples who may have an increased risk of producing a baby with an inherited disorder, but it can't detect all the risks of inherited disorders.
• • Neonatal testing known as the new born blood spot test involves analysing a sample of blood that is taken from pricking a baby's heel. It detects genetic disorders in order to treat them early.
• • Pre-implantation genetic diagnosis (PGD) is used on embryos before implantation. Fertility drugs stimulate the release of several eggs.The eggs are collected and fertilised in a Petri dish. This is known as in vitro fertilisation (IVF). Once the embryos have reached the eight-cell stage, one cell is removed.
• • The cells are tested for the disorder causing alleles. Embryos that don't contain the disorder allele are implanted into the uterus.
• Genetic tests are not available for every possible inherited disorder, and are not completely reliable. They may produce false positive or false negative results, which can have serious consequences for the parents involved.
• A false positive is a genetic test that wrongly detected a certain allele or faulty chromosome. The individual could believe that they have inherited a genetic condition, when they have not.