B1 - Genes

Genes and Chromosomes
1. 1. Most cells In your body have a nucleus. The nucleus contains your genetic material in the form of chromosomes.
2. 2. In most animal cells chromosomes come in pairs, but different species have different numbers of pairs. For example, a human body cell contains 23 pairs of chromosomes and a Guinea pig's body cells contain 32 pairs.
3. 3. Chromosomes carry genes. Different genes control different characteristics. E.g. Eye colour.
4. 4. A gene is a short length of chromosome, which is quite a long length of DNA
5. 5. The DNA is coiled up to form the arms of a chromosome (seen below).
6. 6. There can be different versions of the same gene, which give different versions of a characteristic, like eye colour. The different versions of the same gene are called alleles instead of genes.
Genetic Variations
1. There are three sources of genetic variation:
  1. Gamete formation (making sperm and egg cells)
    1. 1. Gametes are sperm and egg cells, they are found in the ovaries and testes
    2. 2. The body cells they're made from have 23 pairs of chromosomes. In each pair there's one chromosome that was originally inherited from the mother and one that was inherited from the father.
    3. 3. When these body cells split to form gametes, the chromosomes are also split up. This means that gametes end up with half the number of chromosomesof a normal body cell, just 23.
    4. 4. In each gamete, some of your fathers chromosomes are grouped with some of your mother's.
    5. 5. This shuffling up of chromosomes leads to variation in the new generation.
  2. Fertilisation (the gametes join together)
    1. 1. Fertilisation is when the sperm and the egg, with 23 chromosomes each, join to form a new cell with the full 46 chromosomes.
    2. 2. However, in nature fertilisation is random, so you don't know which two gametes are going to join together.
  3. Mutations (changes to Genetic code)
    1. Occasionally a gene may mutate, this can create new characteristics, increasing variation.
2. Most features are determined by both Genes and the Environment:
  1. Health: Some people are more likely to get certain diseases such as cancer, because of their genes. However, lifestyle also affects the risk. E.g. If you smoke you will be much higher risk than a non-smoker.
  2. Intelligence: one theory is that your maximum possible IQ might be determined by your genes, whether you get to it depends on your environment. E.g. your upbringing and school life.
  3. Sporting ability: Again, genes probably determine your potential, but training and a healthy diet is important too.
Genetic Diagrams
1. Genetic Diagrams show the possible genes of offspring
  1. 1. Alleles are different versions of the same gene.
  2. 2. Most of the time you have two of each gene (i.e. two alleles), one from each parent.
  3. 3. If the alleles are different, you have instructions for two different versions of a characteristic (e.g. eye colour), but you only show one of the two. The version of the characteristic that appears is caused by the dominant allele. The other allele is said to be recessive, the recessive allele is only expressed if there is no dominant allele present.
  4. 4. In genetic diagrams letters are used to represent genes. Dominant alleles are always shown with an upper case letter, while recessive alleles are represented with lower case letters.
  5. 5. If your homozygous for a trait you have two alleles the same for that particular gene. E.g. CC or cc.
  6. 7. Your genetic makeup (i.e. the alleles you have for a particular gene) is known as a genotype. The characteristics that these alleles produce (brown eyes) is known as your phenotype.
  7. 6. If your heterozygous for a trait you have two different alleles for that particular gene. E.g. Cc.
2. interpreting, explaining and constructing Genetic diargrams
  1. In a genetic diagram, you show all of the possible alleles for a particular characteristic. There will be two alleles from one parent, and two from the other parent, making four altogether. You then draw lines to show all the possible ways that these alleles could be paired in the offspring. There will be four possible ways, but some or all of them could be repeated.
Sex inheritance and genetic disorders
1. You chromosomes control whether you're male or female
  1. There are 23 matched pairs of chromosomes in every human body cell. the 23rd pair are labelled XY. they're the two chromosomes that decide whether you turn out ale or female.
  2. All men have an X and a Y chromosome, Hence them being labelled XY. The Y chromosome causes male characteristics.
  3. All women have two X chromosomes, hence them being labelled XX. The XX combination causes female characteristics.
2. There's an equal chance of having a boy or girl:
  1. However this 50:50 ratio is only a probability, so if someone had 4 children, then they could all be boys.
3. Genetic disorders are caused by faulty genes
  1. Cystic fibrosis is a genetic disorder that causes the body to produce a lot of thick, sticky mucus in the air passages and in the pancreas.
  2. 1. The allele that causes cystic fibrosis is a recessive allele, 'f' carried by about 1 person in 25.
  3. 2. Because it's recessive, people with only one copy of the allele won't have the disorder, they're known as the carriers.
  4. 3. For a child to have a chance of inheriting the disorder, both parents must either be carriers or sufferers themselves.
  5. 4. As the diagram shows, there is a 1/4 chance of a child having the disorder if both parents are carriers.

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B1 - Genes

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