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Chromosomes are structures found in the nucleus of most cells. They consist of long strands of a substance called deoxyribonucleic acid, or DNA for short. A section of DNA that has the genetic code for making a particular protein is called a gene.
The proteins can either be:
- structural proteins such as the ones found in muscles and hair
- enzymes, such as proteases and other digestive enzymes
Size, Largest to Smallest.
Organism. Organ. Cell. Nucleus. DNA. Chromosome. Gene. Molecule. Atom.
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Human body cells each contain 23 pairs of chromosomes.
Parents pass on their genes to their offspring in their sex cells.
- female sex cells are called egg cells, or ova
- male sex cells are called sperm
A pair of chromosomes carry the same genes, in the same place, on the chromosome. However, there are different versions of a gene called alleles.Sex cells contain one chromosome from each pair. When an egg cell and sperm cell join together, the fertilised egg cell contains 23 pairs of chromosomes. One chromosome in each pair comes from the mother, the other from the father.
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- females are XX
- males are XY
- All normal egg cells produced by a human ovary have an X chromosome
- Half of the sperm carry an X chromosome, and half a Y
A baby's gender is determined by the sperm that fertilises the egg cell. The baby will be a girl if it carries an X chromosome. It will be a boy if the fertilising sperm carries a Y chromosome. 50:50 Chance of boy or girl.
The Y chromosome carries a gene called the ‘sex-determining region Y’ (SRY)
The SRY gene causes testes to develop in an XY embryo.
These produce androgens: male sex hormones. Androgens cause the embryo to become a male.
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People differ in all sorts of ways, even when they are offspring of the same parents. These differences are called variation.
Characteristics, such as height, are determined by several genes working together. They are also influenced by environmental factors. These include:
- physical accidents
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The chromosomes in a pair carry the same genes in the same places. But there are different versions of the same gene.
Different versions of the same gene are called alleles. For example, the gene for eye colour has an allele for blue eye colour and an allele for brown. For any gene, a person may have the same two alleles, or two different ones.
Alleles may be either recessive or dominant.
- A recessive allele only shows if the individual has two copies of it.
- A dominant allele always shows even if the individual only has one copy of it.
If a person has brown eyes, but still carries the allele for blue eyes, they are a carrier of the blue eye allele, so can pass it onto their offspring.
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Cystic fibrosis (CF) is caused by a recessive allele. (f)
People with CF produce abnormally thick and sticky mucus in their lungs and airways. As a result, they are more likely to get respiratory infections. Physiotherapy relieves congestion and antibiotics can fight infection. CF affects the gut and pancreas, so food is not digested efficiently.
You need to inherit two copies of the faulty allele to be born with CF. Just one copy, you are a carrier and will not experience any symptoms. If two carriers have a child there is a one in four chance of it inheriting CF.
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Huntington’s disorder is caused by a dominant allele, written as H. The symptoms usually develop in middle age, and include problems with controlling muscles and forgetfulness.
You only need to inherit one copy of the faulty allele to have Huntington’s disorder, You can inherit Huntington’s disorder if one or both of your parents carry the faulty allele, because it is a dominant allele.
You can show inheritance of the disorder using genetic diagrams. In the example, one parent carries the allele for HD. There is a 50% chance the child will have the disorder.
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