B1 - You and Your Genes

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Genes, Chromosomes and DNA

Nucleus - contains genetic material arranged into chromosomes. Human = 23 pairs

Chromosome - long molecule of DNA coiled up

Gene - short length of a chromosome

Genes control the development of different characteristics and how an organism functions

Each gene is a code for making a certain type of protein. Having different versions of proteins = different characteristics

Structural proteins - Part of things like skin, hair, blood, cytoplasm. Collagen found in tendons, bones, cartilage

Functional proteins - Enzymes are proteins that help with digestion by breaking down food molecules.

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Genotype - All the genes an organism has

Phenotype - Characteristics an organism displays

Characteristics can be controlled...

  • Only by genes, either one gene or several working together. E.g eye colour, dimples
  • Only by environmental factors. E.g how and where you live
  • Both genes and environment. E.g weight

Sex cells have 23 single chromosomes. During sexual reproduction 23 chromosomes in sperm and egg combine.

2 chromosomes in a pair carry the same genes and each gene is found in the same place. Because the two chromosomes in a pair come from different parents, they may have different alleles of the same gene.

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Half a child's chromosomes come from each parent. Look similar but not exact.

When people produce sperm or egg cells, their pairs of chromosomes separate and go into different cells

  • 2 chromosomes in a pair are never identical because they have different alleles
  • When they go into 2 differemt sex cells, each of the cells get different alleles
  • Each of the 23 chromosomes pairs separates independently. Millions of different combinations that can be produced
  • All sex cells produced by one individual will probably be genetically different.
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Homozygous - Two alleles the same for that particular gene

Heterozygous - Two different alleles

Have 2 dominant alleles or 1 dominant and 1 recessive, the characteristic caused by the dominant allele will be shown

Both alleles have to be recessive to show characteristic

In genetic diagrams, dominant = capital letter, recessive = small letter

Genetic diagrams + Punnet sqaures can be used to work out a genetic cross

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Sex Chromosomes

Men = XY, Women = XX chromosomes

Y chromosome carries a gene which makes an embryo develop into a male as it grows.

The gene that makes an embryo into a male causes a specific protein to be produced.

When the embryo's reproductive system begins to develop, this protein causes the development of testes.

The testes produce male sex hormones, which make the rest of the male reproductive system develop.

In females the protein is not produces, so the embryo develops ovaries and the rest of the female reproductive system.

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Genetic Disorders

Some disorders are inherited by one or both parents carrying a fault allele that passes on to their children.

Cystic Fibrosis:

  • Faulty recessive allele. People with only one copy of the allele won't show the symptoms and are known as carriers. Both parents must be carriers or sufferers
  • Symptoms = chest infections, difficulty in breathing/digesting food

Huntington's Disease

  • Caused by a dominant allele.
  • 50% of child inheriting if one parent is a carrier
  • Causes tremors, memory loss, mood changes, poor concentration
  • Symptoms don't normally appear until after the age of 40
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Genetic Testing

Embryos produced using IVF tested for genetic disorders ( Pre-implantation genetic diagnosis). Only healthy are implanted, others discarded. Important if one parent carries alleles for that disorder

Children/Adults checked to see if they carry alleles for genetic disorders. Also can be tested before certain drugs are prescribed to show how it is likely to affect the individual E.g effectiveness and safety


  • Not 100% accurate. Errors due to sample contamination/ misinterpretation. False positve/negative
  • Pregancy not 100% safe. Lead to miscarriage
  • Positive - other members tested? Have children? Terminate preganancy? Cause discrimination by employers/ insurance
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Clones - genetically identical organisms. Same genes and alleles. Difference due to environment.

Asexual reproduction:

  • One parent and offpring are identical to each other and parent
  • Bacteria reproduce by diving into two
  • Horizontal stems (runners) move out from base of plant and form new plants at their tips
  • Bulbs (underground fleshy structures) grow to form a new identical plant.
  • Few animals can reproduce asexually. Eg. Greenfly lay eggs which develop into identical females
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Identical twins = clones. (Cells of an embryo spilt)

  • single egg fertalised by sperm
  • embryo splits into two, two separate embryos devlop
  • two embryos are genetically identical so identical babies are born

Scientists make animal clones in the lab

  • Nucleus of an egg cell is removed - egg cell has no genetic info
  • Nucleus from adult donor cell is inserted in it's place
  • Cell is then stimulated so that it starts dividing as if it was a normal embryo (fertilised egg)
  • Embryo produced is genetically identical to the donor cell
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Stem Cells

Stem cells - unspecialised cells which can develop into different types of cells depending on what instructions they're given.

Embryonic Stem Cells:

  • Unspecialised - found in early embryos
  • Stem cells removed from embryo, then embryo is destroyed
  • Have the potential to turn into any kind of cell

Adult Stem Cells:

  • Unspecialised cells - found in adult animals
  • Involved in maintaining and repairing old and damaged tissues and can specialise into many cells (not all)
  • Stem cells can  be safely removed from adult patients
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Stem cells

Can be used to treat many illnesses

  • Medicine uses adult stem cells to cure disease. Bone marrow contains adult stem cells that can turn into new blood cells
  • Embryonic stem cells can replace faulty cells in sick people. E.g. heart muscle cells for people with heart disease or insulin-producing cells for people with diabetes. Still being researched.
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