B1- Genes

Chromosomes
1. Chromosomes are structures found in the nucleus of most cells. They consist of long strands of a substance called deoxyribonucleic acid (DNA).
2. A section of DNA that has the genetic code for making a particular protein is called a gene.
  1. The proteins can either be...
    1. ....structural proteins such as the ones found in muscles and hair.
    2. ...enzymes, such as proteases and other digestive enzymes.
Variation
1. Individuals differ in all sorts of ways, even when they are offspring of the same parents. These differences are called variation.
2. Most characteristics, such as height, are determined by several genes working together. They are also influenced by environmental factors.
  1. These factors are...
    1. climate
    2. physical accidents
    3. diet
    4. culture
    5. lifestyle
identical twins
1. Identical twins are genetically the same
2. An identical twin who takes regular exercise will have better muscle tone than one who does not exercise. All of the differences that you see between identical twins, for example, in personality, tastes and aptitude, are due to differences in their experiences or environment.
Sex cells
1. When an egg and sperm cell come together, the now fertilised egg contains 23 pairs of chromosomes. Sex chromosomes are responsible for certain genetic traits.
2. When an egg and sperm cell come together, the now fertilised egg contains 23 pairs of chromosomes. Sex chromosomes are responsible for certain genetic traits.
  1. Female sex cells are called egg cells, or ova
  2. Male sex cells are called sperm
3. A pair of chromosomes carry the same genes in the same place, on each chromosome within the pair. However, there are different versions of a gene called alleles. These alleles may be the same (homozygous) on each pair of chromosomes, or different (heterozygous), for example, to give blue eyes or brown eyes.
  1. Which chromosome we get from each pair is completely random. This means different children in the same family will each get a different combination. This is why children in the same family look a little like each other and a little like each parent, but are not identical to them.
4. The highlighted pair of chromosomes are called the sex chromosomes; they are a pair. The longer sex chromosome is called the X chromosome, the shorter one the Y chromosome. Females are XX, Males are XY.
Determination of gender
1. When sex cells form, the pairs of sex chromosomes (XX and XY) are separated
  1. All normal egg cells produced by a human ovary have an X chromosome half the sperm carry an X chromosome, and half a Y.
Higher Tier
1. The Y chromosome carries a gene called the ‘sex-determining region Y’, or SRY for short. 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: without them, the embryo develops into a female.
2. Some people think parents should be able to choose the sex of their future children, especially if they had a child that died, or already have three or four children of the same sex. Other people think we should not be able to choose, because this could affect the balance of males and females in society, or because they believe it is against God or nature.

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B1- Genes

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	Created by 10berryebo over 9 years ago