Cell Division & Inheritance


Human Body Cells

Body cells contain tow sets of 23 chromosomes arranged in pairs (46 in total) 

Chromosomes contain genetic information

Gametes are sex cells. Gametes only have one set of 23 chromosomes 

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During fertilisation, the female & male gametes fuse to produce a zygote - a single body cell with 23 new pairs of chromosomes. In each pair:

  • one chromosome comes from the mother
  • one chromosome comes from the father 

The cell then dividies repeatedly by mitosis  to form a new individual

Variation is caused due to the combinationof genes from the mother & father 

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Mitosis is the division of body cells to produce new cells. Mitosis occurs:

  • for growth
  • for repair
  • in asexual reproduction (cells produced by asexual reproduction contain the same alleles as the parents)

During mitosis:

  • a copy of each chromosome is made
  • the cell then divides once to produce two new body cells
  • the new cells contain exactly the same genetic information as the parent cell 
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Meiosis occurs in the testes & ovaries to produce the gametes (sperm and egg) for sexual reproduction

When a cell divides to form gametes:

  • copies of the genetic material are made
  • the cell then divides twice to form four gametes, each with a single set of chromosomes
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Inheritance of the Sex Chromosome

Of the 23 pairs of chromosomes in the human body, one pair carries the genes that determines sex: 

  • in females, the sex chromosomes are identical and are called the X chromosomes 
  • in males, one sex chromosome (Y) is much shorter than the other (X)

As with all chromosomes, offspring inherit:

  • one sex chromosome from the mother (always an X)
  • one sex chromosome from the father (either an X or Y)

So, the sex of the offspring is decided by whether the ovum (egg) is fertilised by an X-carrying sperm or a Y-carrying sperm 

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Some characteristics are controlled by a single gene. Genes may have different forms, or variations caled alleles. For examples:

  • the gene that controls tongue-rolling ability has 2 alleles - either u can or u can't
  • the gene that controls eye colour has two alleles - blue or brown 

Sexual reproduction gives rise to variation because when gametes join during fertilisation:

  • one allele for each gene comes from the mother 
  • one allele for each gene comes from the father 

In a pair of chromosomes, the alleles for a gene can be the same or different. If they're different:

  • one allele will be dominant
  • one allele will be recessive 

A dominant allele will always control the characteristic; it will express itself even if present on only one chromosome in a pair 

A recessive allele will only control the characteristic if it is present on both chromosomes in a pair 

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Genetic Diagrams

In genetic diagrams you should use:

  • capital letters for dominant alleles 
  • lower case letter for recessive alleles 

When constructing genetic diagrams:

  • clearly identify the alleles of the parents 
  • place each of these alleles in a seperate gamete 
  • join each gamete with the two gametes from the other parent 

Following terms:

  • genotype - combination of alleles that an individual has for a particular gene 
  • homozygous - an individual who carries two copies of the same allele for a particular gene 
  • phenotype - the expression of the genotype (characteristic shown) 
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Stem Cells

Most types of animal cells differentiate & become specialised at an early stage. Many plant cells retain the ability to differentiate throughout their life

Stem cells have the ability to develop into any kind of cell because they haven't yet differentiated. Stem cells in humans are found in:

  • human embryos 
  • adult bone marrow

Human stem cells can be made to differentiate into any type of human cell. These cells could potentailly be used to help treat people with conditions like paralysis. There are some ethical issues surrounding the use of stem cells for scientific research:

  • Against - some people believe that human embryos shouldn't be used bc they're a potential human life 
  • For - some people believe its more important to help people who are already living 

Some embryo stem cells are taken from unwanted embryos from fertility clinics where they would be destroyed if not used for research. Some countries have banned stem cell research 

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The Structure of Chromosomes

Chromosomes are made up pf DNA. A DNA molecule consisits of two long strands that are coliled to form a double helix. 

Each person has unique DNA (apart from identical twins) So, DNA can be used for identification. A gene is a small section of DNA. 

Genes code for a particular characterisitc by providing a code for a combination of amino acids that make up a specific protein 

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Genetic Disorders

Some disorders are inherited, such as:

  • polydactyly
  • cystic fibrosis 

People with polydactlyly have extra fingers or toes. Its caused by a dominant allele, so it can be passed on even if only one person has the disorder.

Cystic fibrosis is a disorder of the cell membranes. It's caused by a recessive allele, so both parents must carry the allele. Because the allele is recessive, the parents can be carriers without having the disorders themselves. 

Embryos can be screened for the alleles that cause genetic disorders. This has enconomic, social & ethical issues. 

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