Biology 1- Inheritance


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  • Created by: georgia s
  • Created on: 04-05-12 15:49


A persons unique characteristics are caused by: 

  • the set of genes they inherited from their parents (nature)
  • the environment in which they develop (nurture)

In the nucleus of out cells, we find 46 chromosomes, that exist in 23 pairs. Each chromosome is made up of a chemical called DNA. Along this DNA chain we find genes. Genes control out physical characteristics.

Genes are sections of DNA molecules that determine inherited characteristics and are in pairs. Genes have different forms called alleles. 

The last pair of chromosomes determines the sex of the human:

  • XX Chromosomes= female
  • XY Chromosomes= male
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Some characteristics, such as eye colour and the shape of the earlobe, are controlled by a single gene. These genes may have different forms.

Different forms of the same gene are called alleles. The gene for eye colour has an allele for blue eye colour and an allele for brown eye colour.

Alleles are dominant or recessive:

  • the characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair (capitol letter for dominant)
  • the characteristic controlled by arecessiveallele develops only if the allele is present onbothchromosomes in a pair (lower case for recessive)

Homozygous- If you're homozygous for a trait you have two alleles the same for that particular gene e.g. CC

Heterozygous- If you're heterozygous for a trait you have two different alleles for that particular gene e.g. Cc

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DNA stands for DeoxyriboNucleic Acid, it is the smallest piece of the genetic puzzle.

DNA makes long chains called chromosomes.

Chromosomes are strands of DNA located in the nuclei

Each gene is responsible for producing protein (or an enzyme) which determines how a cell functions.

DNA is made up of individual units call nucleotides, composed of phosphate group, a sugar and a base.

DNA can be extracted from cells.

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Punnet Square


H is the dominant gene so there is 75% chance that the child will be right-handed. However, there is a 25% chance that the child will inherit both recessive genes from mother and father and be left-handed.

  • HH= Homozygous
  • Hh= Heterozygous 
  • hh= Homozygous
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Cell Division- Mitosis

  • Cell division for growth and repair
  • Can happen in all parts of the body
  • Two identical daughter cells are produced.

Mitosis is when a cell reproduces itself by splitting to form two identical offspring.

1) In a cell that is not dividing, the DNA is all spread out in long strings

2) If the cell gets a signal to divide, it needs to duplicate its DNA-so there's one copy for each new cell. The DNA is copied and form X-shaped chromosomes. Each 'arm' of the chromosome is an exact duplicate of the other.

3) The chromosomes then line up at the centre of the cell and cell fibres pull them apart. The two arms of each chromosome go to opposite ends of the cell.

4) Membranes from around each of the sets of chromosomes. These become the nuclei of the two new cells.

5) Finally the cytoplasm divides, you now have two new cells containing exactly the same DNA-they're identical.

Most cells have a limit to the number of times they can divide. This is called the Hayflick limit.

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Cell Division- Meiosis

Gamete only have half the number of chromosomes as a normal body cell. They are produced by meiosis. In humans it only happend in the reproductive organs. Meiosis produces 4 haploid (single set of unpaired chromosomes.) cells whose chromosomes are NOT identical.

Division 1

1) Before the cell divides, it duplicates its DNA.

2) In the 1st division the chromosome pairs line up in the centre of the cell.

3) They're then pulled apart, so each new cell only has one copy of each chromosome. Some of the father's chromosomes and some of the mother's chromosomes go into each new cell.

4) Each new cell will have a mixture of the mother's and father's chromosomes. Mixing up the genes in this way creates variation in the offspring. This is a huge advantage of sexual reproduction over asexual reproduction

Division 2

5) In the second division the chromosomes line up again in the centre of the cell. The arms of the chromosomes are pulled apart.

6)You get four gametes, each with only a single set of chromosomes in it.

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Genetic Fingerprinting

Each person has DNA that produces the particular proteins needed for life, plus sections of DNA which are unique to the individual.

By cutting a sample of DNA into fragments and comparing it with other samples it's possible to work out if the DNA has come from the same person. This technique is called DNA fingerprinting, and is a technique used to help solve crimes where the suspect may have left a sample of body tissue at the crime scene.

Genetic fingerprinting can be used to identify criminals and paternity testing.

How to produce a DNA fingerprint 

  1. Isolation - separate the DNA from other tissues
  2. Fragmentation - use an enzyme to break the DNA into short lengths
  3. Separation - pass an electric current across a layer of gel which has the DNA fragments at one end. The fragments will move different distances across the gel. This is called gel electrophoresis
  4. Comparison - match the pattern of fragments on the gel with other samples of DNA
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Gene Ownership

Should DNA be stored on a database?


  • Samples can be taken from a crime scene and used to identify suspects.
  • People could identify unknown relatives easily
  • DNA on passports would stop them being replicated


  • If employers accessed the database, they could eliminate groups of people from a job e.g. ethnicity, disability, future illness
  • No privacy- DNA could be accessed unknowingly
  • Expensive to set up and maintain
  • Tragic consequences if information go into the wrong hand e.g. terrorists.
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DNA bases

There are 4 different bases, represented by the letters A,T,C and G. They join together in a set pattern (A to T,C to G)

This makes DNA a double stranded molecule

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Key Terms

Gamete: a sex cell (eggs/sperm)

Zygote: a fertilized egg

Embryo: a developing baby

Genotype: the genes that are inside the cells of a living thing

Phenotype: physical features of a plant/animal you can see e.g. black fur

Karotype: an organised profile of a person's chromosomes.

Dominant alleles: only shows up in the phenotype of a heterozygous individual

Recessive alleles: will only show up in the phenotype of a homozygous individual

Selfing: Self-pollinate; self-fertilize

Heterozygous: a living thing that has two different alleles for a characteristic

Homozygous: a living thing that has two identical alleles for a characteristic

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Key Terms 2

Heterozygote: An individual having two different alleles of a particular gene or genes, and so giving rise to varying offspring.

Homozygote: An individual having two identical alleles of a particular gene or genes and so breeding true for the corresponding characteristic.

DNA: The chemical that chromosomes are made out of

Chromosomes: long pieces of DNA that contain thousands of genes. A normal human cell has 23 pairs of chromosomes; a gamete only has 23 chromosomes.

Genes: instructions found in chromosomes. They code for the sequence of amino acids in our body's proteins.

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Cystic Fibrosis

Cystic Fibrosis is a genetic disorder that is passed onto a child by his/her parents. CF sufferers make lots of mucus that clogs up their airway and makes breathing difficult. They also have difficulty in digesting food. CF is managed through a combination of chest massages, medication and gene therapy 

( C=normal c=CF gene

If both parents are heterozygous then there is 25% chance of a child being born with CF.

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DNA Profiles

A DNA profile can be made from a sample of someone's DNA. This gives us a code (similar to a barcode) which is unique to that person. This DNA profile can be used in situation where confirming identity it important (e.g. solving crimes, paternity tests). It is important that DNA profiles are protected as they may contain sensitive information.

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Gregor Mendel

Gregor Mendel (1822-1884) studied the inheritance of different characteristics in pea plants. He found that when he bred red-flowered plants with white-flowered plants, all the offspring produced red flowers. If he bred these plants with each other, most of the offspring had red flowers, but some had white. This was because the allele for red flowers is dominant, and the allele for white flowers is recessive. .

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A well written set of cards covering inheritance which is found in many GCSE specifications. Try writing a set of flashcards for all the terms  in this section as understanding these is a key part of understanding genetic problems. Check your specification as sometimes this is only found in Higher papers.

Teen Wolf D

Awesome!! It's interesting to study.

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