I/GCSE Biology - Inheritance

• Do you remember what you have learned in I/GCSE Biology regarding DNA?
• In human body cells, one of the 23 pairs of chromosomes is the sex chromosomes
• In females the sex chromosomes are the same (XX) and in males the sex chromosomes are different (XY)
• In males, one is much shorter than the other, the shorter one is called the Y chromosome and the longer one is the X chromosome
• The sex of an individual is decided by whether the ovum (egg) is fertilised by an X-carrying sperm or a Y-carrying sperm
• Look at the diagram showing the 23 pairs of chromosomes of male and female below!

After knowing the sex chromosome in I/GCSE Biology, you should also remember:

• Some characteristics are controlled by a single gene – these are called alleles.
• A dominant allele = an allele which controls the development of a characteristic when it is only on one of the chromosomes
• A recessive allele = an allele which controls the development of characteristics only if the dominant allele is not present
• Capital letters are used to represent dominant alleles and lower case letters are used to represent recessive alleles.

• Understanding what alleles are in I/GCSE Biology, let's move on to monohybrid inheritance!
• When a characteristic is controlled by just a single gene e.g. tongue rolling or eye colour, genetic crosses can be used to investigate the mechanism of inheritance
• This type of inheritance is referred to as monohybrid inheritance

• When cells develop a specialised structure to carry out a specific structure, this is called differentiation.
• Most type of animal cells differentiate at an early stage in life so quickly become muscles, nerves etc.
• Many plant cells retain the ability to differentiate throughout life
• In mature animals, cell division is mainly restricted to cell replacement and repair

When talking about differentiation of cell in I/GCSE Biology, usually we link to stem cells......

• Stem cells are cells in human embryos and adult bone marrow
• These can be made to differentiate into many different types of cells e.g. nerve cell, muscle cell, skin cell
• Treatment with these cells may help conditions such as paralysis

• Some disorders are inherited such as cystic fibrosis and Huntington’s disease.
• Embryos can be screened for the alleles that cause these and other disorders

Huntington’s disease

• A disorder of the nervous system
• Caused by a dominant allele of the gene
• Can be passed on by only one parent who has the disorder

Cystic Fibrosis

• A disorder of cell membranes
• Must be inherited by both parents
• The parents may be carriers of the disorder without actually having the disorder themselves
• It is caused by a recessive allele of a gene

• It's important in I/GCSE Biology to make informed judgements about the social and ethical issues concerning the use of stem cells from embryos in medical research and treatment. Some are listed below.

For the use of stem cells from embryos:

1. The embryos from which the stem cells are taken are grown in laboratories and are only a few days old; many people see them as simply microscopic balls of cells
2. Embryos provide the most useful stem cells: the cells are non-specialised, which means they can become any specific type of cell (adult stem cells are more limited in their potential uses)
3. They can act as a repair kit for damaged tissue
4. Stem cells may be able to treat many diseases and conditions in the future.

Against the use of stem cells from embryos:

• Many people believe it is morally wrong to experiment on embryos (even those grown in the laboratory) as they could all potentially develop into a baby
• Stem cells are cultivated using nutrients from animal sources, which could carry diseases that could be passed on to humans
• People who receive cell transplants through stem cell therapy could be infected with viruses
• Stem cells may turn cancerous

Advantages of embryo screening:

1. Doctors can determine whether a child will have an increased risk of contracting a particular illness or disease
2. It prepares parents for the possibility of a child developing a disorder, disease or illness
3. Carriers of genetic disorders could make informed decisions about whether to have children

References:

1. "3.26: Understand How the Sex of a Person is Controlled by One Pair of Chromosomes, XX in a Female and XY in a Male", https://1.bp.blogspot.com/-N7JqYSfdeLE/XUZkAm5lbXI/AAAAAAAAAX4/9OMWSA1InPAJ5BzRRDOWWeqSI3ez5iyDgCLcBGAs/w1200-h630-p-k-no-nu/Screenshot%2B2019-08-04%2Bat%2B12.49.48%2BPM.png
2. "Specialised Cells", https://studyrocket.co.uk/assets/img/assumeJPG/B_axRIdJrEpq1k9HtdEzdyFB9sXSwqmf2kAxxOu9wQxnqtOrv7U29A6W3NXK1ihJYzQSjeZZGNqarMajxuVg1LznTJZdgfWiqr8IEHsV8U9AG9VRV0S0SKe16jne91hkhIRXhv4m.jpg
3. "Prenatal Screen Detects Fetal Abnormalities",https://www.nature.com/scitable/content/ne0000/ne0000/ne0000/ne0000/117683904/17_1_2.jpg  

And we're done with this topic! Well Done!

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Drafted by Alyssa (Biology)