Download File - Thomas Tallis Science

Inheritance
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Make sure you revise:
Mitosis - cell division
Meiosis – produces sex cells
Genes and chromosomes
Inherited disorders
Embryo screening
• HT: stages of meiosis
Mitosis
parent
cell
mitosis
daughter cells
Body cells divide by mitosis. When a body cell divides by
mitosis:
• Copies of the genetic material are made first
• Then two genetically identical body cells are produced
Higher Tier – Meiosis
and key terms
Chromosomes in body cells
• In body cells the chromosomes
are in pairs
• Body cells have two sets of
chromosomes;
• Sex cells (gametes) have only
one set.
• Sex cells (gametes) are made
by meiosis
Asexual and sexual reproduction
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In asexual reproduction offspring contain the same genes as the parents.
Sexual reproduction gives variation because, when gametes fuse, one of each
pair of alleles comes from each parent.
Chromosomes are made up of DNA. A gene is a small section of DNA.
DNA can be used to identify individuals in a process known as DNA
fingerprinting.
HT: Each gene codes for a particular combination of amino acids which
make a specific protein.
Alleles
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allele for
brown
eyes
allele for
blue
eyes
•Some characteristics are controlled by a single gene.
•Each gene may have different forms of genes called alleles
•An allele which controls the development of a characteristic
when it is present on only one of the chromosomes is a
dominant allele.
•An allele which controls the development of a characteristic
when the dominant is not present is a recessive allele
Determining the sex of a child
• In females the sex chromosomes are the same (XX)
• In males the sex chromosomes are different (XY).
Genetic diagrams/Punnet squares
Eg, the allele for brown eyes (B) is dominant to the allele
for blue eyes (b)
If 2 parents have brown eyes and have the genetic make up Bb,
what would be the chance of them having a blue-eyed child?
B
b
75% chance brown
eyed child
B
25% blue eyed
child
BB
Bb
(brown eyes = (brown eyes =
25%)
25%))
b
Bb
(brown eyes =
25%)
bb
(blue eyes =
25%)
Gregor Johann Mendel
Question
Answer
Who was Gregor
Mendel?
An Austrian monk who discovered how
characteristics were inherited.
What experiments did
he do?
Cross-bred pea plants and counted the different
types of offspring.
What did he discover
from his findings?
That characteristics were inherited in clear and
predictable patterns (idea of inherited ‘factors’).
That some characteristics were dominant over
others and that they did not ‘mix’ together.
Why did people not
accept his ideas at the
time?
Genes and chromosomes had not yet been
discovered so people didn’t understand his theories.
When and why were
Mendel's ideas finally
accepted?
By 1900 people had seen chromosomes through a
microscope. Three scientists found Mendel’s papers
and repeated his experiments leading to further
developments into genetics and Mendel's work.
Punnet squares
What are inherited diseases?
Inherited diseases are diseases caused by faulty genetic
material that is passed on to future generations.
They are sometimes called genetic disorders.
Many inherited diseases are caused by
mutations in DNA, resulting in faulty alleles
that are not properly expressed.
Mutations can be spontaneous or caused
by exposure to mutagens such as
radiation and certain chemicals.
There are over 4,000 known inherited
diseases, although the specific alleles
involved are only known for 25% of
them.
Polydactyly
Polydactyly – having extra fingers or toes – is caused by a dominant allele
of a gene and can therefore be passed on by only one parent who has the
disorder
Cystic fibrosis summary
• A disorder which affects cell membranes in many organs of
the body particularly the lungs and the pancreas
• Caused by a recessive allele so must be inherited by both
parents
Cystic fibrosis
Embryo screening
• Embryos can be screened for the alleles that
cause these and other genetic disorders.