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Genetics
An organism’s characteristics are passed on from generation to generation through inheritance
of genes. Genes are found along the threadlike structures called chromosomes.
Chromosomes - The cell’s nucleus contains chromosomes made from long DNA molecules. The
diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus, and
genes.
Genes are short sections of DNA. Genetically identical cells are produced by a type of cell
division called mitosis. In sexual reproduction, a male gamete fuses with a female gamete to
produce a new cell. This is called fertilization. Gametes are produced by a type of cell division
called meiosis. They contain a single set of chromosomes, whereas body cells contain two sets of
chromosomes.
DNA (deoxyribose nucleic acid) molecules are large and complex. They carry the genetic code
that determines the characteristics of a living thing.
Except for identical twins, each person’s DNA is unique. This is why people can be identified
using DNA fingerprinting. DNA can be cut up and separated, forming a sort of 'bar code' that is
different from one person to the next.
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Sex cells and chromosomes
Human body cells each contain 23 pairs of chromosomes.
Parents pass on their genes to their offspring in their sex cells.
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female sex cells are called egg cells, or ova
male sex cells are called sperm
A pair of chromosomes carries the same genes, in the same place, on the chromosome. However, there are
different versions of a gene called alleles. These alleles may be the same on each pair of chromosomes, or
different, for example, blue eyes or brown eyes.
Sex cells only contain one chromosome from each pair. When an egg cell and sperm cell join together, the
fertilized egg cell contains 23 pairs of chromosomes (46 total). One chromosome in each pair comes from the
mother, the other from the father.
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.
Mitosis
Mitosis is a type of cell division. Mitosis occurs wherever more cells are needed. It produces two new cells that
are identical to each other, and to the parent cell. The process of growth and division is called the cell cycle.
The cell cycle is a series of stages through which the cell passes between divisions and it is composed of 2
stages easily identified through the microscope.
1. Interphase is the period between divisions when nothing seems to be happening
2. Mitosis (nuclear division), when the genetic material is dividing and the chromosomes are visible
The cycle starts as the number of organelles - the different parts of the cell - increases. This is to ensure that
each of the two new cells receives copies of all the organelles
Before a cell divides, its chromosomes
are copied exactly. The DNA molecule is
made of two strands. As each of the two
strands separate, new strands are made
alongside each of them, thereby making
two new copies.
Meiosis
Meiosis is a different kind of cell division. It is used to produce male and female gametes. A human body cell
contains 46 chromosomes arranged in 23 pairs. The gametes are sperm or eggs, and only contain half as many
chromosomes (23). This is why meiosis is sometimes called reduction division.
At fertilization, the nuclei of the sperm and an egg join to form the zygote. The zygote contains 23 pairs of
chromosomes - 23 single chromosomes from the sperm, and 23 single chromosomes from the egg, thereby creating the
correct number of 46 chromosomes for all body cells. It also means the zygote contains a complete set of chromosomes
from each parent.
Types of Inheritance
Inheritance of Sex
Inheritance is the passing on of genetic information from one generation to the next.
Inheritance of sex difference is due to different chromosomes in the male and female
Sex chromosomes
A set of chromosomes can be separated from its cell, spread out on a microscope slide and magnified many
thousands of times. When stained and photographed, they look like this:
Fig 3:Chromosomes of a female
Fig 4: Chromosomes of a male
The highlighted pair of chromosomes are called the sex chromosomes. The longer sex chromosome is called the
X chromosome, the shorter one the Y chromosome.
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females are XX
males are XY
The Punnett square is a diagram that is used
to predict an outcome of a particular cross
or breeding experiment
2 males , 2 females , 1:1 ratio
Sex-Linked traits
The sex chromosomes also carry genes, genes that are carried by either sex chromosome are said to
be sex linked. Sex linked traits or characteristics include sickle cell anaemia, haemophilia and colour
blindness.
Inheritance of Dominance.
If a dominant and recessive gene is present together in an individual, the dominant gene will be
expressed over the recessive one.
Similarities between children and their parents, or sometimes their grandparents have long
recognized. The similarities of similar eye colour, face shape, colour, facial features etc are
called characteristics or traits.
How are traits passed…?
When organisms reproduce, traits are passed or inherited from one generation to the next.
These traits/characteristics are passed from your parents to you via genetic instructions; your
father’s genetic information was passed in sperm cells, your mother’s through her egg cells.
These genetic instructions are located in the nucleus of every cell, stored within chromosomes.
The chromosomes are made up of genes; a gene is a section of a chromosome, that codes for a
specific trait. Hence, Chromosomes are made of tightly wound strands of DNA and proteins.
Chromosomes exist as homologous pairs, the genes on them will be the same but the form of the
gene may be different they are called alleles. e.g. the gene that codes for colour in flower
petals is in the same position on the chromosome that came from the female and male of the
plant. However, one gene codes for purple flowers while the other codes for white flower. Same
characteristic-flower colour but different colours
Some genes are recessive and while some are dominant.
What are dominant genes?
The gene that expresses itself, or overshadows another gene is said to be the powerful and
dominant gene. There is complete dominance.
Geneticists use symbols (letters) to represent the different forms of a gene.
Dominant traits are represented by a capital letter
e.g. if Yellow seeds in peas are dominant…….. Y
Recessive genes (for the same trait) are represented by THE SAME letter but in lower case.
e.g.
Green seeds are recessive……….y
Dominant (yellow) = Y
Recessive (green) = y
Dominant genes hide recessive genes when both are inherited by an organism.
Yy = yellow seeds (yellow is dominant)
The genotype of this pea seed would be Yy and the phenotype of the pea is yellow
A Homozygous (purebred) organism has two of the same genes for a trait.
If the letter t is used represent the gene that codes for height, T = Tall and t = short
TT = (homozygous dominant) TALL
tt = (homozygous recessive) SHORT
A Heterozygous (hybrid) organism has two different genes for a trait.
Tt = (heterozygous dominant) TALL
Some genes are neither dominant nor recessive. These genes show incomplete dominance.
If an organism shows INCOMPLETE DOMINANCE for a trait, the hybrid will show a blend
of the two traits.
In humans, hair texture is such a trait.
Because neither curly nor straight hair is recessive, no lower case letters are used.
e.g. if the letter C represents the gene curly hair and S represents gene for straight hair.
The following genotypes and phenotypes are:
CC = curly hair
CS = wavy hair
SS = straight hair
Co-dominance
Some genes have alleles that are both expressed in the heterozygote individuals
e.g. sickle cell anemia
NN = normal cells
SS = sickle cell
NS = some of each
This person is sick
There are three phenotypes
Normal -Normal individuals have two normal haemoglobin alleles
Sickle cell anaemia- a severe form where all the red blood cells are affected.
Sickle cell anaemia patients have two sickle cell alleles in their genotype
Sickle cell trait- a mild condition where 50% of the red blood cells are affected.
Sickle cell trait individuals are heterozygotes, having one of each allele
Another example of co-dominance is found in ABO blood groups in man.
Type A, Type B, Type AB, Type O
Each blood group is controlled by a different allele. Blood Alleles- A, B, O (IA, IB, I)
A and B are co-dominant
A and B are dominant over O
AA = Type A
AO = type A
BB = Type B
BO = type B
AB = Type AB
OO = type O
LOOK UP ALBINISM,HAEMOPHILIA and
COLOUR BLINDNESS.
Genetic Diagrams
Crosses and Punnett Squares are genetic diagrams which allow us to show what offspring
genotypes and phenotypes could look like. They also allow us to calculate the probability that
certain genotypes and phenotypes will occur.
GENETIC ENGINEERING
Genetic engineering is also called genetic modification or GM. It is not the same as cloning.
Although cloning techniques are used in genetic engineering, the two things should not be
confused. The table shows some of the differences.
http://www.bbc.co.uk/schools/gcsebitesize/science/aqa_pre_2011/evolution/reproductionrev6.shtml
There are strong arguments for and against cloning and genetic engineering. It is possible to produce
genetically modified animals and plants. Sheep that produce human proteins for treating the symptoms of
cystic fibrosis - a disease which causes sufferers to produce abnormally thick and sticky mucus in their
lungs - have been produced, and even tobacco plants that glow in the dark when they need watering.
Some people are excited by the almost limitless possibilities of cloning and genetic engineering, while
some people believe the process is unethical and should be banned. Others are concerned about what
might happen in the future.
Advantages and Disadvantages of Genetic Engineering
Advantages
1. Diseases could be prevented.
2. New treatments/medicines developed e.g. Humalin,production of Insulin by recombinant DNA;
the gene for insulin in humans is inserted into bacteria. The bacteria can make insulin faster
than humans..Stem cell research
3. Production of food; genetically modified foods
RESEARCH DISADVANTAGES.
Vocabulary:
1. Genetics- the study of heredity; heredity is the way genes or traits are passed on from
one organism to the next
2. Homozygous - Pair of identical alleles for a character.
3. Heterozygous - Having 2 different alleles for a character.
4. Phenotype - An organism's outward appearance.
5. Genotype - An organism’s genetic makeup.
6. Punnett Square - A square used to show all the possible combinations of gametes.
7. Pedigree Test - A test in which you look at the offspring of parents to determine the
genotypes of the parents.
8. Variation -In genetics, variation is used to describe characteristics in one individual that
different from the others of the same kind.
9. Chromosomes- threadlike structures make up of DNA.
10. DNA- deoxyribonucleic acid is the hereditary material in humans and almost all other
organisms.