Genetics
Introduction
oSimilarities & differences exist between parents and their
offspring.
oGenetics – study of heredity (inheritance) and variation in
living organisms.
The History of Heredity & Genetics
oGregor Mendel – father of genetics.
oStudied inheritance in pea plants by looking at seven
contrasting characteristics in the plant.
DO NOT LEARN
Mendel experimented with pea plants because:
1. The structure of the flower was simple and
contained both male and female parts.
2. Clear, observable contrasting characteristics
that were easy to record.
3. Quick reproductive cycle.
Mendel’s experiment was scientific because:
1. Carefully selected pea plants that were true
breeding/pure breeding to start with.
2. Studied only 1 characteristic at a time, so results could
be recorded independently.
3. Chose contrasting characteristics that were observable.
4. Chose plants that grew quickly so results were seen in 1
year.
5. Controlled pollination by hand, so self pollination was
prevented.
6. Counted all offspring that grew & recorded all
observable information in a journal.
7. Worked with large sample sizes over many generations.
Terminology
o True breeding / Pure breeding – identical individuals
(parents) that produce offspring similar to them for
the characteristics being studied. Offspring have the
same phenotype as the parents.
o Cross breeding– producing offspring through mating
two purebred individuals that usually come from
different breeds, varieties or even different species.
o Cross breeding results hybrid offspring, e.g., a mule.
o Self Pollination – transfer of pollen from anther to
stigma of the same flower.
o Cross Pollination - transfer of pollen from anther of
one flower to stigma of the same species of flower
on a different plant.
o Phenotype – the observable characteristic (physical
appearance) of an organism.
o Genotype - genetic make-up of an organism.
Represented by 2 letters; each letter represents 1 allele
in the gene pair.
o Genome – complete set of genes of a particular
organism.
o Homozygous – having 2 identical alleles for a
characteristic e.g., TT or tt.
o Heterozygous – having 2 different alleles for a
characteristic e.g., Tt.
o Locus – position of genes / alleles on a chromosome.
o Monohybrid Inheritance - study of the inheritance of 1
pair of contrasting characteristic at a time e.g., tall &
short.
o Complete dominance – having 1 dominant
characteristic and 1 recessive characteristic.
Mendel’s Conclusions
• A characteristic in an organism is determined by 2
factors called genes.
• One gene comes from the mother and one gene comes
from the father. This forms a gene pair on homologous
chromosome pairs.
• A particular gene may occur in two different forms that
affect the same characteristic in different ways.
• Alternative forms of the same genes are called alleles.
• One allele of the gene pair can mask another allele –
this is the dominant allele.
• The allele that is masked and is not visibly expressed in
the organism – is the recessive allele.
Going back to Mendel’s Experiments:
1. No medium –sized plants appeared in F1 or F2
generation.
2. All F1 (1st filial generation) plants were tall.
3. Short plants only appeared in F2 generation
(2nd filial generation).
4. Ratio of tall to short in F2 generation was 3:1.
5. The visible characteristic in F1 – Dominant
i.e., tall. Use a capital letter e.g., T.
6. The hidden characteristic in F1 – Recessive
i.e., short. Use a small letter e.g., t.
Mendel’s Law of Dominance
If two alleles are different, only the dominant
allele will be expressed
Mendel’s Law of Segregation
During meiosis, homologous chromosome pairs
separate from each other, therefore each gamete
receives only one allele from the gene pair.
Mendel’s Results
Can be shown by using:
1. Punnet Square / Genetic Diagrams
Polygenes
o In 1 cell there are 2 alleles (1 pair) for each gene
pair. 1 paternal allele & 1 maternal allele. This is
always the case for simple traits (discontinuous
characteristics).
o Polygenes - certain continuous characteristics in
people e.g., height, skin colour & eye colour are
controlled by more than 1 gene pair at different
loci i.e., more than 1 pair of alleles. This is always
the case for the more complex traits.
o E.g., The genes that control skin colour are found
at many loci.
Sex-Linked Genes
o Recessive genes on X chromosomes called sexlinked genes. They carry genetic disorders.
o Also known as X-Linked genes.
o Sex-linked disorders are usually recessive.
o E.g., of sex-linked disorders:
1. Colour Blindness
2. Haemophilia
3. Muscular Dystrophy
Haemophilia
o Blood clotting disorder i.e., blood does not clot
because a mutation occurred in the making of the
protein that is responsible for blood clotting.
o Normally dominant alleles express the
phenotype, but in certain sex-linked disorders
e.g., haemophilia, the recessive allele expresses
the disorder i.e., small letter of alphabet.
o Haemophilia is caused by a recessive gene on the
X chromosome.
Therefore:
1. Homozygous Dominant – normal (2 capital
letters)
2. Heterozygous – carriers i.e., do not show any
symptoms, but can pass disorder to their
children (1 capital & 1 small letter). NB: only
females can be carriers for sex linked
disorders.
3. Homozygous Recessive – disorder (2 small
letters)
A Sex-Linked Disorder - Haemophilia
Genotypes for Haemophilia
o XHY – Male without haemophilia
o XhY – Male with haemophilia
o XHXH – Female without haemophilia
o XHXh – Female carrier of haemophilia
o XhXh - Female with haemophilia
Examples of Genetic Diagrams for
Haemophilia Crosses
1. Parents:
Phenotype : Haemophiliac
×
father
Genotype:
X hY
×
Gametes:
Xh
Y ×
F1:
25% carrier female
XH
25% haemophiliac female
25% normal male
Xh
25% Haemophiliac male
haemophiliac
carrier mother
XHXh
XH
Xh
Xh
Y
XHXh
XHY
XhXh
XhY
2. Parents:
Phenotype :
Normal
× haemophiliac
father
carrier mother
X HY
×
X HX h
XH
Y ×
XH
Xh
Genotype:
Gametes:
F1:
25% normal female
25% carrier female
25% normal male
25% Haemophiliac male
XH
Y
XH
XHXH
XHY
Xh
XHXh
XhY
Mutations
o Unexpected changes in structure of genes/ DNA.
o Can be harmful (lethal mutations), harmless (neutral
mutations), useful (beneficial mutations).
o Therefore, genotypes and phenotypes will be
altered.
o Mutations can also lead to variation.
o Mutations in somatic cells (body cells) – dangerous
e.g., cancer. Person could die, but the mutation will
not necessarily be passed on.
o Mutations that occur in sex cells can be passed on to
next generation.
Disorders Caused by Point Mutations
1. Sickle Cell Anaemia (Read Over – Do Not Learn)
o Gene that codes for protein (haemoglobin) is mutated.
o 1 amino acid in the haemoglobin protein is altered.
o Normal haemoglobin changed into abnormal
haemoglobin.
o Red blood cells become sickle shaped. These cells carry
less oxygen – person becomes anaemic.
o Person with heterozygous condition – has enough
haemoglobin, therefore does not suffer from sickle cell.
o Advantageous to be heterozygous for this condition if living
in malaria area.
o Malaria parasite cannot survive in sickle cells. Therefore,
these people are resistant to malaria.
2. Albinism (Read Over – Do Not Learn)
o Melanin pigment is a protein.
o In albinos, the recessive gene that codes for
melanin is mutated.
o Is an autosomal recessive disorder.
o Body cannot make enough melanin for eye,
hair or skin colour.
o Heterozygous individuals will be carriers for
albinism.
Chromosome Mutations
o Changes in chromosome structure or number.
1. Changes in Chromosome Structure
During crossing over in meiosis mistakes can
happen. Pieces of chromatids get:
o Deleted
o Duplicated
o Inverted
The Human Genome Project (HGP)
• DNA Sequencing – finding the order of the complete set of
nucleotides along a DNA strand.
• The Human Genome Project was set up to map / work out
the sequence of bases (nucleotides) in all the genes of
humans i.e. identify every single gene that makes up DNA.
• It was set up by James Watson and other scientists.
• Helps with research in medicine, biotechnology,
agriculture and the environment.
• Helps to identify genes that cause inherited diseases and
therefore make it easier to treat these diseases.
• DNA Sequencing provides evidence of relationships
between organisms, e.g. the genome of the chimpanzee is
more than 98% identical to the human genome. This can
help with the study of evolution.
Pedigree Charts (Family Trees)
o Used to show the inheritance of characteristics in
families over many generations.
o Can also be used to determine the possibilities of
a couple transferring a genetic disorder to the
next generation.
o This will then determine if genetic testing and
genetic counselling is required by the couple.
On the chart:
o Circles = females
o Squares = males
o Shaded = has disorder
o Not shaded = no disorder