Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Genetics Revision
Term
Alleles
Genetics
Gene
Genotype
Homozygous
Heterozygous
Phenotype
Dominant
Recessive
Locus (Loci)
Homologous
Chromosomes
Cross
Monohybrid Cross
Dihybrid Cross

Definition
Alternative forms of the same gene
The study of mechanisms which an organism inherits
characteristics from its parents
A segment of DNA on a chromosome that codes for one
polypeptide and controls one characteristic
The combination of alleles found in an individual
The genotype of an organism if both alleles of a gene are the
same e
...
RR and rr
The genotype of an organism if both alleles of a gene are
dissimilar e
...
Rr
The appearance of an organism, determined by the genotype
(and the environment)
The allele, which, in a monohybrid cross, always produces an
effect on the appearance of an organism where present
The allele which produces an effect only when present as an
identical pair i
...
when no dominant allele is present
The specific position on a chromosome where a gene is located
Chromosomes that have the same genes along their length and
pair up during meiosis i
...
a matching pair
The deliberate mating of two organisms in genetic analysis
A breeding experiment involving the inheritance of a single
gene (one pair of alleles)
A breeding experiment involving the inheritance of two
different genes (two pairs of alleles)

Gregor Mendel (1822- 1884) was the first to examine relationships between one generation and the
next in a scientific manner
...
Mendel worked on peas using 4 phenotypes (characteristics);
Dominant
Recessive
Tall
Dwarf
Purple Flowers
White Flowers
Yellow Seed
Green Seed
Round Seed
Wrinkled Seed
Mendel realised that when he crossed a dominant parent phenotype with a recessive parent the F1
generation all had the dominant phenotype
...
When a gamete is produced by meiosis, the alleles are
segregated so that only one allele is included within the gamete
...
Only one member of a pair of factors can be represented in a single gamete
...

Parental Phenotypes Tall
Dwarf
Parental Genotypes TT
Tt
Gametes
T or T
t or t
Fertilisation
Gametes
t
t
T
Tt
Tt
T
Tt
Tt
F1 Genotypes
All Tt
F1 Phenotypes
100% Tall
F1 Phenotypes
F1 Genotypes
Gametes
Fertilisation

F2 Genotypes
F2 Phenotypes

Tall
Tall
Tt
Tt
T or t
T or t
Gametes
T
t
T
TT
Tt
t
Tt
tt
TT, Tt, Tt, tt
Genotype Ratio 1:2:1
3 Tall: 1 Dwarf
75% Tall: 25% Dwarf

This type of inheritance is complete inheritance as the F1 plants look identical to the dominant
parent
...
g
...


Test Crosses
A test cross can be used to find whether an organism showing the dominant phenotype is
homozygous or heterozygous
...
If the dominant unknown organism is homozygous
then the offspring will be heterozygous so all show dominant phenotypes
...
Therefore half of the
offspring will be heterozygous and half homozygous recessive
...
The ratio of long- winged and vestigial- winged flies show whether the long- winged fly was
homozygous or heterozygous
...


Monohybrid
 Homozygous Dominant x Homozygous Recessive
...
:2 Dominant Heterozygous :1 Recessive
Homozygous
Phenotype Ratio; 3 Dominant: 1 Recessive
 Heterozygote x Homozygous Recessive
Genotype Ratio; 1 Heterozygous :1 Recessive Homozygous
Phenotype Ratio; 1 Dominant: 1 Recessive
Dihybrid
 Double homozygous Recessive x Double Homozygous Recessive
100% Double Heterozygous
 Double Heterozygote x Double Heterozygote
9 Double Dominant Phenotype: 3 Dominant and Recessive: 3 Recessive and Dominant: 1
Double Recessive Phenotype
 Heterozygote (Double) x Double Homozygous Recessive
1:1:1:1

Probabilities
Chi Squared Test
Looks at whether differences between expected and actual results have occurred by chance or not
and identifies whether differences are significant
Null Hypothesis
Null hypothesis (Negative hypothesis) assumes differences have occurred by chance
...

To find the probabilities we must;
 Assume genes are on different chromosomes so alleles do not interact with each other
 Probabilities do not apply for Co-Dominance and sex linkage- not independent

Cystic Fibrosis
 CF is most common genetic disease inherited in Britain
 Affects glands secreting bodily fluids- damaging many organs including lungs, pancreas,
digestive tract and reproductive system
 Complex disease causes thick, sticky mucus to be produced which clogs bronchial tubes and
prevents body’s natural enzymes from digesting food- suffer constant chest infections and
malnutrition
 Improved treatments allow longer life expectancy
 1 in 25 is carrier
To Calculate Chi Squared (X2)
1) Hypothesis
2) Expected frequency
3) Observed and expected in table
4) Find (O-E)2
E
5) Add up values in X2
6) Find degree of freedom (n-1) and significance level (0
...
Thus both will
produce an effect on the phenotype, such alleles are called co dominant alleles
...

Some alleles are carried only on the X chromosome and so are described as sex- linked
...

Sex linked diseases are due to chromosomes carried on the X- chromosome
...
Females must have two affected alleles for expression in phenotype
A sex- linked condition is never passed from father to son as the Y chromosome will be
contributed, not the X chromosome with the recessive allele
...
Heterozygous
females for S
...
condition are carriers
...

 May effect a single gene or a whole chromosome
 Most mutations occur in the somatic body cells
 Mutations which occur during the formation of gametes can be inherited
 Mutations are spontaneous random events which may provide a source of material for
natural selection pressures and therefore evolution
 Mutation rates are usually very small therefore mutation has less impact on evolution than
other sources of variation




Generally organisms with shorter life cycles and more frequent meiosis show a greater rate
of mutation
Rate of mutations occurring can be increased by ionising radiation and mutagenic chemicals

Mutagens and Mutation Rate
`Mutations occur naturally but it has been found that the mutation rate is increased if an
organism are exposed to mutagens
...

Mutations Can Occur in Two Ways;
 DNA is not copied properly before cell division- Mistakes made during the copying process
causes the chromosomes to be faulty
...
However this can be a problem if an
important gene is affected
 Chromosomes are damaged and break
...
This can lead to large
changes in the structure of the DNA so may affect a large number of genes
...

Other Sources of Variation;
 Crossing over of chiasmata
 Random assortment
 Fertilisation (Random genes meet)

A change in the number of chromosomes can occur in meiosis
...

Down syndrome is caused by the chromosome 21 being trisomic caused by an error in the ovum
production related to the age of the mother

---