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Revision Notes
Genetics and Evolution:
Basic Genetics
Molecular Genetics
Biodiversity and Evolution
Kendy
Tsang Hin Fung
2
Mo Man Ying
Contents
Chapter
8
Basic Genetics
9
Molecular Genetics
18
Biodiversity and Evolution
37
10
1
1
8
Basic Genetics
In this chapter, you should learn:
Basic genetics:
1.
Mendel’s laws of inheritance
2.
Inheritance in humans (Multiple alleles ABO blood groups, sex linkage and sex determination)
3.
Pedigree analysis
4.
Variations in characteristics (Continuous variation, discontinuous variation and the causes of variation)
A. Revision Notes
1.
Basic concepts in genetics:
„
Nearly all sexually reproducing organisms have paired sets of chromosomes, one set from each parent.
„
The equivalent chromosomes that form pairs during cell division are called homologous chromosomes.
They contain equivalent sets of genes for the same biological features such as eye colour.
„
A gene is the basic unit of inheritance.
„
Locus (plural: loci) is the position where a gene located on chromosome.
„
A gene may have more than one form.
„
Genotype is the genetic make-up of an organism.
„
Phenotype is the characteristics or external appearance of an organism.
between genotype and environment.
It is a segment of DNA which controls hereditary character.
Allele is one of the alternative forms of a gene on a locus.
It is the result of the interaction
3
4
5
2
3
gene 1
a
A
allele a
4
allele A
gene 2
B
B
gene 3
c
c
1
2
1
5
locus is the position where a gene
located on chromosome
homologous chromosomes
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8
Basic Genetics
Homozygous and Heterozygous:
„
„
Homozygous condition is a condition in which both homologous chromosomes possess identical copies of
allele of a gene. The homozygous organism is called homozygote.
Heterozygous condition is a condition in which both homologous chromosomes possess different copies of
allele of a gene. The heterozygous organism is called heterozygote.
a
A
B
B
c
c
homologous chromosomes
Allele expression:
„
The alleles which can fully express themselves in the phenotype even though different alleles are present
(heterozygous conditions) are said to be dominant.
„
The alleles which can only fully express themselves in the phenotype in the absence of different alleles or in
the presence of identical alleles (homozygous conditions) are said to be recessive.
„
Incomplete dominance occurs when neither one of the alleles in heterozygous condition is dominant. The
phenotype of the heterozygote is the intermediate between two homozygotes. For example, a plant with
red flowers and a plant with white flowers may give an offspring with pink flowers.
„
Codominance occurs when both alleles in heterozygous condition can fully express themselves in the
phenotype. Human ABO blood group system is an example.
„
Some genes may have more than two alleles or alternative forms in the population as a whole.
alleles is called multiple alleles. Human ABO blood group system is an example.
„
Sex linkage is the phenotypic expression of allele related to the sex of organism. For example, some of
the genes on X chromosome don’t have corresponding alleles on Y chromosome. Therefore, a single
allele can be expressed in phenotype.
A series of
2. Mendel’s laws of inheritance:
Monohybrid inheritance:
„
„
Inheritance of one well defined characteristic was studied in the experiments.
Based on the results of the study of monohybrid inheritance, Mendel’s first law of inheritance was
formulated.
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8
Basic Genetics
Usually, capital form is used to represent
dominant character while small form is
used to represent recessive character.
Punnett square can also
be used to show the
genotype of offspring
75% of the F2 generation are tall while
15% the F2 generation are dwarf,
phenotypic ratio of offspring is 3:1.
Mendel’s first law (Law of segregation):
„
„
Alleles exist in pairs. The characteristics of an organism
are determined by a pair of alleles of genes.
Alleles separate during gamete formation and enter different
gametes. Each gamete possesses only one of the alleles of
each gene.
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Basic Genetics
Dihybrid inheritance:
„
„
Inheritance of two pairs of contrasting characteristics was studied in the experiments.
Based on the results of the study of dihybrid inheritance, Mendel’s second law of inheritance was
formulated.
R = dominant round
r = recessive wrinkled
Y = dominant yellow
y = recessive green
parents
Law of segregation
gametes
Only one type of
gamete is produced
from each parent.
F1 generation
All F1 generation
must be round and
yellow (RrYy).
F1 gametes
Each individual of F1
generation can produce
four different kinds of
gametes.
(Independent assortment)
F2 generation:
F1 generation self-fertilized: RrYy x RrYy
F1 generation
is
self-fertilized
to give F2
generation
Gametes
RY
RY
RRYY
RRYy
RrYY
RrYy
Round, yellow
Round, yellow
Round, yellow
Round, yellow
Ry
rY
ry
Ry
rY
ry
RRYy
RRyy
RrYy
Rryy
Round, yellow
Round, green
Round, yellow
Round, green
RrYY
RrYy
rrYY
rrYy
Round, yellow
Round, yellow
Wrinkled, yellow
Wrinkled, yellow
RrYy
Rryy
rrYy
rryy
Round, yellow
Round, green
Wrinkled, yellow
Wrinkled, green
4
16 possible
offsprings
produced
from different
combinations
of F1 gametes