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OCR A LEVEL BIOLOGY PAPER 2
NEWLY FORMATTED DOCUMENT
Homozygous
When an organism carries
two copies of the same
alleles.
Heterozygous
When an organism has two
different alleles of the
same gene.
Genotype
Description of an
organism's alleles.
Characteristics of an Phenotype
organism as a result of
the expression of its
genotype and the
environment.
Gene
Section of a chromosome
that codes for a
polypeptide.
Allele
Alternative version of the
same gene.
Gene locus
Location of a gene on a
chromosome.
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Dominant allele
Allele that is always
expressed in the
phenotype.
Allele that is only Recessive allele
expressed in the
phenotype when there
are two of them i.e. in
a
homozygote.
Codominant allele
Both alleles are expressed
in the phenotype.
Multiple alleles
More than 2 alleles for a
particular gene.
Sex linkage
Characteristic or trait
controlled by a gene
found on the sex
chromosomes.
Pair of chromosomes that Homologous chromosomes
carry genes for the same
characteristics, at the same
gene loci.
Autosome
Any chromosome that is
NOT a sex chromosome.
Genes coding for Autosomal linkage
different
characteristics, found
on the
same non-sex
chromosome, are said to
be linked.
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During metaphase I of Independent assortment
meiosis, the arrangement
of one pair of
homologous
chromosomes on the
equator of the spindle
is
independent of the
arrangement of any
other pair of
chromosomes. A key
event that produces
GENETIC VARIATION in
gametes.
Where chromatids twist Crossing over
around each other and
exchange genetic
material. Happens
during prophase I of
meiosis
which increases the
amount of GENETIC
VARIATION in
gametes by producing new
combination of alleles.
OCR A LEVEL BIOLOGY PAPER 2
NEWLY FORMATTED DOCUMENT
Homozygous
When an organism carries
two copies of the same
alleles.
Heterozygous
When an organism has two
different alleles of the
same gene.
Genotype
Description of an
organism's alleles.
Characteristics of an Phenotype
organism as a result of
the expression of its
genotype and the
environment.
Gene
Section of a chromosome
that codes for a
polypeptide.
Allele
Alternative version of the
same gene.
Gene locus
Location of a gene on a
chromosome.
,2|Page
Dominant allele
Allele that is always
expressed in the
phenotype.
Allele that is only Recessive allele
expressed in the
phenotype when there
are two of them i.e. in
a
homozygote.
Codominant allele
Both alleles are expressed
in the phenotype.
Multiple alleles
More than 2 alleles for a
particular gene.
Sex linkage
Characteristic or trait
controlled by a gene
found on the sex
chromosomes.
Pair of chromosomes that Homologous chromosomes
carry genes for the same
characteristics, at the same
gene loci.
Autosome
Any chromosome that is
NOT a sex chromosome.
Genes coding for Autosomal linkage
different
characteristics, found
on the
same non-sex
chromosome, are said to
be linked.
, 3|Page
During metaphase I of Independent assortment
meiosis, the arrangement
of one pair of
homologous
chromosomes on the
equator of the spindle
is
independent of the
arrangement of any
other pair of
chromosomes. A key
event that produces
GENETIC VARIATION in
gametes.
Where chromatids twist Crossing over
around each other and
exchange genetic
material. Happens
during prophase I of
meiosis
which increases the
amount of GENETIC
VARIATION in
gametes by producing new
combination of alleles.
