Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
392
Summary
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Homologous chromosomes are pairs of chromosomes in
a diploid cell that have the same structure and the same
genes, but not necessarily the same varieties of those
genes.
Meiosis consists of two divisions. The first division,
meiosis I, separates the homologous chromosomes, so
that each cell now has only one of each pair. The second
division, meiosis II, separates the chromatids of each
chromosome. Meiotic division therefore produces four
cells, each with one complete set of chromosomes.
Diploid organisms contain two copies of each gene in
each of their cells. In sexual reproduction, gametes
are formed containing one copy of each gene. Each
offspring receives two copies of each gene, one from
each of its parents.
The cells produced by meiosis are genetically different
from each other and from their parent cell. This results
from independent assortment of the chromosomes as
the bivalents line up on the equator during metaphase I,
and also from crossing over between the chromatids of
homologous chromosomes during prophase I.
Genetic variation also results from random fertilisation,
as gametes containing different varieties of genes fuse
together to form a zygote.
An organism’s genetic constitution is its genotype. The
observable expression of its genes is its phenotype.
Different varieties of a gene are called alleles. Alleles
may show dominance, codominance or recessiveness.
An organism possessing two identical alleles of a gene
is homozygous; an organism possessing two different
alleles of a gene is heterozygous. If a gene has several
different alleles, such as the gene for human blood
groups, these are known as multiple alleles. The
position of a gene on a particular chromosome is its
locus.
A gene found on the X chromosome but not on the Y
chromosome is known as a sex-linked gene. Genes that
are close together on a chromosome that is not a sex
chromosome are said to be autosomally linked.
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The genotype of an organism showing dominant
characteristics can be determined by looking at the
offspring produced when it is crossed with an organism
showing recessive characteristics. This is called a test
cross. Monohybrid crosses consider the inheritance of
one gene. Dihybrid crosses consider the inheritance
of two different genes. Different genes may interact to
affect the same phenotypic character. The chi-squared
(χ 2 ) test can be used to find out whether any differences
between expected results and observed results of
a genetic cross are due to chance, or whether the
difference is significant.
Mutation can be defined as an unpredictable change in
the base sequence in a DNA molecule (gene mutation)
or in the structure or number of chromosomes
(chromosome mutation). New alleles arise by gene
mutation. Gene mutations include base substitutions,
deletions or additions.
The Hb S (sickle cell) allele arose by base substitution.
The allele responsible for Huntington’s disease includes
a repeated triplet of nucleotides called a ‘stutter’.
Albinism and haemophilia show the effect on the
phenotype of missing or inactive polypeptides.
‘Structural’ genes code for the proteins required
by a cell for its structure or metabolism, whereas
‘regulatory’ genes control the expression of other
genes. A repressor protein can block the synthesis of a
‘repressible’ enzyme, by binding to the gene’s operator
site. An ‘inducible’ enzyme is synthesised only when its
substrate is present.
The lac operon provides an example of how a
prokaryote can alter the transcription of a cluster of
structural genes coding for enzymes concerned with
lactose uptake and metabolism, depending on whether
or not lactose is present.
Transcription factors in eukaryotes make sure that
genes are expressed in the correct cell, at the correct
time and to the correct extent. In plants, gibberellins
allow gene transcription by causing the breakdown
of DELLA proteins which inhibit the binding of
transcription factors.