9 Genes, chromosomes and
patterns of inheritance
The genetic lottery
• Genetic instructions are packaged in gametes (either eggs
or sperm cells) for transmission from parents to offspring.
• Each parent contributes a set of genetic instructions to its
offspring so that the offspring inherits a double set.
• Genetic instructions consist of genes that are made of DNA.
• The genetic instructions comprise the genotype of an
organism.
• The phenotype is the physical, biochemical or physiological
expression of the genotype.
• In animals, egg and sperm cells are formed in specialised
organs called gonads — ovaries in females and testes in
males.
Chromosomes: gene carriers
Figure 9.11 Various diagrammatic
representations of chromosomes
(a) in the double-stranded
condition,
and (b) in the single-stranded
condition. The double-stranded
condition is a temporary state
formed only during part of a cell
multiplication cycle (mitosis) or
during gamete formation (meiosis).
• Chromosomes are organised collections of genes.
• Each species has a characteristic number of
chromosomes, known as the diploid number,
typically present in body (somatic) cells.
• Human chromosomes in body cells exist in pairs,
normally 23 pairs.
• The 23 pairs of human chromosomes include 22
pairs of autosomes present in both sexes, and
one pair of sex chromosomes, XX in the female
and XY in the male.
• Additional or missing entire chromosomes or
parts of chromosomes typically produce
deleterious effects.
• Various mechanisms of sex determination exist in
Kingdom Animalia.
Meiosis: diploid to haploid
Meiosis: source of variability in
offspring
• During meiosis, gametes with a haploid set of
chromosomes are formed.
• Normal human eggs contain 22 different autosomes and
one X chromosome.
• Normal human sperm contains 22 different autosomes and
either one X chromosome or one Y chromosome.
• Normally only one member of each pair of chromosomes
goes into a gamete.
• Variation between gametes of one person arises from
exchanges during crossing over and from random disjoining
of chromosomes.
• Non-disjunction leads to gametes with abnormal numbers
of chromosomes.
Genes: inherited instructions
• An estimated 20 000 to 25 000 human genes exist.
• Genes are identified in terms of the functions they
control.
• A system of symbols to denote genes exists.
• Excluding mitochondrial genes, genes are located on
chromosomes in the nucleus of eukaryotic cells.
• Each gene occupies a specifi c position or locus on a
chromosome.
• Genes located on the same chromosome form a
linkage group.
• A very small number of genes exist in mitochondria.
Alleles: particular forms of a gene
• One gene can exist in a number of different forms,
called alleles.
• Each different allele is identified by its specific
phenotypic action.
• A person is said to be homozygous when she/he has
two identical alleles and heterozygous when she/he
has two different alleles of one gene.
• Males are hemizygous for both X-linked and Y-linked
genes.
• When an allele is expressed in the phenotype of a
heterozygote organism, the trait controlled by
that allele is dominant.
• The trait controlled by the ‘hidden’ allele in a
heterozygote is recessive.
• When two different alleles are both expressed in
the phenotype of a heterozygous organism, they
are said to show co-dominance.
• The phenotype of an organism can be due to an
interaction between the genotype and the
environment.
Rules of the genetic gamemonohybrid cross
Rules of the genetic game-dihybrid
cross
Figure 9.45 Punnett square
showing the cross of Tracey and
John. Note that, in the overall
summary statement below the
Punnett square, the ‘dash’ symbol
(–) means that the second allele in
the genotype can be either of the
two possible alleles. So, for
example,
A– denotes both AA and Aa.
Test crosses
• Another important type of cross that can be
either monohybrid or dihybrid is a test cross. A
test cross involves one parent that is homozygous
recessive, such as aa or aabb, and is used for the
following purposes:
• to identify if an organism showing a dominant
trait is homozygous or heterozygous
• to establish linkage relationships between genes.
• In reality, today, it is possible to make these
decisions using molecular techniques.
What about linked genes?
Because of equal numbers of each kind of offspring, we
can conclude that the genes
are not linked but are assorting independently
• The chance of various kinds of offspring from
a cross can be identified from knowledge of
the kinds and proportions of gametes that
parents can produce.
• The probability of two independent events is
the product of their separate probabilities.
• When genes are linked, parental gametes will
be produced and, in addition, recombinant
gametes will arise from crossing over.
• The closer two linked genes are, the less the
chance that crossing over will separate them
during meiosis.
• The result of a test cross involving two genes
can identify whether the genes are linked or
not.
• When genes are closely linked and separated
by a known map distance, the results of a test
cross can be predicted.
Family pedigrees: drawing
genetic portraits
Autosomal dominant pattern
Autosomal recessive pattern
X-linked dominant pattern
X-linked recessive pattern
Summary
• A pedigree uses symbols to show the pattern
of appearance of an inherited trait across
several generations.
• Features of the pattern of inheritance of a
trait may allow conclusions to be drawn
regarding its mode of inheritance.
• Common modes of inheritance are autosomal
dominant, autosomal recessive, X-linked
dominant or X-linked recessive.