Download Genes - Unit3and4Biology

Key Knowledge:
meiosis, gamete production,
variation: mutations, genotype, phenotype;
continuous, discontinuous variation
patterns of inheritance in sexually reproducing
organisms:
Chapter 9
Male Gamete Production
 Spermatogenesis
 Produced and stored in
testes
Human Sperm
Human Sperm
3 μm
5 μm
50 μm
Average ejaculate contains ~180 x 106 sperm (~60 x 106 sperm per mL)
Female Gamete Production
 Oogenesis – produced in ovaries
Human Ova
100 μm
Human Gametes
The human ovum has approximately 100,000 times the volume
of a single human sperm.
Ovum
Sperm
Inherited Variation
 Inherited variations are packaged into sperm and
ova (gametes).
 This occurs in the gonads.
 The gonads in males are the testes and the
gonads in females are the ovaries.
 The gametes produced by the gonads are always
haploid.
Variation
 Individuals differ in all sorts of ways, even when
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they are offspring of the same parents.
These differences are called variation.
Most characteristics, such as height, are determined
by several genes working together. They are also
influenced by environmental factors such as climate,
diet and lifestyle.
Variation among individuals is due to meiosis.
During meiosis Anaphase I, the chromosomes of
each pair migrate to opposite poles randomly
(Independent Assortment of Genes).
Variation
 Also during Metaphase I in meiosis, a
recombination of genetic material occurs.
 During this recombination process, a significant
event called crossing-over occurs between sister
chromatids.
 This allows genes to be mixed and hence allow for
greater genetic variation of offspring.
 The last type of variation is mutation. To be
inherited the mutation must occur in the gonads.
Somatic cell mutations are not inherited.
Independent Assortment of Genes
 Random orientations of non-matching chromosomes
lead to independent assortment of genes into
different gametes.
Crossing Over (Recombination)
Crossing-over involves exchange of DNA between
chromatids of sister chromosomes, allowing
genes to recombine.
Errors in Meiosis
 Sometimes during meiosis,
sister chromosomes fail to
separate and therefore you end
up with two copies of a
chromosome instead of the
usual one.
 This is called non-disjunction
and result in aneuploidy (missing
a chromosome) and the reciprocal
polyploidy (more than two
chromosomes) in gametes.
 A is the normal process, B and C
show non-disjunction
What is a Gene?
A unit of heredity made up of a sequence of DNA.
Alleles are alternate forms of a gene.
A whole set of genes in an organism is a genome.
Genes vary in size form about 100 to 2.5 million
base pairs.
 Humans are thought to have 20,000 – 30,000 genes.
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After sequencing of genomes began in 2001, it became
apparent that a more complex organism does not
necessarily have more genes.
Common name
Approx. size of genome (mbp)
Fruit fly
180
Human
3,100
Onion
18,000
Newt
84,000
Amoeba
670,000
The number of genes vary between species.
Common Name
Number of genes
Human
30,000
Mustard plant
27,000
Fruit fly
14,000
Baker’s yeast
6,000
Does size matter?
The genome of the
onion is six times larger
that the human
genome!
Where are genes located?
 Each gene has a particular position, called a locus
(plural=loci), on a specific chromosome.
 p or q denotes which arm of the chromosome. p denotes
the short arm, and q denotes the long arm.
Gene locus
The chromosomal locus of a gene might be written
"6p21.3"
Component
Explanation
6
The chromosome number
p
p=short arm or q=long arm
21.3
The numbers that follow the letter represent
the position on the arm: band 2, section 1, subband 3. The bands are visible under a
microscope when the chromosome is suitably
stained. Each of the bands is numbered,
beginning with 1 for the band nearest the
centromere.
Non-Nuclear Genes
 There are also a very small number of genes located on
the circular DNA of each mitochondria (mtDNA).
 There are many copies of the mtDNA in one
mitochondrion.
Non-Nuclear Genes
 In plants, there are also a very small number of
genes located on the circular pieces of DNA in each
chloroplasts (ctDNA).
Genotype and Phenotype
 Genotype - the genes of a trait
 Homozygous - both alleles of a gene are the same
 QQ stands for a homozygous dominant genotype
(uppercase letters)
 qq stands for a homozygous recessive genotype
(lowercase letters).
 Heterozygous means that each of the alleles are
different (e.g. Qq, IAIB).
 Phenotype refers to the physical or observable traits of
the individual.
 Genotype + Environment = Phenotype
Location of Genes
 Apart from the 22 pairs of autosomes, genes are
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also found on the sex chromosomes (X and Y).
Genes are mainly found on the X-chromosome
and very rarely on the Y-chromosome.
The Y-chromosome contains the SRY gene which
determines ‘maleness’ of an embryo.
Without the SRY gene, individuals develop as
phenotypically female.
XY phenotypic ‘females’ are possible if the SRY gene
or the receptors for the SRY gene product are faulty.
23-22
X-Chromosome Inactivation
 In a normal female, one of the two X-chromosomes
are randomly switched off (and turned into a Barr
Body) in each of the somatic cells (e.g. tortoiseshell
coloured cats)
 X inactivation occurs so that the female, with two Xchromosomes, does not have twice as many active
X-chromosome genes as the male, who only
possess a single copy of the X-chromosome.
 X inactivation in marsupials is always paternal (i.e.
the father’s X is always switched off)
Tortoiseshell Coloured Cats
Only female cats can
be tortoiseshell. The
gene for orange coat
colour is on the X
chromosome. In a
heterozygous cat (Oo)
one of the two X
chromosomes are
randomly inactivated
giving the ‘blotchy’
phenotype.
Expression of Alleles
 Complete dominance – this occurs when the
phenotype of the heterozygous individual displays
the dominant allele (e.g. the heterozygous Qq will
exhibit a widows peak ).
 Incomplete dominance – occurs when there is a
blending of the two alleles in the heterozygous
individual (e.g. an allele for red flowers and an allele
for white flowers results in pink flowers)
 Codominance – this occurs when the heterozygous
individual expresses both alleles equally.
Genotypes and Environment
 Whilst we have found that an individual’s genotype
is largely responsible for their phenotype, it is still
important to keep in mind that the environment
plays a major role too.
 For example, individuals who have the condition
PKU, can live relatively normal lives on a special diet.
 Some flowers (e.g. hydrangea) can change colour
depending on the pH (acidity) of soil.
Genetic Variation
 Traits can be expressed with continuous variation
(e.g. height) or discontinuous (e.g. ABO blood
groups)
Discontinuous Variation
 Discontinuous variation (e.g. Rhesus blood type) due
to single gene affect.
Phenotype
Genotype
Rh+
DD and Dd
Rhdd
Continuous Variation
 Continuous variation usually due to polygene traits
(i.e. more than one gene involved)