Transcript Genetics

GENETIC INHERITANCE
Lesson Objectives
At the end of this lesson you should be able to
1. Give a definition for a gamete
2. Understand gamete formation
3. Give the function of gamete in sexual reproduction
4. Define fertilisation
5. Define allele
6. Differentiate between the terms homozygous and
heterozygous
Lesson Objectives (cont.)
At the end of this lesson you should be able to
6. Differentiate between genotype and phenotype
7. Differentiate between dominant and recessive
8. Show the inheritance to the F1 generation in a cross
involving:
• Homozygous parents
• Heterozygous parents
• Sex determination
• Show the genotypes of parents, gametes and offspring
Sexual Reproduction
• Involves two parents
• Each parent makes reproductive cells
- called gametes
Outline of Fertilisation
• Gametes join together by fertilisation
• Form a diploid zygote
• This develops into an embryo
• Eventually into a new individual
• New individual resembles both parents –
but is not identical to either
What are Gametes?
• Reproductive Cells
• Formed by meiosis
• Contain single sets of chromosomes
- haploid
• Capable of fusion to form zygote
- diploid
• Zygote contains genetic information of both
gametes
Genetics
• The study of heredity.
• Gregor Mendel (1860’s) discovered the fundamental
principles of genetics by breeding garden peas.
Allele - an alternative form of a gene e.g.
gene for height in pea plants has two forms
-tall (T) or small (t).
• Dominant gene: A gene that is expressed in
the heterozygous condition
• e.g. tall Tt.
• Recessive gene: A gene that is expressed only
in the homozygous condition
• e.g. small tt.
• Homozygous: The alleles of a gene pair are
the same e.g. TT, tt - purebreeding.
•
• Heterozygous: The alleles of a gene pair are
different (Tt) – ‘carriers’.
Gregor Mendel
• Father of genetics
• 1857 – began collecting pure
lines of peas
• Chose self-fertilizing peas so
that all offspring look exactly
like their parent
• Mendel chose 7 traits for study
Pea Traits used by Mendel
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Mendel’s Peas
1.Pure lines with easily identifiable traits were
available
2.Peas are self-fertilizing with a flower structure that
minimizes accidental pollination
3.Peas can be artificially fertilized which allows
specific crosses to be made
4.Peas have a short growth period
5.Peas produce large numbers of offspring
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(1) He crossed pure plants with alternative
phenotypes for a single trait
(2) He recorded how many offspring were of
each type
– 1st generation results (F1 generation)
(3) He allowed these offspring to self-fertilize
(4) He again recorded the nature of the offspring –
F2 generation
(5) He did a mathematical analysis
(6) He deduced several principles
(7) He published paper in good scientific journal –
1866
LAW OF SEGREGATION (MENDEL’S FIRST LAW):
• A characteristic is controlled by 2 genes which
separate at gamete formation.
LAW OF INDEPENDENT ASSORTMENT
(MENDEL’S SECOND LAW):
• When gametes are formed, each member of a
pair of alleles may combine randomly with
either of another pair.
Phenotype
• The physical appearance of the organism
• Examples:
1.
tall pea plant
2.
dwarf pea plant
Genotype
The set of genes an individual possesses
Example:
1. tall pea plant
TT = tall (homozygous dominant)
2. dwarf pea plant
tt = dwarf (homozygous recessive)
3. tall pea plant
Tt = tall (heterozygous)
GENETIC CROSS TERMS
• Progeny: offspring produced
• Haploid: having a single set of unpaired chromosomes.
• Diploid: having chromosomes in pairs.
• Mitosis: Produces two identical daughter cells and maintains
chromsome number during cell division of somatic cells.
• Meiosis: Produces four daughter cells and reduces the
chromsome number by half during cell division.
• Gamete: a haploid sex cell.
• A carrier of a trait has one copy of the recessive allele and
therefore shows the dominant allele.
Monohybrid Cross
• a genetic cross in which only one characteristic is
being examined e.g. TT x Tt
Example of Results – Seed Coat
(Smooth seeds vs Wrinkled seeds)
Parents:
• one parent had smooth seeds
• the other wrinkled seeds
Result: F1 all smooth
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Homologous Chromosomes
eye color locus
B = brown eyes
eye color locus
b = blue eyes
This person would
have brown eyes (Bb)
Paternal
Maternal
Meiosis - eye color
B
sperm
B
B
Bb
haploid (n)
b
diploid (2n)
b
b
meiosis I
meiosis II
Monohybrid Cross
• Example: Cross between two heterozygotes
for brown eyes (Bb)
BB = brown eyes
Bb = brown eyes
bb = blue eyes
B
b
B
Bb x Bb
b
female gametes
male
gametes
Monohybrid Cross
B
b
B
BB
Bb
b
Bb
bb
Bb x Bb
1/4 = BB - brown eyed
1/2 = Bb - brown eyed
1/4 = bb - blue eyed
1:2:1 genotype
3:1 phenotype
Punnett square
A Punnett square is used to show the possible
combinations of gametes.
Breed the P generation
• tall (TT) vs. dwarf (tt) pea plants
T
t
t
T
tall (TT) vs. dwarf (tt) pea plants
T
T
t
Tt
Tt
produces the
F1 generation
t
Tt
Tt
All Tt = tall
(heterozygous tall)
Breed the F1 generation
• tall (Tt) vs. tall (Tt) pea plants
T
T
t
t
tall (Tt) vs. tall (Tt) pea plants
T
T
t
TT
Tt
t
Tt
tt
produces the
F2 generation
1/4 (25%) = TT
1/2 (50%) = Tt
1/4 (25%) = tt
1:2:1 genotype
3:1 phenotype
Dihybrid
• A genetic cross where two contrasting traits
are investigated
• Eg: TtYy or TTYY
• Example: Seeds may be yellow or green in
colour. They may also be round or wrinkled
in shape. Let;
•
•
•
•
Y = yellow (dominant)
y = green (recessive)
R = Round (dominant)
r - wrinkled (recessive)
DIHYBRID CROSSES SHOW MENDELS 2ND LAW
• Law of independent assortment (Mendel’s
second law): when gametes are formed, each
member of a pair of alleles may combine
randomly with either of another pair (if
genes are not linked)
• YyRr 
YR
•
meiosis
or
Yr
or
yR
or
yr.
• Inheritance of many human characteristics are in
keep with Mendel’s findings e.g. eye colour,
albinism, cystic fibrosis, blood groups.
• The allele for tongue rolling (R) is dominant to
the allele for non tongue rolling (r). Also the
allele for brown hair (B) is dominant to red
hair (b). Neither of these characteristics is sex
linked.
• Using the punnet square determine the
possible F1 generation genotypes of a cross
between two heterozygous parents
(heterozygous for both characteristics).
• In the fruit fly, Drosophila, the allele for grey body
(G) is dominant to the allele for ebony body (g) and
the allele for long wings (L) is dominant to the allele
for vestigial wings (l). These two pairs of alleles are
located on different chromosome pairs.
• (i) Determine all the possible genotypes and
phenotypes of the progeny of the following cross:
grey body, long wings (heterozygous for both) X
ebony body, vestigial wings.
grey body, long wings (heterozygous for both) X ebony body, vestigial wings.
Incomplete Dominance
• Niether genes are dominant over the other and form
an intermediate phenotype in the heterozygous
offspring.
• Example: snapdragons (flower)
• red (RR) x white (rr)
RR = red flower
rr = white flower
r
r
R
R
Incomplete Dominance
R
R
r
Rr
Rr
produces the
F1 generation
r
Rr
Rr
All Rr = pink
(heterozygous pink)
Pink Flowers?
Co-dominance
• Both alleles are dominant and both express their
phenotype (multiple alleles) in heterozygous
individuals.
• Example: blood
1.
2.
3.
4.
type A
type B
type AB
type O
=
=
=
=
IAIA or IAi
IBIB or IBi
IAIB
ii
Co-dominance
• Example: homozygous male B (IBIB)
x
heterozygous female A (IAi)
IB
IB
IA
IA I B
IA IB
i
IB i
IB i
1/2 = IAIB
1/2 = IBi
Chromosomes and Genetics
 Chromosomes are long pieces of DNA, with supporting
proteins
 Genes are short regions of this DNA that hold the
information needed to build and maintain the body

Genes have fixed locations: each gene is in a particular
place on a particular chromosome

Diploids have 2 copies of each chromosome, one from
each parent. This means 2 copies of each gene.
Linkage
• Linked genes are genes located on the same
chromosome, which tend to be inherited
together.
It is important to note:
• Independent assortment does not occur
between linked genes
Example: Drosophila fruit fly:
• Genes for body colour and wing length are on the
one chromosome i.e. are linked.
• Grey body (G) and long wings (L) are dominant to
black body (g) and vestigial wings (l).G with L and g
with l.
• Parents: GGLL
X
ggll
G
•
•
L
•
• Gametes:GL
•
•
G
•
•
L
• F1 :
•
G
g
g
L
l
l
X
gl
g
l
GgLl
Self-cross (if genes linked):
•
• Parents: GgLl
• Gametes:
GL
• F2:
•
GGLL
X
gl
GgLl
GgLl
GL gl
GgLl
ggll
Sex Determination
Human Chromosomes
• We have 46 chromosomes, or 23 pairs.
• 44 of them are called autosomes and are numbered 1 through
22. Chromosome 1 is the longest, 22 is the shortest.
• The other 2 chromosomes are the sex chromosomes: the X
chromosome and the Y chromosome.
• Males have and X and a Y; females have 2 X’s: XY vs. XX.
Male Karyotype
Female Karyotype
Sex Determination
The basic rule:
If the Y chromosome is present, the person is
male.
If absent, the person is female.
Meiosis
 the X and Y chromosomes separate and go into
different sperm cells:
 ½ the sperm carry the X and the other half carry
the Y.
 All eggs have one of the mother’s X
chromosomes
 The Y chromosome has the main sex-determining
gene on it, called SRY
Sex Determination
• About 4 weeks after fertilization, an embryo that
contains the SRY gene develops testes, the
primary male sex organ.
• The testes secrete the hormone testosterone.
• Testosterone signals the other cells of the embryo
to develop in the male pattern.
• Sex determination
• Autosome: a chromosome other than the sex
chromosomes.
•
• Sex chromosomes: chromosomes that determine the sex
of an individual - XX or XY.
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Parents: Mother
X X
Gametes:X
F1 genotype:
Phenotype:
X
Father
X Y
(Egg) X orY (Sperm)
XX
Female
XY
Male
• The male thus determines the sex of an
offspring.
• Mother gives an X to everyone but father
gives an X or Y chromosome. There is a 50:50
chance that any child will be male/female)
• In man sex-linked genes (i.e. those on the X
chromosome with no corresponding part on
the Y chromosome) include those governing
red-green colour blindness, muscular
dystrophy and haemophilia (inability to clot
blood).
• Females with both recessive genes for
haemophilia do not survive beyond the first
four months of gestation period.
• Parents: Female carrier X
•
•
X HX h
•
• Gametes:
XH X h
•
Male normal
X HY
XH
Y
•
•
•
•
•
•
•
F1
X HX H X HY
X HX h
Xh Y
Female
Male
Female
Male
Normal Normal Carrier Haemophili
25% chance of producing a haemophiliac child
50% chance of producing a haemophiliac son.
It is the mother that determines if the son is
haemophiliac or not since the father always passes
the Y chromosome to his son.
• Colour blindness is caused by a recessive gene on
the X chromosome.
•
• Parents:
• Female carrier
X
Male colour blind
2006 OL Q11
2008 HL Q11
• Haemophilia in humans is governed by a sexlinked allele. The allele for normal blood
clotting (N) is dominant to the allele for
haemophilia (n).
• (i) What is meant by sex-linked?
• (ii) Determine the possible genotypes and
phenotypes of the progeny of the following
cross: haemophilic male X heterozygous
normal female.
Non-nuclear inheritance
Mitochondria and chloroplasts contain their own
DNA, which indicates that they are descendants of
once free-living bacteria.
• At human fertilisation, only the head of the
sperm enters the egg. Each offspring gets a
nucleus from the male parent and a nucleus
plus cytoplasm from the female parent.
Mitochondria are inherited from the female
only. Mitochondrial DNA has been used as a
molecular clock to study evolution. By
measuring the amount of mutation that has
happened the time that has taken for it to
occur can be calculated.
• Mitochondria generate about 90% of the
energy of the cell and other cells that use a
lot of energy would be dependent on them.
Other examples of non-nuclear inheritance
include leaf variegation in snapdragons,
Parkinson’s disease.
Multiple alleles
• More than two different forms of a gene e.g.
blood grouping in man is governed by 3 alleles
A, B and O. Only two of the possible number
of alleles will be in any one organism.
•
Human Blood groups:
• 3 allelic genes A, B and O.
• A and B are co-dominant
• O is recessive.
Genotype
AA
AO
BB
BO
AB
OO
Phenotype
A
A
B
B
AB
O
End