Transcript Genetic Inheritance - leavingcertbiology.net

Genetic Inheritance
Leaving Certificate Biology
Higher Level
Genetic Inheritance
• Humans have 23 pairs of chromosomes
– Each pair of chromosomes are what we cell
‘homologous’ – meaning they contain the same genes
– Therefore, everyone has two copies of every single
human gene – fail-safe mechanism encase one gene
in a cell fails there is another to carry on
– 22 of these pairs are called
autosomes
– 1 pair are called the sex
chromosomes and determine
the sex of the individual
• Male: XY
• Female: XX
Genetic Inheritance
• Humans have 23 pairs of chromosomes
– 22 of these pairs are called autosomes
– 1 pair are called the sex chromosomes and determine
the sex of the individual
• Female: XX
• Male: XY
Genetic Inheritance
Female Karyotype, 46XX
Male Karyotype, 46XY
Genetic Inheritance
• Gamete Formation and Function:
– Gamete: a gamete is a haploid sex cell which
has to fuse with another sex cell of the
opposite sex in order to survive and pass on
its genes to form a new individual
– Fertilisation: fertilisation is the fusion of two
haploid sex cells (gametes) to form a single
diploid cell called the zygote
Genetic Inheritance
• Allele: an allele is a particular form of a
gene (can be represented as a letter, e.g.
S or s; H or h)
• Alleles: alleles are different forms of the
same gene (e.g. S is the dominant form of
the gene whereas s is the recessive form
of the gene)
• Locus: the locus (plural: loci) of a gene is
the position is occupies on a chromosome
Genetic Inheritance
• Genotype: the genotype of an organism
refers to its genetic make-up (e.g. Ss)
• Phenotype: the phenotype of an organism
refers to the physical appearance or
characteristics of that organism (e.g. Ss
can be responsible for a physical
appearance or characteristic)
– Genotype and environmental conditions
together have an effect on the phenotype
Alleles and Loci Before Meiosis
Male heterozygous for
brown eyes
10
2n
B
Female heterozygous for
brown eyes
10
10
b
B
MEIOSIS
10
b
MEIOSIS
Mendel’s First
Law of
Segregation
2n
Alleles and Loci After Meiosis
Male heterozygous for
brown eyes
n SPERM n
10
B
10
Female heterozygous for
brown eyes
n EGGS n
10
b
Half sperm are
B and half are b
B
10
b
Half eggs are B and
half eggs are b
Alleles and Loci After Fertilisation
Male heterozygous for
brown eyes
Female heterozygous for
brown eyes
13
13
b
b
n
FERTILISATION
13
b
2n
13
b
Phenotype of F1:
Blue Eyes
Genetic Inheritance
Genetic Inheritance
• Syllabus:
– “Study of the inheritance to the first filial
generation (F1) of a single unlinked trait in a
cross involving:
• homozygous parents
• heterozygous parents
• sex determination”
Homozygous Parents
Homozygous
BB
F1
Progeny
Genotype
F1
Progeny
Phenotype
Homozygous
x
bb
B
B
b
b
Bb
Bb
Bb
Bb
{
{
All offspring have brown eyes
Heterozygous Parents
Heterozygous
Bb
F1
Progeny
Phenotype
x
Bb
B
b
B
b
BB
Bb
Bb
bb
{
{
F1
Progeny
Genotype
Heterozygous
3:1 – Brown Eyes : Blue Eyes
Heterozygous and Homozygous
Heterozygous
Bb
F1
Progeny
Genotype
F1
Progeny
Phenotype
Homozygous
x
bb
B
b
b
b
Bb
Bb
bb
bb
{
{
1:1 – Brown Eyes : Blue Eyes
Genetic Inheritance of eye colour
(brown and blue eyes)
Sex Determination
• Question 8 (page 170): show by
diagrams why in humans the father
determines the sex of a child.
Male
X Y
X X
2n
Female
2n
MEIOSIS
MEIOSIS
Sex Determination
Possible male
gametes
X
Possible female
gametes
Y
X
X
n
FERTILISATION
X X
Y X
2n
Incomplete Dominance
• Incomplete dominance: incomplete
dominance is where two homologous
alleles are equally expressed and neither
allele is dominant over or recessive to the
other
– The heterozygous genotype produces a
phenotype intermediate between those
produced by the two homozygous genotypes
– An example is flower colour in snapdragons –
red flower crossed with white flower produces
pink flowered offspring
Incomplete Dominance
Parental
phenotypes:
Parental
genotypes:
Gamete
genotypes:
RED FLOWER
WHITE FLOWER
RR
rr
R
Possible fertilisations:
Gamete
genotypes:
Rr
Gamete
phenotypes:
Pink
R
r
r
Rr
Rr
Rr
Pink
Pink
Pink
Gregor Mendel
•
Mendel studied the inheritance of seven
characteristics of pea plants:
1.
2.
3.
4.
5.
6.
7.
Seed shape
Seed colour
Ripe pod shape
Unripe pod colour
Flower position
Flower colour
Height
Gregor Mendel
•
These 7 characteristics were chosen because
each has only two clearly contrasting qualities:
1.
2.
3.
4.
5.
6.
7.
Seed shape: round/smooth vs wrinkled (RR vs rr)
Seed colour: yellow vs green (YY vs yy)
Ripe pod shape: inflated vs constricted (II vs ii)
Unripe pod colour: green vs yellow (GG vs gg)
Flower position: axial vs terminal (AA vs aa)
Flower colour: purple vs white (PP vs pp)
Height of stem: tall vs dwarf (TT vs tt)
Gregor Mendel
• Mendel used pea plants because they
have several advantages over other
plants:
–
–
–
–
they have a short life cycle
relatively easy to cultivate
could be grown in large numbers
are capable of self-pollination and
fertilisation
Gregor Mendel
• Mendel developed separate populations
of pea plants, each a pure breed
(homozygous) for a particular quality
– e.g. for height, Mendel developed purebred
(homozygous) tall pea plants and purebred
(homozygous) dwarf pea plants (this took a
long time to achieve as Mendel had to check
that the purebred tall plants always produced
100% tall offspring and ditto for dwarf pea
plants
Gregor Mendel
• Mendel kept strict records of his results
• Mendel converted the results of his many
crosses into simple ratios that gave him
an insight into mechanism of inheritance
and led to his two famous laws of
genetics
Gregor Mendel’s First Cross
• Mendel carried out 2 consecutive crosses:
Phenotypes:
Genotypes:
TALL
x
DWARF
TT
F1 generation: Tt
tt
Tt
x
Tt
Tt
SELF-FERTILISATION
F2 generation:
TT ; Tt ; Tt; tt
3 tall:1 dwarf
Mendel’s First Law of Genetics:
Law of Segregation
• He repeated this cross for the other six
characteristics of pea plants and
consistently came up with the same ratio
3:1
• Mendel worked backwards and came up
with the Law of Segregation
Mendel’s First Law of Genetics:
Law of Segregation
• Law of Segregation: each cell contains
two factors for each trait, these factors
separate during the formation of gametes
so that each gamete contains only one
factor from each pair of factors. At
fertilisation the new organism will have
two factors for each trait, one from each
parent.
Dihybrid Crosses
• Having worked out the mechanism governing
the inheritance of one characteristic, Mendel
then proceeded to study the simultaneous
inheritance of two different characteristics, e.g.
height and seed shape
• Again Mendel began his dihybrid cross with
purebreds for the characteristics he wanted to
study
• Mendel knew from his monohybrid crosses that
tall (T) and round seed (R) are dominant, so
dwarf (t) and wrinkled seed (r) are recessive
Dihybrid Crosses
Phenotypes: Tall, round
Genotypes:
Gametes:
F1 generation:
x
Dwarf, wrinkled
TTRR
ttrr
TR
tr
TtRr
Dihybrid Crosses
• Mendel understood the results of the F1
generation from a cross of parents
homozygous dominant and homozygous
recessive because the only offspring that
could be produced from this cross was
heterozygous individuals – because each
individual could only produce ONE type of
gamete due to the fact that they were
homozygous for the two traits studied in
this cross
Dihybrid Crosses
• The dihybrid cross between parent plants
heterozygous for both traits posed a
problem – how are the gametes made
from the genotype: TtRr?
• Mendel’s solution to the problem of
gamete formation involving more than one
characteristic is Mendel’s Second Law:
The Law of Independent Assortment
Mendel’s Second Law of Genetics:
Law of Independent Assortment
• Law of Independent Assortment:
members of one pair of factors separate
independently of members of another pair
of factors at gamete formation
Independent Assortment
Parents:
Gametes:
Gametes
Tall, Round (TtRr) x Tall, Round (TtRr)
TR; Tr; tR; tr
TR
Tr
tR
tr
9:3:3:1
TR
TTRR TTRr TtRR TtRr
9/16 Tall, Round
Tr
TTRr
TTrr
TtRr
Ttrr
3/16 Tall, Wrinkled
tR
TtRR
TtRr
ttRR
ttRr
3/16 Dwarf, Round
tr
TtRr
Ttrr
ttRr
ttrr
1/16 Dwarf, Wrinkled
NOTE: The tall, wrinkled (TTrr andTtrr genotypes), dwarf, round (ttRR and ttRr
genotypes) and dwarf, wrinkled (ttrr genotypes) progeny are called
recombinants because they differ to the parental genotypes and phenotypes
Independent Assortment
Parents:
Gametes:
Tall, Round (TtRr)
x
x
TR; Tr; tR; tr
Dwarf, Wrinkled (ttrr)
tr
Gametes
tr
TR
TtRr
1/4 Tall, Round
Tr
Ttrr
1/4 Tall, Wrinkled
tR
ttRr
1/4 Dwarf, Round
tr
ttrr
1/4 Dwarf, Wrinkled
1:1:1:1
NOTE: The tall, wrinkled (TTrr andTtrr genotypes) and dwarf,
round (ttRR and ttRr genotypes) progeny are called
recombinants because they differ from the parental
genotypes and phenotypes
Non-Linked v Linked
• The genes governing the traits studied by
Mendel were found to be ‘non-linked’ meaning
that each trait studied was on a separate
chromosome to another trait
• Note: non-linked genes are on different
chromosomes and so will undergo independent
assortment and therefore are true to Mendel’s
Second Law
• ‘Linked’ alleles (linkage): linked alleles are
those genes found on the same chromosome
Linked Genes
• Linked genes are the genes that are
present on the same chromosome
• Note: genes are said to be tightly linked if
they are close together on the same
chromosome – tightly linked genes tend
not to follow Mendel’s Second Law of
Independent Assortment
Linked Genes
Non-linked;
Genotype: RrTt
R
Linked;
Genotype: RrTt
r
T
t
R
r
T
t
Example of Linked Genes
• In maize: C (coloured seed) is dominant over c
(colourless seed) and S (full seed) is dominant
over s (shrunken seed)
• Firstly, a heterozygous coloured, full seed
(CsSs) maize plant is crossed with a
homozygous recessive colourless, shrunken
seed (ccss) maize plant
• Secondly, two heterozygous coloured, full seed
(CcSs) maize plants are crossed
• Note: the genes for coloured seed and full seed
are linked tightly
1. Parent phenotypes:
Coloured Full
2. Parent genotypes:
CcSs
C
c
S
x
x
Colourless Shrunken
ccss
c
c
s
s
s
3. Meiosis
C
c
c
4. Gamete
genotypes:
S
s
s
5. Possible
random
fertilisations:
6. F1 progeny
phenotypes:
C
c
c
c
S
s
s
s
Coloured
Full
Colourless
Shrunken
1:1
1. Parent phenotypes:
x
x
Coloured Full
CcSs
C
c
2. Parent genotypes:
S
Coloured Full
CcSs
C
c
s
S
s
3. Meiosis
C
c
C
c
4. Gamete
genotypes:
S
s
S
s
5. Possible
random
fertilisations:
6. F1 progeny
phenotypes:
C
C
C
c
C
c
c
c
S
S
S
s
S
s
s
s
CCSS
Coloured
Full
CcSs
Coloured
Full
CcSs
Coloured
Full
ccss
Colourless
Shrunken
3:1
Ratio of Offspring Between NonLinked and Linked Genes
• Ratio of genotypes of gametes from an
non-linked cross are different from those
produced by a linked cross
Non-Linked Cross
PARENTS: RrTt x
Linked Cross
RrTt
GAMETES: RT; Rt; rT; rt x RT; Rt; rT; rt
F1:
RRTT
RRTt
RrTT
RrTt
RRTt RrTT
RRtt RrTt
RrTt rrTT
Rrtt
rrTt
9:3:3:1
RrTt
RrTt x
RrTt
Rrtt
rrTt
rrtt
RT; rt
x
RT; rt
RRTT
RrTt
3:1
RrTt
rrtt
Ratio of Offspring Between NonLinked and Linked Genes
• Ratio of genotypes of gametes from an
non-linked cross are different from those
produced by a linked cross
Non-Linked Cross
rrtt
PARENTS:
RrTt x
GAMETES:
RT; Rt; rT; rt x rt
F1 OFFSPRING: RrTt; Rrtt; rrTt; rrtt
1:1:1:1
Linked Cross
rrtt
RrTt x
RT; rt
x
rt
RrTt and rrtt
1:1
Sex Linkage
• Sex linkage is where a characteristic is controlled by a
gene on an X chromosome
• Sex-linked genes can also be said to be X-linked
• The X chromosome carries many more genes (~800
more genes) than the Y chromosome
• Recessive genotypes for particular traits that are Xlinked therefore occur more frequently in males than in
females
• Females have a pair of genes governing each trait – if
one gene is faulty, then she has a second one to cover
for it
• However, if a gene is faulty on the X chromosome of a
male then he may not have a second one to cover and is
more likely to suffer an X-linked genetic defect
Common Sex-Linked Traits
• Colour vision: gene controlling colour
vision has two alleles: N (normal) and n
(colour-blind)
• Blood clotting: gene controlling blood
clotting has two alleles: N (normal) and n
(haemophiliac)
– Haemophilia is the inability to clot blood and a
haemophiliac therefore suffers from persistent
bleeding if the deficient protein factor needed
is not taken
Haemophilia
Haemophilia (cont.)
• There are three possible female genotypes for a
sex-linked trait (e.g. haemophilia) and only two
for males:
– Females:
– Males:
NN; Nn; nn
N–;n–
• Heterozygous female for this trait is called a
‘carrier’ – she will pass on this defective allele to
50% of her gametes (egg cells) and thus 50% of
her children
• There are six possible crosses for this trait:
Haemophilia (cont.)
FEMALE
MALE
1.
NN
N–
2.
NN
n–
3.
Nn
N–
4.
Nn
n–
5.
nn
N–
6.
nn
n–
Haemophilia (cont.)
Parental phenotypes:
Parental genotypes:
Normal (carrier)
female
XN Xn
Gametes:
XN
Random fertilisations:
F1 progeny genotypes:
Xn
Normal male
XN Y–
XN
XN
Y–
XN
XNXN
XNY–
Xn
XNXn
XnY–
XNXN; XNY–; XNXn; and XnY–
F1 progeny phenotypes: Normal female; Normal male;
Normal carrier female; and
haemophiliac male
Y–
Non-Nuclear Inheritance
• DNA is also found in the mitochondrion – it is
also found in the chloroplast
• The DNA found in these organelles is described
as non-nuclear
• Mitochondria and chloroplasts replicate
themselves in a process similar to binary fission
• NOTE: male gametes only pass on a haploid
nucleus at fertilisation whereas the female
gamete (egg cell) passes on a haploid nucleus
and the cytoplasm – which includes
mitochondria and chloroplasts
Non-Nuclear Inheritance (cont.)
• Therefore, non-nuclear inheritance (i.e. the
mitochondrial DNA) is by way of the
female gamete ONLY
• Non-nuclear genes show a non-Mendelian
pattern of inheritance