Download Genetic Crosses

Chpt. 17
Genetic Crosses
Gregor Mendal is
known as the
father of
genetics!!!
General Definitions
• Genetics: is the study of inheritance.
• Somatic Cells: are all cells except sex cells
e.g. Cheek, liver, muscle, blood, leaf, stem
etc.
• Gametes (sex cells): are haploid cells that
are capable of fusion.
• Fertilisation: is the union of two gametes
to form a single called a zygote.
Genetic Crosses – Definitions
• Alleles: are different forms of the same
gene.
Eg. Rabbits may have long ears (L) or
short ears (l). So, the gene for ears has
two alleles L or l.
• Dominant: means that the allele prevents
the recessive allele working.
Genetic Crosses – Definitions
Examples of dominant traits:
Genes for
Brown eyes
Ear lobes
Dimples in chin
Freckles
Curly hair
Tongue rolling
Human Traits
Genetic Crosses - Definitions
• Recessive: means that the allele is
prevented from working by a dominant
allele.
• Examples include:
- Cystic fibrosis
- Albinism
- Haemophilia
Eg. A rabbit with the alleles Ll has long ears
because the recessive allele for short ears (l) is
prevented from working by the dominant allele for
long ears (L).
Genetic Crosses – Definitions
• Genotype: means the genetic make up of an
organism, i.e. the genes that are present.
E.g. The possible genotypes for ear lengths
in rabbits are LL, Ll, ll
Genetic Crosses – Definitions
• Phenotype: means the physical make-up or
appearance, of an organism.
- Eg. The phenotypes of the rabbits are
either long ears or short ears.
- It is the interaction of the genes
(genotype) with the environment that
produces the phenotype:
Phenotype = Genotype + Environment
Genetic Crosses – Definitions
• Homzygous (Pure Breeding): means that both alleles
are the same. i.e. The genotypes LL and ll are both
homozygous.
Homozygous genotypes
phenotype
genotype
Genetic Crosses - Definitions
• Heterozygous (Hybrid): means that the alleles are
different. i.e. the genotype Ll is heterozygous.
Heterozygous genotype
phenotype
genotype
Genetic Crosses - Definitions
• Progeny: refers to offspring that are
produced:
F1 progeny means the first generation of
offspring (first filial generation).
Genetic Crosses
Points to note:
• a pair of alleles is present in the cells
of an organism for each characteristic.
• only one allele for each characteristic
is carried in each gamete.
Example 1:
phenotype
Homozygous Brown
x
genotype
genotype
phenotype
?
Homozygous Blue
Example 2:
phenotype Heterozygous Brown
x
Heterozygous Brown
genotype
genotype
phenotype
?
?
?
?
Genetic Crosses – Questions
Question 1:
In rabbits, long ears(L) are dominant over short
ears (l). Show the genotypes and phenotypes for
the offspring of a cross between two rabbits
whose genotypes are (LL) and (ll).
Solution 1:
Parents Genotypes
LL
Genotypes of Gametes L
x
ll
x
l
F1 Genotype of Offspring
Ll
F1 Phenotype of offspring All long ears
Genetic Crosses – Questions
When answering genetic questions a punnett square is
used for the more difficult questions.
Punnett Square: is a grid used to show the ratio of the
genotypes of the first generation of offspring (progeny) in
a genetic cross.
Six sample questions will be completed involving the
use of a Punnett Square (see attached leaflet)
Incomplete Dominance
Incomplete dominance (codominance): means that
neither allele is dominant or recessive. Both alleles
work in the heterozygous condition to produce an
intermediate phenotype.
• Incomplete dominance is relatively rare.
Example 1:
In shorthorn cattle:
- the genotype RR produces a red coat
- the genotype rr produces a white coat
- The heterozygous genotype Rr gives a roan coat
( patches of red and patches of white)
Incomplete Dominance
Example 2:
In snapdragons (flower):
- the genotype RR produces red flowers
- the genotype rr produces white flowers
- the heterozygous genotype Rr produces pink
flowers
Example 2:
Example 2 Contd.:
Pedigree Studies
Pedigree: is a diagram showing the genetic history of a
group of related individuals.
See pg. 168 Question 7
Sex Determination
• Normal body cells in humans contain 46 chromosomes:
- 44 chromosomes – have genes that do not
control sexuality – called autosomes.
- 2 chromosomes – control sexuality –
heterosomes.
• Autosomes – control features independent of whether
a person is male or female:
- skin colour
- number of arms
- formation of saliva
- production of digestive enzymes
Sex Determination
•Heterosomes - these two chromosomes are the X and Y
chromosomes.
- male body cells are XY
- female body cells are XX
The arrangement of XX for females and XY for males has
the following two consequences:
- it is the male who determines the sex of the child.
- the ratio of male to female births should be 1:1.
Sex Determination in Other Species
The pattern of sex determination in some species is the
reverse of that in humans.
For example in birds, moths and butterflies:
- male body cells – XX
- female body cells – XY
As a result of the this it is the female who determines
the sex of the offspring.
Mendel’s Experiments
(Higher Level)
• Gregor Mendel – ‘The Father of Genetics’
• Research carried out around 1860 but ignored until
1900’s
• Mendel’s work resulted in two basic laws of
inheirtance – ‘Mendel’s 1st and 2nd laws’
Mendel’s 1st Law – Law of Segregation
• Inherited traits are controlled by two alleles
• The alleles separate at gamete formation, with each
gamete only having one allele
Example:
Consider a species of plant whose height is controlled
by a pair of alleles Tt. When gametes are formed only
one allele can enter each gamete i.e. the alleles
segregate
Tt
T
t
Pair of gametes each
having only one allele
Mendel’s 1st Law – Law of Segregation
Behaviour of chromosomes according to Mendel’s 1st
law:
• Diploid organisms – chromosomes occur in
homologous pairs.
• Pairs of alleles occupy same position on a homologous
pair.
• During meiosis, homologous chromosomes separate
and go into different cells.
• As a result pairs of alleles also separate
T
T
t
Meiosis
t
Each cell has only
one chromosome
and one allele
Monohybrid Cross: involves the study of a single
characteristic e.g. eye colour, seed shape etc.
Dihybrid Cross: involves the study of two
characteristics at the same time e.g. studying plant
size (tall or small) and pod colour (green or yellow) at
the same time.
Mendel’s 2nd Law – ‘Law of Independent Assortment’
When gametes are formed .....
either of a pair of alleles ......
is equally likely .......
to combine with either of another pair of alleles .......
Example:
AaBb
A can combine with either B or b
a can combine with either B or b
Four types of gamete can form
AB Ab aB ab
Mendel’s 2nd Law – ‘Law of Independent Assortment’
Behaviour of chromosomes according to Mendel’s 2nd
Law:
• At gamete formation...
• either of a pair of homologous chromosomes....
• is equally likely..............
• to combine with either chromosome of a second
homologous pair
Dihybrid Crosses
Example 1:
In pea plants, tall plant (T) is dominant over small plant
(t). In addition, green pod (G) is dominant over yellow
pod (g).
A tall plant with green pods (homozygous for both
traits) is crossed with a small plant with yellow pods.
a) Why is this a dihybrid cross?
b) Show by diagrams the genotypes and phenotypes
of the progeny of this cross.
Example 2:
In guinea pigs, black coat (B) is dominant to white coat
(b). Also short hair (S) is dominant to long hair (s).
a) Show the genotypes and phenotypes of the F1
progeny for a cross involving a black coated, shorthaired guinea pig (heterozygous for both traits) and a
white-coated, long-haired animal.
a) State the expected ratio of the offspring
Example 3:
A homozygous purple-flowered, short stemmed plant
was crossed with a red flowered, long stemmed plant.
All the F1 offspring were purple-flowered with short
stems.
a) State the dominant and recessive traits.
b) Explain, using diagrams, why the F1 plants all had
the same phenotypes
c) Give the expected ratios if an F1 plant is selfed.
Mendel’s 2nd Law – Expected Ratios
Case 1 (Dihybrid Cross):
Heterozygous
X
(both characteristics)
Homozygous Recessive
Ratio of offspring = 1 : 1 : 1 : 1
Independent Assortment has occurred!!!
Case 2 (Dihybrid Cross):
Heterozygous
X
(both characteristics)
Heterozygous
(both characteristics)
Ratio of offspring = 9 : 3 : 3 : 1
Independent Assortment has occurred!!!
Linkage
• Means that both genes are present on the same
chromosome
• Linked genes are passed on together
Linked Genes
No Linkage
R and S
R
r
r and s
S
s
Pair of alleles
Homologous
Pair
T
t
T and t
Linkage
• Linkage contradicts Mendel’s 2nd law of independent
assortment (see following examples)
• Much less variation in offspring
*Note: The position a genes occupies on a chromosome
is called the locus
Linkage Example 1:
Draw simple chromosome diagrams to illustrate the
following cells. In each case show the gametes that
might be produced.
a) The genes are not linked and the genotype is
AaBb.
b) The genes are linked (A to B and a to b) and
the genotype is AaBb.
Linkage Example 2:
Draw simple chromosome diagrams to show each of the
following cells. In each case indicate the gametes that each
cell might produce.
a) Genotype RrSs, genes are not linked
b) Genotype RrSs, genes are linked (R to S and r to s)
c) The genes are not linked, the cell is homozygous
for R and heterozygous for S
d) The genes are linked, and the cell is homozygous
dominant for both genes.
Linkage Example 3:
Show the expected genotypes of the progeny for the
following crosses, AaBb x aabb:
a) where there is no linkage
b) where the genes are linked, A to B and a to b
Sex Linked (X Linked)
• Sex chromosomes in humans:
X – carries a large number of genes
Y – much shorter than X and carries
very few genes
• Sex linkage means that a characteristic is controlled by
a gene on an X chromosome.
• Examples of traits controlled by a gene on the X
chromosome:
Colour blindness
Haemophilia
Duchenne muscular dystrophy
• In sex linked characteristics, the recessive phenotype is
more likely to occur in males i.e. males suffer more
often from sex linked characteristics.
Note: in males there is no corresponding allele on the Y
chromosome
Sex Linked (X Linked)
• Females tend to be carriers - they carry the gene for
the trait but do not suffer from the disease
Carrier Mother & a Colour Blind Father
Parents Genotype NXXn
Gametes
NX
nX
nXY
nX
Y
Offspring Genotype NXnX
NXY
n XnX
Offspring Phenotype:
female
carrier
male female
normal CB
nXY
male
CB
Colour-blindness
• A colour-blind female inherits the colour-blind gene
from her mother( a carrier) as well as from her father both parents must have the gene.
• For a boy to be colour-blind, it is necessary only that his
mother is a carrier. This is far more common and the
reason why far more boys are colour-blind than girls
Haemophilia
• Haemophilia – is a bleeding disorder caused by the
lack of a particular blood protein
• Caused by a gene located on the X chromosome:
- allele (N) for production of clotting
protein is dominant.
- recessive allele (n) does not carry the
correct genetic code for the production
of the protein.
• More common in males, extremely rare in females
Non-nuclear Inheritance
• Extra-nuclear genes are present as small circles of
DNA in mitochondria and chloroplasts (both of which
reproduce by themselves passing on their genes)
• Since, pollen does not contain these organelles and
mitochondria are in the tail of the sperm, only the
head joins with the egg, this means that
mitochondria and chloroplasts are inherited from
the female in the cytoplasm of the egg.
• Mitochondrial DNA (mtDNA) in humans is inherited
only from the mother, subsequently, a number of rare
human disorders are inherited only from the mother
because they are controlled by extra-nuclear genes
located on mtDNA.
Mitochondrial Eve
Please read over paragraph relating to this topic
pg. 179 in your book.