Download Genetics

Genetics
SBI 3U
Examples
• Dihybrid Cross
• Two traits Therefore TWO pairs of alleles
• Alleles can be either Dominant or Recessive
Questions Involving More Than One Trait
In mice, grey coat colour, G, is dominant to
white coat, g, and long tail, T, is dominant
to short, t. What fraction of F1 mice would
you predict to have grey coats and short
tails when a male ggTt parent is crossed
with a female GGtt parent?
Given: G is dominant to g. T is dominant to t. Therefore:
GG or Gg = grey coat gg = white coat TT or Tt =long tail tt = short
tail
Required:
•
The fraction of F1 mice expected to have grey coats and
short tails;
•
that is, to be of genotypes GGtt or Ggtt
Analysis:
• The cross is ggTt X GGtt.
• Produce a Punnett square.
Solution:
The ggTt parent produces two types of gametes gT and gt
The GGtt parent produces only one type of gamete – Gt
The dimensions of the Punnett square are 2 X 1.
Statement:
In the F1 mice that are produced, half of the mice will be expected to have
the genotype Ggtt and as a result, half of the F1 mice will exhibit the grey
coat and short tail phenotype combination.
Incomplete Dominance
Under dominance the dominant
allele can hide the expression of a
recessive allele in the
heterozygous condition.
However, in the determination of
some traits, the different alleles of
a gene may be expressed in the
heterozygous
condition to produce an
intermediate phenotype. When
neither gene is completely
dominant over the other, we say
that there is incomplete
dominance. One of the clearest
examples of incomplete
dominance is found in the colours
of snapdragon flowers.
Co-dominance
A slightly different form of
inheritance is observed in
horses and shorthorn cattle
where two alleles are
expressed at the same time.
“Roan” coat Colour - a blend
of red and white.
The two alleles have been expressed at the same time, a type
of inheritance termed co-dominance. In this condition,
neither allele dominates the expression of the other. Capital
letters with superscripts or simply capitals are often used to
represent genotypes when co-dominance is in effect.
A roan calf (RW or CRCW) results from crossing a white
parent (WW or CWCW) with a red parent (RR or CRCR).
RR X WW
Gametes
R
W
Paternity Testing
• Blood groups can be used to disprove the
paternity of a baby.
• Example Mum Blood Group A and Dad Blood
Group B have a child Blood Group A. “The
man says this is not my child.” Is he correct?
• Mums Blood group could be AA or AO
• Dad could be BB or BO.
• Given that A and B are codominant the child
could be:
A
O
B
O
AB
BO
AO
OO
Baby could have any of the four blood groups so using this
method it would be impossible to prove or disprove the
paternity of the father.
New Scenario
• Mum is blood group A and Dad is blood group
O. Baby is blood group B. Dad says: “not my
child.” Is he correct?
A
O
O
AO
OO
He’s right. Not his.......
O
AO
OO