Download Mendel's genetics - Life Sciences 4 All

How to do genetics crossings
SIMPLE MONOHYBRID CROSSES
INVOLVING A SINGLE PAIR OF
CONTRASTING ALLELES OF A
SPECIFIC TRAIT
• DOMINANT AND RECESSIVE
• INCOMPLETE DOMINANCE
• CO-DOMINANCE
1
Introduction (dominance &
recessive characteristics)
Study the information given in the
question. Look for the following aspects:
1. The trait with the contrasting alleles
2. Which allele is dominant and which
one is recessive
3. The genotype of the two parents
(homozygous or heterozygous)
4. The phenotype of the two parents
5. What is the extra question? (ratio,
probability, numbers)
QUESTION
 Flies with heterozygous grey bodies were crossed with
flies with black bodies. Grey bodies were dominant over
black bodies. Use the letters G and g to represent a
genetic cross to show the F1 genotypes and phenotypes.
1. Trait:
4. Recessive:
Colour of bodies
Grey and black
Grey (G)
black (g)
5. Genotype of Grey
heterozygous Gg
2. Contrasting alleles:
3. Dominant:
parent:
6. Genotype of black
parent:
Not given
When is a recessive
characteristic expressed?
 Can always be deducted:
 There is only one combination of
recessive alleles that will make the
recessive characteristic to be
visible/functional/expressed …
 the homozygous recessive alleles (two
recessive alleles)
 In this case: gg
4
Using a template to do a crossing
Have to provide the examiner the
following:
 Phenotype and Genotype of the two
parents
 meiosis and the gametes formed
 Fertilisation and the punnett square
 F1 ratio of genotypes and phenotype
5
General Marking guidelines
√ Gametes
G
G
g
Gg
Gg
g
Gg
gg
6
Memo for question
P1 Phenotype Grey body x Black body √
Genotype
Gg x gg √
Meiosis
gametes
Correct gametes √
F1
G
g
g
Gg
Gg
g
Gg
gg
Phenotype: 3 Grey : 1 black √
Genotypes: 3 Gg : 1 gg √
F1 & P1 : √
Meiosis and fertilisation: √
max 7
7
Some important notes
 In answering the questions about a genetic crossing
always use this format/template described in the
previous slide.
 Use a punnett square to do the crossings of the
gametes – less confusing
 Use the generations (P1 and F1) as well as the
processes (meiosis and fertilisation) in the correct
places – can obtain two marks even if your crossing
is incorrect.
 This format/template is applicable for all
crossings – also for dihybrid crossings
8
Answering an incomplete dominance
crossing
 No dominance or recessive alleles.
 The homozygous offspring or two different parents
have two contrasting characteristics.
 The heterozygous offspring or parents have an
characteristic which is in between (intermediate) to
the two contrasting characteristics of the different
homozygous organisms
 Any information about an intermediate/in between
value refers to incomplete dominance.
 Use for both contrasting alleles capitals e.g. R = Red
and W = white. RW = pink (intermediate colour).
 Use the same template/format for the crossing.
9
Incomplete dominance question
A red flower snapdragon is crossed with a white
flower snapdragon. Use the symbols R for red
flowers and W for white flowers.
The heterozygous for this trait is a pink flower.
Use a genetic crossing to illustrate the possible
genotype and phenotype ratio of the F1.
Memo for question
P1 Phenotype Red flower x White flower √
Genotype
RR x WW √
Meiosis
gametes
Correct gametes √
F1
R
R
W
RW
RW
W
RW
RW
Phenotype: All pink flowers√
Genotypes: all heterozygous / RW√
F1 & P1 : √
Meiosis and fertilisation: √
max 7
11
Answering an co-dominance
crossing
 Both the contrasting alleles for a trait are equally
expressed.
 The homozygous offspring or two different parents
have two contrasting characteristics.
 In the heterozygous offspring or parents both
contrasting characteristics appear separately / are
equally expressed.
 For example: Inheritance of blood groups A and B
 Genotype for blood groups A and B = IA, IB
 Note: Blood group O (genotype = ii) is recessive to both
blood group A and B.
 Another example: cattle with red fur coat crosses white
fur coat, produce calves that are roan.
12
Co-dominance question
A man with a homozygous blood group A is
crossed with a woman with a homozygous blood
group B. Use a genetic crossing to illustrate the
possible ratio of the phenotype and genotype of
the offspring.
Memo for question
P1 Phenotype Blood group A x Blood group B √
Genotype
IAIA x IBIB√
Meiosis
gametes
IB
IB
IA
IAIB
IAIB
Correct gametes √
IA
IAIB
IAIB
F1
Phenotype: All blood group AB√
Genotypes: all IAIB √
F1 & P1 : √
Meiosis and fertilisation: √
max 7
14
Co-dominance question
A bull with a red fur coat (R ) is crossed with a
cow with a white fur coat (W ) and produces
calves that are roan. Use a genetic crossing to
illustrate the possible ratio of the phenotype and
genotype of the offspring.
Memo for question
P1 Phenotype Red fur coat x white fur coat √
Genotype
RR x WW√
Meiosis
gametes
W
W
R
RW
RW
Correct gametes √
R
RW
RW
F1
Phenotype: All calves roan√
Genotypes: all RW √
F1 & P1 : √
Meiosis and fertilisation: √
max 7
16
Sex-linked crossing
Introduction – sex chromosomes
• Females have two X chromosomes
• If a woman has a rare recessive allele on the
one X chromosome, she is likely to have a
dominant allele on the second chromosome
• Males have a X and a Y chromosome
• The Y chromosome is much shorter and
carries a different set of genes
• If a man has a rare recessive allele on his X
chromosome, his phenotype will show it
because there is no second X chromosome to
carry a dominant allele that could hide it.
17
Sex-linked crossing question
Two examples prescribed, namely, haemophilia
and red-green colour blindness. Both are
recessive sex-linked genes carried on the X
chromosomes.
Haemophilia
An unaffected father is married to a
heterozygous mother for haemophilia. Use a
genetic cross to illustrate the possible genotype
and phenotype ratio of the offspring. Use the
symbols XH, Xh
Memo for question
P1 Phenotype Unaffected male x heterozygous female √
Genotype
XH Y x XH Xh √
Meiosis
gametes
Correct gametes √
XH
Y
XH
XHXH
XH Y
Xh
XHXh
Xh Y
F1
Phenotype: 2 unaffected females, 1
unaffected male, 1 affected male√
Genotypes: 1 homozygous and 1 heterozygous
female, 1 XH Y, 1 Xh Y male, √
F1 & P1 : √
Meiosis and fertilisation: √
max 7
19
Some notes
 Males are affected or unaffected. Only one X
chromosome
 Females can be a carrier. Have heterozygous alleles
on the two X chromosomes.
 Females very seldom have the recessive genes on
both X chromosomes.
20