Genetics - worksheet answers
Glossary:
Allele: different forms of a gene; ex. blue is one of the alleles for the gene that codes for eye
colour.
Carrier: when you have the allele for a certain characteristics and are able to pass it on, but
do not show it due to its recessiveness (ability to be masked by another dominant allele).
Chromosome: the nuclear material made up of DNA that contains all of the information that
codes for the characteristics of an organisms; it contains 100s of genes; there are 23 pairs of
chromosomes in a human cell.
Gamete: an egg or sperm cell that contains half the number of chromosomes of a normal cell.
Genotype: the genetic makeup of an individual (what you HAVE).
Homologous pair: the complimentary chromosomes, one from each of the parent gametes
that code for the same characteristics.
Meiosis: the process of nuclear replication with two divisions performed by the sex cells in the
gonads, to produce haploid gametes.
Phenotype: the physical make-up of an individual (what you SEE).
Sex-linked characteristics: characteristics coded for by genes that are found on the part of
the X (female) chromosome that do not have comparable gene locations on the Y (male)
chromosome.
Questions:
1.
purpose of the process
MITOSIS
growth, repair, replace,
maintenance
type of cells involved
(location in the body)
# of cells produced
name and general # of chromosomes
in the parent cells
name and general # of chromosomes
in the daughter cells
MEIOSIS
produces haploid gametes
2
2
4
2
diploid
-
2N
diploid
-
2N
diploid
-
2N
haploid
-
2N
2.
One on each homologous chromosome from each parent gamete. (See homologous pair
definition.)
3.
They are recessive traits and had previously been masked by the dominant form of that trait.
Both parents were carriers.
1
4.
a) dominant/recessive
- there are two forms of the gene and two phenotypes, but only one is ever shown; if the form
is shown in either the homozygous (both the same) or the heterozygous (both different) state,
then it is dominant; it the form is shown only in the homozygous state (there is no dominant
form to mask it), then it is recessive
ex.
for the trait eye colour: the brown gene (B) is dominant over the blue gene (b)
genotype
phenotype
BB
brown
Bb
brown
bb
blue
b) incomplete dominance
- there are two forms of the gene, but there are three possible phenotypes - a new one
completely different from the original two; neither one is dominant over the other; the form of
the gene is only seen if it is homozygous
ex.
for the trait flower colour: the white gene (W) is incompletely dominant to the red gene
(W'), with the intermediate colour being pink
genotype
phenotype
WW
white
WW'
pink
W'W'
red
c) co-dominance
- there are two forms of the gene, and there are three possible phenotypes - one that is a
combination of the original two; neither one is dominant over the other; both are shown when it
is heterozygous
ex.
for the trait of human blood type: the A gene is co-dominant with the B gene
genotype
phenotype
AO
A
AA
A
BO
B
BB
B
AB
AB
OO
O
** NOTE: there are multiple alleles (not just two) for blood type and the third allele is recessive
to the other two
d) sex-linked
- the genes for sex-linked traits are found on the X chromosome and must be shown as such,
with a corresponding Y gene as the homologous pair; these traits are usually recessive, which
means if they are present in a male then they are shown, and are only shown in a female if
they are in the homozygous condition
ex.
the gene for haemophiliac (a blood disorder): the gene for normal blood (H) is
dominant to the gene for haemophilia (h)
genotype
phenotype
XHXH
female - normal
XHXh
female - normal, carrier
XhXh
female - haemophiliac (rarely found)
XHY
male - normal
XhY
male - haemophiliac
2
5.
a)
genotype
TT
Tt
tt
phenotype
tall
tall
short
b)
t
t
T
Tt
Tt
T
Tt
Tt
1. genotype: all heterozygous tall
2. phenotype: all tall
c)
d)
t
t
•
•
6.
T
Tt
Tt
t
tt
tt
genotypes: 50% heterozygous
tall, 50% (homozygous) short
phenotypes: 50% tall, 50%
short
a)
G
g
G
GG
Gg
g
Gg
gg
g
g
G
Gg
Gg
g
gg
gg
G
g
G
GG
Gg
G
GG
Gg
b)
c)
7.
a)
T
t
Genotype
GG
Gg
gg
•
•
T
TT
Tt
t
Tt
tt
genotypes: 25% homozygous tall; 50%
heterozygous tall, 25% (homozygous) short
phenotypes: 75% tall, 25% short
There was a 25% possibility of a black-bodied fly and
in 5664 flies, there probably should have been at least
one; therefore these are not the parent genotypes.
There was a 75% possibility of a black-bodied fly and
in 5664 flies, there probably should have been at least
one; therefore these are not the parent genotypes.
There was no possibility of a black-bodied fly and in
5664 flies there wasn't any; therefore these are one
possibility of parent genotypes - the other being GG x
GG.
Phenotype
green
green
yellow
i)
Genotypic ratio (GG:Gg:gg) is 0:4:0
g
g
G
Gg
Gg
G
Gg
Gg
Phenotypic ratio (green: yellow) is 4:0
NOTE: ratios can and should be reduced to 0:1:0 and
1:0. All subsequent ratios will be reduced where
possible
ii)
Genotypic ratio (GG:Gg:gg) is 1:2:1
g
g
G
Gg
Gg
G
Gg
Gg
Phenotypic ratio (green: yellow) is 3:1
3
8.
a)
Genotype
RR
Rr
rr
Phenotype
red
red
white
i)
Genotypic ratio (RR:Rr:rr) is 1:1:0
R
r
R
RR
Rr
R
RR
Rr
r
r
R
Rr
Rr
r
rr
rr
Phenotypic ratio (red: white) is 1:0
ii)
Genotypic ratio (RR:Rr:rr) is 0:1:1
Phenotypic ratio (red: white) is 1:1
9.
a)
b)
Homozygous tasters (PP) and non-tasters: 100% - all children will be Pp (tasters)
Both heterozygous tasters: 75% - it will be a 3:1 ratio of tasters (PP and Pp) to nontasters (pp)
10.
a)
This is incomplete dominance, as there are three different phenotypes and only two
alleles.
Since neither allele is dominant over the other, both must be the same kind of letter
(upper case); and since they refer to the same characteristic they must be the same letter;
therefore to distinguish between them you must put a 'prime' beside one of them.
Genotype
Phenotype
RR
Red
RRʼ
Pink
RʼRʼ
White
b)
c)
d)
Red and pink
R
Rʼ
R
RR
RRʼ
Both pink
R
RR
RRʼ
R
Rʼ
50% red and 50% pink
11.
a)
b)
c)
12.
a)
b)
R
RR
RRʼ
25% red, 50% pink and 25% white
This is a co-dominance.
Since neither allele is dominant over the other they both must be the same kind (upper
case).
i) Red cattle and white cattle are crossed. - 100% roan cattle (RW)
ii) Two roan cattle are crossed. - 1:2:1 (red:roan:white)
Sex-linked inheritance
i) XHXh -female, normal blood-clotting (carrier)
ii) XHY - male, normal-clotting
iii) XhXh - female, haemophiliac
iv) XhY - male, haemophiliac
c)
Rʼ
RRʼ
RʼRʼ
v) XHXH - female, normal blood-clotting
1. XHY
2. XHXh
3. XhY
4. XHY
4
5. XHXh
6. XhY
13.
Since they have an AB child, the parents had to have had the A and B alleles to be able to
pass them on; and since they have an O child, genotype OO, the parents each had to have
had the O allele to be able to pass it on; therefore, the genotypes of the parents were AO and
BO.
A
AB
AO
B
O
5
O
BO
OO