Download Chapter 14: Genotype, phenotype and crosses Key questions

Chapter 14: Genotype, phenotype and crosses
Key questions
1
A gene is a series of nucleotides that code for a characteristic or instruction. An allele is a variation of that
characteristic.
2
A genotype is the genetic detail (alleles) of an individual’s characteristic(s) being investigated. The phenotype
of the individual is the trait that a given genotype expresses. The environment also plays a role in the
expression of the phenotype.
3
In humans, hemizygous individuals are male because they only have one X chromosome, i.e. half the number
of X chromosomes in females (hemi = half).
4
In some cases the phenotype of an organism is not visible to the naked eye and may require a microscope or
further testing. Blood groups, colour blindness and vitamin D resistant rickets require biochemical
investigation.
5
Both genetic and environmental factors contribute to the phenotype of individuals. For example, the presence
of skin pigment is controlled by genes, but the degree of colour or tan of the skin can also be influenced by the
amount of exposure an individual has to the sun.
6
Ee—heterozygous; ee—homozygous; EE—homozygous; eE—heterozygous
7
In blowflies Ww and WW, both individuals will have the same eye colour or phenotype even though they
have different genotypes. This results when the expression of one allele is dominant over another. In this case,
W refers to red eye colour and w refers to white. If Ww genes are present, the W gene masks the expression of
w gene.
8
A recessive trait is only expressed in a homozygote. In an example of seed shape, RR or Rr show round seed
phenotypes, while the genotype rr expresses oval seeds. Therefore, the expression of the R allele is dominant
to the expression of the r allele.
9
a
i A red-eyed individual would have the genotype Rr or RR.
ii A white-eyed individual would have the genotype rr.
b
The phenotype for the heterozygote would be red because only one dominant allele needs to be present to
mask the recessive allele for white.
10 Complete dominance occurs when the heterozygote form of a genotype as well as the dominant homozygote
genotype is expressed as the same phenotype. In incomplete dominance, the heterozygote genotype results in
a blending of the two homozygote traits. An example of incomplete dominance is found in some flower
colours. A cross involving pure-breeding red and white snapdragons produces pink flowers.
11 Codominance occurs when both alleles are expressed equally. In the ABO blood grouping, the AB blood
group produces both antigen A and antigen B on the red blood cell surface. Incomplete dominance differs
from codominance in that no blending occurs in codominance.
12 The ABO blood grouping system in humans is controlled by a single gene, but there are more than two alleles
for the gene, which results in more than two phenotypes. The alleles are IA, IB and i. Genotypes IA IA and IAi
generate a phenotype of blood type A. Genotypes IBIB and IBi generate a phenotype of blood type B. Genotype
IAIB generates a phenotype of blood type AB. Blood type O has the genotype ii.
13 a
b
An example of a single gene that affects multiple phenotypic characters is found in the vinegar fly. In a
mutant variety, a gene called lozenge affects eye structure, sensory bristles on the antennae, and taste
receptors. Also affected were claws, a section of the brain, a group of blood cells involved in the immune
system and sperm storage in females resulting in infertility.
A single gene can affect multiple characteristics if the gene is a regulatory one, as is the lozenge gene.
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14 Identical twins often have visible differences due to environmental influences such as diet and exercise.
15 Yes, monozygote twins have identical genotypes, as do plants produced by asexual reproduction or by
cuttings.
16 Studies of the IQ of identical twins raised in different homes from birth have provided evidence that
environmental factors, such as upbringing and education, play a vital role in the phenotypes of the individuals.
Twins have also been used to investigate the contribution of genes to behavioural traits such as personality,
handedness and alcoholism.
17 Due to the absence of the enzyme phenylalanine hydroxylase, an accumulation of the amino acid
phenylalanine in the tissues of individuals with phenylketonuria results in impaired development that leads to
mental retardation. Controlling the environmental factor of diet in these individuals by deleting phenylalanine
(most protein foods) overcomes the problem of phenylalanine accumulation and normal development occurs.
18 A monohybrid cross is a cross between two individuals with respect to a single gene locus. For example, eye
colour in sheep blowfly is controlled by a single gene with two alternative alleles, W: red eye and w: white
eye. A cross between two individuals with respect to this single gene is a monohybrid cross. For example:
WW
ww all offspring have genotype Ww and phenotype red eyes.
Ww Ww ¾ offspring have phenotype red eyes (genotype either WW or Ww) and ¼ offspring have
phenotype white eyes (genotype ww)
19 a
b
A pure-breeding blowfly has two identical alleles such as WW (red eye) or ww (white eye). When two
pure-breeding strains, such as homozygous red-eyed blowflies, breed (WW WW) the offspring have
the same genotype and phenotype as the parents.
Geneticists use specific names to identify between a series of crosses involving individuals and offspring
of the same family:
•
parental (P) generation—WW
ww
•
first filial (F1) generation—all offspring are Ww
•
second filial (F2) generation—Ww
Ww (a cross of F1 individuals).
c
Individuals of the F1 generation are all red-eyed because the genotype Ww is heterozygous red eye. The
expression of the W allele is dominant and therefore masks the expression of the allele w.
d
The phenotypes in the F2 generation occur in the ratio of 3 red eye : 1 white eye because the parents are
heterozygous: each carries one red-eye allele and one white-eye allele. When a cross occurs, the
probability that a parent will pass on a white allele is 50%. The probability that a parent will pass on a W
allele is also 50%. Therefore, the probability that a WW will occur is 25%, that a Ww will occur is 50%
and that a ww will occur is 25%. So the total probability that WW or Ww (red) will occur is 75% and
that ww (white) will occur is 25%. As red eye is dominant, only one allele needs to be present for the
phenotype to be expressed.
20 a
parental (P) generation—dark red snapdragons
P generation genotype—R1R1
white snapdragons
R2R2
P gametes—R1 and R2
F1 generation
Gametes
R1
R1
R2
R1R2
R1R2
R2
R1R2
R1R2
F1 genotype—all R1R2
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F1 phenotype—all pink
b
The genotypic and phenotypic ratios in the F2 generation are as follows:
(F1) P generation—R1R2
R1R2
P gametes—1/2 R1, 1/2 R2
1/2 R1, 1/2 R2
F2 generation
Gametes
1/2 R1
1/2 R2
1/2 R1
1/4 R1R1
1/4 R1R2
1/2 R2
1/4 R1R2
1/4 R2R2
F2 genotypes—1/4 R1R1, 1/2 R1R2, 1/4 R2R2
F2 phenotypes—1 dark red : 2 pink : 1 white
21 a
A 3:1 ratio in the F2 generation suggests that the variation in the trait is controlled by two alleles of a
single autosomal gene. In this case, three-quarters of the progeny would display the dominant trait.
b
A 1:2:1 ratio in the F2 generation suggests that two alleles are involved and that partial dominance or
codominance is the mode of inheritance.
22 A monohybrid cross involves a cross that deals with only one characteristic, such as hairline or, in plants, stem
length. A dihybrid cross involves a cross that deals with two characteristics at the same time, such as eye
colour and length of bristles. These characteristics can be found on the same chromosome or on separate
chromosomes.
23 Independent assortment is the principle of segregation of homologues to opposite poles within the nucleus of
the cell. During meiosis the homologues line up at the equator during Metaphase 1. The homologue
chromosomes/chromatids containing the gene being investigated can move to either pole of the cell. Each
homologous pair of chromosomes in the cell can move to either end of the cell independently of the
movement of other homologues.
24 a
b
Number of different
phenotypes
Number of different
genotypes
Parents
2
2
F1
1
1
F2
4
8
9:3:3:1
25 A cross between AABB and aabb individuals results in all offspring with genotype AaBb and the dominant
phenotype. F1 individuals can each produce four different kinds of gametes: AB, Ab, aB, ab. A cross between
these individuals can be tracked using a Punnett square.
AB
Ab
aB
ab
AB
AABB
AABb
AaBb
Aabb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
9/16 of the offspring have at least one allele that confers the dominant phenotype for each characteristic; 3/16
have at least one allele that confers the dominant phenotype for the first characteristic combined with
homozygous recessive for the second (recessive phenotype); 3/16 are homozygous recessive for the first
characteristic (recessive phenotype) and have at least one allele that confers the dominant phenotype for the
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77
second characteristic; 1/16 are homozygous recessive at both gene loci, resulting in recessive phenotype for
both characteristics. Overall, this represents a 9:3:3:1 ratio.
26 a
b
Testcrosses are used to determine whether individuals with a dominant phenotype are heterozygous or
homozygous for that dominant trait.
A backcross is a cross between the F1 and either one of the pure-breeding parental strains. The difference
between a testcross and a backcross is that a testcross involves a cross between the individual in question
and the parent with the homozygous recessive genotype.
27 The genotypes of the F1 guinea pigs are 1/4 BB, 1/2 Bb, 1/4 bb. The phenotypes of the cross would be
3 black : 1 white.
28 The genotypes and phenotypes of the F1 guinea pigs and a backcross to the pure-breeding black coat colour
are 1/2 BB and 1/2 Bb. The phenotypes would be all black.
Chapter review questions
1
a
F: free lobes; f: attached lobes
b
Three possible genotypes: FF, Ff, ff. Two phenotypes: free lobes: FF and Ff; attached lobes: ff
c
Homozygous man with free lobes: FF; Heterozygous woman: Ff
F
F
F
FF
FF
f
Ff
Ff
All of the children will have free lobes, although expect 50% to be homozygous and 50% to be
heterozygous.
d
Two people who both have free lobes could have children with attached lobes if they are heterozygous.
F
f
F
FF
Ff
f
Ff
ff
Expect 3/4 offspring to have genotype that confers dominant phenotype of free lobes (either homozygous
dominant or heterozygous) and 1/4 to have attached lobes (homozygous recessive).
e
2
The chance of having a child with attached lobes is 25% or 0.25 or 1:3.
B: black coat (dominant); b: white coat (recessive)
B
B
B
BB
Bb
b
Bb
bb
Expect a genotypic ratio of BB:Bb:bb of 1:2:1, which equates to a phenotypic ratio of ¾ black offspring: ¼
white offspring, ie. 3:1.
3
a
i Cc would represent curly-wing flies and cc would represent wild type. The curly-wing flies must be
heterozygotes because the F1 do not all show this dominant trait.
ii At 25°C the temperature allows both traits to be expressed without the effect of the environment.
iii The difference in results between 25°C and 18°C suggests that low temperatures reduce the effect of
the curly-winged allele. Therefore, some individuals with the Cc genotype will express the recessive wild
type trait or curly-wing flies will have a lower ability to survive low temperatures, therefore dying before
they mature.
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