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biology topic 7 * > inheritance > Flashcards

inheritance Flashcards

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AP® Biology flashcards Biology 101 flashcards
1
Q

What is meant by the term genotype?

A

Genetic constitution of an organism

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2
Q

What is meant by the term phenotype

A

The expression of this genetic constitution (genotype)
and its interaction with the environment

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3
Q

What are alleles and how do they arise?

A

Variations of a particular gene (same locus) → arise by mutation (changes in DNA base sequence)

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4
Q

How many alleles of a gene can be found in diploid organisms?

A

● 2 as diploid organisms have 2 sets of chromosomes (chromosomes are found in homologous pairs)
○ But there may be many (more than 2) alleles of a single gene in a population

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5
Q

Describe the different types of alleles

A

Dominant allele
Always expressed (shown in the phenotype)

Recessive allele
Only expressed when 2 copies present (homozygous recessive)
/ NOT expressed when dominant allele present (heterozygous)

Codominant alleles
Both alleles expressed / contribute to phenotype (if inherited together)

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6
Q

What is meant by the terms homozygous and heterozygous?

A

Homozygous
Alleles at a specific locus (on each homologous chromosome) are the same

Heterozygous
Alleles at a specific locus (on each homologous chromosome) are different

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7
Q

What do monohybrid and dihybrid crosses show

A

● Monohybrid cross - inheritance of one phenotypic characteristic coded for by a single gene
● Dihybrid cross - inheritance of two phenotypic characteristics coded for by two different genes

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8
Q

The dominant allele for the grey gene
(G) in horses results in a coat colour
turning progressively grey.
The recessive allele (g) results in the
normal coat colour being maintained
(non-grey phenotype).
A non-grey female is crossed with a
heterozygous male.
Draw a genetic diagram to show the
expected ratio of phenotypes in the
offspring.

A

Parental phenotypes
Non-grey Grey
Parental genotypes
gg Gg
Gamete genotypes
g (and g) G and g
Genetic diagram
see notes
Offspring genotypes
Gg, gg
Offspring phenotypes Grey, non-grey
Ratio
1:1

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9
Q

In one snail species, shell colour is
controlled by a gene with 3 alleles. The
shell may be brown, pink or yellow. The
allele for brown (C^B) is dominant to the
other two alleles. The allele for pink (C^P)
is dominant to the allele for yellow (C^Y).
A cross between two pink-shelled
snails produced only pink-shelled and
yellow-shelled snails.
Use a genetic diagram to explain why.

A

Parental phenotypes
Pink-shelled Pink-shelled
Parental genotypes
C^PC^Y C^PC^Y
Gamete genotypes
C^P and C^Y
C^P and C^Y
Genetic diagram
see notes
Offspring genotypes
and phenotypes
C^PC^P, C^PC^Y - pink-shelled
C^YC^Y - yellow-shelled

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10
Q

The inheritance of the ABO blood
groups in humans is controlled by
three alleles of a single gene, I^A, I^B and
I^O. The alleles I^A and I^B are codominant,
and the allele I^O
is recessive to I^A and
recessive to I^B.
Two heterozygous parents plan to
have a child. One has blood group A
and the other has blood group B.
Use a genetic diagram to show all the
possible genotypes and the ratio of
phenotypes expected in their offspring.

A

Parental phenotypes
Blood group A
Blood group B
Parental genotypes
I^A I^O
I^B I^O
Gamete genotypes
I^A and I^O
I^B and I^O
Genetic diagram
see notes
Offspring genotypes
I^A I^B, I^A I^O, I^B I^O, I^O I^O
Offspring phenotypes
AB, A, B, O
Ratio 1:1:1:1

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11
Q

Explain the evidence from a pedigree diagram which would show that the
allele for [named phenotype] is dominant

A

● [Named phenotype] parents [n & n] have child [n] WITHOUT [named phenotype]
● So both parents [n & n] must be heterozygous / carriers of recessive allele
○ If it were recessive, all offspring would have [named phenotype]

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12
Q

Explain the evidence from a pedigree diagram which would show that the
allele for [named phenotype] is recessive

A

● Parents [n & n] WITHOUT [named phenotype] have child [n] WITH [named phenotype]
● So both parents [n & n] must be heterozygous / carriers of recessive allele

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13
Q

In fruit flies, the allele for grey body (G)
is dominant to the allele for ebony
body (g) and the allele for normal
wings (N) is dominant to the allele for
vestigial wings (n). These genes are not
linked.
Vestigial-winged flies, heterozygous
for grey body colour, were crossed with
ebony-bodied flies, heterozygous for
normal wings.
Complete the genetic diagram to show
all the possible genotypes and the
ratio of phenotypes expected in the
offspring of this cross.

A

Parental phenotypes
Grey body,vestigial wings
Ebony body,normal wings
Parental genotypes
Ggnn ggNn
Gamete genotypes
Gn and gn
gN and gn
Genetic diagram
see notes
Offspring genotypes
GgNn, Ggnn, ggNn, ggnn
Offspring phenotypes Grey, normal;
grey, vestigial;
ebony, normal;
ebony, vestigial
Ratio
1:1:1:1

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14
Q

In a species of flowering plant, the T allele
for tallness is dominant to the t allele for
dwarfness. In the same species, two
alleles C^R
(red) and C^W
(white) code for
the colour of flowers.
When homozygous red-flowered plants
were crossed with homozygous
white-flowered plants, all the offspring
had pink flowers. A dwarf, pink-flowered
plant was crossed with a heterozygous
tall, white-flowered plant.
Complete the genetic diagram to show
all the possible genotypes and the ratio
of phenotypes expected in the offspring
of this cross.

A

Parental phenotypes Dwarf, pink-flowered
Tall, white-flowered
Parental genotypes
ttC^RC^W
TtC^WC^W
Gamete genotypes
tC^R and tC^W
TC^W and tC^W
Genetic diagram
see notes
Offspring genotypes
TtC^RC^W,
ttC^RC^W,
TtC^WC^W,
ttC^WC^W
Offspring phenotypes
Tall pink, dwarf pink,
tall white, dwarf white
Ratio
1:1:1:1

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15
Q

What is a sex-linked gene?

A

A gene with a locus on a sex-chromosome (normally X)

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16
Q

Explain why males are more likely to express a recessive X-linked allele

A

This assumes males are XY and females are XX, as in humans. In some organisms, it is swapped. In these cases,
females (XY) would be more likely to express a recessive X-linked allele

● Females (XX) have 2 alleles → only express recessive allele if homozygous recessive / can be carriers
● Males (XY) have 1 allele (inherited from mother) → recessive allele always expressed

17
Q

A single gene that’s carried on the
X chromosome controls the
presence of hair on the skin of
cattle. Its dominant allele causes
hair to be present and its recessive
allele causes hairlessness.
A male and a female with hair
have two hairless female offspring.
What is the probability that their
next calf born is hairless?

A

Parental phenotypes
Female with hair
Male with hair
Parental genotypes
X^HX^h
X^HY
Gamete genotypes
X^H and X^h
X^H and Y
Genetic diagram
see notes
Offspring genotypes
X^HX^H,
X^HX^h,
X^HY,
X^hY
Offspring phenotypes Female with hair (2),
male with hair, male hairless
Probability
0.25 / 25% / ¼ / 1 in 4

18
Q

A gene on the X chromosome
controls fur colour in cats. The
allele G codes for ginger fur and
the allele B codes for black fur.
These alleles are codominant.
Heterozygous females have
patches of both so their phenotype
is described as tortoiseshell.
A tortoiseshell female was crossed
with a black male.
Use a genetic diagram to show all
the possible genotypes and the
ratio of phenotypes expected in
the offspring of this cross.

A

Parental phenotypes Tortoiseshell female
Black male
Parental genotypes
X^GX^B
X^BY
Gamete genotypes
X^G and X^B
X^B and Y
Genetic diagram
see notes
Offspring genotypes
X^GX^B,
X^BX^B,
X^GY,
X^BY
Offspring phenotypes Tortoiseshell female, black female, ginger male, black male
Ratio
1:1:1:1

19
Q

In fruit flies, a gene for body colour
has a dominant allele for grey
body (G) and a recessive allele for
black body (g). A gene for eye
colour has a dominant allele for
red eyes (R) and a recessive allele
for white eyes (r) and is located on
the X chromosome.
A heterozygous grey-bodied,
white-eyed female fly was crossed
with a black-bodied, red-eyed
male fly. Complete a genetic
diagram to show all the possible
genotypes and the ratio of
phenotypes expected in the
offspring from this cross.

A

Parental phenotypes
Grey-bodied, white-eyed female
Black-bodied, red-eyed male
Parental genotypes
GgX^rX^r
ggX^RY
Gamete genotypes
GX^r and gX^r
gX^R and gY
Genetic diagram
see notes
Offspring genotypes
GgX^RX^r,
ggX^RX^r,
GgX^rY
and ggX^rY
Offspring phenotypes Grey-bodied red-eyed female,
black-bodied red-eyed female,
grey-bodied white-eyed male,
Black-bodied white-eyed male
Ratio
1:1:1:1

20
Q

Explain the evidence from a pedigree diagram which would show that the
allele for [named phenotype] on the X-chromosome is recessive

A

● Mother [n] WITHOUT [named phenotype] has child [n] WITH [named phenotype]
● So mother [n] must be heterozygous / carrier of recessive allele

21
Q

Explain the evidence from a pedigree diagram which would suggest that
[named recessive phenotype] is caused by a gene on the X chromosome

A

Only males tend to have [named recessive phenotype].

22
Q

Explain the evidence from a pedigree diagram which would show that the
gene for [named phenotype] is not on the X chromosome

A

● [Named phenotype] father [n] has daughter [n] WITHOUT [named phenotype]
● Father [n] would pass on allele for [named phenotype] on X chromosome so
daughter [n] would have [named phenotype]
OR
● [Named phenotype] mother [n] has son [n] WITHOUT [named phenotype]
● Mother [n] would pass on allele for [named phenotype] on X chromosome so
son [n] would have [named phenotype]

23
Q

Explain how autosomal linkage affects inheritance of alleles

A

● Two genes located on same autosome (non-sex chromosome)
● So alleles on same chromosome inherited together
○ Stay together during independent segregation of homologous chromosomes during meiosis
● But crossing over between homologous chromosomes can create new combinations of alleles
○ If the genes are closer together on an autosome, they are less likely to be split by crossing over

24
Q

In fruit flies, the genes for body colour and for wing development are not
on the sex chromosomes. The allele for grey body colour, G, is dominant
to the allele for black body colour, g. The allele for long wings, L, is
dominant to the allele for short wings, l.
A cross was carried out between flies with grey bodies & long wings
(heterozygous for both genes) and flies with black bodies & short wings.
The result of this cross was 225 offspring with a grey body & long wings
and 220 with a black body & short wings. Explain these results.

A

● The two genes are linked
/ autosomal linkage
● No crossing over occurs /
genes are close together
● So only GL and gl
gametes produced / no
Gl and gL gametes
produced / no Ggll and
ggLl offspring produced

25
Q

What is epistasis?

A

Interaction of (products of) non-linked genes where one masks / suppresses the expression of the other.

26
Q

Describe when a chi-squared (X^2) test can be used

A

● When determining if observed results are significantly different from expected results (frequencies)
○ Eg. comparing the goodness of fit of observed phenotypic ratios with expected ratios
● Data is categorical (can be divided into groups eg. phenotypes)

27
Q

Suggest why in genetic crosses, the observed phenotypic ratios obtained in
the offspring are often not the same as the expected ratios

A

● Fusion / fertilisation of gametes is random
● Autosomal linkage / epistasis / sex-linkage
● Small sample size → not representative of whole population
● Some genotypes may be lethal (cause death)

28
Q

Describe how a chi-squared value can be calculated

A

sum of
(O-E)^2 / E

29
Q

Describe how a chi-squared value can be analysed

A
  1. Number of degrees of freedom = number of categories - 1 (eg. 4 phenotypes = 3 degrees of freedom)
  2. Determine critical value at p = 0.05 (5% probability) from a table
  3. If X^2 value is [greater / less] than critical value at p < 0.05
    ● Difference [is / is not] significant so [reject / accept] null hypothesis
    ● So there is [less / more] than 5% probability that difference is due to chance

biology topic 7 * (5 decks)

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