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Chapter 6: Genetics and Heredity
BLM 6.1–1
Mendel’s Experiments
Phenotypes (in his experiments)
Genotypes (in his explanations)
P1 generation – Parental cross
P1 - homozygous tall
! purebred short
Purebred tall
TT
⇓
! tt
⇓
F1 generation – all tall (hybrids)
F1 cross – tall
! homozygous short
F1 generation – all heterozygous Tt
! tall
F1 cross – Tt
⇓
! Tt
– hybrid cross
⇓
F2 generation – 3:1 ratio
F2 generation – 1:2:1 ratio
– 3/4 tall, 2/4 short
– 1/4 TT: 2/4 Tt: 1/4 tt
Mendel’s Laws
Law of Segregation – Members of a pair of alleles for a given trait are segregated (separated) when
gametes are formed.
Illustration: If a parent plant has the genotype Tt, then it will produce two types of gametes, T and t.
Each gamete contains only one of the alleles for plant height.
Law of Independent Assortment – When considering two or more pairs of characteristics at a time,
each pair of genes shows dominance and segregation independently of the other.
Illustration: If a parent plant has the genotype TtRr, then it will produce four different types of gamete
combinations in equal numbers, ¼ TR, ¼ Tr, ¼ tR, ¼ tr.
A 2 Χ 2 Punnett Square for Mendel’s Hybrid Cross
Coding – T: tall, t: short
Tt ! Tt
Tt
Tt
T 1/2
t 1/2
T 1/2
TT 1/4
Tt 1/4
t 1/2
Tt 1/4
tt 1/4
Phenotype Ratio – 3:1 (Tall:short)
Genotype Ratio – 1:2:1 (TT:Tt:tt)
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Chapter 6: Genetics and Heredity
BLM 6.1–2
Mendel’s Dihybrid Cross
Coding
Plant height
T – tall
t – dwarf
Seed type
R – smooth
r – wrinkled
P1 cross – TTRR ! ttrr
F1 genotypes – all TtRr
F1 cross – TtRr ! TtRr
Gamete combinations possible for each parent
¼ TR
¼ Tr
¼ tR
¼ tr
Punnett Square
TtRr
TtRr
TR 1/4
Tr
tR
tr
TR 1/4
Tr 1/4
tR 1/4
tr 1/4
TTRR 1/16 •
TTRr 1/16
TtRR 1/16
TtRr 1/16
•
•
•
TTRr 1/16
TTrr 1/16
TtRr 1/16
Ttrr 1/16
•
°
•
°
TtRR 1/16
TtRr 1/16
ttRR 1/16
ttRr 1/16
•
•
×
×
ttRr 1/16
ttrr 1/16
×
Δ
1/4
1/4
1/4
TtRr 1/16
Ttrr
•
1/16
°
Resulting Phenotype Ratio – 9:3:3:1
• 9/16 Tall with smooth seeds (both dominant traits)
° 3/16 Tall with wrinkled seeds (one dominant, one recessive trait)
× 3/16 Dwarf with smooth seeds (one recessive, one dominant trait)
Δ 1/16 Dwarf with wrinkled seeds (both recessive traits)
Copyright © 2002 Pearson Education Canada Inc.
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Chapter 6: Genetics and Heredity
BLM 6.2–1
Genetics Problems
Show all of your work when answering the following questions. Use Punnett squares whenever
possible.
1. In humans, the allele A for pigment formation is dominant to the allele a for inability to form
pigment; aa individuals are albino. Determine the expected genotype and phenotype ratios expected
from a cross between two individuals heterozygous for this trait.
2. In humans, tasting, T, is dominant to non-tasting, t. Determine the expected genotypic and
phenotypic ratio resulting from a cross between:
a) a homozygous taster and a non-taster
b) two heterozygous tasters
c) a heterozygous taster and a homozygous taster
3. In mice, G for grey coat is dominant to g. The gg individuals are black. Two grey mice are mated to
produce nine grey mice and two black mice. Explain these results.
4. In pea plants, tall (T) is dominant to dwarf (t) and smooth seeds (S) is dominant to wrinkled
seeds(s). Use a Punnett square to determine what fraction of the offspring produced in each of the
following crosses you would expect to be tall with wrinkled seeds.
a) TtSs ! TtSs (this is a dihybrid cross, use your ratios!)
b) TtSs ! ttss
c) TtSs ! TTSs
d) TtSs ! Ttss
e) TtSs ! ttSs
f) TTss ! ttSS
5. 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 resulting from each of the following crosses would you expect to have grey
coats and short tails?
a) Ggtt ! Ggtt
b) ggtt ! GGtt
c) GgTt ! GgTt
d) GgTt ! ggTt
6. In a certain organism, two traits are determined by two pairs of alleles, each of which shows
dominance. One trait is determined by some combination of the alleles A and a. The other trait is
determined by some combination of the alleles B and b, which are located on a different
chromosome from A and a. Construct a complete Punnett square to show the results of a cross
between two parents with genotypes Aabb (parent #1) and AaBb (parent #2). Then determine each
of the following:
a) What fraction of the offspring is expected to have the same genotype as parent #1?
b) What fraction of the offspring is expected to have the same phenotype as parent #2?
c) What fraction of the offspring is expected to be dihybrid?
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Chapter 6: Genetics and Heredity
BLM 6.3–1
Genetics After Mendel Summary Sheet
Incomplete Dominance
Incomplete dominance occurs when neither gene is completely dominant over the other. In these cases,
the heterozygous individuals usually show intermediate expression, a phenotype part way between the
phenotypes of the two different homozygous genotypes.
Illustration: flower colour in snapdragons
RR – homozygous red
rr – homozygous white
RR ! rr – F1 all Rr – pink
F1 cross - Rr ! Rr – cross involving two pink flowered plants (results below)
Rr
Rr
R 1/2
r 1/2
R 1/2
RR 1/4
Rr 1/4
r
Rr 1/4
rr
1/2
1/4
Genotype ratio – 1:2:1 (RR:Rr:rr)
Phenotype ratio – 1:2:1 (Red:Pink:White)
Co-dominance
Two alleles are expressed at the same time; neither allele dominates the expression of the other. The
human blood group system ABO shows co-dominance. It also shows multiple allelism, where there are
more than two alleles possible for a given gene. Multiple allelism results in a larger number of possible
genotypic combinations and a greater variety of phenotypes.
Illustration: co-dominance and multiple allelism in the ABO blood group system
Genotype
Phenotype
IAIA or IAi
Type A
IBIB or IBi
Type B
IAIB
Type AB
ii
Type O
Multifactorial Inheritance
Many traits in humans show a very wide range of phenotypic expression. This wide range of expression
is the result of genetic influence from genes found at many loci. When many genes interact in this way
to determine a phenotype, the trait is termed polygenic. In addition, geneticists have recognized that
these genes may be expressed differently under different external conditions. In these cases, the
environment has some contribution to the phenotype of an individual. Phenotypes that are influenced
by the environment and are polygenic tend to show continuous distribution.
Copyright © 2002 Pearson Education Canada Inc.
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Chapter 6: Genetics and Heredity
BLM 6.3–2
Name:_____________________________________________
__
Linkage Worksheet
Four different farmers plant a crop with seeds that have been obtained from the same type of cross. In
each case, the seeds were obtained from the cross AaBb ! aabb. Each farmer, however, uses a different
species or strain of plant for their crop. The phenotypes of the plants that grow from the seeds and their
ratio are recorded for each farmer’s crop. Explain each of the following phenotype ratios.
Farmer #1: Of the 10 000 plants grown, 2600 exhibit both dominant traits, 2400 exhibit one dominant
and one recessive trait, 2550 exhibit the other combination of recessive and dominant, and 2450 exhibit
both recessive traits. Use the Punnett square below to explain this farmer’s results.
AaBb
aabb
Explanation:
Farmer #2: Of the 10 000 plants grown, 5500 exhibit both dominant traits and 4500 exhibit both
recessive traits.
Explanation:
Farmer #3: Of the 10 000 plants grown, 5200 exhibit one dominant and one recessive trait, and 4800
exhibit the opposite combination of one recessive and one dominant trait.
Explanation:
Farmer #4: Of the 10 000 plants grown, 4500 exhibit both dominant traits, 4500 exhibit both recessive
traits, 500 exhibit one recessive and one dominant trait, and 500 exhibit the opposite combination of one
dominant and one recessive trait.
Explanation:
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Chapter 7: Genetics and Society
BLM 7.1–1
Name:_____________________________________________
__
X-Linkage Problems
1. Briefly explain why X-linked traits appear in males far more often than in females.
2. If Fred is the only member of his family with hemophilia, what are the chances that a newborn
sibling (brother or sister) will also be a hemophiliac? Show your explanation.
3. In humans, the gene for normal colour vision is dominant to the gene for colour-blindness. This trait
is X linked. The gene for tasting is dominant to the gene for non-tasting. A non-tasting woman who
is a carrier for colour blindness is married to a heterozygous tasting normal male. Produce a
complete Punnett square in order to determine the chance that they will produce each of the
following children.
____ a son
____ a non-tasting son
____ a non-tasting colour-blind son
____ a tasting colour-blind daughter
4. A few sex-linked traits in humans are located on the Y chromosome and are dominant. What pattern
of inheritance would you expect to see in these traits? Show your explanation.
Copyright © 2002 Pearson Education Canada Inc.
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Chapter 7: Genetics and Society
BLM 7.2–1
Pedigrees
Some of the commonly used symbols in pedigrees
Autosomal Recessive Inheritance
Both parents of an affected individual
must be heterozygous.
Affected individuals may not appear in every generation
(that is, the trait may skip generations).
Both males and females are affected
in equal numbers.
Copyright © 2002 Pearson Education Canada Inc.
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Chapter 7: Genetics and Society
BLM 7.2–1
Pedigrees (continued)
Autosomal Dominant Inheritance
Half of the children of an
affected parent are expected
to be affected.
The trait is transmitted only
by affected individuals and
does not skip generations.
Males and females are
affected in equal numbers.
X-Linked Recessive Inheritance
No father-to-son transmission (affected
father to carrier daughter
and, in turn, to half of her sons).
Predominantly males are affected;
affected females are extremely rare.
The trait skips generations.
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