Biology 12
Definitions• Allele - alternative form of a gene• Chromosome – chunk of DNA• DNA – the genetic information in a nucleus• Gene – section of chromosome that controls one characteristic• Dominant – the allele that is expressed in a heterozygote• Co-dominant – both alleles are expressed in a heterozygote• Recessive – the allele that is not expressed (hidden) in the heterozygote• Autosomal – carried on one of the non-sex chromosomes (autosomes)• Sex-linked – carried on one of the sex chromosomes (X or Y)• Multiple alleles – more than 2 alleles exist for 1 characteristic• Genotype – alleles present in an organism• Phenotype – alleles expressed in an organism (ie its appearance)• Heterozygous – has 2 different alleles for a characteristic• Homozygous – has 2 alleles the same for a characteristic• Hybrid – has 2 different alleles for a characteristic• Pure-breeding – has 2 alleles the same for a characteristic• Hemizygous – has only 1 allele for a characteristic eg males with sex
linked condition
Genetics problems 1Tabby is dominant to black coat
colour in cats. Predict the genotype and phenotype ratios expected from a cross between a hybrid tabby and a black cat.
Tongue rolling is dominant to non-tongue rolling in humans. Predict the genotype and phenotype ratios expected from a cross between a heterozygous tongue roller and a non tongue roller.
Short hair is dominant to long hair colour in cats. Predict the genotype and phenotype ratios expected from a cross between a hybrid short-haired cat and a long hair cat.
Astigmatism is dominant to normal vision in humans. Predict the genotype and phenotype ratios expected from a cross between a person heterozygous for astigmatism and a normally sighted person.
Genetics problems 1Tabby is dominant to black coat colour in
cats. Predict the genotype and phenotype ratios expected from a cross between a hybrid tabby and a black cat.
Astigmatism is dominant to normal vision in humans. Predict the genotype and phenotype ratios expected from a cross between a person heterozygous for astigmatism and a normally sighted person.
T = tabby t = blackCross is Tt x ttGenotypes ½ Tt, ½ ttPhenotypes ½ tabby, ½ black
Tongue rolling is dominant to non-tongue rolling in humans. Predict the genotype and phenotype ratios expected from a cross between a heterozygous tongue roller and a non tongue roller.
Short hair is dominant to long hair colour in cats. Predict the genotype and phenotype ratios expected from a cross between a hybrid short-haired cat and a long hair cat.
t t
T Tt
ttt Tt
tt
h h
H Hh hh
h Hh hh
t t
T Tt tt
T Tt tt
a a
A Aa aa
a Aa aa
T = tongue roller t = non tongue rollerCross is Tt x ttGenotypes ½ Tt, ½ ttPhenotypes ½ tongue roller, ½ non tongue roller
H = short h = longCross is Hh x hhGenotypes ½ Hh, ½ hhPhenotypes ½ short, ½ long
A = astigmatism a= normal visionCross is Aa x aaGenotypes ½ Aa, ½ aaPhenotypes ½ astigmatism, ½ normal
Genetics problems 2
Predict the genotypes and phenotypes expected from crossing a white flowered plant and a red flowered plant
Predict the genotypes and phenotypes expected from crossing a pink flowered plant and a white flowered plant
Predict the genotypes and phenotypes expected from crossing a pink flowered plant and a red flowered plant
Predict the genotypes and phenotypes expected from crossing 2 pink flowered plants
Red flower colour is co-dominant with white flower colour in snap dragons, with the heterozygote showing pink flowers
Genetics problems 2
Predict the genotypes and phenotypes expected from crossing a white flowered plant and a red flowered plant
Predict the genotypes and phenotypes expected from crossing a pink flowered plant and a red flowered plant
Red flower colour is co-dominant with white flower colour in snap dragons, with the heterozygote showing pink flowers
Predict the genotypes and phenotypes expected from crossing a pink flowered plant and a white flowered plant Predict the genotypes and phenotypes
expected from crossing 2 pink flowered plants
FRFR = red, FWFW = white, FR FW = pink
FWFW x FRFR all FWFR pink
FWFW x FW FR
FRFR x FW FR
FRFW x FR FW
FRFW
FR FW
FRFR FR
FR FRFR FRFW
FR
FW
FWFW FRFW
FW
FW
FRFW FWFW
FR FW
FRFR FRFW FR
FWFW FW FRFW
½ FRFR = red, ½ FRFW = pink
½ FRFR = red, ½ FRFW = pink 1 FRFR = red : 2 FR FW = pink : 1 FWFW = white
Genetics problems 3
What genotypes and phenotypes are expected if a haemophiliac male marries a homozygous normal female?
What genotypes and phenotypes are expected if a haemophiliac male marries a heterozygous female?
What genotypes and phenotypes are expected if a normal male marries a heterozygous female?
What is the probability of having a son with haemophilia, if a haemophiliac male marries a heterozygous female?
Haemophilia is a disease carried as a sex-linked recessive trait.
Genetics problems 3
What genotypes and phenotypes are expected if a haemophiliac male marries a homozygous normal female?
What genotypes and phenotypes are expected if a normal male marries a heterozygous female?
Haemophilia is a disease carried as a sex-linked recessive trait.
What genotypes and phenotypes are expected if a haemophiliac male marries a heterozygous female?
What is the probability of having a son with haemophilia, if a haemophiliac male marries a heterozygous female?
XHXH = normal female, XHXh = carrier female, XhXh = haemophiliac femaleXHY = normal male, XhY = haemophiliac male
XHXH x XhY XHXh x XHY
XHXh x XhYXHXh x XhY
Y
XH
Xh
XH XH XH
XHXH XHXh
XHY
XH
Xh
XHXh XHXh
XH Xh
XHXhXh XhXh
Xh
Xh XHXh XhXh
XHY XHY Y XhY
Y XHY XhY Y XHY XhY
All females carriers XHXh all males normal XHY
½ females are XHXh = carrier, ½ females are XhXh = haemophiliac ½ males are XHY = normal, ½ males are XhY = haemophiliac
Probability is 1/4
PedigreesThese are diagrams that show the inheritance of characteristics
within familiesThey allow us to determine the pattern of inheritance and predict
characteristics of future offspring Conventions include
Autosomal dominant recessive crossesPedigrees will show 2 phenotypes.Crossing a homozygous dominant with a homozygous recessive leads to
heterozygous offspring with the dominant traitCrossing 2 heterozygous individuals leads to offspring which could show
either trait
B_
bb
B_
Bb
BB
Bb Bb
bbB_
Autosomal co-dominant crossesPedigrees will show 3 phenotypes.Crossing a homozygous dominant with a homozygous recessive
leads to heterozygous offspring with the intermediate traitCrossing 2 heterozygous individuals leads to offspring which could
show any of the three phenotypes
SBSB SWSW
SBSWSBSW
SBSBSBSWSBSW
SWSW
Determining patterns of inheritance 1
• Autosomal recessive • Heterozygotes don’t show trait so it
may skip generations• If both parents have it, all offspring
will have it• Parents don’t have to have it
• Autosomal dominant• Heterozygotes do show trait so it
can’t skip generations• Even if both parents have it,
offspring don’t have to show it• At least one parent must have it
• Autosomal co-dominant• 3 phenotypes are present
Sex linked inheritance
Males and females have different chromosomesMales can only show 2 phenotypes (ie males
can not be carriers)Females can show 3 phenotypes (if codominant)
or 2 phenotypes (if dominant recessive, with a carrier)
Males and females will show different patterns of inheritance
Determining patterns of inheritance 2In sex-linkage mothers pass to
sons, fathers pass to daughters
• Sex-linked recessive • Mainly in males• Females can only have it if father
has it and mother is at least a carrier
• Affected females pass it to all their sons
• Sex-linked dominant• Fathers pass to all daughters• Mothers pass to half their sons
• Sex-linked co-dominant• Only females can show
intermediate trait
Reading pedigrees
Autosomal recessive
Autosomal dominant
Sex linked recessive
Autosomal recessive
Pedigree problem 1
a) How many males are there?b) How many females are there?c) How many males have long fur?d) How many females have short fur?e) What type of inheritance is this?f) Write the most likely genotype of each individual on the pedigree.g) Which individuals are definitely homozygous?h) Which individuals are definitely heterozygous?i) How could you confirm the genotypes of the short haired cats in the
fourth generation?j) What offspring would you expect from a cross of III1 and II6?
Pedigree problem 1
a) How many males are there? 9b) How many females are there? 9c) How many males have long fur? 4d) How many females have short fur? 3e) What type of inheritance is this? Autosomal recessivef) Write the most likely genotype of each individual on the pedigree.g) Which individuals are definitely homozygous? Shaded onesh) Which individuals are definitely heterozygous? Unshaded ones with
shaded children and/or a shaded parenti) How could you confirm the genotypes of the short haired cats in the
fourth generation? Test cross (breed to long haired cat)j) What offspring would you expect from a cross of III1 and II6? Ll x ll =
50% Ll (short hair) 50% ll (long hair)
L = short furl = long fur
LL LL
LL LL LL LL
Ll Ll LlLl
LlLl
ll ll
ll ll ll ll
ll
L l
llLll
l Ll ll
Pedigree problem 2
a. What is the pattern of inheritance is this?b. Write the genotype for each individual.c. What offspring would you expect if you mated individual
III 1 to:i) III 2?ii) III 3?iii) I 1?
Pedigree problem 2
a. What is the pattern of inheritance is this? codominanceb. Write the genotype for each individual.c. What offspring would you expect if you mated individual
III 1 to:i) III 2? x =ii) III 3? x = ½ : ½ iii) I 1? x =
CSCS = spottedCDCD = darkCDCS = sable
CSCS
CSCS
CDCSCDCS CDCS
CDCS
CDCS
CDCD
CDCD
CDCD
CDCD
CDCD CDCS
CDCDCSD
CD
CDCD
CD CS
CSCS
CDCS
CDCD
CDCD
CDCD
CDCS
CSCS CDCS
CDCSCDCD
Pedigree problem 3
a) Is this trait dominant or recessive? Give reasons to justify your answer.
b) Is this trait autosomal or sex-linked? Give reasons to justify your answer.
c) Which individuals are definitely homozygous?d) Which individuals are definitely heterozygous?e) What is the probability of individuals II2 and II8 having white
coated offspring? Show all working.
Pedigree problem 3
a) Is this trait dominant or recessive? Give reasons to justify your answer. recessive
b) Is this trait autosomal or sex-linked? Give reasons to justify your answer. autosomal
c) Which individuals are definitely homozygous? Shaded onesd) Which individuals are definitely heterozygous? Unshaded ones with
shaded children and/or a shaded parente) What is the probability of individuals II2 and II8 having white coated
offspring? Show all working. Bb x bb = ½ Bb black ½ bb white
B = blackb = white
Pedigree problem 4
a) Is this trait dominant or recessive? Give reasons to justify your answer.
b) Is this trait autosomal or sex-linked? Give reasons to justify your answer.
c) Which individuals are definitely heterozygous?d) What is the probability of individuals II1 and II7 have white
coated offspring? Show all working.
Pedigree problem 4
a) Is this trait dominant or recessive? Give reasons to justify your answer. dominant
b) Is this trait autosomal or sex-linked? Give reasons to justify your answer. autosomal
c) Which individuals are definitely heterozygous? Shaded ones with unshaded children and/or an unshaded parent
d) What is the probability of individuals II1 and II7 have white coated offspring? Show all working. rr x rr = all rr dark
r = darkR = white
Pedigree problem 5
a) Is this trait dominant or recessive? Give reasons to justify your answer.
b) Is this trait autosomal or sex-linked? Give reasons to justify your answer.
c) Which individuals are definitely homozygous?
d) Which individuals are definitely heterozygous?
e) Individuals II8 and II9 are expecting another child. What is the probability it will be normal? Show all working.
f) Individual II1 is marrying a woman from a family which is free of this disease. What is the probability of their children having this disease? Show all working.
Pedigree problem 5
a) Is this trait dominant or recessive? Give reasons to justify your answer. Recessive : d = disease, D = normal
b) Is this trait autosomal or sex-linked? Give reasons to justify your answer. Sex linked
c) Which individuals are definitely homozygous? Shaded femalesd) Which individuals are definitely heterozygous? Unshaded females with
shaded children and/or a shaded parente) Individuals II8 and II9 are expecting another child. What is the probability it
will be normal? Show all working. XDXd x XdY = 50% won’t have diseasef) Individual II1 is marrying a woman from a family which is free of this
disease. What is the probability of their children having this disease? Show all working. XDY x XdXd = all XdXd or XdY = 0% with disease
XD Xd
Xd XDXd XdXd
Y XDY XdY
Pedigree problem 6
a) Is this trait dominant or recessive? Give reasons to justify your answer.
b) Is this trait autosomal or sex-linked? Give reasons to justify your answer.
c) Which individuals are definitely homozygous?
d) Which individuals are definitely heterozygous?
e) Individuals II8 and II9 are expecting another child. What is the probability it will not have the disease? Show all working.
Pedigree problem 6
a) Is this trait dominant or recessive? Give reasons to justify your answer. Dominant : D = disease, d = normal
b) Is this trait autosomal or sex-linked? Give reasons to justify your answer. Sex linked
c) Which individuals are definitely homozygous? Unshaded femalesd) Which individuals are definitely heterozygous? Shaded females with
unshaded children and/or an unshaded parente) Individuals II8 and II9 are expecting another child. What is the
probability it will not have the disease? Show all working. XDY x XDXd = 25% chance of normal child
XD
XDXDXD
Xd
XDXd
Y XDY XdY