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