Biology 2B

Pedigrees

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

SBSW SBSW

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 inheritanceMales 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 pedigreesAutosomal recessive

Autosomal dominant

Sex linked recessive

Autosomal recessive

PedigreesIdentify the most probable mode of inheritance for these:

PedigreesIdentify the most probable mode of inheritance for these:

Autosomal dominant Autosomal recessive

Sex linked recessiveAutosomal or sex linked dominant

Pedigrees 2

Identify the modes of inheritance possible for these

Pedigrees 2

Identify the modes of inheritance possible for these

Autosomal dominantAutosomal recessive

Autosomal dominantAutosomal recessiveSex linked dominant

Autosomal dominantAutosomal recessive

Autosomal dominantAutosomal recessiveSex linked recessive

Genetics

Genetics

RecessiveSex linked

RecessiveAutosomal

DominantSex linked

DominantAutosomal

Pedigrees

Pedigrees

Autosomal dominantAutosomal recessive

Sex-linked recessive Autosomal recessive

Genetics problem 1

This pedigree shows the inheritance of a rare characteristic in guinea pigsa) Identify the type of inheritance shown.

b) Identify the genotypes ofi) I2 ii) II1iii) III3 iv) III7

c) What is the probability that the next offspring of individuals II1 and II2 will have this characteristic?

d) What is the probability that the next offspring of individuals II5 and II6 will have this characteristic?

Genetics problem

This pedigree shows the inheritance of a rare characteristic in guinea pigsa) Identify the type of inheritance shown. Autosomal recessive

b) Identify the genotypes ofi) I2 nn ii) II1 Nniii) III3 N_ iv) III7 Nn

c) What is the probability that the next offspring of individuals II1 and II2 will have this characteristic? Nn x Nn ¼ NN, ½ Nn, ¼ nn, so 25%

d) What is the probability that the next offspring of individuals II5 and II6 will have this characteristic? Nn x nn ½ Nn, ½ nn, so 50%

WATP short answers 2

Explain whether this trait is sex-linked or autosomal, dominant or recessive. Give reasons in your answer.

Explain how you could determine the genotype of individual B.

What genotypes and genotypes would you expect to see from a cross between individuals A and C?

A B C

WATP short answers 2

Explain whether this trait is sex-linked or autosomal, dominant or recessive. Give reasons in your answer.

Autosomal(1)-Second generation individual IV would have the condition if it was sex linked, sex-linked conditions are always passed on from affected mother to son.(1) Recessive(1)-Second generation individuals I and II neither have condition but son does. If it were dominant, this would not be possible.(1)

Explain how you could determine the genotype of individual B.A test cross can be used to determine the unknown genotype of an organism when the trait

is determined by an autosomal gene.(1) You must cross the unknown individual with a homozygous recessive individual.(1) If the unknown individual was homozygous dominant then all of the offspring will have the dominant trait.(1) If the unknown individual was heterozygous then half of the offspring will be dominant and the other half will be recessive.(1)

What genotypes and genotypes would you expect to see from a cross between individuals A and C?

Nn x Nn ¼ NN, ½ Nn, ¼ nn; ¾ would not show it, ¼ would

A B C

a) What is the mode of inheritance for this condition? (2 marks)

b) Outline the evidence in the pedigree that provides definite support for your answer in part (a) above. (3 marks)

c) What is the chance that another child of individuals 5 and 6 will be affected? Show some working. (2 marks)

d) Despite a clear understanding of how this condition is inherited, it continues to appear in populations and families. Give a reason for its continued appearance. (1mark)

Human Biology Q43

The pedigree drawn below represents the inheritance of a rare genetic condition that affects normal liver functions. Individuals who inherit this condition experience no symptoms until they become middle-aged (40-55 years). At this point in their life, affected individuals will present with severe digestive and blood composition problems.

a) What is the mode of inheritance for this condition? (2 marks) Autosomal recessiveb) Outline the evidence in the pedigree that provides definite support for your answer

in part (a) above. (3 marks) Recessive – individuals 5 & 6 don’t have it but individual 11 doesAutosomal – individual 1 has it, but her son 4, doesn’t - individual 11 has it and her father 6 doesn’t

Human Biology Q43

The pedigree drawn below represents the inheritance of a rare genetic condition that affects normal liver functions. Individuals who inherit this condition experience no symptoms until they become middle-aged (40-55 years). At this point in their life, affected individuals will present with severe digestive and blood composition problems.

c) What is the chance that another child of individuals 5 and 6 will be affected? Show some working. (2 marks)

N n 1/4N NN Nnn Nn nn

d) Despite a clear understanding of how this condition is inherited, it continues to appear in populations and families. Give a reason for its continued appearance. (1mark)

It can skip generations as it is recessiveIt doesn’t appear until later in life – often after the affected individual has had offspring (and passed on

the gene)

Human Biology Q43The pedigree drawn below represents the inheritance of a rare genetic condition that affects normal liver functions. Individuals who inherit this condition experience no symptoms until they become middle-aged (40-55 years). At this point in their life, affected individuals will present with severe digestive and blood composition problems.

e) What is the chance that Jane is a carrier? (1 mark)

f) What is the chance that Jane will have an affected daughter? (1 mark)

g) What is the chance that Jane will have an affected child? (1 mark)

Human Biology Q43

A family with a history of haemophilia (X-linked recessive) has the following pedigree:

e) What is the chance that Jane is a carrier? (1 mark) ½ (mother is carrier)f) What is the chance that Jane will have an affected daughter? (1 mark)

0 – her husband is unaffected

g) What is the chance that Jane will have an affected child? (1 mark) If she is a carrier, she has ¼ chance of affected childShe has ½ chance of being a carrierHer chance = ¼ x ½ = 1/8

Human Biology Q43

A family with a history of haemophilia (X-linked recessive) has the following pedigree:

Normal = XH Haemophilia = Xh

XH

XH

Xh

Y

XH XH

XH

XH Xh

XhY Y

Carrier female

Normal male

a) What is the probability that individual III-4 is heterozygous? Show your working. (2 marks)

b) List all the individuals for whom genotype is uncertain. (2 marks)

c) If individual III-2 married and had children with an affected person, what is the chance that their child will have the condition? Show your working. (2 marks)

d) A second student examined this pedigree and made the statement: "There is evidence on the pedigree that this condition could also be inherited as a sex linked recessive condition". Is this statement correct or incorrect? Use evidence from the pedigree to support your answer.

(2 marks)

Human Biology Q46The pedigree shown below represents the inheritance of a genetic condition that affects normal vision. A student examined this pedigree and concluded that the condition must be inherited in an autosomal fashion. On the basis of this information, answer the questions below. In your analysis, use the letters N or n.

a) What is the probability that individual III-4 is heterozygous? Show your working. (2 marks)

100% - she doesn’t show the condition so must have N, and mother has the condition, so must have given her a n

b) List all the individuals for whom genotype is uncertain. (2 marks)II4, II5, II7, III2

Human Biology Q46The pedigree shown below represents the inheritance of a genetic condition that affects normal vision. A student examined this pedigree and concluded that the condition must be inherited in an autosomal fashion. On the basis of this information, answer the questions below. In your analysis, use the letters N or n.

N = normal, n = disease

If individual III-2 married and had children with an affected person, what is the chance that their child will have the condition? Show your working. (2 marks)

His parents are both Nn, so he has 2/3 chance of being NnIf he is Nn his children have ½ chance of being nn, as their mother is nn

d) A second student examined this pedigree and made the statement: "There is evidence on the pedigree that this condition could also be inherited as a sex linked recessive condition". Is this statement correct or incorrect? Use evidence from the pedigree to support your answer.

(2 marks)

No – II6 has it and her father doesn’t - II6 has it and her son doesn’t

Human Biology Q46The pedigree shown below represents the inheritance of a genetic condition that affects normal vision. A student examined this pedigree and concluded that the condition must be inherited in an autosomal fashion. On the basis of this information, answer the questions below. In your analysis, use the letters N or n.