Pedigrees and Sex Chromosomes (Lecture 7) Flashcards
How are the genders represented?
Male = square
Female = circle
Sex unknown or unspecified = diamond
What is an unaffected person and an affected person represented by?
Unaffected person = empty shape
Affected person = filled in shape
How is an obligate carrier (carries the gene but does not have the trait) and an asymptomatic carrier (unaffected at this time but may later exhibit the trait) represented?
Obligate carrier (carries the gene but does not have the trait) = shape with a dot inside
Asymptomatic carrier (unaffected at this time but may later exhibit the trait) = straight line through the shape
How are multiple persons and the deceased persons represented?
Multiple persons = shape with a number in it
Deceased person = diagonal line through the shape
How are proband ( first affected family member coming to attention to a geneticist) and family history of person unknown represented?
Proband ( first affected family member coming to attention to a geneticist) = P with an arrow towards the shape
Family history of person unknown = question mark in shape
How is one family represented? (parents and 3 children, one boy and two girls in birth order)
How is adoption represented?
Brackets enclose adopted person; dashed line denotes adoptive parents; solid line denotes the biological parent
How are twins represented?
How is consanguinity (mating between related persons) represented?
Each generation in a pedigree is identified by what?
A roman numeral
Children in each family are listed ______ to _____ in birth order
Children in each family are listed left to right in birth order
What are 5 pedigree characteristics of autosomal recessive traits?
- Usually appears in both sexes with equal frequency.
- Tends to skip generations.
- Affected offspring are usually born to unaffected parents.
- When both parents are heterozygous, approximately one-fourth of the
offspring will be affected.
- Appears more frequently among the children of consanguineous
marriages.
What are 6 pedigree characteristics of autosomal dominant traits?
- Usually appears in both sexes with equal frequency.
- Both sexes transmit the trait to their offspring.
- Does not skip generations.
- Affected offspring must have an affected parent unless they possess a
new mutation.
- When one parent is affected (heterozygous) and the other parent is
unaffected, approximately half of the offspring will be affected.
- Unaffected parents do not transmit the trait.
- From the pedigree, albinism appears to be a recessive or dominant trait?
- What are S traits?
- Is R a carrier?
- What is the probability that T will be Albino?
- From the pedigree, albinism appears to be a recessive trait
- S is therefore homozygous recessive for c
- R has siblings with albinism, so there is some risk that R is a carrier (heterozygous Cc)
- The risk of T having albinism therefore depends on two factors: the probability that R is a carrier AND the probability that R transmits the c allele to T if he is.
Among F1 offspring without albinism, 2/3 are heterozygous, or carriers,
calculate the risk that child T has albinism
Calculation for risk that child T has albinism:
The risk of T having albinism therefore depends on two factors: the probability that R is a carrier: Pr(Aa)= 2/3
and the probability that R transmits the allele to T if he is: Pr(a) = 1/2
Therefore:
Pt(aa) = Pr(Aa) x Pr(a) x Ps(a)
= 2/3 x 1/2 x 1
= 2/6
= 1/3
Recessive or dominant?
Genotypes of A,B,C,D,E,G,I,J
The probability that H is a carrier
The probability that K will be affected
A and B must both be heterozygous
since they have an affected daughter but are not themselves affected
C and D must both be heterozygous
since they have an affected daughter but are not themselves affected
E is homozygous
Assume F is homozygous dominant (rare trait marrying into family)
G is homozygous recessive
H therefore has a 2/3 probability of being a carrier and, if he is a carrier, the chance of him passing on the recessive allele is 1⁄2
I must be a heterozygote because her mother (E) is homozygous and therefore the chance of “I” passing on the recessive allele is 1⁄2
The probability that the child will have the trait is
Pk(gg) = Ph(Gg) x Ph(g) x Pi(g)
Pk (gg) = 2/3 x 1/2 x 1/2
Pk (gg) = 1/6
Human Sex Chromosomes
The tips of the arms of the X and Y chromosomes share a small region of what?
The tips of the arms of the X and Y chromosomes share a small region of homology (red).
Human sex chromosomes
Almost none of the genes in the X chromosome have what in the Y chromosome?
We are only concerned with__________ traits when it comes to sex-linkage.
Almost none of the genes in the X chromosome have counterparts in the Y chromosome.
We are only concerned with recessive traits when it comes to sex-linkage.
Why are X-linked recessive alleles only expressed in males?
X-linked recessive alleles are expressed in males because males have only one X chromosome
Why is a mutation seen in the male’s phenotype?
A mutation is seen in the male’s phenotype because Y does not carry an allele of this gene.
An X chromosome present in a male in one generation must be transmitted to…..
An X chromosome present in a male in one generation must be transmitted to a female in the next generation, and in the generation after that can be transmitted back to a male.
Separation of the Sex Chromosomes
Meiosis in a female result in only?
Meiosis in a male results in what ratio of X-bearing and Y-bearing sperm?
Random fertilization results in an expected ratio of what female and male progeny?
Meiosis in a female results in X-bearing eggs only.
Meiosis in a male results in a 1:1 ratio of X-bearing and Y- bearing sperm.
Random fertilization results in an expected ratio of 1/2 XX (female) and 1/2 XY (male) progeny.
Heterozygous Cross: female carrier and normal male
Among progeny from a heterozygous carrier female, how many of the daughters are expected to be heterozygous carriers, and how many of the sons are expected to be affected?
Among progeny from a heterozygous carrier female, half of the daughters are expected to be heterozygous carriers and half the sons are expected to be affected.
Affected male must pass mutant X chromosome to daughter.
Among progeny from a normal female with an affected male, how many the daughters are expected to be heterozygous carriers, and how many of the sons are expected to be normal?
Among progeny from a normal female with an affected male,all of the daughters are expected to be heterozygous carriers, and all of the sons are expected to be normal
X-linked recessive traits appear more frequently in?
An affected male does not pass the trait to his ____ but he can pass the allele to a ______ who is ________ and passes it to _____ who are.
X-linked recessive traits appear more frequently in?
An affected male does not pass the trait to his sons but he can pass the allele to a daughter who is affected and passes it to sons who are.
For a rare X-linked recessive trait, most affected individuals are?
The offspring of an affected ____ are usually not affected.
The sisters of an affected male can have affected _____.
The heterozygous _______ of affected males can have affected _____.
For a rare X-linked recessive trait, most affected individuals are male.
The offspring of an affected male are usually not affected.
The sisters of an affected male can have affected sons.
The heterozygous daughters of affected males can have affected sons.
1) I-1 is a carrier (has a son who is affected and the father is not affected)
2) 100% ( I-1 has two mutant chromosomes)
3) 50% chance II-3 is a carrier (one chromosome from dad who is unaffected and 1 from mom who has two mutant chromosomes)
4) 50% she will pass it on to her daughter
5) 50%
6) 50%
7) 50%
The probability that II-3 inherits the mutant allele is 1⁄2 and if she is a carrier, the probability that she passes it on to her daughter (III-1) is 1⁄2 and if this daughter is a carrier, the probability that she passes it on is 1⁄2. If they have a boy, his chance is (1⁄2 X 1⁄2 x 1⁄2)= 1/8. If they have a girl, there is a 1/8 chance that she is a carrier.
What are the chances they have a boy? 1⁄2. So the overall chance that they have a child with hemophilia is 1/16 (1/8 X 1⁄2)
