Patterns of inheritance

Patterns of Inheritance

What are the different ways in which a genetic condition can be inherited?

Autosomal

Sex Linked

Sex linked is carried on the sex chromosomes either X or Y

Carried on any other chromosome (chromosomes 1-22).

INHERITANCE


Autosomal

Sex Linked INHERITANCE

Autosomal

dominant

Autosomal RecessiveOne mutated

copy of the gene in each

cell is sufficient for a person to be affected



Autosomal


Autosomal

dominant

Autosomal RecessiveOne mutated

copy of the gene in each


Both copies of the gene in each cell have the

disorder. Typically are not seen in every generation of an affected family.



Autosomal


Autosomal

dominant

Autosomal Recessive

X-linked recessiv

e

X-linked Dominan

t

Y-linked

One mutated copy of the gene in each




X-linkedCaused by mutations in genes on the X chromosome. (no male-to-male transmission)

In females, a mutation in one of the two

copies of the gene is sufficient to cause the disorder. In males, a mutation in the only copy of the gene in each cell causes the

disorder



Autosomal


Autosomal

dominant

Autosomal Recessive

X-linked recessiv

e

X-linked Dominan

t







disorder

X-linked

In males (only one X chromosome), one altered

copy of the gene in each cell is sufficient to cause the

condition. In females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to

cause the disorder.

Caused by mutations in genes on the X chromosome. (no male-to-male transmission)

Y-linked



Autosomal


Autosomal

dominant

Autosomal Recessive

X-linked recessiv

e

X-linked Dominan

t







disorder

X-linked

In males (only one X chromosome), one altered

copy of the gene in each cell is sufficient to cause the

condition. In females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to

cause the disorder.

Caused by mutations in genes on the X chromosome. (no male-to-male transmission)

Y-linked disorders are rare

Y-linked

SEX LINKED DISORDERS

Why males have a higher risk to suffer from an x-linked disorder?

Males are at a much greater risk for inheriting sex-disorders because they only inherit one X, so if the X has the allele for the disorder, they will suffer

from the disorder.

Color blindness

Myopia

Night blindness

Hemophilia


Punnett squares are used to predict the outcome of sex-linked inheritance.

Recessive disorders: Xb

Dominant disorders: XB



XNXN

Knowing that hemophilia is a recessive X-linked recessive Of the following genotypes, identify the male and females between normal, carriers or affected.

XNXn XnXn XNY XnYNormal

female Carrier

female Affected

female Normal

male Affectedm

ale


Hemophilia is a recessive X-linked disorder. What is the probability of a couple having a hemophiliac child if the man does not have hemophilia and the woman is a carrier?

♀: ♂:

XNXn

XNY

XN Xn

XN

Y

XN

XN

Xn

Xn

XN

Y

XN

Y

♀ ♂

Affected

Unaffected


Color-blindness is caused by a recessive allele on the X sex chromosome. What chance of a colour-blind child in the cross between a color blind male and a carrier mother? (X-linked recessive disorder)

♀: ♂:

XNXn

XnY

XN Xn

Xn

Y

XN

XN

Xn

Xn

Xn

Y

Xn

Y

♀ ♂

Affected

Unaffected

PATTERNS OF INHERITANCE

PATTERNS OF INHERITANCE

A pedigree is a genetic family tree that shows how prevalent a trait is in a family unit from generation to generation.They are often used to track the expression of genetic conditions and disorders.

Squares represent males and circles

females.

A coloured in shape means that person has the trait in question.

A half coloured in shape means that they are carrying an allele for a recessive trait.

AUTOSOMAL DOMINANT INHERITANCE

3. An affected person has at least a 50% chance of transmitting the dominant allele to each offspring.

Autosomal means not on the sex chromosomes.

1. Every affected person should have at least one affected parent.

2. Males and females should be equally often affected.



EXAMPLESProgeria (caused by a mutation) in which the person ages very rapidly. They die before they can reproduce.



EXAMPLESHuntington’s Disease in which the central nervous system starts to break down around the age of 30.

AUTOSOMAL RECESSIVE INHERITANCE

3. Two affected parents will have affected children 100% of the time.

1. An affected person may not have affected parents. Parents would be carriers.

2. Affects both sexes equally. Can appear to skip generations.

Refers to those situations where two recessive alleles result in a trait being expressed.



EXAMPLESAlbinism is a genetic condition which is the loss of pigment in hair, skin and eyes



EXAMPLESTay Sachs is a genetic disorder which is a build up of fatty deposits in the brain, eventually proving to be fatal.

X-LINKED RECESSIVE INHERITANCE

3. Homozygous recessive females can arise only from matings in which the father is affected and the mother is affected or a carrier.

1. Affected males will transmit the allele to all daughters, but not to sons.

2. Males are affected more often than females. Ratio of 8:1.

X-LINKED RECESSIVE INHERITANCE

EXAMPLESHemophilia which is the inability of the blood to clot properly.Duchenne Muscular Dystrophy which causes progressive and degenerative muscle weakness.

X-LINKED DOMINANT INHERITANCE

3. All the daughters of an affected male will be affected but none of the sons

1. Twice as many females are affected as males.

2. Usually half the children of an affected female will be affected, regardless of sex.

X-LINKED DOMINANT INHERITANCE

EXAMPLESVitamin D resistant rickets which can lead to bone deformities, particularly in the lower limbs (bowed legs).

LET’S PRACTICE

Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases

LET’S PRACTICE

1. Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases

CA B

Sex-Linked, Recessive:• Trait is able to skip

generations• Males are

predominantly affected

Autosomal, Recessive:• Trait is able to skip

generations• No major sex-bias in

expression

Autosomal, Dominant:• Trait cannot skip

generations• No major sex-bias in

expression

LET’S PRACTICE

2. Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases

Autosomal or sex-linked?

Dominant or recessive?

DOMINANT: A and B are both affected but have produced unaffected (D&F). Therefore A and B must have been carrying recessive healthy alleles.If it were recessive, it would need to be homozygous to be expressed in A & B – and then all offspring would be homozygous recessive.

AUTOSOMAL: Male C can only pass one X chromosome. If it were carried on X, daughter H would be affected by the dominant allele

Tip: Don’t get hung up on the number of individuals with each phenotype – each reproductive event is a matter of chance. Instead focus on possible and impossible genotypes.

LET’S PRACTICE

3. A person can either have two eyebrows or one fused eyebrow called unibrow. Analysis of pedigree charts to deduce the pattern of inheritance of the eyebrow gene.

Autosomal or sex-linked?

AUTOSOMAL: Male 3 can only pass one X chromosome. It means that he is normal. If it was x-linked (XNY) he will pass a healthy dominant trait to all his daughters and therfore they will be protected. However #10 is sick.

Dominant or recessive?

RECESSIVE: non affected parents (8 and 9) have children with the trait. It means that both are carriers.

LET’S PRACTICE

4. Identify the genotypes of the following individuals using the pedigree above. (homozygous dominant, homozygous recessive, heterozygous)

Homozygous recessiveHeterozygousHomozygous recessiveHeterozygous

III-3:II-1: I-1:II-4:

Science

Patterns of inheritance