SEX DETERMINATION

• The sex of an individual is determined by the sex chromosomes contributed to the zygote by the sperm and the egg

SEX DETERMINATION

• An egg can donate an X
• A sperm can donate an X or Y
• Therefore the sperm determines the sex of a child

SEX-LINKED INHERITANCE

• Some traits are located on the sex chromosomes, so the inheritance of these traits depends on the sex of the parent carrying the trait.

SEX-LINKED INHERITANCE

• Most known sex-linked traits are X-linked (carried on the X chromosome). This is probably because the X chromosome is much larger than the Y chromosome.

SEX-LINKED DISORDERS

• Some sex-linked traits are associated with disorders.
• Most are found on the X chromosome, Y-linked disorders are rare.
• 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.
• Recessive lethal X-linked traits result in death.

EXAMPLES OF SEX-LINKED TRAITS and DISORDERS

• Male pattern baldness, red-green colour blindness, myopia, night blindness, hemophilia

SEX-LINKED INHERITANCE

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

Assume the trait is X-linked unless told otherwise!

Most disorders are recessive, some are dominant, the question will tell you.

A “carrier” is a female who is heterozygous for the trait.

EXAMPLE

​

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

EXAMPLE

Sex determination

• TED Ed - Sex Determination

Patterns of Inheritance

Pedigrees

• 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.

Pedigrees

• 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

• Autosomal means not on the sex chromosomes.
• Refers to those situations in which a single copy of an allele is sufficient to cause expression of a trait.

Autosomal Dominant Inheritance

• 1. Every affected person should have at least one affected parent.
• 2. Males and females should be equally often affected.
• 3. An affected person has at least a 50% chance of transmitting the dominant allele to each offspring.

Autosomal Dominant Inheritance�Examples

• Progeria (caused by a mutation) in which the person ages very rapidly. They die before they can reproduce.
• Huntington’s Disease in which the central nervous system starts to break down around the age of 30.

Autosomal Recessive Inheritance

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

Autosomal Recessive Inheritance

• 1. An affected person may not have affected parents. Parents would be carriers.
• 2. Affects both sexes equally. Can appear to skip generations.
• 3. Two affected parents will have affected children 100% of the time.

Autosomal Recessive Examples

• Albinism is a genetic condition which is the loss of pigment in hair, skin and eyes.
• Tay 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

• Refers to those situations where a recessive allele on the X chromosome can lead to a trait/condition or disorder.

X – linked Recessive Inheritance

• Males are affected more often than females. Ratio of 8:1.
• Affected males will transmit the allele to all daughters, but not to sons.
• Homozygous recessive females can arise only from matings in which the father is affected and the mother is affected or a carrier.

X – linked Recessive Disorders

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

X – Linked Dominant Inheritance

• Refers to situations where a single dominant allele on the X chromosome can lead to a trait/condition.
• Very uncommon.

X – Linked Dominant Inheritance

• 1. Twice as many females are affected as males.
• 2. Usually half the children of an affected female will be affected, regardless of sex.
• 3. All the daughters of an affected male will be affected but none of the sons.

X – Linked Dominant Example

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

PEDIGREES

• Chart showing genetic relationships between members of a family

​

• Squares represent males, circles females

​

• Colour shows infected person, ½ shaded shows carrier