Human Genetics

It’s all in the…

Inheritance• Parents and offspring often

share observable traits.

• Grandparents and grandchildren may share traits not seen in parents.

• Why do traits disappear in one generation and reappear in another?

A pedigree is a chart that shows how a trait and the genes that control it are

inherited within a family.

Genetic predictions

What is the chance that Ellen’s child has a sickle cell anemia allele (a)?

Ellen Michael

?

Ellen and Michael’s parents must be carriers.

A a

A

a

AA Aa

Aa aa

Ellen is not affected andcannot carry aa genotype

Chance child inherits sickle cell allele = 1/2

Overall chance child carries sickle cell allele from Ellen = 2/3 x 1/2 = 1/3

Chance Ellen is a carrier = 2/3

Ellen’s brother Michael hassickle cell anemia, an autosomal recessive disease.

Autosomal recessive inheritance

• Heterozygous individuals carry the recessive allele but exhibit the dominant phenotype.

• Males and females are equally affected and may transmit the trait.

• May skip generations.

Sex chromosomes determine gender

•Human males are the have two different sex chromosomes, (XY).

•Human females only have one (XX).

What determines gender?

Number of X chromosomes, or presence of Y chromosome?

X0 Female with Turner syndrome

Male with Klinefelter syndromeXXY

X-linked traits

Males• One X chromosome

• Inherited from mother

• Two possible genotypes• XNY• XnY

• Have trait or do not have trait- they can’t be a carrier!

Females• Two X chromosomes

• Inherited from both parents

• Three possible genotypes• XNXN

• XNXn

• XnXn

• Heterozygous individuals are carriers of recessive traits- they can pass it down to their offspring- especially their male children!

X-linked recessive traits

• Always expressed in males if the male inherits the recessive gene from the mother.

• Affected males: Inherited from mother

• Females need to be homozygous recessive to show the trait

• Affected females : Fathers affected or mother had the trait or carried it!

X-linked dominant inheritance

Expressed in females with one copy.

Males are often more severely affected.

Typically associated with miscarriage or lethality in males.

BLOOD TYPE

• Human blood type genes are an example of a codominant trait- meaning a trait with more than one set of alleles.

Phenotype : A B AB O

Genotype : AO, AA BO, BB AB OO