Human Inheritance

Single Gene Traits• Many Human traits are controlled by a

single gene with one dominant and one recessive allele

• This yields two distinct phenotypes for the three different genotypes (TT, Tt, tt)– I.e.: Widow’s peak vs. straight hairline or tall

and short

• When two alleles are Co-dominant, they produce an intermediate phenotype that is some mix of the homozygous parents

• Also known as Incomplete Dominance

Incomplete Dominance

• Produces a phenotype that appears to be a mix between the 2 homozygous parents– It’s not a mix like paint, there are lots of

“spots” which look like one parent or the other – The distribution and size of these can give

different looking combinations in different organisms that can look like a mixture

– The Punnett Squares for incomplete dominance are different as well

• Both letters are capitals but one was an added apostrophe called prime

• For example– Cross of a red (FF) four

o’clock flowers with white (F’F’) ones

– The results are pink (FF’) flowers.

– The pink is really lots of small red and white dots intermixed.

F F

F’ FF’ FF’

F’ FF’ FF’

Multiple Alleles

• Some single gene traits have more than two alleles – Multiple Alleles, which will yield more than three phenotypes– Blood type has three alleles (A, B & O) and

thus 4 main blood types: A, B, AB, O– O is recessive to A and B but they are co-

dominant with each other, thus giving AB.– A person with A-type blood has AA or AO– B-type would be BB or BO and O-type is OO

Polygenic (Multi-Gene) Traits

• Some Human traits show a large number of phenotypes because the traits are controlled by many genes. The genes act together to produce a single trait

• Height, eye hair, skin color are examples.• This allows for numerous combinations of genes

and alleles and thus many variations in the phenotypes

• Other environmental factors, such as diet, can contribute as well, affecting height etc.

Traits and Chromosomes

• Gender (sex) is controlled by chromosomes: female = XX male = XY

• Male parent donates an X or a Y and thus determines the offspring’s sex.

• Genes on the X and Y chromosomes are called sex-linked genes and the resulting traits are also sex-linked.

• The Y chromosome does not carry alleles for these traits.

Sex-linked Traits• Since males have only one X chromosome, they

are more likely to have a sex linked trait that is controlled by a recessive allele.

• This is because they can have only one recessive allele and not have a dominant allele mask the trait.

• Red-Green colorblindness is an example.• A Carrier is someone who has one recessive

and one dominant allele. • A Carrier does not have the trait but can pass it

to her offspring• Only females can be carriers of sex-linked traits

because males only have one allele (on the X-Chromosome).

Sex Linked Recessive TraitsFather does not have trait (XHY)

XH Y

XH XHXH XHY

Xh XHXh XhY

XH Y

Xh XHXh XhY

Xh XHXh XhY

Mother is a carrierSome sons will have trait

Mother has the traitAll sons will have trait

Sex Linked Recessive TraitsFather does have trait (XhY)

Xh Y

XH XHXh XHY

Xh XhXh XhY

Xh Y

Xh XhXh XhY

Xh XhXh XhY

Mother is a carrierSome children could have it

Mother has the trait.All children will have trait

Sex Linked Recessive TraitsFather does have trait (XhY)

Xh Y

XH XHXh XHY

XH XHXh XHY

Mother does not have trait but all daughters are carriers!

Pedigree

• In studying inheritance patterns, geneticist trace traits through many generations

• To do this they use a Pedigree– A pedigree is a chart or “family tree” that

tracks which members of a family have a particular trait

• Note that inheritance patterns don’t always follow the predicted probabilities from Punnett squares!