Download Human Inheritance

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 codominant 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 XChromosome).
Sex Linked Recessive Traits
Father does not have trait (XHY)
XH
Y
XH
Y
XH
XHXH XHY
Xh
XHXh
XhY
Xh
XHXh XhY
Xh
XHXh
XhY
Mother is a carrier
Some sons will have trait
Mother has the trait
All sons will have trait
Sex Linked Recessive Traits
Father does have trait (XhY)
Xh
Y
Xh
Y
XH
XHXh XHY
Xh
XhXh
XhY
Xh
XhXh XhY
Xh
XhXh
XhY
Mother is a carrier
Some children could have it
Mother has the trait.
All children will have trait
Sex Linked Recessive Traits
Father 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!