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Modern Genetics
Human Inheritance
Patterns of Inheritance
• Some traits are controlled by a single gene with two alleles
– One dominant, one recessive with 2 distinct phenotypes
– Example: widow’s peak
• Some traits are controlled by single genes with multiple
alleles
– Three or more forms of a gene that code for a single trait
– Example: blood type
• There are 4 main types of blood- A, B, AB and O. A and B are
dominant, O is recessive.
• The allele for A is written IA , B is written IB and O is written i.
• Other traits are controlled by many genes
– At least 4 genes control height in humans. The genes act together
as a group to produce a single trait.
– Skin color is another example of a human trait controlled by many
genes
Sex Chromosomes
• Sex chromosomes carry genes that determine whether a
person is a male or a female. They also carry genes that
determine other traits.
• Girl or boy?
– Sex chromosomes are the only chromosomes that do not always
match.
– If you are a girl, they do match XX
– If you are a boy, they do not match XY
Sex Linked Genes
• Genes on the X and Y chromosome are often
called sex linked.
– Red-green color blindness
• Like other genes, sex linked genes carry
dominant and recessive alleles. Any allele on an
X chromosome, recessive or dominant, will
produce that trait in a male.
• Because males only have one X chromosome,
they are more likely than a female to have a sex
linked trait that is controlled by a recessive allele.
Colorblindness
• Colorblindness is a sex linked trait controlled by a recessive
allele on the X chromosome.
• Many more males have the trait than females.
• Carrier- a person who has one recessive allele for a trait
and one dominant.
– The carrier can pass on the trait, but does not have the trait.
Environment
• The effects of your genes are often influenced by the
environment- organisms surroundings.
• Height is a good example:
– A diet lacking protein, minerals or
certain vitamins can prevent a person
from growing as tall as might be possible
• Environmental factors can also
effect human skills, such a
playing an instrument or muscle
coordination.