Human Inheritance
Key Concepts
1. What are some patterns of
inheritance in humans?
2. What are the functions of the sex
chromosomes?
3. What is the relationship between
genes and the environment?
Key Terms
Multiple alleles
 Sex chromosomes
 Sex-linked genes
 carrier

Patterns of Human Inheritance

Human traits are controlled by:
◦ single genes with two alleles
◦ others by single genes with multiple alleles.
◦ Still other traits are controlled by many genes
that act together
Single Gene with Two Alleles
Have 1 dominant and 1 recessive allele
 Have 2 distinctly different phenotypes
 Ex. Widow’s Peak

◦ Smile dimples – Draw a Punnett Square that
shows a cross between two heterozygotes for
smile dimples
Single Genes with Multiple Alleles
Some traits have more than 2 alleles
although a person can have only 2 of
those alleles because chromosomes exist
in pairs. Each chromosome in a pair
carries only 1 allele for each gene
 Ex. Human blood type –
3 alleles A, B, O
A and B are codominant
O is recessive

Traits Controlled by Many Genes
Produce a large variety of phenotypes
 Genes act together as a group to produce
a single trait
 Ex. Height – at least 4 genes

◦ Skin color – at least 3 genes
Question: In our face trait lab most of the
children had dark colored
skin. What was the
assumption about the
parents that caused this
result?
The Sex Chromosomes
One of the 23 pairs of chromosomes that
carries the genes that determine whether
a person is male or female
 Also carries genes that determine other
traits

Girl or Boy?
The only chromosome
pair that does not always
match
 Girl – two sex
chromosomes match
(X,X)
 Boy –don’t match, one is
an X other is a Y

◦ The Y chromosome is
smaller than the X
Sex Chromosomes and Fertilization
Egg cells all carry an X chromosome
 Sperm cells carry either an X or a Y
 When a sperm cell with an X
chromosome fertilizes an egg cell, the egg
has 2 X’s = girl
 When a sperm cell with a Y chromosome
fertilizes an egg cell, the egg has an X and
a Y = boy

Sex-linked Genes
Genes for some human traits are carried
on the sex chromosomes
 Traits controlled by sex-linked genes are
called sex-linked traits
 EX. Red-green colorblindness
 Most of the genes on the X chromosome
are not on the Y chromosome
 An allele on the X may not have a match on
the Y

Sex-Linked Genes
Sex-linked genes can have dominant and
recessive alleles
 In females a dominant allele on one x will
mask a recessive on the other X
 In males, there is no matching allele on
the Y to mask a recessive allele on the X
 Any trait on the X chromosome in males
(even a recessive trait) will produce the
trait in a male who inherits it.

Inheritance of Colorblindness
Colorblindness is a trait controlled by a
recessive allele on the X chromosome
 Many more males have colorblindness
than females
 Females can be carriers. They have one
dominant trait for normal color vision and
one recessive trait for colorblindness.
They have normal color vision

Red-Green Colorblindness
Inheritance of Colorblindness
If the mother passes on the X
chromosome to a son, he will be
colorblind
 If she passes it onto a daughter, she will
also have an X chromosome from her
father. If her father has normal color
vision, then she will inherit the dominant
allele for normal color vision from him
and she will have normal color vision

Test for Red Green Colorblindness
Inheritance of Colorblindness
Father XCY
XC
Y
XC
XCXC
XCY
XCXc
XcY
Mother
XCXc
Xc
The Effect of Environment
Many of a person’s characteristics are
determined by an interaction between
genes and the environment
 Several genes work together to determine
height
 Environment affects height – a poor diet
can prevent a person from reaching their
potential
 Environmental factors can also affect
human skills – Ex. Playing a musical
instrument – need instruction to play well
