Chapter 6.1 notes

1. Chapter 6: Modern Genetics
How are traits inherited in people?

2. Section 1:
Human Inheritance
What are some patterns of inheritance in
humans?
What are the functions of the sex chromosomes?
What is the relationship between genes and the
environment?
 7.2 A typical cell of any organism contains genetic
instructions that specify its traits. Those traits may be
modified by environmental influences.
 7.2.c Students know an inherited trait can be
determined by one or more genes.

3. Patterns of Human Inheritance
• Some human traits are controlled by single
genes with two alleles, one dominant and
one recessive (Mendelian genetics).
• Others by single genes with multiple alleles.
• Still many traits are controlled by many genes
that act together.

4. Single Genes With Two Alleles
• Two alleles, one
dominant, one recessive
• Two distinct phenotypes
• Phenotype= Physical
appearance

5. Single Genes With Multiple Alleles
• A person can still only carry
two alleles – one allele on
each chromosome.
• Blood type is determined by
a single gene with three
alleles
• .
• The alleles for blood types A
and B are codominant, O is
recessive.
Multiple alleles = three or more forms of a gene for a
single trait

6. Traits Controlled By Many Genes
• Show a very large variety of phenotypes.
• Genes act together as a group to control a single trait.
•Height
•Skin color
•Eye color
•Hair color

9. WHY DO SOME TRAITS EXHIBIT A
LARGE NUMBER OF PHENOTYPES?
the traits are controlled by many genes

10. The Sex Chromosomes
• Sex chromosomes = The
sex chromosomes carry
genes that determine
whether a person is male
or female.
• They also carry genes that
determine other traits.

11. Girl or Boy?
As this Punnett square shows, there
is 50% chance that a child will be a
girl and 50% chance that a child will
be a boy.
The sex chromosomes are the only
chromosomes that do not always
match.
What sex will the
offspring be if a sperm
with a Y chromosome
fertilizes an egg?

12. Sex Chromosomes and Fertilization
• All eggs carry one X
chromosome.
• Half of a male’s
sperm cells carry an X
chromosome, while
half carry a Y
chromosome.

13. Sex-Linked Genes
• The genes for some human traits are
carried on the sex chromosomes (X).
• Sex-linked genes
• Traits controlled by sex-linked genes are
called sex-linked traits.
1. Unlike most chromosomes pairs, the X and Y chromosome have different genes.
2. Most of the genes on the X chromosome are not on the Y chromosome.
3. In females, a dominant allele on the X chromosome will mask a recessive allele on
the other X chromosome.
4. But in males, there is usually no matching allele on the Y chromosome to masks
the allele on the X chromosome.
5. As a result, any allele on the X chromosome will produce the trait in a male who
inherits it.
6. So males are much more likely than females to have sex-linked traits that are
controlled by a recessive allele.

14. Inheritance of Colorblindness
• Red-green colorblindness is a
sex-linked trait.
• It is controlled by a recessive
allele on the X chromosome.
• Carrier = a person who had
one recessive allele for a trait
and one dominant allele.
• A girl who receives only one
recessive allele (written Xc) for
red-green colorblindness will
not have the trait.
• However, a boy who receives
one recessive allele will be
colorblind.
What allele combination would a
daughter need to be colorblind?

16. WHAT IS THE SEX OF A PERSON WHO IS
A CARRIER FOR COLORBLINDNESS?
Reading Checkpoint

17. The Effect of Environment
• Many of an organism’s characteristics are
determined by an interaction between genes
and the environment.

18. HOW CAN ENVIRONMENTAL FACTORS
AFFECT A PERSON’S HEIGHT?
Reading Checkpoint