Human Heredity

Note Expectations:
◦ Cell phones and electronics are not in use.
◦ You are taking the notes.
◦ You are helping the people at your table to
answer the questions.
◦ You are prepared to answer the questions.
I can describe the chromosomes found in
a human cell.
 I can explain why a karyotype is useful.
 I can explain how the sex of a baby is
determined.
 I can give examples of sex linked traits.
 I can explain why sex linked traits are
more common in males.

How many chromosomes does a human
cell have?
 Where did your chromosomes come
from?

A
human cell has 46 chromosomes.
 A karyotype is a picture of
chromosomes lined up in pairs
from largest to smallest.
Predict how a karyotype would be useful.
 What are the sex chromosomes?
 Predict what you think the purpose of a
sex chromosome is.
 What combination of sex chromosomes
codes for a male? A female?

Karyotypes are useful for two reasons:
1. To determine the sex of a baby.
2. To determine if the baby has a
chromosomal disorder.
 Two of the 46 chromosomes in a human
cell are sex chromosomes.

 Sex
chromosomes determine the
sex of an individual.
 Females have the genotype XX.
 Males have the genotype XY.
 The X chromosome is much larger
than the Y chromosome and
carries a lot more information.
Who determines the sex of a baby, the
male or female? Why do you think so?
 Predict how you would fill in the Punnett
Square to determine the sex of a baby.
 What chromosome do you think is bigger,
the X or Y? Why?

 The
X chromosome is much larger
than the Y chromosome and carries a
lot more information.
 Males actually determine the sex of a
baby.
 Draw the Punnett Square:
 Genes
located on the X and Y
chromosomes are called sex-linked
genes.
 Most sex linked genes are found on
the X chromosome because it is
larger.
 There are more than 100 sexlinked genetic disorders found on
the X chromosome.
If males only have one X chromosome,
will dominant and recessive alleles
matter? Why or why not?
 Give an example of a sex-linked trait.

 Males
have just one X
chromosome, so all X linked
alleles are expressed.
 Color blindness is a recessive
allele that is much more common
in males than females.
 Females
will only be color blind if
they have 2 recessive alleles.
 However, a male with just one
recessive allele will be color
blind.
 Other
sex-linked traits include:
hemophilia and muscular
dystrophy
 The remaining 44 chromosomes
in a human cell are autosomes.
 Most human traits are polygenic,
meaning they are controlled by
many genes.
Are genes the only thing that determine
your phenotype? Why or why not?
 What other factors could influence your
traits?

 Many
traits are also influenced by
environmental factors like
nutrition and exercise and are
not simply genetic.
 There are also chromosomal
disorders.
When do chromosomes separate?
 Predict what would happen if
chromosomes did not separate correctly.

 Most
chromosomal disorders
occur as a result of nondisjunction
during meiosis.
 Nondisjunction means not coming
apart.
 If nondisjunction occurs, various
numbers of chromosomes may
occur in gametes, resulting in a
chromosome disorder.
 Down’s
syndrome is a result of
an extra 21st chromosome.
 There are also sex chromosome
disorders where there is an extra
or missing chromosome.
Review Questions
What is a karyotype and why is it useful?
 What are the two sex chromosomes?
What combination of sex chromosomes
results in a male and female?
 Explain why sex linked traits are more
common in males.
 What is nondisjunction? What is the
result of nondisjunction?
 Draw the Punnett Square to determine
the sex of a baby.

PEDIGREE NOTES

Note Expectations:
◦ Cell phones and electronics are not in use.
◦ You are taking the notes.
◦ You are helping the people at your table to
answer the questions.
◦ You are prepared to answer the questions.
I can explain why pedigrees are useful.
 I can draw a pedigree.
 I can determine whether a trait is
dominant or recessive based on a
pedigree.
 I can determine whether a trait is sex
linked or autosomal based on a pedigree.

What is a pedigree?
 When you draw a pedigree, how are
males represented? Females?
 How do you show that someone has a
trait?

A
pedigree is a chart of the
genetic history of a family over
several generations.
 Females in a pedigree are
represented by a circle.
 Males in a pedigree are
represented by a square.
Predict how a pedigree could be useful.
 What are the sex chromosomes?
What are the autosomal chromosomes?

 Pedigrees
can show an autosomal
or an X-linked disease.
 X-linked disorders are carried by
the X chromosome.
 Autosomal disorders are carried by
one of the autosomal
chromosomes.
Will males or females be more likely to
have an autosomal disorder? Why?
 Will males or females be more likely to
have a sex-linked disorder? Why?

 X-linked
diseases show up mostly
in males.
 This is due to the fact that males
have only one X chromosome.
 If the disorder is found in a 50:50
ratio of males to females, it is
autosomal.
Does a parent have to have a recessive
trait in order for the children to have it?
Why or why not?
 Does a parent have to have a dominant
trait in order for the children to have it?
Why or why not?

 If
the disorder is dominant, one
of the parents must have it.
 If the disorder is recessive, the
parents may not show the
disorder.
How can a child end up with a recessive
trait if neither of the parents have the
trait?
 Review: What does homozygous mean?
Give an example.
 Review: What does heterozygous mean?
Give an example.

 Recessive
disorders can be
hidden by parents if they have the
heterozygous phenotype.
How to read a pedigree:
Female with a trait
Male with a trait
Married
Children
Twins
Female carrying a trait

Male carrying a trait
Review Questions
What is a pedigree?
 Why is a pedigree useful?
 Why do sex-linked traits show up more
often in males?
 What is the ratio for an autosomal
disorder of male to female in a pedigree?
 How is it possible for a child to have a
recessive trait if neither of the parents
have it? Explain your answer.

SEX LINKED TRAITS

Note Expectations:
◦ Cell phones and electronics are not in use.
◦ You are taking the notes.
◦ You are helping the people at your table to
answer the questions.
◦ You are prepared to answer the questions.
I can draw Punnet Squares for sex linked
traits.
 I can predict the outcome of genetic
crosses involving sex linked traits.
 I can explain why sex linked traits occur
more often in males.

How many X chromosomes does a
female have? To have a recessive trait, how
many recessive alleles does a female need
on the X chromosomes?
 How many X chromosomes does a male
have? To have a recessive trait, how many
recessive alleles does a male need on the
X chromosomes?

 Females
have 2 X chromosomes,
so they must have 2 recessive
alleles to show a recessive trait.
 Males only have 1 X
chromosomes, so they will show
all recessive traits no matter
what.
 When
doing punnett squares
involving sex linked genes, the
trait is written as a superscript.
 What is a superscript?
 Predict how you would write an
allele on the X chromosome.
 Colorblindness
is a recessive trait
(cc)
◦ Normal Female:
◦ Female who is colorblind:
◦ Normal Male:
◦ Male who is colorblind:
 You
fill in the Punnett Square just
like before, but now you use the
X and Y chromosomes.

Why do you need to include the X and Y
chromosome when doing Punnett
Squares with sex linked traits?

Example: A colorblind male mates with a
female who carries the trait but has
normal vision. Show the Punnett Square
for this cross.
 Since
females have two X
chromosomes, one X
chromosome is usually shut off in
cells.
 The shut off X chromosome
forms a dense region in the
nucleus known as a Barr body.

Are calico cats normally male or female?
Why?
X
chromosome inactivation is
the reason you can have calico
cats, and they are only female.
A female who is colorblind marries a man
with normal vision. Colorblindness is a
recessive trait. What percent of their
children will be colorblind?

Hemophilia is a recessive trait that does
not allow a person’s blood to clot. A
normal man marries a woman who is
heterozygous for the trait. What percent
of their children will have hemophilia?
Review
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

Demonstrate how you would write the genotype
for a female who is colorblind. Remember that
colorblindness is found on the X chromosome
and is recessive.
What chromosomes are sex linked disorders
found on?
How many recessive alleles does a female need
to have a sex linked disorder? Why?
How many recessive alleles does a male need to
have a sex linked disorder? Why?
What is X chromosome inactivation? Give an
example.