Download Modern Genetics - Mrs. Thomas` Science Classroom

Chapter 4, Section 1
Patterns of Human Inheritance
Traits are controlled by:
• A single gene with 2 alleles.
OR
• A single gene with multiple alleles.
OR
• Many genes that act together.
Patterns of Human Inheritance
Single genes with 2 alleles:
You learned about this. This is when a trait is controlled by
two alleles that have completely different phenotypes.
For example, the gene for a widow’s peak is controlled by
2 alleles. You either have a widow’s peak or you don’t.
So, you could have any combination of W and w.
Remember this?
Patterns of Human Inheritance
Single genes with multiple alleles:
Some traits are controlled by one gene that has more
than 2 alleles.
For example, blood type is controlled by one gene with 3
alleles. Your alleles could be I A, IB, a combination of the
two, or ii.
So, there is one gene (the letter I) and three alleles (I A, IB,
or i).
Patterns of Human Inheritance
Traits that are controlled by M-A-N-Y genes
Height and skin color are traits that are controlled by
many genes.
The genes work together as a group to produce a single
trait.
The Sex Chromosomes
You have 23 pairs of chromosomes in your body.
One of those pairs is the sex chromosome that carries
genes that determine whether you are a boy or a girl.
The sex chromosomes carry genes that determine other
traits too.
The Sex Chromosomes
Girl- XX
Boy- XY
Sex chromosomes are the only chromosomes
that don’t always match.
The Sex Chromosomes
Sex Chromosomes and Fertilization
All eggs carry X chromosomes because that is all women
have (XX).
Half of a man’s sperm cells carry X chromosomes and the
other half carry Y chromosomes because men have X and
Y.
The Sex Chromosomes
Where do baby girls come from?
When an X sperm cell fertilizes an egg a baby girl will be
formed (2 X chromosomes).
Where do baby boys come from?
When a Y sperm cell fertilizes an egg a baby boy will be
formed (X chromosome from egg, Y chromosome from
sperm)
The Sex Chromosomes
Sex-Linked Genes
Genes on the X and Y chromosomes are often called sexlinked genes.
Why?
Because the alleles of the genes are passed from parent
to child on a sex chromosome.
The Sex Chromosomes
Sex-Linked Genes
Sex linked traits are traits that are controlled by sexlinked genes.
A sex-linked trait is red-green colorblindness.
The Sex Chromosomes
Sex-Linked Genes
Recall: male sex chromosomes don’t match (XY), so X and
Y chromosomes each have different genes.
That means that an allele on the X chromosome may not
have a matching allele on the Y chromosome.
The Sex Chromosomes
Sex-Linked Genes
Sex-linked genes have dominant and recessive alleles,
just like other genes.
In females, the dominant gene on one X will mask a
recessive allele on the other X.
In males, since X and Y don’t match, even if there is a
recessive allele the trait will show up.
Colorblindness
• The trait for colorblindness is controlled by a
RECESSIVE allele on the X chromosome.
• A carrier for a trait is someone who has one recessive
allele and one dominant allele for a trait, but doesn’t
have the trait.
• A carrier for colorblindness is not colorblind, she is
someone who can pass the trait on to offspring.
Colorblindness
• Many more males than females have colorblindness.
WHY???
• Because…the gene for color vision is on the X
chromosome, not on the Y chromosome, so…..
• There isn’t an allele on the Y chromosome that will
mask the colorblindness allele on the X chromosome.
The Effect of the Environment
• Many of a person’s characteristics are determined by
an interaction between genes and the environment.
• For example, a person’s height can be influenced by
diet.
• Skills can also be affected by environmental factors,
such as playing a musical instrument.