CHAPTER 6, SECTION 1:

HUMAN INHERITANCE

OBJECTIVES Identify some patterns of inheritance in

humans. Describe the function of the sex

chromosomes. Explain the relationship between genes

and the environment.

PATTERNS OF HUMAN INHERITANCE Some human traits are controlled

by single genes with two alleles, and others by single genes with multiple alleles. Still others are controlled by many genes that act together.

SINGLE GENE WITH TWO ALLELES Several human traits are controlled by a

single gene with one dominant allele and one recessive allele.

These traits have two distinctly different phenotypes.

This type of inheritance includes the following traits: attached/free earlobes and a widow’s peak.

SINGLE GENES WITH MULTIPLE ALLELES Some human traits are controlled by a

single gene that has more than two alleles (called multiple alleles).

Multiple alleles is three or more forms of a gene that code for a single trait.

Even though a gene can have multiple alleles, a person can only carry two of those alleles because chromosomes exist in pairs. Each chromosome in a pair only carries one allele for each gene.

…CONTINUED… Human blood type is controlled by a

gene with multiple alleles:

There are four main blood types: A, B, AB, and O. Three alleles control the inheritance of blood types.

Blood type:

Combination of Alleles:

A IAIA or IAiB IBIB or IBiAB IAIBO ii

…CONTINUED… The allele for blood types A and B are

codominant. The alleles for blood type O is recessive. When two codominant alleles are

inherited, neither allele is masked, thus a person who inherits both an A allele and a B allele, has the AB blood type.

TRAITS CONTROLLED BY MANY GENES Some human traits show a large number

of phenotypes because the traits are controlled by many genes.

These genes act together as a group to produce a single trait.

At least four genes control height in humans, so there are many possible combinations of genes and alleles.

…CONTINUED… Skin, eye and hair color are other

human traits that are controlled by multiple alleles.

THE SEX CHROMOSOMES The sex chromosomes are one of the 23

pairs of chromosomes in each body cell. The sex chromosomes carry genes

that determine whether a person is male or female. They also carry genes that determine other traits.

GIRL OR BOY? The sex chromosomes are the only

chromosome pair that do not always match.

If you are a girl, your sex chromosomes match: XX.

If you are boy, your sex chromosomes do not match: XY.

SEX CHROMOSOME AND FERTILIZATION All eggs carry one X chromosome

because all females have two X chromosomes.

Each sperm has either an X or a Y chromosome (but not both) because all males have an X and a Y chromosome.

Half of a male’s sperm will have an X chromosome and half will have a Y chromosome.

HOW GENDER IS DETERMINED If a sperm with an X chromosome

fertilizes an egg, the baby will be a girl. If a sperm with a Y chromosome

fertilizes an egg, the baby will be a boy.

SEX LINKED GENES The genes for some traits are carried on

the sex chromosomes. Genes on the X and Y chromosomes are

often called sex-linked genes because their alleles are passed from parent to child on a sex chromosomes.

Sex linked traits include red-green color blindness. A person with this trait cannot distinguish between red and green.

HOW SOMEONE WITH RED GREEN COLOR BLINDNESS SEES

X VS. Y Unlike most chromosome pairs, the X

and Y chromosomes have different genes.

Most of the genes on the X chromosome are not on the Y chromosome, so an allele on an X chromosome may not have a corresponding allele on the Y chromosome.

DOMINANT AND RECESSIVE Like other genes, sex-linked genes can

have dominant and recessive alleles. In females, a dominant allele on one X

chromosome will mask a recessive allele on the other X-chromosome so the organism will not be affected in her phenotype.

…CONTINUED… In males, there is no matching allele on

the Y chromosome to mask the allele on the X chromosome, so as a result, any allele on the X chromosome (even a recessive one) will produce the trait in a male who inherits it.

Males are much more likely than females to have a sex-linked trait that is controlled by a recessive allele

INHERITANCE OF COLORBLINDNESS Colorblindness is a trait controlled by a

recessive allele on the X chromosome. Many more males than females have

red-green colorblindness because the female would have to receive recessive alleles from both mom and dad.

Males only have to receive a recessive allele from mom in order to be red-green colorblind.

THE EFFECT OF ENVIRONMENT Many of an organism’s

characteristics are determined by an interaction between genes and the environment.

Genes are not the factors that influence height.

Someone’s diet can affect a person’s height.

A diet lacking in protein, certain minerals, or certain vitamins can prevent someone from growing as tall as might be possible.