Human Human InheritanceInheritance

& & PedigreePedigree

Patterns of Human Patterns of Human InheritanceInheritanceHuman 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

◦All genes are made up of the DNA sequences on your chromosomes.

A Single Gene with Two A Single Gene with Two AllelesAlleles

Have 1 dominant and 1 recessive allele

Have 2 distinctly different phenotypes

Examples:◦Widow’s Peak and straight hair line◦Smile dimples and no dimples

Hybrid vs. Purebred Hybrid vs. Purebred Hybrids=Heterozygous- Have two different alleles for a gene: Rr

Purebreds=Homozygous- Have the same allele for a gene:

RR or rr

Incomplete Incomplete DominanceDominanceWhen the offspring of two homozygous parents show an “in-between”

phenotype.

Example:

Purebred white flowers

Purebred red flowers

Makes “in-between” color pink flowers

Multiple AllelesMultiple AllelesAny trait that is controlled by many alleles.

Example: Blood Type

Single Genes with Single Genes with Multiple AllelesMultiple AllelesSome traits have more than 2 alleles even though 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

PolyPolygenic genic InheritanceInheritanceWhen many genes act to control

a trait.

Poly=many

Examples:◦ hair color◦eye color◦skin color

Traits Controlled by Many Traits Controlled by Many GenesGenesProduce a large

variety of phenotypes

Genes act together as a group to produce a single trait

Examples: ◦Height – at least 4 genes◦Skin color – at least 3 genes

The Sex ChromosomesThe Sex ChromosomesOne 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?Girl or Boy?Sperm cells carry either an X or a Y

Egg cell alwaysalways carries an X chromosome

Girl – two sex chromosomes match (X,X)

Boy –don’t match (X,Y)(The Y chromosome is smaller than the X!!)

Sex-Linked GenesSex-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.

Example: Red-green colorblindness

Inheritance of Inheritance of ColorblindnessColorblindnessColorblindness is a trait controlled by a

recessive allele on the X chromosome

More males have colorblindness than females.

Females can be carriers. If they have one dominant trait for normal color vision and one recessive trait for colorblindness. They have normal color vision.

Red-Green Red-Green ColorblindnessColorblindness

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

Inheritance of Inheritance of ColorblindnessColorblindness

XCY

XC

Xc

MotherXCXc

Father XCY

XCXC XCY

XCXc XcY

The Effect of EnvironmentMany 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

PedigreePedigree One important tool that geneticists use to trace the

inheritance of traits in humans is a pedigree.

A pedigree is a chart or “family tree” that tracks which members of a family have a particular trait.

Pedigree Pedigree ChartChart A circle represents a female

A square represents a male

A completely shaded square or circle indicates that the person has the trait.

A circle or square that is NOT shaded indicates that a person neither has the trait nor is a carrier.

A half-shaded square

indicates that a person is a

carrier.