Chapter 12 Section 4

Beyond Mendelian Heredity

Adapted from Holt Biology 2008

Chapter 12 Section 4: Beyond Mendelian Heredity

Key Vocabulary Terms

Polygenic Character

A character that is influenced by more

than one gene

Codominance

A phenotype that results when both

alleles for a gene are fully expressed

Linked

In genetics, describes two or more genes

that tend to be inherited together

Incomplete Dominance

An offspring has a phenotype that is

intermediate between the

traits of its two parents

Multiple Alleles

• Genes that have three or more possible alleles.

• Multiple alleles exist for hair color in mice.

• Only two alleles for a gene can be present in one individual.

Pleiotropy

One gene affects two or more traits.

Various

Many kinds of…

Carrier

Carriers have alleles for a disorder but do not show symptoms.

Carriers can pass the allele for the disorder to their offspring.

Albinism

unable to produce the pigment that gives dark color to skin, eyes, and hair.

Without this pigment, the body may appear white or pink. A recessive allele causes albinism.

Sex-Linked Gene

A sex-linked gene is located on either an X or a Y

chromosome, but most are located on the X

chromosome.

The sex chromosomes, X and Y, carry genes for many

characters other than gender.

Adapted from Holt Biology 2008

Chapter 12 Section 4:

Beyond Mendelian Heredity

Notes

Pedigree

A pedigree diagram shows how a trait or disorder is inherited through many generations.

It shows 3 things:

1. Sex-Linked Genes

Genes that occur only on an X or a Y chromosome

2. Dominant or Recessive traits

3. Hetero- or Homo- zygous traits

Sex-Linked Genes

The gene for

hemophilia and

colorblindness is

only found on the X

chromosome.

Symbols for sex-linked traits

Hemophilia is a recessive disorder

Xh X chromosome with hemophilia gene

X X chromosome with no hemophilia gene

Y Y chromosome

Symbols for sex-linked traits

• Genotypes

Xh Xh female with hemophilia

Xh X female carrier – no hemophilia

X X female - no hemophilia

Xh Y male with hemophilia

X Y male with no hemophilia

Male hemophiliac with a female carrier (X = normal female, Xh = hemophilia, Y =

normal male)

Male Hemophiliac

Female carrier

hemophilia

X Xh XY

Xh Xh Xh Y

Female Carrier Normal Male

Female hemophiliac Male hemophiliac

Xh Y

X

Xh

Xh Y

X Xh

7

Can a normal color vision male and female have a son who is color-blind? Yes!

Show the Punnett square.

Male

Female carrier

X X XY

Xc X Xc Y

Female Normal Male

Female carrier Male color-blind

X Y

X

Xc

X Y

X Xc

6

Polygenic Inheritance

– many genes control a single trait (e.g. human height or skin color)

Incomplete Dominance

Neither allele is dominant; the

heterozygote shows an intermediate phenotype

A red snapdragon crossed with a white snapdragon results in

a pink snapdragon.

Multiple Alleles

• for a trait - three or more alleles occur within a population.

ABO blood groups A and B alleles are dominant to O A and B are codominant

Blood Types