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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