Download Human Pedigree

A Summary of Mendel's Principles
• Genes are passed from parents to their offspring.
• If two or more forms (alleles) of the gene for a single
trait exist, some forms of the gene may be dominant
and others may be recessive.
• In most sexually reproducing organisms, each adult has
two copies of each gene. These genes are segregated
from each other when gametes are formed.
• The alleles for different genes usually segregate
independently of one another.
Beyond Mendel
• Some alleles are neither dominant nor
recessive, and many traits are controlled by
multiple alleles or multiple genes.
Beyond Mendel
• Incomplete Dominance
– When one allele is not
completely dominant over
another it is called
incomplete dominance.
– In incomplete dominance,
the heterozygous
phenotype is between the
two homozygous
phenotypes.
Beyond Mendel
• Codominance
– both alleles contribute to the phenotype.
– In certain varieties of chicken, the allele for black
feathers is codominant with the allele for white
feathers.
– Heterozygous chickens are speckled with both
black and white feathers. The black and white
colors do not blend to form a new color, but
appear separately.
Beyond Mendel
• Multiple Alleles
– Genes that are controlled by more than two
alleles
– An individual can’t have more than two alleles.
However, more than two possible alleles can exist
in a population.
– A rabbit's coat color is determined by a single
gene that has at least
four different alleles.
Beyond Mendel
• Polygenic Traits
– Traits controlled by two or more genes are said to
be polygenic traits.
– Skin color in humans is a
polygenic trait controlled
by more than four
different genes.
Human Pedigree
A pedigree chart can help trace the genotypes
and phenotypes in a family.
A pedigree can be used to determine whether
people carry recessive alleles.
Pedigree Symbols
= Male
= Female
= Affected Male
= Affected Female
= Male Carrier
= Female Carrier
= Deceased Male
= Deceased Female
= Marriage
= Offspring
Using Phenotypes to Determine
Genotypes
If about the same number of males and females
have the gene then the gene is on an
autosome.
If more males than females have the phenotype,
then the gene is on the X-chromosome
Tracing Autosomal Genes
A person who does show the trait must be
homozygous recessive
People without the trait are either homozygous
dominant or heterozygous
Tracing Autosomal Genes
Two parents that are recessive can only have
children that do have the trait
Two parents that do not have the trait can have
children that have the trait or children that do
not have the trait (Homozygous or
heterozygous)
Tracing Sex-Linked Genes
A male with a sex-linked trait will pass on the
allele to all of his daughters and none of his
sons
If a female has the trait, she must be
homozygous recessive
Tracing Sex-Linked Genes
Males that have the trait have one recessive
allele and one Y chromosome
Females who are carriers will not show the trait
Practice
Sam and Sean recently got married.
They have two children (a boy and a girl).
Sean is colorblind.
Sean and Sam’s son is colorblind, but their
daughter is not colorblind.
Sean has two brothers, and neither of them are
colorblind.
Sam has one sister and one brother. Her brother
is colorblind, but her sister is not.
XcY
Sean
XcY
XCXc
Sam
XCXc
1. What is Sean’s genotype? ________________
2. What is Sam’s genotype? _____________
3. What is the genotype of the daughter? ____________________
4. What is the genotype of the son? ____________